The potential user impact of changes (e.g. patches or config changes) made on the Cloud infrastructure (by the Cloud provider) is a sore point in the Cloud value proposition (see Hoff’s take for example). You have no control over patching/config actions taken by the provider, any of which could potentially affect you. In a traditional data center, you can test the various changes on specific applications; you don’t have to apply them at the same time on all servers; and you can even decide to skip some infrastructure patches not relevant to your application (“if it aint’ broken…”). Not so in a Cloud environment, where you may not even know about a change until after the fact. And you have no control over the timing and the roll-out of the patch, so that some of your instances may be running on patched nodes and others may not (good luck with troubleshooting that).
Unfortunately, this is even worse for PaaS than IaaS. Simply because you seat on a lot more infrastructure that is opaque to you. In a IaaS environment, the only thing that can change is the hardware (rarely a cause of problem) and the hypervisor (or equivalent Cloud OS). In a PaaS environment, it’s all that plus whatever flavor of OS and application container is used. Depending on how streamlined this all is (just enough OS/AS versus a traditional deployment), that’s potentially a lot of code and configuration. Troubleshooting is also somewhat easier in a IaaS setup because the error logs are localized (or localizable) to a specific instance. Not necessarily so with PaaS (and even if you could localize the error, you couldn’t guarantee that your troubleshooting test runs on the same node anyway).
In a way, PaaS is squeezed between IaaS and SaaS on this. IaaS gets away with a manageable problem because the opaque infrastructure is not too thick. For SaaS it’s manageable too because the consumer is typically either a human (who is a lot more resilient to change) or a very simple and well-understood interface (e.g. IMAP or some Web services). Contrast this with PaaS where the contract is that of an application container (e.g. JEE, RoR, Django).There are all kinds of subtle behaviors (e.g, timing/ordering issues) that are not part of the contract and can surface after a patch: for example, a bug in the application that was never found because before the patch things always happened in a certain order that the application implicitly – and erroneously – relied on. That’s exactly why you always test your key applications today even if the OS/AS patch should, in theory, not change anything for the application. And it’s not just patches that can do that. For example, network upgrades can introduce timing changes that surface new issues in the application.
And it goes both ways. Just like you can be hurt by the Cloud provider patching things, you can be hurt by them not patching things. What if there is an obscure bug in their infrastructure that only affects your application. First you have to convince them to troubleshoot with you. Then you have to convince them to produce (or get their software vendor to produce) and deploy a patch.
So what are the solutions? Is PaaS doomed to never go beyond hobbyists? Of course not. The possible solutions are:
- Write a bug-free and high-performance PaaS infrastructure from the start, one that never needs to be changed in any way. How hard could it be? ;-)
- More realistically, narrowly define container types to reduce both the contract and the size of the underlying implementation of each instance. For example, rather than deploying a full JEE+SOA container componentize the application so that each component can deploy in a small container (e.g. a servlet engine, a process management engine, a rule engine, etc). As a result, the interface exposed by each container type can be more easily and fully tested. And because each instance is slimmer, it requires fewer patches over time.
- PaaS providers may give their users some amount of visibility and control over this. For example, by announcing upgrades ahead of time, providing updated nodes to test on early and allowing users to specify “freeze” periods where nothing changes (unless an urgent security patch is needed, presumably). Time for a Cloud “refresh” in ITIL/ITSM-land?
- The PaaS providers may also be able to facilitate debugging of infrastructure-related problem. For example by stamping the logs with a version ID for the infrastructure on the node that generated the log entry. And the ability to request that a test runs on a node with the same version. Keeping in mind that in a SOA / Composite world, the root cause of a problem found on one node may be a configuration change on a different node…
Some closing notes:
- Another incarnation of this problem is likely to show up in the form of PaaS certification. We should not assume that just because you use a PaaS you are the developer of the application. Why can’t I license an ISV app that runs on GAE? But then, what does the ISV certify against? A given PaaS provider, e.g. Google? A given version of the PaaS infrastructure (if there is such a thing… Google advertises versions of the GAE SDK, but not of the actual GAE runtime)? Or maybe a given PaaS software stack, e.g. the Oracle/Microsoft/IBM/VMWare/JBoss/etc, meaning that any Cloud provider who uses this software stack is certified?
- I have only discussed here changes to the underlying platform that do not change the contract (or at least only introduce backward-compatible changes, i.e. add APIs but don’t remove any). The matter of non-compatible platform updates (and version coexistence) is also a whole other ball of wax, one that comes with echoes of SOA governance discussions (because in PaaS we are talking about pure software contracts, not hardware or hardware-like contracts). Another area in which PaaS has larger challenges than IaaS.
- Finally, for an illustration of how a highly focused and specialized container cuts down on the need for config changes, look at this photo from earlier today during the presentation of JRockit Virtual Edition at Oracle Open World. This slide shows (in font size 3, don’t worry you’re not supposed to be able to read), the list of configuration files present on a normal Linux instance, versus a stripped-down (“JeOS”) Linux, versus JRockit VE.
[UPDATED 2009/10/22: For more on this (in an EC2-centric context) see section 4 ("service problem resolution") of this IBM paper. It ends with "another possible direction is to develop new mechanisms or APIs to enable cloud users to directly and automatically query and correlate application level events with lower level hardware information to better identify the root cause of the problem".]
[UPDATES 2012/4/1: An example of a PaaS platform update which didn't go well.]