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+---
+stage: enablement
+group: pods
+comments: false
+description: 'Pods: Container Registry'
+---
+
+This document is a work-in-progress and represents a very early state of the
+Pods design. Significant aspects are not documented, though we expect to add
+them in the future. This is one possible architecture for Pods, and we intend to
+contrast this with alternatives before deciding which approach to implement.
+This documentation will be kept even if we decide not to implement this so that
+we can document the reasons for not choosing this approach.
+
+# Pods: Container Registry
+
+GitLab Container Registry is a feature allowing to store Docker Container Images
+in GitLab. You can read about GitLab integration [here](../../../user/packages/container_registry/index.md).
+
+## 1. Definition
+
+GitLab Container Registry is a complex service requiring usage of PostgreSQL, Redis
+and Object Storage dependencies. Right now there's undergoing work to introduce
+[Container Registry Metadata](../container_registry_metadata_database/index.md)
+to optimize data storage and image retention policies of Container Registry.
+
+GitLab Container Registry is serving as a container for stored data,
+but on it's own does not authenticate `docker login`. The `docker login`
+is executed with user credentials (can be `personal access token`)
+or CI build credentials (ephemeral `ci_builds.token`).
+
+Container Registry uses data deduplication. It means that the same blob
+(image layer) that is shared between many projects is stored only once.
+Each layer is hashed by `sha256`.
+
+The `docker login` does request JWT time-limited authentication token that
+is signed by GitLab, but validated by Container Registry service. The JWT
+token does store all authorized scopes (`container repository images`)
+and operation types (`push` or `pull`). A single JWT authentication token
+can be have many authorized scopes. This allows container registry and client
+to mount existing blobs from another scopes. GitLab responds only with
+authorized scopes. Then it is up to GitLab Container Registry to validate
+if the given operation can be performed.
+
+The GitLab.com pages are always scoped to project. Each project can have many
+container registry images attached.
+
+Currently in case of GitLab.com the actual registry service is served
+via `https://registry.gitlab.com`.
+
+The main identifiable problems are:
+
+- the authentication request (`https://gitlab.com/jwt/auth`) that is processed by GitLab.com
+- the `https://registry.gitlab.com` that is run by external service and uses it's own data store
+- the data deduplication, the Pods architecture with registry run in a Pod would reduce
+  efficiency of data storage
+
+## 2. Data flow
+
+### 2.1. Authorization request that is send by `docker login`
+
+```shell
+curl \
+  --user "username:password" \
+  "https://gitlab/jwt/auth?client_id=docker&offline_token=true&service=container_registry&scope=repository:gitlab-org/gitlab-build-images:push,pull"
+```
+
+Result is encoded and signed JWT token. Second base64 encoded string (split by `.`) contains JSON with authorized scopes.
+
+```json
+{"auth_type":"none","access":[{"type":"repository","name":"gitlab-org/gitlab-build-images","actions":["pull"]}],"jti":"61ca2459-091c-4496-a3cf-01bac51d4dc8","aud":"container_registry","iss":"omnibus-gitlab-issuer","iat":1669309469,"nbf":166}
+```
+
+### 2.2. Docker client fetching tags
+
+```shell
+curl \
+  -H "Accept: application/vnd.docker.distribution.manifest.v2+json" \
+  -H "Authorization: Bearer token" \
+  https://registry.gitlab.com/v2/gitlab-org/gitlab-build-images/tags/list
+
+curl \
+  -H "Accept: application/vnd.docker.distribution.manifest.v2+json" \
+  -H "Authorization: Bearer token" \
+  https://registry.gitlab.com/v2/gitlab-org/gitlab-build-images/manifests/danger-ruby-2.6.6
+```
+
+### 2.3. Docker client fetching blobs and manifests
+
+```shell
+curl \
+  -H "Accept: application/vnd.docker.distribution.manifest.v2+json" \
+  -H "Authorization: Bearer token" \
+  https://registry.gitlab.com/v2/gitlab-org/gitlab-build-images/blobs/sha256:a3f2e1afa377d20897e08a85cae089393daa0ec019feab3851d592248674b416
+```
+
+## 3. Proposal
+
+### 3.1. Shard Container Registry separately to Pods architecture
+
+Due to it's architecture it extensive architecture and in general highly scalable
+horizontal architecture it should be evaluated if the GitLab Container Registry
+should be run not in Pod, but in a Cluster and be scaled independently.
+
+This might be easier, but would definitely not offer the same amount of data isolation.
+
+### 3.2. Run Container Registry within a Pod
+
+It appears that except `/jwt/auth` which would likely have to be processed by Router
+(to decode `scope`) the container registry could be run as a local service of a Pod.
+
+The actual data at least in case of GitLab.com is not forwarded via registry,
+but rather served directly from Object Storage / CDN.
+
+Its design encodes container repository image in a URL that is easily routable.
+It appears that we could re-use the same stateless Router service in front of Container Registry
+to serve manifests and blobs redirect.
+
+The only downside is increased complexity of managing standalone registry for each Pod,
+but this might be desired approach.
+
+## 4. Evaluation
+
+There do not seem any theoretical problems with running GitLab Container Registry in a Pod.
+Service seems that can be easily made routable to work well.
+
+The practical complexities are around managing complex service from infrastructure side.
+
+## 4.1. Pros
+
+## 4.2. Cons