> ## Documentation Index
> Fetch the complete documentation index at: https://backstage.spotify.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Entity References

> How to express references between entities

Entities commonly have a need to reference other entities. For example, a
[Component](./descriptor-format#kind-component) 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
[kind](./descriptor-format#apiversion-and-kind-required),
[namespace](./descriptor-format#namespace-optional), and
[name](./descriptor-format#name-required). 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.

## String References

This is the most common alternative and is used in almost all circumstances.

The string is of the form `[<kind>:][<namespace>/]<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 theme={"theme":{"light":"github-light","dark":"dracula"}}
# Example:
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.

## Compound References

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.