13장. Red Hat Quay as a proxy cache for upstream registries


With the growing popularity of container development, customers increasingly rely on container images from upstream registries like Docker or Google Cloud Platform to get services up and running. Today, registries have rate limitations and throttling on the number of times users can pull from these registries.

With this feature, Red Hat Quay will act as a proxy cache to circumvent pull-rate limitations from upstream registries. Adding a cache feature also accelerates pull performance, because images are pulled from the cache rather than upstream dependencies. Cached images are only updated when the upstream image digest differs from the cached image, reducing rate limitations and potential throttling.

With Red Hat Quay cache proxy, the following features are available:

  • Specific organizations can be defined as a cache for upstream registries.
  • Configuration of a Quay organization that acts as a cache for a specific upstream registry. This repository can be defined by using the Quay UI, and offers the following configurations:

    • Upstream registry credentials for private repositories or increased rate limiting.
    • Expiration timer to avoid surpassing cache organization size.
  • Global on/off configurable via the configuration application.
  • Caching of entire upstream registries or just a single namespace, for example, all of docker.io or just docker.io/library.
  • Logging of all cache pulls.
  • Cached images scannability by Clair.
  • Caching of all layers when an image is pulled from a proxied repository, which helps ensure that Clair can scan all images and that images remain pullable even if the upstream registry becomes unavailable.

13.1. Proxy cache architecture

The following image shows the expected design flow and architecture of the proxy cache feature.

Proxy cache overview

When a user pulls an image, for example, postgres:14, from an upstream repository on Red Hat Quay, the repository checks to see if an image is present. If the image does not exist, a fresh pull is initiated. After being pulled, the image layers are saved to cache and server to the user in parallel. The following image depicts an architectural overview of this scenario:

Pulled image overview

If the image in the cache exists, users can rely on Quay’s cache to stay up-to-date with the upstream source so that newer images from the cache are automatically pulled. This happens when tags of the original image have been overwritten in the upstream registry. The following image depicts an architectural overview of what happens when the upstream image and cached version of the image are different:

Updating opposing layers overview

If the upstream image and cached version are the same, no layers are pulled and the cached image is delivered to the user.

In some cases, users initiate pulls when the upstream registry is down. If this happens with the configured staleness period, the image stored in cache is delivered. If the pull happens after the configured staleness period, the error is propagated to the user. The following image depicts an architectural overview when a pull happens after the configured staleness period:

Staleness pull overview

Quay administrators can leverage the configurable size limit of an organization to limit cache size so that backend storage consumption remains predictable. This is achieved by discarding images from the cache according to the frequency in which an image is used. The following image depicts an architectural overview of this scenario:

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