Caching

Each container built in a Factory publishes the build cache layers to our private registry, hub.foundries.io. Subsequent builds pull/import from this cache. This provides efficient, incremental container builds. This exporting and importing of build cache layers uses built-in features of Docker Buildx.

While the cache is useful, there are a few things to note:

• The build cache for each container image is branch specific
• It is architecture specific
• It can be invalidated for a few reasons, such as source files changing or a base image update.

Cache Invalidation

When trying to understand why a cache has been invalidated, there is not any tools that can assist. That said, in a Factory, each container build automatically uses the Docker build context to create a list of md5sums of all source files used.

Each Compose App generates an artifact named <compose-app-name>-md5sum.txt. This can then be used to generate a diff between builds, assisting in understanding what files may have changed, and how the cache may have been effected.

Generally, each line of your Dockerfile creates an image layer with a corresponding cache layer. If source files change, or any direct modification to the Dockerfile occurs, it will invalidate the cache from that point forward. Due to the importance of these concepts towards building Compose Apps, it is recommended to read the Dockerfile best practices..