Managing Computers with Automation

Using Interviews to Improve Software Quality

Stunning Quality

Back in the 90s I was involved with about 100 other people in a project to develop a new voice mail system – software, hardware and firmware. The hardware was 100% new design, and the software was about 70% new. Along the way we stumbled into something that improved our end quality in a way that can reasonably be described as stunning.

A few months away from our first controlled introduction (CI) or beta site – and things didn't really feel right – lots of the system was best described as squishy – that is, not really solid. They passed unit tests, and the system tests worked continually for days or weeks without failures, but then something would fail. These squishy components included key subsystems like voice record/play – vital to the system. For a voice mail system, this is having a database for critical data that usually stores the data. Not good enough.

About this time, our second level manager (2LM) declared that everyone will work 60 hours a week on site until our CI. As it turns out, my job had been to lead the device driver team (we had about 18 new UNIX device drivers – and I was the only experienced UNIX device driver writer) – and all of these were quite solid – for reasons worthy of another article. But returning to this article... As was my personal style, I had been winding down and celebrating no longer being able to break anything no matter how hard I tried by spending time in what is sometimes called Beneficial Scholastic (BS) discussions – avoiding anything that looked like work for a few days. So, when our 2LM declared that we were all to work 60 hours a week, I wondered what I could possibly do with 60 hours a week. So I asked her what we were supposed to do with those 60 hours. Her reply? Do the same as you are doing – only more so. Some of you may know me, and know I have a certain fondness for BS discussions – but I was certain that not even I could spend 60 hours a week in BS – not to count that it would be a bit on the unproductive side.

Squishy Code

I knew there was a good bit of squishy code out there – you could feel it in the air – in the hallway and water-cooler talk. If I was going to spend 60 hours a week on-site I wanted it to be for something that mattered. I wanted to get the voice play/record code fixed – now that would matter. But it wasn't my code – and deciding to start fixing or writing tests for someone else's code is definitely not the kind of thing to endear you to others. I also knew that this wasn't the only squishy code out there. So I decided to try another approach.

In parts of this company (like many) you can occasionally run into a shoot-the-messenger mentality hiding not very far down. I had an idea on how to redirect our effort in a much more profitable way – but I didn't want to get shot as the bearer of bad news. So I asked our 2LM - “If I knew how we should be spending our time most effectively, would you want to know?”. To her credit, she replied “Of course!”.

What Needs Doing?

So I went off to carry out my idea. My idea was simple – I interviewed most of the people in the organization and set the emotional stage before asking them a few questions, wrote down the answers and put them into a short (5-page) report.

For the purposes of this posting, imagine March 23 was the cut date for our furst customer. My interviews with the project engineers went something like this:

Imagine it's the morning of March 23. You come in to work. Most of the offices are dark. It's very quiet. Those people who are here, just came to print copies of their resumes. We failed. Why did we fail? Then I asked – What should we do to prevent this? Followed by the important question – Can I use your name?

Interestingly enough every single person said yes to the last question. By far the most common answers to the first question were of the form “My software caused us to fail”. The most common answers to the second question were of the form “We need to test it unreasonably – beat the excrement out of it”.

This was an interesting set of results – indicative, in my opinion of a great organization. People understood the gravity of what they were trying to do, they took responsibility for their own bugs, and they knew what to do to fix it, and very much wanted to.

I wrote up the memo, and added a graph to it – since my management (at all levels) loved graphs. Now, it isn't a graph that any mathematician would love – it was more of an illustration – but this is OK I was showing it to management, not to mathematicians. The original graph is lost to antiquity – but I've drawn an approximation to it below.

This graph represents the universal truth that “some things are better than other things” - and also the common truth that some things are good enough, but some things are not. There are basically two lines on the graph separated by a small margin. This would be the result of applying uniform effort across the project. Note that few things rise up to cross the “good enough” line. My proposal then, was to apply our efforts only in the areas that were below the “good enough” line, and fill in only the areas below the line – indicated by light blue in the illustration. Seems perfectly obvious. In fact, it is perfectly obvious. What wasn't obvious was which things weren't good enough.

Since I was still skeptical that I wouldn't be a victim of “kill the messenger”, I went back to my 2LM and asked her if she still wanted to know what we should be doing – she said “Of course”. Since I was still concerned about shoot-the-messenger, I handed her my memo, and went home to hide where I would be hard to find. By the next day, I was dying of curiosity and went to see how my memo was taken. I looked and looked but could find no managers at all. I finally got up the nerve to ask my 2LM's assistant where everyone was – she said they were all off-site discussing the “Robertson memo”. I was completely floored. This was a very surprising result to me. To my shock and the everlasting credit of my 2LM, she reorganized the entire project around the recommendations I had collected in my memo.

If you take staff off the good enough things, and put them on the things that aren't good enough, then lots of normal rules on who does what get broken down. People are by definition, going to be working on areas that aren't “theirs”. And because it's being done over the whole project, it's by definition not a condemnation of anyone in particular, and is thereby socially acceptable.

More Testing – The Right Kind in the Right Places

Not all the suggested things were testing, but most were. The project had good unit tests. It had good system tests. Many of the recommendations for new tests turned out to be automated and merciless subsystem tests – which were subsequently nicknamed Bamm-Bamm tests. Since I had done the interviews and written the memo, I claimed first choice on which area to work in. I chose to work on the voice play/record testing. I wrote a set of unreasonable, merciless automated tests, which set up the hardware in cross-connected mode to allow for monitoring, grabbed the voice APIs at a low level and exercised them randomly. Play, skip ahead, skip back, speed up, slow down, record, play a touch tone, etc – all randomly, and without any kind of rhyme, rhythm, or restraint. This setup also could use more voice channels than were visible on the real system – and it didn't have to be connected to a switch – meaning it could run without tying up so much expensive test hardware, and it could produce more load than could ever possibly exist in the real system. The first bug it found reproduced itself reliably in 5 minutes – which took at least a week to reproduce itself in the system test environment.

Once the voice subsystem could run these tests for an hour without a failure, no one could ever find any more bugs. This from a subsystem where previously it had taken weeks to reproduce a problem – even once. Like the first bug, these subsequent bugs found reproduced themselves in a few minutes. This is a huge difference. You could reproduce a problem a few times, create a fix, and try it out all in a morning – instead of a few months.

This kind of result was certainly dramatic, but there were many others performing similar work on other weak subsystems. This all sounds wonderful, but what was the actual result? Our 2LM decided to delay our CI by a week, to let us finish our testing and fixing. Our CI site was already a heavy user of our voice mail systems – and we were going to replace their current system with the new one – which would not look any different to them. So, they already had a culture of significant use of voice mail, and they would hit the system hard on the first Monday after the cutover.

And Then A Miracle Occurs...

We installed the system and migrated their data over to it, and then everyone sat around and watched – waiting to respond to that first crash – which we assumed would come by 10:30 their local time. But that crash didn't come – and it didn't come – and it still didn't come. We were all pretty shocked. We assumed that they must have had a company holiday – so we pulled the traffic logs and compared them to their previous weeks of traffic – they were doing what they had always done – and it was just working. It was fully 6 weeks later before this heavy user of voice mail had their first complaint of any kind. And it was 6 months before our first crash in the field. Given how much new hardware and software there was, this was more than a little surprising. This reliability continued on and certainly appeared to be better than any product put out by this company (one known for reliable products), but more interesting confirmation of this came years later.

For a variety of internal political reasons, there were really only two major feature releases of this product – and after that it went mostly into maintenance mode – except for recording new languages and getting certified in new countries. However, a number of years later, some of the chips on the board weren't going to be available any more. Given the political issues surrounding the project, I assumed that would be the end of it – after all, the corporate hierarchy had tried to kill it numerous times, and it was a niche product in a small part of our portfolio. As it turns out, the product was incredibly profitable – and in spite of the efforts to minimize it and kill it, had become responsible for a very significant portion of the profit of our division – and politics or no, somehow no one was willing to leave all that money on the table. Of course, because of the political issues, all the development staff had fled to places that weren't going to kill their careers.

The Results Over Time

To keep this revenue stream going, the company had to gather up some of the original staff to create an updated version of the product. We had a new VP who didn't know about all the political issues of the past, and got together the new team in a meeting to learn about this project. He asked “Who all is working on the product now?” One person raised his hand, then someone said, “Yeah, but he's so good, he only works on it half time”. Of course, everyone laughed. But it was the truth – this 100-person software/hardware/firmware project which was out there in the field making tons of money had been maintained completely by one person half-time for the last 5 years. It simply worked - all the time – almost without fail. The only known bug was due to a hardware design issue relating to where some signal paths ran. About once a year one particular DSP would crash because of crosstalk in this set of signal paths. No one would let him redesign the board to fix it – so he put in a simple work around to catch it quickly and restart the DSP. This was really the only outstanding issue of any substance in the product. Of course, the redesign also performed more cost reductions as a result of 10 years of hardware evolution, so that it became even more profitable than it had been before.

When I saw the stunned look on that VP's face when he realized that despite the laughter this wasn't a larger in-joke – I really realized the magnitude of what we had accomplished. It was written all over his face that he really didn't believe that it was possible for a popular and profitable product of this size to work so well that it almost never needed fixing – for more than five years. It isn't exactly a common occurrence – “unheard of” probably isn't too strong a term.

Questions and Reflections

A series of interesting questions come out of this – Which things that we did made the difference? Why did they make such a difference? Why were they necessary? If this was a great organization, why didn't everyone already know all these things and act accordingly? How much of the answers to the “what to do about it” were influenced by my personal assumptions?

I don't have definitive answers to these questions – but I have thought about them a lot since this happened – since I was as stunned by the outcome as anyone. Although we all recognized how good the quality seemed to be at this first release, the political implications of it all for our own personal careers took first priority – and most of us forgot about it and got caught up in surviving and starting on our next things.

There are several things you can observe about this which seem interesting – first of all, this process helped identify what really needed doing – and reminded people of what they mostly already knew – which things worked, and which ones didn't.

Secondly, because the information came from the staff, it has inherent credibility – when someone says their own software isn't too good, it has more credibility than if someone is complaining about someone else's software. These complaints came from expert sources.

Thirdly, each of the areas has information from experts on what they thought would most likely result in success.

Fourthly, the resulting plan to take all these things seriously, resulted in a radical reallocation of staff – breaking down normal social barriers like “this is my code, stay out”, and replacing them with more of a community approach – where the areas with troubles got more help. This last point is worth elaborating some more. A normal project profile looks a bit like a normal distribution – with a small amount of staff at the beginning, a large amount at the middle, with the staff tapering off at the end. However, the truth is, that at the beginning and ends of those curves, that there is a good bit of underutilized staff – that is people who don't really have enough to do. I was personally a case in point – I had finished my code, but I was still on the project – part of the staffing curve – but not being utilized. Naturally, I wasn't about to tell anyone – I had put in incredible effort and hours over a period of years to get to this point. One of the things this technique does is locate that underutilized staff and put them to work where their help is most needed – in a way that causes minimal turf issues.

To be honest, I won't say there were no turf issues – but they were minimal and quickly overcome because the entire management structure supported these changes. The person I gave my testing tool (and bug reports to) wasn't completely happy that I'd found these issues – but he understood the benefits and after a bit, was OK with all this – especially since in the process I'd given him a tool to reproduce and fix problems very quickly – in the end making his life much easier.

Where Did The Staff (Money) Come From?

One of the more interesting realizations about this technique is that it cost very little additional money over what was already being spent for the project. How can that be, when clearly a lot more testing was getting done?

The answer is pretty simple, we redirected staff from those parts of the project that were above the line to places where things were below the line. Note that these people weren't writing code in those other areas, they were writing test code, which requires a good bit less knowledge of the particular subsystem under test. The staff already knew the project, how it worked, and had a basic understanding of most or all the subsystems – so they became productive quickly.

The fact that there was unused staff was an interesting thing – and I suspect is reasonably common towards the end of a project. Most of the people on my subteam were in a similar situation to mine – they had worked hard and gotten their code done right, and were now waiting for someone to bring them more bugs. I'm sure we weren't alone – people whose subsystems were above the imaginary “good enough” line were often in this situation. Below is a graph which I believe illustrates this situation reasonably well.

This represents the basic idea that staff needs rarely match staff availability. This particular graph represents the happy situation that eventually peak staff needs are met. The key realization is that in the latter phases of the project, there comes a situation where staff available to the project actually exceeds current needs. When individuals find themselves in this situation, they rarely call attention to it, but just try and keep busy and hope no one notices. In the case of our project, we chose to redeploy this staff to create new test mechanisms and solve other problems as identified by the informal survey I took.

Trying This Out For Yourself

If one were to try and reproduce this as an experiment, here are the things which seem to me to be most relevant to such an attempt:

• Wait until the project is near enough completion, so that there is the possibility of having some underutilized staff that can be redirected, and a probability that the development staff has a good idea where the important problem areas are.

• Perform interviews asking people what they think needs to happen. The person doing these interviews needs to understand development and be seen as an honest broker – either an outside development consultant or a respected developer with a reputation for honesty and respect of their peers. The emotional content of the method I used may have helped break people out of normal ways of thinking, but the exact formulation is probably not that important. Although having someone who can provide guidance in providing solutions to problems is not ideal interview technique, but it may be desirable in terms of getting good results.

• Follow the recommendations that are gathered – trying to take them seriously. In my experience, reorganizing the project to move staff around will likely be necessary.

Needless to say, if you decide to repeat this experiment, I'd be interested in hearing about it.

High availability apps in the IBM Cloud

Dima Rekesh and I wrote an HA-in-the-cloud article that has recently appeared on IBM's DeveloperWorks.  Article abstract is below

The new features of the IBM® Cloud enable application developers and architects to eliminate single points of failures in applications. This article provides a detailed guide on those features. It includes a discussion of the approach the IBM Cloud takes (added support for virtual IP addresses); how to prepare your cloud instances to take advantage of this feature; how to set up a highly available website; and how to test that site.

Read more...

A network discovery proposal for highly scaleable monitoring with O(1) overhead

In my previous article on scalable monitoring, I described an interesting and highly scalable low level infrastructure for monitoring servers and services.  Although that proposed architecture would work without network topology information, it worked best with a certain minimal amount of network topology information - which machines were connected to which switches, and what subnets these various servers are connected to.

This article proposes a high-level design for collecting and managing this network topology information.

Basic information Outline

The previous article described a three-level ring structure with each ring representing a different level in the network hierarchy.  Although other topologies are possible, and some may even be especially interesting, this article will expand on the three-level ring topology.  These three levels were (from top to bottom):

• Collections of subnets
• Collections of switches providing a given subnet
• Collections of hosts attached to switches

Towards this end, there are a few types of data items that need to be collected.  These include:

• A (hostid, switchid) tuple.  Depending on the protocol (CDP, LLDP) a given switch might have different types of switchids.  One might be the fully qualified domain name of the switch, and another might identify itself by the base MAC address of the switch.
• A set of one or more (switchid, subnet-description) pairs collected per-host.  For IPv4, the subnet description can be a standard CDL address/mask format.  I don't know what the right terminology is for a corresponding IPv6 physical network segment.

This information is sufficient for creating a basic network topology map from which one can create the 3-level ring structure as described in my previous blog post.

Some Considerations

For both LLDP and CDP, the client cannot solicit an announcement - you just have to wait for it.  The default interval for CDP announcements is one per minute.  This creates some complications in handling newly booted hosts since there is no way to tell when, if ever, an LLDP or CDP packet will arrive.  In addition, since one can move switch ports around easily, the affiliation of a given host with a given switch can change dynamically.

This implies that a general design for managing network topology must be prepared for a host to have no switch identification, for it to be supplied later, and for it to change.

Minion Design Outline

As you will recall from my previous article, I use the term minion to refer to a server which sends and receives heartbeats and reports failures.  For a minion, the job is relatively simple.  Here are a few elements of what it has to do beyond the tasks identified in the previous article:

• On initialization, send a startup packet containing network segment information for the host.
• Forward LLDP or CDP packets to the overlords when it first hears one and when something significant changes in the packets it hears.  Significant is meant to include changing of the identity of the switch, what port the host is connected to, and perhaps a small number of other conditions.  Minions only have to understand a small number of fields in CDP or LLDP packets

Overlord Topology Design Outline

Overlord is a term I used in my first article on this topic to refer to a host that is performing a management function.  There may be many overlord machines performing a variety of functions, and they may be in a cluster or other method of enhancing reliability.  For the purposes of this article, I will describe a single node implementation of the topology management function.

For managing topology, an overlord has to perform the following functions:

• Receive announcements from minions announcing their startup.  When this happens, the overlord should initially add it to its subnet ring as described below.
• Receive update CDP/LLDP packets from hosts and depending on how things have changed, one or more of the following actions can be taken:
  • Move it from the subnet ring into the ring for its switch.
  • Move it from one intra-switch ring to another
  • Create new higher level ring(s) for its switch and/or subnet.
  • Assign it a position in a higher-level ring.
  • Take away its position in a higher-level ring.
  • Give its position in higher level ring(s) to other nodes.
  • Remove subnets and/or switches from the active configuration.
  • Update the active configuration database with the forwarded CDP or LLDP packet.
• Receive "apparent death of host" notices from minions announcing that another minion has apparently died.  It will need to take one or more of the following actions when this happens.
  • Put a message in syslog for the death of this host
  • Send a detailed message to a higher-level management entity.
  • Close the intra-switch ring this host was a member of (if any) by sending the dead machine's neighbors directives telling them to send heartbeats to each other instead of the dead machine.
  • If the ring had two nodes before, move the surving node to its subnet ring.
  • Assign its role in higher level rings to another eligible host.
  • Remove the host information from the active configuration.
  • Remove a subnet or switch from the active configuration.

A few special cases need to be accounted for:

• A two-node ring - each only needs to send the other a single packet per interval - not two.
• A configuration with only one minion.

Although there are a number of cases to be taken care of, the details resemble the well-known algorithms for inserting and deleting from doubly-linked circular lists.  Of course, this case is more complex because things might fail in the middle of these various rearrangements.  What comes to mind are finite state machines to handle timeout and other failure conditions appropriately.

No doubt there are things I've overlooked, but this should be enough to demonstrate feasiblity and approximate level of difficulty.

Really Big Clusters: A Scalable membership proposal

This blog entry is a bit different than previous entries - I'm proposing some enhanced capabilities to go with the LRM and friends from the Linux-HA project.  I will update this entry on an ongoing basis to match my current thinking about this  proposal.

This post outlines a proposed server liveness ("membership") design which is intended to scale up from a dozen or so servers to tens of thousands of servers to be managed as an entity.

Scalability depends on a lot of factors - processor overhead, network bandwidth, and network load.  A highly scalable system will take all of these factors into account.  From the perspective of the server software author (for example, me), one of the easiest to overlook is network load.  Network load depends on a number of factors - number of packets, size of the packets, how many switches or routers it has to go through, and how many endpoints will receive the packet.  To best accomplish this task, it is desirable that the majority of "normal" traffic be network topology aware.  To scale up to very large collections of computers, it is also necessary that as much as possible be monitored as locally as possible.  In addition, since switching gear is not optimized for multicast packets, and multicast packets consume significant resources when compared to unicast packets, it is desirable to avoid using multicast packets during normal operation.

The Basic Concept - network aware liveness

Although the LRM in Linux-HA is not network-enabled, it tries to minimize monitoring overhead by distributing the task of monitoring resources to the machine providing the resource, and only reporting failures upstream.

To extend this idea of local monitoring into system liveness monitoring, one might imagine a standard 48-port network switch with 48 servers attached to it.  If one were to choose a server to act as proxy for monitoring the servers on that switch, then the other 47 nodes on the switch would send that unicast heartbeats to that node.  That node in turn would report on failure-to-receive-heartbeat events from the other 47 nodes on the switch.

Although this is simple to understand, it isn't ideal either - the work of detecting failures within a switch is  concentrated in a single node - and what happens when a switch fails.  So, let's try a different idea...

What if instead of having a hierarchy, the nodes within a switch arranged in a sort of doubly linked circular list of nodes in a switch - where each node sent heartbeats to the node "ahead" of or "behind" it on the list - making a ring-like structure.    This would spread the work for detecting failures across to more machines. For the 48-port switch, then each would send a heartbeat to the node with the next lower port number, and to the node with the next higher port number.  The first port would send heartbeats to port 2 and port 48, and port 48 would send heartbeats to ports 47 and 1. Since each node is monitored by two other nodes, monitoring has no single point of failure.   This design  is different from normal communication rings because there is no token being passed around the whole ring - just heartbeats going to a single node ahead and a single node behind.  So, conventional thoughts about latency in rings don't apply here.

If one were to implement this in the context of a cloud-like infrastructure, or a monitoring environment it is likely to be desirable to also run the LRM (or something like it) on these same machines so one can perform more detailed monitoring and/or effect changes to the managed servers.

Some Tentative Terminology

In order to facilitate discussion, I'll use the following terms (which are, of course, subject to change):

• Overlord - the policy-aware management entity at the top of the food chain.  The Overlord layer might be a cloud management layer, it might be a System management package like IBM director or Nagios, or a high-performance scientific cluster management entity, or some other entity interested in managing large numbers of computers.  The term overlord is used to refer to a management capability - not necessarily implemented in a single computer.
• Minion - a monitoring node which is listening to heartbeats from other nodes and reporting failure-to-receive-heartbeat events to an Overlord.  The minions at the lowest level in the system are only part-time minions - with the rest of their resources being spent on performing other tasks.  Dedicated minion servers should be capable of monitoring thousands (or tens of thousands) of other minions.  It is anticipated that the minion function would be performed by some software which also acts as a network proxy for an LRM client.  Minions send heartbeats, listen for heartbeats, report problems up the line, and act as LRM proxies.

Datagram Protocols

I anticipate the need for two different datagram transmission protocols, UDP (unreliable datagram protocol) and RDS (reliable datagram service).  UDP would be used for heartbeats, and RDS for communication between the Overlords and the LRM proxies (regardless of role).  It is expected that digital signatures are sufficient for heartbeats, but encryption may be desirable for some Overlord communication.

For this application, it is anticipated that the reliable datagram communication would not be high bandwidth, or likely need congestion control.  Other options in place of RDS would be an application protocol over UDP, or something like Qpid (AMQP).  I suspect any of these would work nicely.

LRM Proxy Design Goals

Here are a few design goals which are good to keep in mind during detailed design and implementation:

• Continue the policy-free approach that the LRM implements.  Although this implementation is intended to be topology aware, the LRM proxy code should rely on directions from is Overlord to tell it where and how often to send heartbeats, which heartbeats to expect (and how often), and other configuration information.  The Overlord is be the entity that "understands" the overall heartbeat topology.  Nevertheless, it is possible that the LRM proxy may gather some low-level network configuration for its Overlord.
• Assume a hostile network environment, and design communications accordingly.
• Minimize resource consumption - CPU, memory, network bandwidth should be held to a minimum.  Although this component is inherently lightweight, it will exist on every server in the collection - so resource waste is multiplied by thousands or tens of thousands of instances.
• Minimize latency.  Even if there is a hierarchy of watchers watching for failures doesn't mean that reporting problems should go back through the hierarchy.
• Minimize membership workload on non-dedicated minions.
• Try to design the API so that it is flexible enough and stable enough that version mismatches are unlikely to cause problems.
• Keep it simple.  The LRM proxy is a simple concept, the detailed design and implementation should be simple as well.

LRM Proxy Design Outline

The LRM proxy would implement the following capabilities:

• Send multicast (or broadcast) packets on boot-up to announce our presence
• Receiving requests from authenticated Overlords.  Requests currently anticpated include:
  • Requests to forward an encapsulated request to the LRM
  • Requests to send heartbeats to a certain address at a certain rate
  • Requests to stop sending heartbeats to a certain address
  • Requests to expect heartbeats from a certain address at a certain rate
  • Requests to stop expecting heartbeats from a certain address
  • Requests to add an address of an Overlord
  • Requests to delete an address of an Overlord
  • Requests to register a new Overlord authentication key
  • Requests to drop an Overlord authentication key
  • Requests to change the outgoing LRM proxy authentication key
  • Requests to stop sending boot-up multicasts
• Sending failure-to-receive-heartbeat messages to Overlords
• Sending LRM return result messages back to the registered Overlords

As implied above, at startup time, each LRM proxy would send out a multicast or broadcast packet announcing it was now up and ready to accept requests.  It will continue to send them until it receives an authenticated message from an overlord telling it to stop.  From then on the normal proxy API would be employed.

Some Thoughts about Heartbeating Topologies

This design could also be extended to create multiple levels of bidirectional rings between all the switches on a subnet, and between all the subnets.   The point is not that this is an optimal topology, but that the design of the LRM proxy component is topology-independent - it should support any unicast-based topology.

Below is an illustration of what a multi-level ring topology might look like.

For this smallish network, there are 54 servers.  12 of them send and receive heartbeats from 4 peers.  The remaining 42 have two peers - averaging 2.4 connections per server.  In a larger network, this number would be closer to 2.

If one wanted a higher degree of redundancy, one could imagine two shifted rings - with each node talking to node n-2, n-1, n+1 and n+2.  This would increase the number of packets sent and receive per interval to 4.  This could be used at all levels, or at selectively at higher levels.

There are no doubt other interesting topologies one could imagine as well.

 Advantages to this LRM proxy design

Here are a few advantages to this design:

• The LRM proxy design is simple and straightforward.
• Once a few releases have gone out, it is likely that the LRM proxy code and API will be stable for a long time.  This is a significant advantage for a component which is deployed on tens of thousands of computers.  It also simplifies upgrades, since it would be a very rare occurrence that an LRM proxy update would have to be performed in order to update the Overlord software.
• The LRM code will not have to change in order to fine tune the network resource usage.  Since where to send packets, and who to send them to and how often to send them are commands that come from its Overlord, the LRM proxy neither knows nor cares what the topology is.
• Although this was designed with network topology-awareness as a goal, it is not necessary that an Overlord implement topology-awareness, either initially, correctly, or at all.  The LRM proxy concept will function correctly without topology awareness, it will simply consume more network resources than necessary.

Potential problems with this design

• Providing switch topology information to the Overlord(s) is a potential difficulty.  There are some protocols implemented by some switches which provide information similar to what's required.  Further investigation is required to determine how difficult discovery and auto-configuration of network topology will be. The Link Layer Discovery Protocol (LLDP) may be of help here.
  
• The scope of the problem being addressed is very large.  The addition of layers of monitoring will likely introduce difficulties in debugging problems. Infrastructure for determining the origin of anomolous behavior should be provided.
• The flexibility in setting up monitoring topologies adds complexity to the system (even though it keeps it out of the LRM proxy).
• There is the possibility that the Overlord will receive conflicting reports regarding the liveness of a particular node.  This potential lack of consensus and timing issues will add complexity to the corner cases in the Overlord.

Open Issues

• There are many possible topologies for handling the upper layer Minion workload distribution.  It is not yet clear what the set of good choices are, and what the tradeoffs are between those different topologies.
• It is possible that it will prove desirable for this same infrastructure to collect statistical information for the Overlords.  Conversations with experts in high-performance computing and examination of tools like Ganglia will likely prove helpful in better understanding this problem.  What I have in mind is some amount of periodic piggybacking of data on the heartbeats, and then aggregation by nodes in minion roles, and periodic uplinks of this data to the overlords.  The goal would be to reduce the number of uplinks packets while having minimal impact on the timeliness of the data.  Not sure if this is a good idea or not.
• What about IPv6?

Concluding Thoughts

This proposal only provides liveness information and distributed control for a large collection of managed servers.  In many respects, this is perhaps the easiest component to scale up.  It is a long way from a complete cloud infrastructure, clustering software, or enterprise-scope system management package.  Scaling the Overlord components above this layer to the same standard of size is a very interesting and challenging task.  I suspect that this design is sufficiently scalable that it is likely that other components in the system architecture are likely to be the limiting factors in system scaling.

An Update

I have now posted a follow-on article outlining some ideas for the "overlord" portion of the infrastructure.

Methods for relocating network connectivity

Methods for redirecting network connectivity

When a client talks to a service over a network, and the server providing the service fails, or the service needs to be moved for administrative reasons, what methods are available for redirecting network references to that service refer to a different server?

This post outlines the methods that I know of for doing this.  But note the word redirect - that word is the key to all these methods.  These different methods are ways of redirecting various layers in the networking stack.  So let's first look at what happens for a normal IPv4 network connection over ethernet, and what all layers are involved, and what all places there are to redirect (or reroute) the network traffic to another server.

For simplicity's sake, we will ignore load balancers - both in hardware form, and DNS-level load balancers, and we assume a modern "switched" network.

Information Routing

What happens when a client establishes a connection to a service on a server?

Here is a brief synopsis of how connections get established.

1. The client is given or obtains from configuration information, bookmarks, etc. a name of the server running that service.
2. The client consults a DNS server to translate the server/service name into a 32-bit IPv4 address.
3. The client holds onto this name/IPv4 mapping in order to optimize future references to the server name.  DNS lookup libraries normally do this themselves, but some applications also perform their own address caching.
4. The client then examines the IPv4 address, and determines which interface and gateway to send it out on on the basis of its local configuration and the IPv4 address itself.
5. The client OS then sends out an ARP packet to determine the 48-bit Media Access Control (MAC) address of the gateway, or the server itself, if the client and server are on the same subnet.  It may have this MAC/IP correspondence cached from earlier packets it had received from the server.
6. The client OS sends out the packet to the MAC address determined in step 5 over the interface selected in step 4.
7. At some earlier time, the switch network will have "learned" which switch port the corresponds to the selected MAC address.  It does this by observing which port sends packets for that given MAC address.
8. The switch network then routes the packet to the chosen MAC address on the subnet (this could either be the MAC address of the gateway or the server - as discussed earlier).
9. If the server is on the same subnet as the client, it receives the packet and examines the packet to see if the destination IP address is one it provides.  If it does, then all is well.  If it does not, then the packet is dropped.    So ends the "same subnet" case.
10. Assuming the server is not on the same subnet as the client...
11. The gateway receives the packet, and examines its routing table to decide where to route the packet to.  This is determined by the routing protocol the gateway is running - for example, OSPF or BGP.
12. The "network cloud" routes the packet to the "final gateway" on the same subnet as the destination server.  As before, this is determined by the various routing protocol(s) along the way from the first gateway to the last one.  (This explanation is similar to the "then a miracle occurs" in the middle of a math proof).
13. The final gateway sends out an ARP packet to determine the MAC address of the destination server.  It is typically cached for a few minutes up to an hour.
14. The final gateway then sends the packet out to the MAC address above over the selected interface based on the routing protocol it is running.
15. The destination server receives the packet and examines the packet to see if the destination IP address is one it provides.  If it does, then all is well.  If it does not, then the packet is dropped.

There are several address transformations that transform from one conceptual address space into another lower level address space.  These are:

• Translation from "conceptual knowledge" of the server to the DNS name of the server.
• Translation from the DNS server name to the destination IPv4 address
• Translation of the destination IPv4 address to the destination gateway using routing information.
• Translation from the destination IPv4 address to the destination MAC address.
• Translation from MAC address to destination switch port.

Each of these transformations is a place where a redirection can occur.

• The conceptual knowledge layer can be redirected by telling all clients to switch to a new server name.
• The DNS layer can be redirected by updating DNS entries.
• The network routing layer can be redirected by updating routing information in the network and pushing out the new route information.
• The IPv4->MAC layer can be redirected by updating the ARP information and forcing the various ARP caches to be updated.
• The MAC->switch port can be redirected by updating MAC addresses and forcing the switch network to learn the new MAC->switch correspondence.

Subsequent sections present detailed explanations of how to perform these various kinds of redirections.

Conceptual Knowledge Layer

There is no universal automated to update the conceptual knowledge layer - nevertheless, server relocations are sometimes handled at this layer.  One can use automated client update tools to update client configuration files, one can use word-of-mouth, email, or any number of ad hoc tools.  This is the least commonly used method for redirection on failure.  Arguably, since it is hard to automate, it doesn't have much place on a blog on managing with automation.

DNS layer

Updating the DNS layer can be easily automated.  The advantages are - it's universal, and little or no prior preparation has to occur, and no server/network political boundaries have to be dealt with, and the two servers don't have to be on the same subnet.  The disadvantages are - not all clients use DNS addresses, Client OS DNS caching can interfere, Client software itself can interfere by caching the address outside DNS.  Even with Dynamic DNS, it can take minutes to hours for changes to propagate and the new server address become known (and usable) to all clients.  If the client application caches the address itself, then client applications have to be restarted.  This last subcase can be difficult to automate.

Network routing layer

If a server fails, routing can be used to redirect the traffic for the failed server to another server on a different physical access segment.  The advantages of this are - the two servers don't have to be on the same network segment, routing protocols are designed to deal with this kind of situation.  The disadvantages include - if the IP address is public, then  you have to move over at least 256 addresses at a time, there are often political boundaries making it hard for servers to automatically update network routing information, the additional routes for handling a large number of such movable addresses may slow down the routers involved.

ARP layer

When a server fails, another server can bring up the IP address of the dead service, update the ARP cache (typically using gratuitous ARPs - sometimes called ARP spoofing) and packets destined for the now-dead server go to the live one.  The advantages include:  IP address takeover can occur in less than a second, there is well-tested software for doing this, most organizations have a good method for allocating and managing additional IP addresses.  The disadvantages include:  The two servers have to be on the same network segment, some organizations lock down their network gear to make this "impossible" (which it doesn't - it just slows it down), and it typically increases the number of IP addresses needed by the servers and services.

MAC->switch port layer

MAC address takeover is a technique where a given network card is given multiple MAC addresses - one for an administrative address, and one for each group of independently-failable services.  Retraining the switches to understand which switch port services the given MAC address is accomplished by simply sending any IPv4 packet with the new MAC address.  The advantages include:  Takeover can be very fast and quite reliable.  The disadvantages include: the two servers have to be on the same network segment, some organizations lock down their network gear to make this "impossible" (which it doesn't - it just slows it down), and it typically increases the number of MAC addresses needed by the servers and services, organizations almost never have methods for allocating and managing MAC addresses like they do IP addresses.

Which Method is "Best"?

I have heard it said that when you ask an engineer a question the answer is always the same regardless of the question - "It Depends".  So it is here...

• For servers on the same VLAN (or network segment) - IP address takeover (IP address spoofing) is the most common.
• For servers on different VLANs or network segments:
• • Network route updating - if politically and technically feasible
  • DNS updating
  • Updating the conceptual knowledge layer

Note that there are some circumstances in which there is no easy answer.  You may have to change your network configuration, solve political problems, buy an external netblock, or execute various other combinations of uncomfortable or difficult steps.

Using virtualization to provide "HA at wholesale"

Traditionally, the way people have implemented high availability is by using a high-availability management package like Linux-HA[1], then configure it in detail for each application, file system mount, IP address and so on.  This traditional method works quite well, but can be a bit labor intensive - particularly when using custom or uncommon applications.  You may have to understand the structure of your applications, write some resource agents[2], debug them, and test them in detail.  In addition, every time you change your mount structure, or other details you've told your HA system, you have to be sure and update your HA configuration to match - or it might not fail over correctly the next time.

When you have good resource agents, your HA system will also recover from application failures - by restarting applications that have failed.  This is a good thing.  On the other hand, this is enough work that virtually no one runs all their applications in an HA configuration.  It's just too much work for most applications.  I call this traditional boutique-like method "HA at retail".  It works well, but it is a little costly to set up and maintain all the details just so.

With virtualization, another approach is possible, and (big surprise), I call it "HA at wholesale".  In this paradigm, instead of needing to write scripts for each type of application, you just have one resource agent - one for managing a virtual machine.  You also don't need to know the structure of the applications - the OS still starts them in whatever way it has been starting them all along.  Wow, this sounds good - less work, fewer chances for errors!  As expected, there is still no such thing as a free lunch here - you do wind up with some disadvantages.

For example, you can no longer easily detect the failure of an application.  In addition, if an application fails, the only thing you can do about it is reboot the entire virtual machine.   Inevitably, this takes longer than just restarting the failed application.

So, HA at wholesale has these properties:

• Simple enough that you can implement it for every machine
• Works well for hardware failures
• When coupled with hardware predictive failure analysis[3] and smart HA software, outages can sometimes be completely avoided.
• Can't easily detect or recover from application failures
• The only thing you can do about any failure is reboot the virtual machine

HA at retail has these properties:

• It is complex enough that you need to limit how broadly you apply it in your environment
• Works well for hardware failures
• It can easily detect and recover from application failures
• Individual applications can easily be restarted - and don't require a reboot

HA software like Linux-HA[1] can manage either type of environment.  In an ideal world, one would like to be able to do both in the same software infrastructure.


[1] http://linux-ha.org/
[2] http://linux-ha.org/ResourceAgent
[3] http://www-05.ibm.com/hu/termekismertetok/xseries/dn/pfa.pdf

Watch that basket!

The computing industry has lots of trends, numerous buzzwords, and a number of hot topics.  Sometimes these are in conflict with each other, or at least start out that way...  But, in the end, there are often good ways to harmonize all these various things.

Let's wander into virtual machine territory again today.  If you have gone to the trouble to create a bunch of virtual machines, the chances are you hope to do a little server consolidation - because when that's properly done it can save you some money.

This sounds good, and indeed has lots of good things going for it.  It's buzzword compliant, it's green, it saves you green (money).  What's not to like?

To see what you might not like if this is all you do, let's take an example to make it obvious...

If you put all your virtual machines on one physical server, then if that server fails, you lose all your virtual machines.  If you put ten virtual machines on one server, then the impact of that server crashing is roughly ten times as great as if a single server crashed.    If you work at it, you might be able to consolidate the ten most critical virtual machines onto a single server - and bring your entire data center to a halt with just one crash - bringing a suddenly much more personal meaning to the term "shock and awe"

This is not typically what people are looking for in their data center - and could easily be one of those career-limiting mistakes that you'd like to avoid - unless you already have your next job lined up.

This falls under the "putting all your eggs into one basket" way of doing business.  This part of a famous quote - but not the whole quote.  Mark Twain said "Put all your eggs in the one basket and --- WATCH THAT BASKET"[1].  So, to follow Mark Twain's advice, we need to not just put our eggs into one basket, we also need to watch that basket.

As most of you already know, watching servers and services is most commonly done by high-availability software - something like Linux-HA[2].  A properly configured HA system will watch the basket for you, and keep the worst from happening to your basket, your servers or your career.

As you can see, doing virtualization for reasons of consolidation doesn't make much sense unless you also add management software (HA software or otherwise) to watch your basket of virtual machines for you.

In the end, it's easy to see that all these things are connected - virtualization, server consolidation, power savings (green computing), availability management, and you want to manage them all.

[1] http://herbison.com/herbison/broken_eggs_watch.html
[2] http://linux-ha.org/

Virtual machine snapshots considered (nearly) worthless...

With apologies to Edgar Dijkstra...

Usually when people talk about virtual machine snapshotting, they include with it snapshotting both the server and any filesystems its directly connected to.  Although this is more complex than just snapshotting the virtual machine, it isn't that hard.

This works in some very narrow technical sense for some few cases, but it involves loss of data in every case.  If you take a checkpoint every 30 minutes (or every 5 or whatever), then all the updates made during that period of time, are lost when you restore this snapshot and its storage to a consistent (but old) state.  This means that all the checks you deposited during that time, or all the bonuses your boss put you in for during that time, or the books you ordered, or whatever, are lost.  Lost to the point that they probably have to be restored manually - to the tune of great customer dissatisfaction.

In addition, if this application has connections as a client, or as a server to other servers or clients, then although the application and its immediately mounted storage are now consistent, but unless you do simultaneous snapshots between this virtual machines and all the world it is connecting with (some of which may be outside your enterprise), and then restore your entire world to this older state, then there are likely to be many client/server connections which will no longer work - because the client and server are in mutually inconsistent states.

The worst case of this is if you have a Service Oriented Architecture, where any given server is only a small part of the overall service - every service has connections to something else all the time, and to make matters worse, the clients and/or servers are often outside your own enterprise.

And, of course, don't forget that you lost transactions in the process too.  So, a reboot interval of 1 to 3 minutes sounds really good by comparision.  Because all you'll lose in that case is transactions that were not yet committed - which are many fewer than the number of transactions lost by backing up to the previous checkpoint.

As an example of a common special case where this obviously doesn't work, imagine that the server in question is a file server.  So, you restore the virtual machine and all its storage (the file server) to some older state.  Now all the connected applications which _thought_ they had committed some particular piece of work (a spreadsheet, a database transaction) - just had all that work undone.  And, depending on the file server protocol and the application, bad things will happen - certainly loss of data, and probably some of the applications will create corrupt data - since updates they thought they'd made are now gone -  unbeknownst to them.  This corrupt data can cause any number of problems - inability to make further updates, cascading application crashes - these are all possibilities.

Or what if it's a client of a file server?  The file server is a separate machine (possibly virtual, possibly real, possibly an appliance).  Then you can't put its storage state back to a known state - without restoring all its clients back to the same consistent state - and if you somehow did, then _all_ of them now suffer data loss.

Not a very pretty picture.

There are some few cases where you can isolate the application from the "real world" and snapshot the whole "mini-enterprise" in a synchronous way.  Those are mostly limited to large scale scientific applications.  Given how hard it is to make them more available in any other way, this is a good thing.  But, its a practice with narrow applicability.  After reading the paragraphs above, perhaps you can see why...

A Complete Cluster Stack for Linux

Recently, I've had some folks ask me offline what exactly would a “complete” Linux cluster stack look like.  That's a good question, and this posting is intended to address that question.

So let's start with – what kind of cluster?  For the purposes of this posting, I'm primarily talking about a full-function high-availability enterprise-style cluster, not primarily a load balancing cluster, and not a high-performance scientific (Beowulf-style) cluster.

A few caveats before proceeding – much of what I'll reference below will be relative to the Linux-HA[1] framework, but the concepts are easily translated to any other clustering framework one might have in mind.

It's also worth noting that not every application, nor every configuration needs every component.   Adding unnecessary components adds complexity, and complexity is the enemy of reliability.

Many of the components (cluster filesystem, DLM) are primarily needed by cluster-aware applications.  Note that at this time (early 2008) very few applications are cluster-aware.

The Full Cluster Stack Exposed!

Below is a picture of the full cluster stack – which I'll describe in more detail later.  For the most part, the components higher up in the picture build on the components lower down in the picture.  To simplify the drawing, I didn't add all the who-uses-whom lines that one might want to make a detailed study of this subject.

 

Cluster Comm - Intracluster  communications

The most basic component any cluster needs is intracluster communications. There are a variety of different possibilities, but guaranteed packet delivery is a requirement.  Linux-HA has its own custom comm layer for doing this.  It's not perfect, but it works.  At one time in the past, we provided support for the AIS cluster APIs, and if you use OpenAIS today, then you can still have a reasonable cluster using Linux-HA and providing compatible support for the AIS protocols.  As will become clear, it's not a perfect configuration, but it's a reasonable one.  (Of course, like everything, it can always be improved even more)

Very large clusters (hundreds to tens of thousands of nodes) will likely require a different communication protocol, since most guaranteed delivery multicast protocols don't scale that high. 

Nevertheless, in an ideal world, all cluster components and cluster-aware applications would sit on top of the same set of communications protocols.

Membership – who's in the cluster?

Looking to the right of the cluster comm box on our architecture chart, you'll see the membership box.  The next basic function that a cluster has to provide is membership services.  Membership closely related to communication – since a simplistic view of membership is just who we can communicate with.  It is highly desirable that everyone in the cluster be able to communicate with everyone else. It's the job of the membership layer to provide this information to the cluster.

When your communication fails in weird ways, it's the job of the membership layer to present a view of the cluster that makes sense – in spite of the weird kinds of failures that might be going on.

If we eventually wind up with multiple kinds of communications methods, then we'll also have multiple ways of becoming a member.

Linux-HA (with or without OpenAIS) supports the AIS membership APIs.

Like I mentioned for communication, in an ideal world,  all cluster components would use exactly the same membership information.  However, it is important to note that the membership one uses must be computed using the communication method being used by the application.  So, unless every cluster-aware application uses both the common communication method and the common membership, it risks getting its membership out of sync with respect to its communication and other components using other communication methods.  In many cases, this can't be avoided.  Methods for coping with such discrepencies are discussed in more detail at the end of this post.

Fencing

Fencing is the ability to “disable” nodes not currently in our membership without their cooperation  Many of you will remember having discussed this in some detail in an earlier post[2]. As I explained in more detail there, fencing  is vital to ensure safe cluster operation

Our current implementation is STONITH[3]-based - STONITH == Shoot The Other Node In The Head

Quorum

Quorum is a topic we've talked about extensively in a couple of earlier posts [2] [4]. Quorum encapsulates the ability to determine whether the cluster can continue to operate safely or not

Quorum is tied closely to both fencing and membership.  In practice, as we discussed before, it is often highly desirable to implement multiple types of quorum.  Linux-HA currently provides multiple implementations and can provide more through plugins.  Like membership and communication, it is desirable for all cluster components to use the same quorum mechanism.  All the interesting and legal ways that quorum can interact with fencing and membership and the communication layer are too detailed for this posting.

Cluster Filesystems

Cluster filesystems allow multiple machines to sanely mount the same filesystem at the same time.  This is a great boon to parallel applications.  Cluster filesystems typically don't use the network or another server involved when doing bulk I/O. Each node mounting the filesystem is normally expected to have access to the data.  This typically requires a SAN.

Typically, this is done to improve performance, but convenience and manageability are common secondary goals  Cluster filesystems are related to, but distinct from, network filesystems like NFS and CIFS. On Linux, there are several cluster filesystems available including

• Red Hat's GFS

• Oracle's OCFS2

• IBM's GPFS

• etc.

Normally, when they're being used for performance reasons, cluster-aware applications are required.  You can't typically just run 'n' copies of your favorite cluster-ignorant application and have it work.  The filesystem won't scramble the data, but your application typically will.  It goes without saying that high-performance cluster filesystems run in the kernel – unlike all the other items we've talked about before.  Because of the high-performance, it's common for a cluster filesystem to have its own communication and membership code – not using the typically userland communications and membership code.  Since membership isn't high bandwidth or really low latency information, it is possible to feed membership from a user-space membership layer into the kernel.  Of course, then the membership and the communications layer are out of sync.  It is arguable whether this is an improvement or not.

Cluster filesystems typically need cluster lock managers – described in the next section.

DLM - Distributed Lock Manager

A DLM (Distributed Lock Manager) provides locking services across the cluster, and it's an interesting piece of code to implement them – particularly the error recovery. 

To some degree, DLMs are analogous to System V semaphores but – cluster-aware.  In addition, they provide much more sophisticated API and semantics.  Although DLM APIs are fairly well understood, there is no formal standard, so switching from one to another can be annoying.  Red Hat has a reasonable kernel-based DLM which they use with GFS.  DLMs commonly have their own separate communications and membership code.  The comments about getting membership from user-space and having them be potentially different from cluster filesytems also apply here.

Cluster Volume Managers

You might think that you really don't need a cluster-aware volume manager.  Sometimes you might be right.  More often, if you thought that, you'd be wrong.   A cluster volume manager is just like a regular volume manager – only cluster aware.  This is to keep different nodes from getting inconsistent views of the layout of a set of disks or volumes.   The current cluster-aware volume managers are EVMS and CLVM.  Only CLVM is expected to survive into the long term.

The big challenges for cluster volume managers are high-performance mirroring and snapshots.  These operations are potentially very difficult to implement right and fast.  Cluster-aware volume managers often have both kernel and user-space components.  The membership inconsistency issues here are similar to those for cluster filesystems and the DLM.

CRM - Cluster Resource Manager

Every HA cluster has something like a CRM, but they may divide up these functions differently.  Our CRM is a policy-based decision maker for what should run where – handling failed services and failed cluster nodes.

The CRM is similar to UNIX/Linux startup init scripts – it starts everything up – but across a cluster following some policies, and managing failures.

The Linux-HA CRM is arguably the best cluster resource manager around today – at least in terms of flexibility and power.  It has usability issues, and can be extended, but those are solvable.

The Linux-HA CRM function is largely divided between the PE and TE – which are described below.

PE - Policy Engine

The Policy Engine is a key component of the CRM and does two distinct things.

• It determines what should run where (cluster layout)

• It creates a graph of actions of how to get from the current state of affairs to the new desired state

This graph of actions is then given to the TE (described below).

The system would have more flexibility if he PE were split into two parts for these two functions, and supported plugins for the cluster layout function. 

It currently isn't aware of resource cost, nor of absolute resource limits and load balancing considerations, which complicate optimal placement.   Those would be good things to add to it in the future.  Having plugins for doing resource placement would also be a highly useful and desirable thing.

TE - Transition Engine

Receives a graph of actions to perform from the policy engine, then uses the LRM proxy to communicate with the LRMs to carry out the actions

Its main jobs are action sequencing, error detection and reporting

CIB - Cluster Information Base

The CIB manages information on cluster configuration and current status.  The cluster configuration includes the configuration and policies as defined by the system administrator.

Its key difficulty is to keep a consistent copy replicated across the cluster, resolving potential version differences.

All the data it manages is XML, and the CIB has a minimal knowledge of the structure of this XML.

LRM - Local Resource Manager

In the Linux-HA architecture, a local resource manager runs on every machine and carries out the tasks given to it. Everything that gets done gets carried out by the LRM.  Examples are:

• start this resource

• stop this resource

• monitor this resource

• migrate this resource

• etc.

The LRM provides interface matching to the various kinds of resources through Resource Agents.  The Linux-HA LRM supports several classes of Resource Agents.

The LRM is not at all cluster-aware.  It can support an arbitrary number of clients, one of which is the LRM communications proxy (below).

LRM Communications Proxy

The LRM proxy communicates between the CRM and the LRMs on all the various machines.  This function is currently built into the CRM.  This architectural decision was based on expedience more than anything else.

To support larger clusters this needs to be separated out, made more scalable, and more flexible.  This would allow a large number of LRMs to be supported by a small number of LRM proxies.   In large systems, this would probably use the ClusterIP capability to provide load distribution (leveling) across multiple LRM proxies.

Init - Initialization and recovery

This code does really three things:

• Sequences the startup of the cluster components

• Recovers from component failures (restart or reboot)

• Sequences the shutdown of all the various cluster components

This is currently provided by Linux-HA and bundled with the Linux-HA communications code.  This likely needs to be separated out to a separate proxy function (process) in the future.

Infrastructure

The Linux-HA infrastructure libraries (“clplumbing”) does a wide variety of things.  A few samples include:

• Inter-Process Communication

• Process management

• Event management and scheduling

• Many other miscellaneous functions

Surprisingly, these libraries amount to about 20K lines of code.

Quorum Daemon

The quorum daemon is an unusual daemon, because it's the only daemon we have that's intended to run outside the cluster proper.  It is instrumental in solving certain knotty quorum problems – especially for:

• 2-node clusters (very common)

• Split-site (disaster recovery) clusters

This was discussed extensively in previous postings [4] and [5].

Management Daemon

Provides Complete Authenticated Configuration and Status API.  This includes both information contained in the CIB, and also information about the communications configuration and so on.

The management daemon is used by the:

• GUI

• SNMP agent

• CIM agent

Clients are authenticated using PAM, and all communications is via SSL, so its clients can safely be outside cluster, or even outside a firewall.  This daemon should provide different levels of authorization depending on the authenticated user, and should log its actions in a format suitable for Sarbanes-Oxley (SOX) auditing purposes.

GUI

Provides a Graphical User Interface providing configuration and status information.  It also supports creating and configuring the cluster.  Note that at the present time, there are a number of useful cluster configurations it cannot create.

CIM and SNMP agents

The CIM and SNMP agents provide CIM and SNMP management interfaces for systems management tools.  The CIM interface supports status updates and configuration changes, whereas the SNMP interfaces only report status.

Disadvantages of this architecture

For a variety of reasons, kernel space doesn't have access to user-space cluster communications or membership.

As a result, both the DLM and most cluster filesytems implements their own membership and communications.

This is in contradiction to the “ideal world” statements earlier.  This can result in some odd cases where one communication method is working in a particular case, but another method is not.  This results in differences in membership – which can have bad effects.

Why this might not be quite as bad as it seems

One reason why one might not worry about this as much as one might, is because it's a problem which one can't make go away.  A cluster system will always have to interface with software packages which do their own communication, and compute their own membership for a variety of usually good reasons.  As a result, this is a problem which we can't make go away.  Instead we have to deal with it effectively.  There are basically two cases to consider:

1. The “Main” membership thinks that node X should not be in the cluster, whereas the “Other” membership thinks it should be.

2. The “Other” membership thinks that node X should not be in the cluster, whereas the “Main” membership thinks it should be.

Let's take these two cases one at a time:

Case 1:

If the main membership thinks node X is not in the cluster, then it will simply not start any resources on node X.  This takes care of the problem.

Case 2:

If the “Other” membership discovers that a particular node should be dropped from its view of membership, and it can inform the CRM not to start its resources on that machine, then the local view of this membership from the perspective of the resources it deals with is effectively made to exclude these Other-errant nodes.  In the Linux-HA CRM this is easily done having the Other-resources write node attributes to cause those nodes to be excluded, and the rules would then be written to exclude those nodes from consideration for running Other-related resources.

Although Case 2 isn't pretty, it works, and no amount of wishing and hoping is likely to ever make this kind of problem go away in the general case - particularly when one involves proprietary applications  So, even if there is some membership discrepancy, it can is always possible to manage it appropriately assuming you can get a tiny bit of cooperation from the application.

References

[1] http://linux-ha.org/
[2] http://techthoughts.typepad.com/managing_computers/2007/10/split-brain-quo.html
[3] http://linux-ha.org/STONITH
[4] http://techthoughts.typepad.com/managing_computers/2007/10/more-about-quor.html
[5] http://techthoughts.typepad.com/managing_computers/2007/11/quorum-server-i.html

How Managed Virtualization (including HA) conflicts with System Management

Managed Virtualization Versus System Management

In an earlier post[1], I talked about a couple of kinds of virtualization, comparing two of them and highlighting their strengths.  This posting discusses how virtualization can confuse and confound conventional systems management - both automated and manual, and gives some thoughts on how to deal with it.

We all know that virtualization is a GoodThing(TM).  Therefore, it can't really have any disadvantages, can it?  <tongue-in-cheek-off> Unfortunately, it does have disadvantages.  The great strength of virtualization is its ability to break the ties between a service or operating system and the server which implements its service.  Many software systems and a good number of human beings find this confusing.  If I want to reboot a physical server, what services or operating systems will be disrupted by the reboot?

Conversely, if I want to do something to the machine that's running a particular service, which machine do I have to log into?  If you're running both service virtualization (conventional HA like Linux-HA[2]) on top of server virtualization (ala Xen or VMware), then you have a doubly difficult task - first you have to figure out which virtual machine is running a service, then you have to figure out which physical machine is running that particular virtual machine.

This can be really annoying and can easily result in system administrators[3] making mistakes either in the middle of the night, or when under pressure (which all sysadmins know is pretty much all the time).

Remember - Complexity is the Enemy of Reliability.   This is just another example of my favorite phrase at work.

And, if you want to have server monitoring software which tries to figure out whether a service is stopped and have it restart it, then it can also get confused by the fact that all these stupid servers and services are always moving around.  They just won't stay put!  Back in the olden days, you logged into a server and you edited the inittab, and you always knew what hardware it was running on and what server it was.  Now, with virtualization, and especially with virtualization management software, you never know what's where.

A Recipe for Chaos and Conflict

Your HA software and/or your virtualization management software can move things around on you.  Imagine that you have these four kinds of things in your data center:

• High-Availability (HA/service-virtualization) management software

• Virtualization management software

• System management monitoring software

• Human system administrators

This is a recipe for chaos, interspersed with the occasional career-limiting disaster. It's this kind of thing that leads system administrators to pull their hair out, and keep their resumes up to date.  None of these is bad by itself, in fact, each is a GoodThing(TM).  But they don't normally play well with each other. In typical myopic software design fashion, each of these layers is usually unaware of the other layers (except, of course for the last (human) layer - who has to make up for all the poor integration).

In addition, since the software layers typically aren't aware of all this wonderful virtualization going on, they can't really deal with the picture reliably.  They don't know what should be happening where, because it isn't fixed.  The various virtualization management packages keep changing things!

So, what's a body to do?  As far as I know, there are two basic options.

1. Integrate the four layers of management with each other using things like CIM[4] and SNMP[5]

2. Empower your HA software to also manage the server virtualization of your data center

Integration of Layers

Virtually every data center (sadly, pun intended) has a variety of server types and a variety of operating systems, and a variety of management software.  They mostly don't play well with each other.  Almost the only way to get them to play together - even if imperfectly - is to have them talk together using industry standard protocols.

Today, that means using SNMP or CIM.   Here is my personal view on the characteristics of these two protocols for your consideration.

• SNMP - widely deployed - implemented in a truly compatible way, but far too weak for a job this hard.  SNMP is great for grabbing statistics, checking whether a server or router is up and what kind of load it is seeing in great detail.  Anything much beyond this, and the MIBs become 100% vendor-specific - meaning that cross-vendor integration breaks down - basically completely.  For HA clustering or virtualization management or worse yet the combination of the two - forget it.

• CIM - widely deployed in expensive disk subsystems - but rarely deployed outside that.  It has newly developed models for virtualization and clustering, but like most standards they're mostly lowest-common-denominator standards, and unfortunately not widely deployed.  For example, Linux-HA[2] implements CIM, but unfortunately Linux-HA has tremendous power and capability which CIM can't begin to model.  So, this winds up being only possible to model using vendor-specific extensions - greatly weakening the possible integrations.

Now, I'm not saying that these two protocols are useless - far from it. Without open standards like CIM and SNMP, the prospect truly is hopeless.   But I am saying that integrating them in the typical-for-the-industry highly-heterogeneous data center is a challenge, and the more layers there are to integrate, the bigger the challenge.  Since standards necessarily trail industry practice, the more "bleeding edge" the topic (i.e., HA clustering or virtualization) and the more powerful the underlying tool (like Linux-HA), the greater the mismatch.

Here we have two bleeding edge topics and four layers.  Yikes!  Surely there must be some kind of alternative to this somewhat-unattractive mess.

Decrease The Layers and Let Them Manage Themselves

As I mentioned in my earlier virtualization posting, some HA packages (like Linux-HA) can also manage virtualization simultaneously.  So, one way of dealing with this is to let (or extend) your service virtualization product also manage your server virtualization.  One advantage of this approach is that service virtualization software (HA software) is comparatively mature technology, minimizing the risk.

Unfortunately, this doesn't yet go all the way in solving the problem either.  There are a few things that should change to make this really work well. These include

• Support much larger HA clusters - hundreds to thousands of nodes.  In an ideal world, you'd really like fewer of these HA/virtualization clusters as you can get.  Today you'd typically have to have one of these clusters for every 8-32 physical servers - which makes an awfully lot of these things to manage in a data center containing hundreds or thousands of servers.

• Integrate with many virtualization layers - Such a product would need to integrate with Xen, IBM System Z, IBM System P, Linux KVM, VMware, and future virtualization layers like the one promised by Microsoft.   This isn't rocket science, but by the time you're done, it will be some work.

• Support monitoring and controlling services inside the virtual machine - Otherwise you haven't really integrated the two layers - and you wind up running some HA software inside some of the virtual machines.  Again, this isn't rocket science, but it will require some work[1] for each operating system you want to manage services for.

• Integrate with provisioning systems - so that you can add and delete virtual machines and allocate disk to them and their applications with fewer possibilities for error, and more automation.

None of these items are technically difficult, and none of them are prohibitively expensive to implement.  Given that I'm the project leader for Linux-HA, and Linux-HA is one of the most capable HA products around, you might imagine that some of these thoughts are on my mind for our future  ;-).  Of course, that doesn't eliminate the necessity for integration with the remaining layers above, which is why Linux-HA implements both CIM and SNMP.  This allows the virtualization management infrastructure to actively and autonomically manage  servers and services, while letting it bubble up events (especially those it can't automatically recover from) to the management consoles and humans via protocols like SNMP and/or CIM.

Conclusions

Virtualization technologies add complexity to the data center along with the benefits they bring, and in the process may render the existing management facilities less than useful.  However, if HA and Virtualization management are performed by a single entity, and open standards like CIM and SNMP are used, systems can be active the problems can be minimized.

See Also

Preparing for Virtual Management http://www.itbusinessedge.com/blogs/dcc/?p=276

References

[1] http://techthoughts.typepad.com/managing_computers/2007/09/virtualization-.html
[2] http://linux-ha.org/
[3] http://linux-ha.org/SysAdmin
[4] http://www.dmtf.org/standards/cim/
[4] http://en.wikipedia.org/wiki/Common_Information_Model_%28computing%29
[5] http://en.wikipedia.org/wiki/Simple_Network_Management_Protocol