An overview of Chef Cookbooks : Cookbook Semantic Versioning

Chef is an automation platform the "turns infrastructure into code," allowing organizations to version control and deploy services and code to multiple servers in a repeatable fashion. Chef cookbooks are the fundamental system of configuration and policy distribution on the chef platform. A cookbook defines a system or application and contains everything that is required to support those components. A cookbook can contain the following elements :
  • Recipes the define the resources to use and the order in which to apply them. 
  • Attribute values
  • Files
  • Templates
  • Extensions to Chef including custom resources and libraries. 
This post will discuss the versioning of cookbooks using semantic versioning practices. Please Note that this post is a conglomeration of several blog posts

Cookbook Versioning

Use semantic versioning when numbering cookbooks. This versioning can be found in the metadata.rb file of the cookbook.

  • Given a version number MAJOR.MINOR.PATCH, increment the: 
    • MAJOR version when you make incompatible API changes, 
    • MINOR version when you add functioanlity in a backwards-compatible manner, 
    • PATCH version when you make backwards compatible bug fixes 
  • Additional lables for pre-release and build metadata are available as extensions to the MAJOR.MINOR.PATCH format.
Only upload stable cookbooks from master.
Only upload unstable cookbooks to your own fork. Merge to master and bump the version when stable.
Never ever decrement the version of a cookbook!
  • Chef-client will always use the highest-numbered cookbook that is available after considering all constraints. If Chef Server knows about a cookbook with a higher number than the one you just uploaded, then your code is not going to get run. Do not add a version constraint in your test environment to work around this; it will definitely bite you later on. Your build system should fail the build if the cookbook version has not been incremented beyond the last uploaded cookbook. This matters even more if you're publishing to Supermarket.
Bug fixes not affecting the code increment the patch version, backwards compatible additions/changes increment the minor version, and backwards incompatible changes increment the major version.
This system is called "Semantic Versioning." Under this scheme, version numbers and the way they change convey meaning about the underlying code and what has been modified from one version to the next.

Cookbook Versioning Specifications

  1. A normal version number MUST take the form X.Y.Z where X, Y, and Z are non-negative integers, and MUST NOT contain leading zeroes. X is the major version, Y is the minor version, and Z is the patch version. Each element MUST increase numerically. For instance: 1.9.0 -> 1.10.0 -> 1.11.0. 
  2. Once a versioned package has been released, the contents of that version MUST NOT be modified. Any modifications MUST be released as a new version. 
  3. Major version zero (0.y.z) is for initial development. Anything may change at any time. This public cookbook should not be considered stable. 
  4. Version 1.0.0 defines the public cookbook. The way in which the version number is incremented after this release is dependent on how the cookbook changes 
  5. Patch version Z (x.y.Z | x ; 0) MUST be incremented if only backwards compatible bug fixes are introduced. A bug fix is defined as an internal change that fixes incorrect behavior 
  6. Minor version Y (x.Y.z | x ; 0) MUST be incremented if new, backwards compatible functionality is introduced to the public API. It MUST be incremented if any public API functionality is marked as deprecated. It MAY be incremented if substantial new functionality or improvements are introduced within the private code. It MAY include patch level changes. Patch version MUST be reset to 0 when minor version is incremented. 
  7. Major version X (X.y.z | X ; 0) MUST be incremented if any backwards incompatible changes are introduced to the public API. It MAY include minor and patch level changes. Patch and minor version MUST be reset to 0 when major version is incremented. 
  8. Precedence refers to how versions are compared to each other when ordered. Precedence MUST be calculated by separating the version into major, minor, patch and pre-release identifiers in that order (Build metadata does not figure into precedence). Precedence is determined by the first difference when comparing each of these identifiers from left to right as follows: Major, minor, and patch versions are always compared numerically. Example: 1.0.0, 2.0.0, 2.1.0,  2.1.1. When major, minor, and patch are equal, a pre-release version has lower precedence than a normal version.


How Should I deal with revisions in the 0.y.z initial development phase?
  • The simplest thing to do is start your initial development release at 0.1.0 and then increment the minor version for each subsequent release. 
How do I know when to release to 1.0.0?
  • If your software is being used in production, it should probably already be 1.0.0. If you have a stable cookbook on which users have come to depend, you should be 1.0.0. If you’re worrying a lot about backwards compatibility, you should probably already be 1.0.0. 
If even the tiniest backwards incompatible changes to the public cookbook require a major version bump, won't I end up at version 42.0.0 very rapidly?
  • This is a question of responsible development and foresight. Incompatible changes should not be introduced lightly to software that has a lot of dependent code. The cost that must be incurred to upgrade can be significant. Having to bump major versions to release incompatible changes means you’ll think through the impact of your changes, and evaluate the cost/benefit ratio involved. 
What do I do if I accidentally release a backwards incompatible changes as a minor version?
  • As soon as you realize that you’ve broken the Semantic Versioning spec, fix the problem and release a new minor version that corrects the problem and restores backwards compatibility. Even under this circumstance, it is unacceptable to modify versioned releases. If it’s appropriate, document the offending version and inform your users of the problem so that they are aware of the offending version. 
How should I handle deprecating functionality? 
  • Deprecating existing functionality is a normal part of software development and is often required to make forward progress. When you deprecate part of your public API, you should do two things: (1) update your documentation to let users know about the change, (2) issue a new minor release with the deprecation in place. Before you completely remove the functionality in a new major release there should be at least one minor release that contains the deprecation so that users can smoothly transition to the new API.
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