Puppet, Chef, Deterministic Ordering and the Much Maligned DSL

This morning I came across a post entitled Puppet versus Chef: 10 reasons why Puppet wins. The post attempts to explain the differences between Chef and Puppet and why Puppet is superior. The post wasn’t great IMHO, personally I thought it was fairly poorly reasoned and made some, potentially accurate, but throughly unsubstantiated claims. Leaving aside the issues with the post itself though, it did prompt an interesting comment thread, particularly comments between Opcode’s CTO Adam Jacob and Reductive Lab’s Teyo Tyree (links are to the respective comments - Adam’s and Teyo’s reply). I’m going to quote Teyo’s comment in full because I think it answers a lot of question that people have had about some of the key differences between Puppet and Chef - dependency modelling and DSL:

There is a misstatement in your assessment of Puppet’s dependency handling. You express Chef’s ordering as deterministic and imply that Puppet is in someway non-deterministic. This is not the first time you have implied this publicly, so I thought I should bring some clarity to your misstatement. The actual differentiation is procedural ordering versus a dependency graph. Puppet provides a graphing model for ordering versus a procedural model. Sure, you get procedural ordering for free with Ruby, it seems easier, and I am aware that this was a design decision for you guys. We also know that you were frustrated by “having” to express dependencies in Puppet in order to ensure consistent ordering. Properly expressed dependencies in Puppet provide ordering where you care to have it. Procedural ordering is implicit even if you don’t care. This is a BIG difference, perhaps the fundamental difference between Puppet and Chef and one that was designed into Puppet because of experiences we had trying to cope with a large code base of procedurally order scripts to manage an enterprise infrastructure. Yeah we were using make, yeah that was crazy, crazy but informative. Your omission is related to your design decision to avoid dependency graphing, which you yourself have admitted has some major downsides, namely the inability to provide a reasonable dry-run mode, http://bit.ly/4Gcz7G. Frankly, I don’t know how you develop with out a dry-run mode, but hey I am a sysadmin not a developer. Without a graph of resource dependencies, we would have no way of separating concerns. Consider the use case of implementing security standards. Ideally, you would want any given configuration run to bring your system into complete compliance. That sounds great but would you really want security policies not to be implemented because some earlier procedure was unable to succeed, say because it was pulling in data from a source that was not available. So here is the difference in a nutshell. Puppet generates a catalog of dependent resources. This catalog is shipped to the clients and acted on by the Resource Abstraction Layer (RAL). On the other hand Chef, ships the required Ruby code for any node’s configuration and orders the execution of that code procedurally. These are the core differences. The DSL issues become moot if you consider Shadow Puppet or the Ruby DSL that we are developing as part of Puppet’s next release. The real difference, and IMHO Puppet’s advantage is our resource model and it’s dependency graphs versus a monolithic procedural chunk of Ruby code delivered to every client. Here are some derived advantages of our model and a little love for the much maligned declarative external DSL: 1) Graphing base branch independence. Parts of a catalog can be implemented more often than others. That is to say, we can tag certain resources to be checked and reconfigured more often. Additionally, parts of a configuration can be meaningfully checked but not acted on (See Adam’s discussion of noop http://bit.ly/4Gcz7G). Our customers/community love this and without a graph I don’t see how it is possible. 2) Cross host dependencies. Our data model passes dependencies into our catalog caching system, so cross host dependencies can be resolved as well. This isn’t currently available but the framework exists and we intend to take advantage of it. 3) Failures are contained. Critical parts of a configuration run are not excluded because of non-dependent failures. 4) Low barrier to entry for non-rubyist. Non- rubyist can take advantage of the specification language out of the gate and Ruby developers can take advantage of the current Puppet plugin API and the future Ruby DSL, so everyone gets to use their favorite hammer. 5) Don’t like our DSL, don’t like Ruby? Because we are only generating a catalog from the configuration language it should be fairly straight forward for anyone to generate a catalog using whatever language they choose and the RAL would be able to act on it. Come on Python people you know you want to generate catalogs with Python. 6) I can’t run Ruby on my switch! Because we are using a data model for resources, devices that can’t/don’t have access to Ruby could still use the catalog as a basis for configuring themselves. Routers, switches, firewalls, could all be configured using the same specification language independent of how the specification is implemented, but with the resource model intact. Finally, I think that you were a little hard on John about his comments on Chef being Rails focused. Certainly he misspoke, but the truth is that Chef development has been focused on web-application rollout in fairly homogeneous environments. Sure you can use Chef to manage the initial deployment of a web application, but in environments where you may have lots of teams utilizing compute resources for various application architectures, Puppet’s resource model shines. Security administrators can develop their Puppet manifests and not need to worry that security policies are not going to be applied because the DBA teams manifests failed. Operations teams can run Puppet in noop mode persistently and be notified if their configuration is out of compliance. Developers can make sure that the infrastructure they need for successful application deployment is available without having to worry that the security policy failed to be applied. Everyone gets to be friendlier with one and other and perhaps even get to the pub earlier on occasion.

Disclosure & Disclaimer - I am Puppet’s Release Manager and obviously heavily involved with the Reductive Lab’s team and the project. My opinions are my own and not representative of my employer or Reductive Labs.

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