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You can use Kubernetes annotations to attach arbitrary non-identifying metadata to objects. Clients such as tools and libraries can retrieve this metadata.
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You can use either labels or annotations to attach metadata to Kubernetes objects. Labels can be used to select objects and to find collections of objects that satisfy certain conditions.
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In contrast, annotations are not used to identify and select objects. The metadata in an annotation can be small or large, structured or unstructured, and can include characters not permitted by labels.
The keys and the values in the map must be strings. In other words, you cannot use numeric, boolean, list or other types for either the keys or the values.
usecase
Here are some examples of information that could be recorded in annotations:
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Fields managed by a declarative configuration layer. Attaching these fields as annotations distinguishes them from default values set by clients or servers, and from auto-generated fields and fields set by auto-sizing or auto-scaling systems.
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Build, release, or image information like timestamps, release IDs, git branch, PR numbers, image hashes, and registry address.
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Pointers to logging, monitoring, analytics, or audit repositories.
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Client library or tool information that can be used for debugging purposes: for example, name, version, and build information.
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User or tool/system provenance information, such as URLs of related objects from other ecosystem components.
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Lightweight rollout tool metadata: for example, config or checkpoints.
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Phone or pager numbers of persons responsible, or directory entries that specify where that information can be found, such as a team web site.
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Directives from the end-user to the implementations to modify behavior or engage non-standard features.
Instead of using annotations, you could store this type of information in an external database or directory, but that would make it much harder to produce shared client libraries and tools for deployment, management, introspection, and the like.
