Project: Crossplane (Orchestration & Management / Scheduling & Orchestration)
File: functions.md

A Composition Function is a kind of Crossplane extension. A Function is called to instruct Crossplane how a Composite Resource (XR) should be composed of other resources. This document specifies how a Function MUST behave. Refer to the Composition Functions [design document] for broader context on the feature. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. A Function MUST implement a gRPC `FunctionRunnerService` server. A Function SHOULD implement the latest available version of this service - e.g. `v1beta1`. The authoritative definition of this service can be found at the following URL. https://github.com/crossplane/crossplane/tree/master/apis/apiextensions/fn/proto A Function MUST copy the tag field from a RunFunctionRequest's RequestMeta message to the ResponseMeta tag field of the corresponding RunFunctionResponse. A Function MUST NOT modify the tag field. A Function MUST NOT inspect or otherwise depend on the tag's value. A Function SHOULD specify the ttl field of each RunFunctionResponse. The value of the ttl field MUST reflect how long Crossplane may use a cached version of the response for an identical RunFunctionRequest. A Function MUST pass through any desired state for which it does not have an opinion from the RunFunctionRequest desired field to the corresponding RunFunctionResponse desired field. A Function MAY mutate any desired state for which it has an opinion, including adding to, updating, or deleting from the desired state. For example a Function may: Add a new composed resource to the desired state. Add a new field to an existing composed resource in the desired state. Remove a composed resource from the desired state. etc... A Function MUST do so intentionally. Put otherwise, remember that Functions are run in a pipeline. A Function MUST propagate the desired state passed to it by previous Functions. A Function MUST NOT specify desired composite resource spec or metadata fields. A Function MAY only specify desired composite resource status fields. A Function MUST NOT specify desired composed resource status fields. A Function MAY only specify non-status desired composed resource fields, for example fields under the top-level metadata, spec, or data fields. A Function SHOULD NOT specify a metadata.name for desired composed resources. Crossplane will generate an appropriate

A Function SHOULD specify the crossplane.io/external-name annotation for its desired composed resources in order to influence the external name of those resources. A Function SHOULD avoid conflicting with existing observed or desired composed resources, for example by attempting to add an entry to the desired composed resources array with a name that already exists in either that array or the observed composed resources array. A Function MUST only return a Fatal result if it intends to terminate the entire Function pipeline, including preventing any Functions that would normally run after it from running. A Function MUST NOT return a Warning or Normal result every time it is called. A Function SHOULD return Warning or Normal results to indicate transitions - e.g. when entering a warning or normal state. A Function MUST support the following command-line flags: `--debug` (Default: `false`) - Enable debug logging. `--insecure` (Default: `false`) - Disable gRPC transport security. `--tls-certs-dir` - A directory containing mTLS server certs (tls.key and tls.crt), and a CA used to verify clients (ca.crt). A Function MUST support reading the `--tls-certs-dir` flag from the `TLSSERVERCERTS_DIR` environment variable. A Function MUST serve insecurely if both `--tls-certs-dir` and `--insecure` are specified. A Function MUST listen for gRPC requests on TCP port 9443, regardless of whether they are using mTLS transport security or have transport security disabled. A Function MUST enable gRPC transport security unless the `--insecure` flag is explicitly specified. A Function MUST be packaged according to the [xpkg Specification]. A Function SHOULD make use of the `io.crossplane.xpkg: base` OCI annotation to use a single OCI image to deliver both its package metadata (`package.yaml`) and Function binary. A Function MUST have a name beginning with `function-`. This name MUST appear in its package metadata, and SHOULD be used elsewhere, for example its GitHub repository. A Function MUST NOT assume it is deployed in any particular way, for example that it is running as a Kubernetes Pod in the same cluster as Crossplane. A Function MUST NOT assume it has network access. A Function SHOULD fail gracefully if it needs but does not have network access, for example by returning a Fatal result. A Function SHOULD use the latest version of the SDK for its language, for example https://github.com/crossplane/function-sdk-go.