Composition Revisions

This guide discusses the use of “Composition Revisions” to safely make and roll back changes to a Crossplane Composition. It assumes familiarity with Crossplane, and particularly with Compositions.

A Composition configures how Crossplane should reconcile a Composite Resource (XR). Put otherwise, when you create an XR the selected Composition determines what managed resources Crossplane will create in response. Let’s say for example that you define a PlatformDB XR, which represents your organisation’s common database configuration of an Azure MySQL Server and a few firewall rules. The Composition contains the ‘base’ configuration for the MySQL server and the firewall rules that is extended by the configuration for the PlatformDB.

There is a one-to-many relationship between a Composition and the XRs that use it. You might define a Composition named big-platform-db that is used by ten different PlatformDB XRs. Usually, in the interest of self-service, the Composition is managed by a different team from the actual PlatformDB XRs. For example the Composition may be written and maintained by a platform team member, while individual application teams create PlatformDB XRs that use said Composition.

Each Composition is mutable - you can update it as your organisation’s needs change. However, without Composition Revisions updating a Composition can be a risky process. Crossplane constantly uses the Composition to ensure that your actual infrastructure - your MySQL Servers and firewall rules - match your desired state. If you have 10 PlatformDB XRs all using the big-platform-db Composition, all 10 of those XRs will be instantly updated in accordance with any updates you make to the big-platform-db Composition.

Composition Revisions allow XRs to opt out of automatic updates. Instead you can update your XRs to leverage the latest Composition settings at your own pace. This enables you to canary changes to your infrastructure, or to roll back some XRs to previous Composition settings without rolling back all XRs.

Using Composition Revisions

When you enable Composition Revisions three things happen:

  1. Crossplane creates a CompositionRevision for each Composition update.
  2. Composite Resources gain a spec.compositionRevisionRef field that specifies which CompositionRevision they use.
  3. Composite Resources gain a spec.compositionUpdatePolicy field that specifies how they should be updated to new Composition Revisions.

Each time you edit a Composition Crossplane will automatically create a CompositionRevision that represents that ‘revision’ of the Composition - that unique state. Each revision is allocated an increasing revision number. This gives CompositionRevision consumers an idea about which revision is ’newest’.

Crossplane distinguishes between the ’newest’ and the ‘current’ revision of a Composition. That is, if you revert a Composition to a previous state that corresponds to an existing CompositionRevision that revision will become ‘current’ even if it is not the ’newest’ revision (i.e. the most latest unique Composition configuration).

You can discover which revisions exist using kubectl:

1# Find all revisions of the Composition named 'example'
2kubectl get compositionrevision -l

This should produce output something like:

2example-18pdg   1          False     4m36s
3example-2bgdr   2          True      73s
4example-xjrdm   3          False     61s

A Composition is a mutable resource that you can update as your needs change over time. Each CompositionRevision is an immutable snapshot of those needs at a particular point in time.

Crossplane behaves the same way by default whether Composition Revisions are enabled or not. This is because when you enable Composition Revisions all XRs default to the Automatic compositionUpdatePolicy. XRs support two update policies:

  • Automatic: Automatically use the current CompositionRevision. (Default)
  • Manual: Require manual intervention to change CompositionRevision.

The below XR uses the Manual policy. When this policy is used the XR will select the current CompositionRevision when it is first created, but must manually be updated when you wish it to use another CompositionRevision.

 2kind: PlatformDB
 4  name: example
 6  parameters:
 7    storageGB: 20
 8  # The Manual policy specifies that you do not want this XR to update to the
 9  # current CompositionRevision automatically.
10  compositionUpdatePolicy: Manual
11  compositionRef:
12    name: example
13  writeConnectionSecretToRef:
14    name: db-conn

Crossplane sets an XR’s compositionRevisionRef automatically at creation time regardless of your chosen compositionUpdatePolicy. If you choose the Manual policy you must edit the compositionRevisionRef field when you want your XR to use a different CompositionRevision.

 2kind: PlatformDB
 4  name: example
 6  parameters:
 7    storageGB: 20
 8  compositionUpdatePolicy: Manual
 9  compositionRef:
10    name: example
11  # Update the referenced CompositionRevision if and when you are ready.
12  compositionRevisionRef:
13    name: example-18pdg
14  writeConnectionSecretToRef:
15    name: db-conn