There are few optional tasks that need to be executed by the administrator from time to time or during the initial configuration.
If a package is no longer downloadable by APT clients then its files are also not referenced in any index file and can be removed. This rule also applies to most volatile files from the distribution metadata. For example, Debian's Release file references some Packages and Sources files or Diff-Index file, and those do reference most other non-volatile files (binary packages, source packages, index diffs, ...).
To run the cleanup action manually visit the report page in a browser and trigger the Expiration operation there.
There are different flags configuring the parameters of this tracking described below. Usually just the filename is sufficient to consider a file in the cache as a valid (downloadable) file. This is ok in most cases but sometimes leads to false positives, i.e. when another repository in the cache refers to a file with the same name but the reference to the original location is gone. On the other hand there can be cases where the assignment to different repositories happened by mistake and administrator would like to merge repositories later on.
For most files the checksum values are also provided in the index files and so the file contents can be validated as well. This requires reading of the whole cache archive to generate local checksums. It should also not be done when apt-cacher-ng is being used (file locking is not used here).
Usually it's necessary to bring various index files (Release,Sources,Packages,Index) in sync with the repository. This is necessary because apt works around the whole file download by fetching small patches for the original file, and this mode of operation is not supported yet by apt-cacher-ng (and might still be unreliable). When this synchronization fails, the index files might be incomplete or obsolete or damaged, and they might no longer contain references to some files in the cache. Abortion of the cleanup process is advisable in this case.
There is also a precaution mechanism designed to prevent the destruction of cache contents when some volatile index files have been lost temporarily. The results of cache examination are stored in a list with the date when the particular files became orphaned. The removals are only executed after few days (configurable, see configuration file) unless they are removed from this list in the meantime.
Parameters of Expiration:
In additional to the default scan run, there are some "Direct Action" buttons in the Web frontend. It's possible to see the temporary list of files that have been identified as orphaned (unreferenced), and it's possible to delete all files from that list immediately. To be used carefully!
A program called acngtool is shipped with the package. Among other functionality, this tool effectively implements a HTTP client which operates like a human who runs the expiration manually (see above). If needed, it would read the admin credentials and unix socket file path from the local configuration files. On Debian installations it is called by the file /etc/cron.daily/apt-cacher-ng so it should run automatically as a daily cron task. The results are usually not reported unless an error occurs, in which case some hints are written to the standard error output (i.e. sent in cron mails).
The operator script can take some options from the environment, see below. The default operation mode is calling the expiration operation with default parameters and with credentials from local system's apt-cacher-ng installation. However, this can be changed with ACNGREQ variable.
ACNGIP=10.0.1.3
HOSTNAME=localOrPublicName
ACNGREQ=cgiparameters
Sometimes it makes sense to keep a couple of versions of (Debian) packages even after they have been removed from remote source. It is possible to set an exceptional rule for package files which follow the naming and versioning scheme of .deb-packages. This extra handling is configured by the KeepExtraVersions options which tells how many of the top-latest versions shall be kept. The cache system needs the dpkg program and sufficient CPU power (depending on the option value).
Sometimes it's needed to remove all files from a distribution, i.e. when a new release became Stable and older package files are still lying around. In perfect conditions the reference tracking described above should take care of it and remove them soon.
However, this solution will fail if the release files are still available on the server AND apt-cacher-ng learned their real location (i.e. the code name instead of not the release state name) and so they are refreshed during regular expiration.
After all, if the old release is no longer used by local cache users then the extra disk usage becomes a problem. This problem will go away after many months when the old release files are finally deleted on the servers, then the package expiration will start complaining for some days (the expiration delay) and only then the finally unreferenced files will be removed.
To speed up this process, the local administrator can remove the index files of the old distribution release from the archive. Either the top-level "Release" files, or even the whole index file trees relevant for certain releases.
To make this task easier, there is a special helper mode in the cleanup task of the maintenance page. Run this manually (and maybe enable extra verbosity on the main page) and use the wizard on the bottom of the log page to delete particular index files.