Project: Backstage (App Definition and Development / Application Definition & Image Build)
File: references.md

id: references title: Entity References description: How to express references between entities Entities commonly have a need to reference other entities. For example, a entity may want to declare who its owner is by mentioning a Group or User entity, and a User entity may want to declare what Group entities it is a member of. This article describes how to write those references in your yaml entity declaration files. Each entity in the catalog is uniquely identified by the triplet of its , , and . But that's a lot to type out manually, and in a lot of circumstances, both the kind and the namespace are fixed, or possible to deduce, or could have sane default values. So in order to help the writer, the catalog has a few tricks up its sleeve. Each reference can be expressed in one of two ways: as a compact string, or as a compound reference structure. This is the most common alternative and is used in almost all circumstances. The string is of the form `<name>`. That is, it is composed of between one and three parts in this specific order, without any additional encoding: Optionally, the kind, followed by a colon Optionally, the namespace, followed by a forward slash The name The name is always required. Depending on the context, you may be able to leave out the kind and/or namespace. If you do, it is contextual what values will be used, and the relevant documentation should specify which rule applies where. All strings are case insensitive. ```yaml apiVersion: backstage.io/v1alpha1 kind: Component metadata: name: petstore namespace: external-systems description: Petstore spec: type: service lifecycle: experimental owner: group:pet-managers providesApis: petstore internal/streetlights hello-world ``` The field `spec.owner` is a reference. In this case, the string `group:pet-managers` was given by the user. That means that the kind is `Group`, the namespace is left out, and the name is `pet-managers`. In this context, the namespace was chosen to fall back to the value `default` by the code that parsed the reference, so the end result is that we expect to find another entity in the catalog that is of kind `Group`, namespace `default` (which, actually, also can be left out in its own yaml file because that's the default value there too), and name `pet-managers`. The entries in `providesApis` are also references. In this case, none of them needs to specify a kind since we know from the context that that's the only kind that's supported here. The second entry specifies a namespace but the other ones don't, and in this context, the default is to refer to the same namespace as the originating entity (`external-systems` here). So the three references essentially expand to `api:external-systems/petstore`, `api:internal/streetlights`, and `api:external-systems/hello-world`. We expect there to exist three API kind entities in the catalog matching those references. Note that the remarks above in regards to shortening (leaving out kind and/or namespace) only apply for the entity input YAML data. In protocols, storage systems, or when referring to entities externally, the entity ref always consists of all three parts. This is a more verbose version of a reference, where each part of the kind-namespace-name triplet is expressed as a field in an object. You may see this structure used in Backstage code, but it should normally not be used in any form of protocol or between plugins and/or external systems. For those cases, instead prefer to use the string form which has clearer semantics and can be transported more easily since it's just a string.