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Cache

The apk library has the option to cache apk files when downloading. This can provide dramatic speedups when installing packages, especially when you have a large number of packages to install.

This is completely independent of the target FS where you install the packages themselves. You might have multiple install runs, all using similar packages. For example, if you need busybox-1.36.2-r0.apk for multiple installs, you only would need to download it once.

The cache is not enabled by default. It only is enabled by providing the WithCache() option to the New() function.

Cache Location

The cache location will be in any provided directory. If you wish to use the default directory, you can pass "" to the WithCache() option:

a, err := apk.New(
    apk.WithCache(""),
)

No WithCache() option disables use of the cache.

Note that the default changes by platform, and is determined by os.userCacheDir().

Which Repositories are Cache

Only remote repositories are cached. Those referenced using local filesystems, e.g. ./packages/foo, are not cached, as they provide no value in caching.

Cache Structure

The cache directory provided is the root of the cache. Underneath that directory you will find the following structure.

There is a directory for each repository used. Because remote repositories can contain invalid directory characters, the name is URL-encoded. For example, a repository https://dl-cdn.alpinelinux.org/alpine/v3.14/main would be encoded as https%3A%2F%2Fdl-cdn.alpinelinux.org%2Falpine%2Fv3.14%2Fmain

Underneath each repository directory is a directory that looks identical to the repository. There is a directory for each architecture, inside of which is an .apk file for each package.

When a file is retrieved, if available, the etag header is saved alongside the file. if it is available, it is saved in a file <filename>.etag. This is used to determine if the file has changed.

Behaviour if no local etag is available depends on how it was called:

  • APKINDEX.tar.gz - we assume that it can change, and thus no etag found locally means always retrieve it.
  • .apk files - we assume that they do not change, and thus no etag found locally means the file is accepted as is.