From Graduate to Maintainer: My Journey Solving Sealer’s Image Caching Challenges

Overview

Sealer is an open‑source cloud‑native tool (originally from Alibaba Cloud, now a CNCF sandbox project) that packages, distributes and runs distributed software on Kubernetes clusters. The project focuses on cluster image management.

Image Module Responsibilities

Cluster image cache – reuse image layers to accelerate builds.

Cluster dependency image cache – automatically cache all container images required by a cluster without user intervention.

Cluster image sharing – push/pull/save/load of cluster images similar to Docker.

Technical Challenges

Caching all dependent container images during the build process, especially for offline clusters.

Providing a proxy for private container registries because Docker daemon does not support registry mirrors for non‑Docker‑Hub registries.

Investigation

The Docker engine ( moby) and the distribution registry ( distribution) source code were examined. The registry’s pull‑through cache supports only a single remoteurl configuration, which is insufficient when a cluster depends on images from multiple registries.

proxy:
  remoteurl: https://registry-1.docker.io
  username: [username]
  password: [password]

Solution

Incremental configuration items were added to:

Allow Docker to proxy any container registry.

Enable the registry to cache multiple remote repositories without requiring explicit user configuration (manual overrides remain possible).

The enhanced capabilities are illustrated in the diagram:

Impact

Within a few months of open‑sourcing, Sealer gained its first production user (Zhengcai Cloud). Refactoring moved all image‑related operations into a dedicated module and introduced a filesystem abstraction layer, improving code modularity and reducing the risk of accidental file manipulation.

Future Work

Performance optimization for faster and more stable delivery.

Architectural refactoring to decouple modules and lower the entry barrier for contributors.

Expand the ecosystem by encouraging community contributions to cluster image building.

Improve quick‑start guides and increase developer participation.

Collaborate with related projects such as openyurt (https://github.com/openyurtio/openyurt) and sealos (https://github.com/labring/sealos).

References

Sealer repository: https://github.com/sealerio/sealer

Docker private‑registry mirror issue: https://github.com/moby/moby/issues/18818

Docker source: https://github.com/moby/moby

Distribution source: https://github.com/distribution/distribution