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https://issues.apache.org/jira/browse/MNG-7389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17472319#comment-17472319
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Thomas Skjølberg commented on MNG-7389:
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Thanks for the input, your suggested solution seems straigthforward.
Just to be clear, while disc space is cheap, the limiting factor is typically
the network capacity and to some extent CPU (compress / decompress). Usually CI
docker images are downloaded and executed, then Maven cache is restored on top
of them as part of CI workflow logic. So the opportunity here is to reduce cost
of CI, reduce wait time for developers and speed up deploy pipelines.
I do not have any expectations of implementation timeline or participants, I am
just attempting to point out a relativly low-hanging fruit which will save time
and money every day, albeit more for some systems than others. Using our own
(CircleCI) systems as an example, building an app takes perhaps 10 minutes.
Something like 1 in 5 commits update the pom file, resulting in the build
taking about 1 minute longer (to transfer artifacts). So that is (very
approximately) a 2% improvement; as an optimization this is not great, but fair
enough.
> Incremental .m2 cache cleanup for CI
> ------------------------------------
>
> Key: MNG-7389
> URL: https://issues.apache.org/jira/browse/MNG-7389
> Project: Maven
> Issue Type: New Feature
> Components: Dependencies
> Reporter: Thomas Skjølberg
> Priority: Minor
>
> One or more popular continous integration are unable to properly manage the
> .m2 repository cache, resulting in wasted resources in the form of increased
> CI runtime and bandwidth consumption.
> *CircleCI cache behaviour:*
> - immutable cache entries
> - default behaviour is to wipe the cache each time a pom file is modified
> (i.e. using pom hash as a cache key)
> - cache entries TTL > weeks
> So CircleCI always has a cache containing only the necessary artifacts, but
> has to download all dependencies every time the pom file changes.
> *Github Actions cache behaviour*
> - (effectively) mutable cache entries
> - incremental cache (if it gets too big, it is wiped).
> - cache entries TTL 1 week
> So Github actions work well if the cache entries expire from time to time,
> otherwise the cache keeps growing.
> *Summary*
> Perhaps this does not look so bad at first glance, but for a project under
> active development, with a lot of artifacts, the pom file changes often. For
> example we have apps with 100 dependencies and automatic dependency bumping
> via Renovate, in addition to an hierarchy of libraries.
> Key takeaways; time is wasted
> - saving caches in CI
> - loading cache in CI
> - loading artifacts from external artifact store
> This happens quite a lot. From the artifact store perspective, this probably
> multiplies the load by a factor of 10.
> Possible solution: A way to define a "transaction" for artifact use, i.e.
> 1. run command to mark start of transaction
> 2. run one or more maven commands
> 3. run command to mark end of transaction, deleting artifacts not in use.
> For reference, Gradle has the same problem.
> Proof of concept:
> * CircleCI : [https://github.com/entur/maven-orb]
> * Github actions: [https://github.com/skjolber/tidy-cache-github-action]
> The implementation uses instrumentation to record artifact access, then
> delete the artifacts not recorded.
> *Alternatives:*
> I did try the last-accessed file timestamp first, turns out most CI
> filesystems are mounted without that option. However it should also be
> possible to update the modified timestamp and/or add read access to some
> existing metadata file.
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