k3s/docs/adrs/embedded-registry.md

Package spegel Distributed Registry Mirror

Date: 2023-12-07

Status

Accepted

Context

Embedded registry mirror support has been on the roadmap for some time, to address multiple challenges:

• Upstream registries may enforce pull limits or otherwise throttle access to images.
• In edge scenarios, bandwidth is at a premium, if external access is available at all.
• Distributing airgap image tarballs to nodes, and ensuring that images remain available, is an ongoing hurdle to adoption.
• Deploying an in-cluster registry, or hosting a registry outside the cluster, put significant burden on administrators, and suffer from chicken-or-egg bootstrapping issues.

An ideal embedded registry would have several characteristics:

• Allow stateless configuration such that nodes can come and go at any time.
• Integrate into existing containerd registry mirror support.
• Integrate into existing containerd image stores such that an additional copy of layer data is not required.
• Use existing cluster authentication mechanisms to prevent unauthorized access to the registry.
• Operate with minimal added CPU and memory overhead.

Decision

• We will embed spegel within K3s, and use it to host a distributed registry mirror.
• The distributed registry mirror will be enabled cluster-wide via server CLI flag.
• Selection of upstream registries to mirror will be implemented via the existing registries.yaml configuration file.
• The registry API will be served via HTTPS on every node's private IP at port 6443. On servers this will use the existing supervisor listener; on agents a new listener will be created for this purpose.
• The default IPFS/libp2p port of 5001 will be used for P2P layer discovery.
• Access to the registry API and P2P network will require proof of cluster membership, enforced via client certificate or preshared key.
• Hybrid/multicloud support is out of scope; when the distributed registry mirror is enabled, cluster members are assumed to be directly accessible to each other via their internal IP on the listed ports.

Consequences

• The size of our self-extracting binary and Docker images increase by several megabytes.
• We take on the support burden of keeping spegel up to date, and supporting its use within K3s.