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---
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title: Composition Revisions
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toc: true
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weight: 260
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indent: true
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---
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This guide discusses the use of "Composition Revisions" to safely make and roll
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back changes to a Crossplane [`Composition`][composition-type]. It assumes
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familiarity with Crossplane, and particularly with
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[Composition][composition-term].
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> Composition Revisions are an __alpha feature__. They are not yet recommended
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> for production use, and are disabled by default.
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A `Composition` configures how Crossplane should reconcile a Composite Resource
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(XR). Put otherwise, when you create an XR the selected `Composition` determines
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what managed resources Crossplane will create in response. Let's say for example
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that you define a `PlatformDB` XR, which represents your organisation's common
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database configuration of an Azure MySQL Server and a few firewall rules. The
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`Composition` contains the 'base' configuration for the MySQL server and the
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firewall rules that is extended by the configuration fo the `PlatformDB`.
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There is a one-to-many relationship between a `Composition` and the XRs that use
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it. You might define a `Composition` named `big-platform-db` that is used by ten
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different `PlatformDB` XRs. Usually, in the interest of self-service, the
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`Composition` is managed by a different team from the actual `PlatformDB` XRs.
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For example the `Composition` may be written and maintained by a platform team
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member, while individual application teams create `PlatformDB` XRs that use said
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`Composition`.
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Each `Composition` is mutable - you can update it as your organisation's needs
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change. However, without Composition Revisions updating a `Composition` can be a
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risky process. Crossplane constantly uses the `Composition` to ensure that your
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actual infrastructure - your MySQL Servers and firewall rules - match your
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desired state. If you have 10 `PlatformDB` XRs all using the `big-platform-db`
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`Composition`, all 10 of those XRs will be instantly updated in accordance with
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any updates you make to the `big-platform-db` `Composition`.
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Composition Revisions allow XRs to opt out of automatic updates. Instead you can
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update your XRs to leverage the latest `Composition` settings at your own pace.
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This enables you to [canary] changes to your infrastructure, or to roll back
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some XRs to previous `Composition` settings without rolling back all XRs.
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## Enabling Composition Revisions
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Composition Revisions are an alpha feature. They are not yet recommended for
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production use, and are disabled by default. Start Crossplane with the
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`--enable-composition-revisions` flag to enable Composition Revision support.
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```console
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kubectl create namespace crossplane-system
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helm install crossplane --namespace crossplane-system crossplane-stable/crossplane --set args='{--enable-composition-revisions}'
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```
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See the [getting started guide][install-guide] for more information on
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installing Crossplane.
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## Using Composition Revisions
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When you enable Composition Revisions three things happen:
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1. Crossplane creates a `CompositionRevision` for each `Composition` update.
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1. Composite Resources gain a `spec.compositionRevisionRef` field that specifies
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which `CompositionRevision` they use.
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1. Composite Resources gain a `spec.compositionUpdatePolicy` field that
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specifies how they should be updated to new Composition Revisions.
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Each time you edit a `Composition` Crossplane will automatically create a
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`CompositionRevision` that represents that 'revision' of the `Composition` -
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that unique state. Each revision is allocated an increasing revision number.
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This `CompositionRevision` consumers an idea about which revision is 'newest'.
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Crossplane distinguishes between the 'newest' and the 'current' revision of a
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`Composition`. That is, if you revert a `Composition` to a previous state that
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corresponds to an existing `CompositionRevision` that revision will become
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'current' even if it is not the 'newest' revision (i.e. the most latest _unique_
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`Composition` configuration).
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You can discover which revisions exist using `kubectl`:
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```console
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# Find all revisions of the Composition named 'example'
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kubectl get compositionrevision -l crossplane.io/composition-name=example
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```
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This should produce output something like:
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```console
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NAME REVISION CURRENT AGE
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example-18pdg 1 False 4m36s
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example-2bgdr 2 True 73s
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example-xjrdm 3 False 61s
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```
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> A `Composition` is a mutable resource that you can update as your needs
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> change over time. Each `CompositionRevision` is an immutable snapshot of those
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> needs at a particular point in time.
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Crossplane behaves the same way by default whether Composition Revisions are
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enabled or not. This is because when you enable Composition Revisions all XRs
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default to the `Automatic` `compositionUpdatePolicy`. XRs support two update
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policies:
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* `Automatic`: Automatically use the current `CompositionRevision`. (Default)
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* `Manual`: Require manual intervention to change `CompositionRevision`.
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The below XR uses the `Manual` policy. When this policy is used the XR will
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select the current `CompositionRevision` when it is first created, but must
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manually be updated when you wish it to use another `CompositionRevision`.
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```yaml
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apiVersion: example.org/v1alpha1
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kind: PlatformDB
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metadata:
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name: example
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spec:
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parameters:
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storageGB: 20
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# The Manual policy specifies that you do not want this XR to update to the
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# current CompositionRevision automatically.
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compositionUpdatePolicy: Manual
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compositionRef:
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name: example
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writeConnectionSecretToRef:
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name: db-conn
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```
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Crossplane sets an XR's `compositionRevisionRef` automatically at creation time
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regardless of your chosen `compositionUpdatePolicy`. If you choose the `Manual`
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policy you must edit the `compositionRevisionRef` field when you want your XR to
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use a different `CompositionRevision`.
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```yaml
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apiVersion: example.org/v1alpha1
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kind: PlatformDB
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metadata:
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name: example
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spec:
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parameters:
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storageGB: 20
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compositionUpdatePolicy: Manual
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compositionRef:
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name: example
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# Update the referenced CompositionRevision if and when you are ready.
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compositionRevisionRef:
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name: example-18pdg
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writeConnectionSecretToRef:
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name: db-conn
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```
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[composition-type]: ../concepts/composition.md
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[composition-term]: ../concepts/terminology.md#composition
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[canary]: https://martinfowler.com/bliki/CanaryRelease.html
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[install-guide]: ../getting-started/install-configure.md
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