---
stage: enablement
group: Tenant Scale
description: 'Cells: Container Registry'
---

<!-- vale gitlab.FutureTense = NO -->

This document is a work-in-progress and represents a very early state of the
Cells design. Significant aspects are not documented, though we expect to add
them in the future. This is one possible architecture for Cells, 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.

# Cells: 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 Cells architecture with registry run in a Cell 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 Cells 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 Cell, 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 Cell

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 Cell.

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 Cell,
but this might be desired approach.

## 4. Evaluation

There do not seem any theoretical problems with running GitLab Container Registry in a Cell.
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