Saturday, August 04, 2012

Multiple Concurrent Versions

Multiple Concurrent Versions is the term I use to describe a practice for deploying multiple versions of a web app at the same time. Technically, the concept applies to any kind of deployment bundle, but Java web apps make a good example.

Here are the basic precepts of the practice:
  1. We assume that the thing being deployed, the deployment bundle, is represented as files and folders.
  2. Physically moving the deployment bundle to the deployment target (the runtime environment) is a distinct step from activating a particular version.
  3. The deployment target will, at any given time, contain multiple versions of the same deployment bundle: 
    • old versions that used to be in production 
    • old versions that never made it to production 
    • the current production version 
    • future versions that are very close to production ready
  4. Multiple versions are concurrently deployed and accessible at the same time via version specific urls:

    The current or default version can be accessed via a version independent url:
  5. Activation is the process of marking a particular version as the default version. 
    • Changing the default version from an older to a newer version is called a rollout. 
    • Changing the default version from a newer to an older version is called a rollback.
  6. Each deployment bundle version should have a cryptographic identity (i.e. sha).
  7. Concurrent versions should be isolated. That is, a bug in version 8 shouldn't be able to kill version 7. 

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