Cache Docker images

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You can configure DTR to have multiple caches. Users can then configure their DTR user accounts to specify which cache to pull from. This way users can pull Docker images from a cache that is geographically closer to them or that allows them faster downloads.

How DTR caches works

You start by deploying one or more DTR caches.

Since caches have to contact DTR for authorizing user requests, and requests are chained from one cache to the next until the request reaches DTR, you should avoid creating cache trees that have more than two levels. This can make the image pull slower than having no cache at all.

After you’ve deployed the caches, users can configure which cache to pull from on their DTR user profile page. This allows users to choose which cache to use for faster image pulls.

In this example, users can go to their DTR profile page, and configure their user account to use the US cache. Then, when using the docker pull <dtr-url>/<org>/<repository> command to pull an image, the following happens:

  1. The Docker client makes a request to DTR which in turn authenticates the request
  2. The Docker client requests the image manifest to DTR. This ensures that users will always pull the correct image, and not an outdated version
  3. The Docker client requests the layer blobs to DTR, which redirects the request to the cache configured by the user
  4. If the blob exists on the cache it is sent to the user. Otherwise, the cache pulls it from DTR and sends it to the user

When a user pushes an image, that image is only available in DTR. A cache will only store the image when a user tries to pull the image using that cache.

Deploy a DTR cache

You can deploy a DTR cache on any host that has Docker installed. The only requirements are that:

  • Users need to have access to both DTR and the cache
  • The cache needs access to DTR

Log into the host using ssh, and create a config.yml file with the following content:

version: 0.1
storage:
  delete:
    enabled: true
  filesystem:
    rootdirectory: /var/lib/registry
http:
  addr: :5000
middleware:
  registry:
      - name: downstream
        options:
          blobttl: 24h
          upstreams:
            - originhost: https://<dtr-url>
          cas:
            - /certs/dtr-ca.pem

This configures the cache to store the images in the directory /var/lib/registry, exposes the cache service on port 5000, and configures the cache to delete images that are not pulled in the last 24 hours. It also defines where DTR can be reached, and which CA certificates should be trusted.

Now we need to download the CA certificate used by DTR. For this, run:

curl -k https://<dtr-url>/ca > dtr-ca.pem

Now that we’ve got the cache configuration file and DTR CA certificate, we can deploy the cache by running:

docker run --detach --restart always \
  --name content-cache \
  --publish 5000:5000 \
  --volume $(pwd)/dtr-ca.pem:/certs/dtr-ca.pem \
  --volume $(pwd)/config.yml:/config.yml \
  docker/dtr-content-cache:<version> /config.yml

You can also run the command in interactive mode instead of detatched by replacing --detached with --interactive. This allows you to see the logs generated by the container and troubleshoot misconfigurations.

Now that you’ve deployed a cache, you need to configure DTR to know about it. This is done using the POST /api/v0/content_caches API. You can use the DTR interactive API documentation to use this API.

In the DTR web UI, click the top-right menu, and choose API docs.

Navigate to the POST /api/v0/content_caches line and click it to expand. In the body field include:

{
  "name": "region-us",
  "host": "http://<cache-public-ip>:5000"
}

Click the Try it out! button to make the API call.

DTR knows about the cache we’ve created, so we just need to configure our DTR user settings to start using that cache.

In the DTR web UI, navigate to your user profile, click the Settings tab, and change the Content cache settings to use the region-us cache.

Now when you pull images, you’ll be using the cache. To test this, try pulling an image from DTR. You can inspect the logs of the cache service, to validate that the cache is being used, and troubleshoot problems.

In the host where you’ve deployed the region-us cache, run:

docker logs content-cache

Configure the cache

The DTR cache is based on Docker Registry, and uses the same configuration file format. Learn more about the configuration options.

The DTR cache extends the Docker Registry configuration file format by introducing a new middleware called downstream that has three configuration options: blobttl, upstreams, and cas:

# Settings that you would include in a
# Docker Registry configuration file followed by

middleware:
  registry:
      - name: downstream
        options:
          blobttl: 24h
          upstreams:
            - originhost: <Externally-reachable address for the origin registry>
              upstreamhosts:
                - <Externally-reachable address for upstream content cache A>
                - <Externally-reachable address for upstream content cache B>
          cas:
            - <Absolute path to upstream content cache A certificate>
            - <Absolute path to upstream content cache B certificate>

Below you can find the description for each DTR cache-specific parameter.

Parameter Required Description
blobttl no The TTL for blobs in the cache. This field takes a positive integer and an optional suffix indicating the unit of time. If this field is configured, "storage.delete.enabled" must be configured to true. Possible units are:
  • ns (nanoseconds)
  • us (microseconds)
  • ms (milliseconds)
  • s (seconds)
  • m (minutes)
  • h (hours)
If you omit the suffix, the system interprets the value as nanoseconds.
cas no A list of absolute paths to PEM-encoded CA certificates of upstream registries.
originhost yes An externally-reachable address for the origin registry, as a fully qualified URL.
upstreamhosts no A list of externally-reachable addresses for upstream registries for cache chaining. If more than one host is specified, pulls from upstream content caches will be done in round-robin order.
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