Server-Side Apply

Crossplane can use server-side apply to sync claims with composite resources (XRs), and to sync composite resources with composed resources.

When Crossplane uses server-side apply, the Kubernetes API server helps sync resources. Using server-side apply makes syncing more predictable and less buggy.

{{<hint "tip">}} Server-side apply is a Kubernetes feature. Read more about server-side apply in the Kubernetes documentation. {{</hint>}}

Use server-side apply to sync claims with composite resources

When you create a claim, Crossplane creates a corresponding composite resource. Crossplane keeps the claim in sync with the composite resource. When you change the claim, Crossplane reflects those changes on the composite resource.

Read the [claims documentation]({{<ref "./claims">}}) to learn more about claims and how they relate to composite resources.

Crossplane can use server-side apply to keep the claim in sync with the composite resource.

Use the --enable-ssa-claims feature flag to enable using server-side apply. Read the [Install Crossplane documentation]({{<ref "../software/install#feature-flags">}}) to learn about feature flags.

If you see fields reappearing after you delete them from a claim's spec, enable server-side apply to fix the problem. Enabling server-side apply also fixes the problem where Crossplane doesn't delete labels and annotations from the composite resource when you delete them from the claim.

{{<hint "important">}} When you enable server-side apply, Crossplane is stricter about how it syncs a claim with its counterpart composite resource:

• The claim's metadata syncs to the composite resource's metadata.
• The claim's spec syncs to the composite resource's spec.
• The composite resource's status syncs to the claim's status.

When you enable server-side apply Crossplane doesn't sync the composite resource's metadata and spec back to the claim's metadata and spec. It also doesn't sync the claim's status to the composite resource's status. {{</hint>}}

Use server-side apply to sync claims end-to-end

To get the full benefit of server-side apply, use the --enable-ssa-claims feature flag together with composition functions.

When you use composition functions, Crossplane uses server side apply to sync composite resources with composed resources. Read more about this in the [composition functions documentation]({{<ref "./compositions#how-composition-functions-work">}}).

graph LR
  A(Claim) -- claim server-side apply --> B(Composite Resource)
  B -- function server-side apply --> C(Composed Resource)
  B -- function server-side apply --> D(Composed Resource)
  B -- function server-side apply --> E(Composed Resource)

When you use server-side apply end-to-end there is a clear, predictable propagation of fields from claim to composed resources, and back:

• metadata and spec flow forwards, from claim to XR to composed resources.
• status flows backwards, from composed resources to XR to claim.

{{<hint "important">}} When you use composition functions, Crossplane is stricter about how it syncs composite resources (XRs) with composed resources:

• The XR's metadata syncs to the composed resource's metadata.
• The XR's spec syncs to the composed resource's spec.
• The composed resource's status syncs to the XR's status.

When you use composition functions Crossplane doesn't sync the composed resource's metadata and spec back to the XR's metadata and spec. {{</hint>}}

When Crossplane uses server-side apply end-to-end to sync claims with composed resources, it deletes fields from a composed resource's spec when you delete fields from the claim's spec.

When Crossplane uses server-side apply end-to-end it's also able to merge claim fields into complex composed resource fields. Objects and arrays of objects are examples of complex composed resource fields.

{{<hint "tip">}} Crossplane can only merge complex fields for resources that use server-side apply merge strategy OpenAPI extensions. Read about these extensions in the Kubernetes server-side apply documentation.

If you find that Crossplane isn't merging managed resource fields, raise an issue against the relevant provider. Ask the provider maintainer to add server-side apply merge strategy extensions to the managed resource. {{</hint>}}