Dependency Proxy

The Dependency Proxy is a pull-through-cache for registry images from DockerHub. This document describes how this feature is constructed in GitLab.

Container registry

The Dependency Proxy for the container registry acts a stand-in for a remote container registry. In our case, the remote registry is the public DockerHub registry.

flowchart TD id1([$ docker]) --> id2([GitLab Dependency Proxy]) id2 --> id3([DockerHub])

From the user’s perspective, the GitLab instance is just a container registry that they are interacting with to pull images by using docker login gitlab.com

When you use docker login gitlab.com, the Docker client uses the v2 API to make requests.

To support authentication, we must include one route:

To support docker pull requests, we must include two additional routes:

These routes are defined in gitlab-org/gitlab/config/routes/group.rb.

In its simplest form, the Dependency Proxy manages three requests:

  • Logging in / returning a JWT
  • Fetching a manifest
  • Fetching a blob

Here is what the general request sequence looks like for the Dependency Proxy:

sequenceDiagram Client->>+GitLab: Login? / request token GitLab->>+Client: JWT Client->>+GitLab: request a manifest for an image GitLab->>+ExternalRegistry: request JWT ExternalRegistry->>+GitLab : JWT GitLab->>+ExternalRegistry : request manifest ExternalRegistry->>+GitLab : return manifest GitLab->>+GitLab : store manifest GitLab->>+Client : return manifest loop request image layers Client->>+GitLab: request a blob from the manifest GitLab->>+ExternalRegistry: request JWT ExternalRegistry->>+GitLab : JWT GitLab->>+ExternalRegistry : request blob ExternalRegistry->>+GitLab : return blob GitLab->>+GitLab : store blob GitLab->>+Client : return blob end

Authentication and authorization

When a Docker client authenticates with a registry, the registry tells the client where to get a JSON Web Token (JWT) and to use it for all subsequent requests. This allows the authentication service to live in a separate application from the registry. For example, the GitLab container registry directs Docker clients to get a token from https://gitlab.com/jwt/auth. This endpoint is part of the gitlab-org/gitlab project, also known as the rails project or web service.

When a user tries to sign in to the dependency proxy with a Docker client, we must tell it where to get a JWT. We can use the same endpoint we use with the container registry: https://gitlab.com/jwt/auth. But in our case, we tell the Docker client to specify service=dependency_proxy in the parameters so can use a separate underlying service to generate the token.

This sequence diagram shows the request flow for logging into the Dependency Proxy.

sequenceDiagram autonumber participant C as Docker CLI participant R as GitLab (Dependency Proxy) Note right of C: User tries `docker login gitlab.com` and enters username/password C->>R: GET /v2/ Note left of R: Check for Authorization header, return 401 if none, return 200 if token exists and is valid R->>C: 401 Unauthorized with header "WWW-Authenticate": "Bearer realm=\"http://gitlab.com/jwt/auth\",service=\"registry.docker.io\"" Note right of C: Request Oauth token using HTTP Basic Auth C->>R: GET /jwt/auth Note left of R: Token is returned R->>C: 200 OK (with Bearer token included) Note right of C: original request is tested again C->>R: GET /v2/ (this time with `Authorization: Bearer [token]` header) Note right of C: Login Succeeded R->>C: 200 OK

The dependency proxy uses its own authentication service, separate from the authentication managed by the UI (ApplicationController) and API (ApiGuard). Once the service has created a JWT, the DependencyProxy::ApplicationController manages authentication and authorization for the rest of the requests. It manages the user by using GitLab::Auth::Result and is similar to the authentication implemented by the Git client requests in GitHttpClientController.

Caching

Blobs are cached artifacts with no logic around them. We cache them by digest. When we receive a request for a new blob, we check to see if we have a blob with the requested digest, and return it. Otherwise we fetch it from the external registry and cache it.

Manifests are more complicated, partially due to rate limiting on DockerHub. A manifest is essentially a recipe for creating an image. It has a list of blobs to create a certain image. So alpine:latest has a manifest associated with it that specifies the blobs needed to create the alpine:latest image. The interesting part is that alpine:latest can change over time, so we can’t just cache the manifest and assume it is OK to use forever. Instead, we must check the digest of the manifest, which is an Etag. This gets interesting because the requests for manifests often don’t include the digest. So how do we know if the manifest we have cached is still the most up-to-date alpine:latest? DockerHub allows free HEAD requests that don’t count toward their rate limit. The HEAD request returns the manifest digest so we can tell whether or not the one we have is stale.

With this knowledge, we have built the following logic to manage manifest requests:

graph TD A[Receive manifest request] --> | We have the manifest cached.| B{Docker manifest HEAD request} A --> | We do not have manifest cached.| C{Docker manifest GET request} B --> | Digest matches the one in the DB | D[Fetch manifest from cache] B --> | HEAD request error, network failure, cannot reach DockerHub | D[Fetch manifest from cache] B --> | Digest does not match the one in DB | C C --> E[Save manifest to cache, save digest to database] D --> F E --> F[Return manifest]

Workhorse for file handling

Management of file uploads and caching happens in Workhorse. This explains the additional POST routes that we have for the Dependency Proxy.

The send_dependency method makes a request to Workhorse including the previously fetched JWT from the external registry. Workhorse then can use that token to request the manifest or blob the user originally requested. The Workhorse code lives in workhorse/internal/dependencyproxy/dependencyproxy.go.

Once we put it all together, the sequence for requesting an image file looks like this:

sequenceDiagram Client->>Workhorse: GET /v2/*group_id/dependency_proxy/containers/*image/manifests/*tag Workhorse->>Rails: GET /v2/*group_id/dependency_proxy/containers/*image/manifests/*tag Rails->>Rails: Check DB. Is manifest persisted in cache? alt In Cache Rails->>Workhorse: Respond with send-url injector Workhorse->>Client: Send the file to the client else Not In Cache Rails->>Rails: Generate auth token and download URL for the manifest in upstream registry Rails->>Workhorse: Respond with send-dependency injector Workhorse->>External Registry: Request the manifest External Registry->>Workhorse: Download the manifest Workhorse->>Rails: GET /v2/*group_id/dependency_proxy/containers/*image/manifest/*tag/authorize Rails->>Workhorse: Respond with upload instructions Workhorse->>Client: Send the manifest file to the client with original headers Workhorse->>Object Storage: Save the manifest file with some of it's header values Workhorse->>Rails: Finalize the upload end

Cleanup policies

The cleanup policies for the Dependency Proxy work as time-to-live policies. They allow users to set the number of days a file is allowed to remain cached if it has been unread. Since there is no way to associate the blobs with the images they belong to (to do this, we would need to build the metadata database that the container registry folks built), we can set up rules like “if this blob has not been pulled in 90 days, delete it”. This means that any files that are continuously getting pulled will not be removed from the cache, but if, for example, alpine:latest changes and one of the underlying blobs is no longer used, it will eventually get cleaned up because it has stopped getting pulled. We use the read_at attribute to track the last time a given dependency_proxy_blob or dependency_proxy_manifest was pulled.

These work using a cron worker, DependencyProxy::CleanupDependencyProxyWorker, that will kick off two limited capacity workers: one to delete blobs, and one to delete manifests. The capacity is set in an application setting.