Re: How maven repositories are resolved?

[Debraj Manna <su...@gmail.com>]

Thanks John for such a detailed explanation. Few doubts

For release version assuming empty local repo.
> - Build order this time alpha, bravo, charlie
> - Alpha needs A, and Aa is downloaded as it's in central repo and that
> was 1st repo in the order list


When you say central repo is the first repo in the order list , how is
order determined? Is it because central repo is defined before company repo
in the settings.xml ?

What is the "cache" you are referring to in your answer?


On Thu, Jul 30, 2020 at 1:33 AM John Patrick <nh...@gmail.com> wrote:

> The answer depends on the following;
>
> 1) is it a release version or snapshot version. As if the release
> version is in local repo then no remote repo's are checked.
> 2) order your pom's are executed
>
> I've simplified your profiles with central repo and company repo, and
> Aa, Ab and Ac are the same GAV but it's just so in the explanation
> it's easier to know what A i'm talking about.
>
> settings
> - central repo (contains Aa)
> - company repo (contains Ab)
>
> parent-pom
> - alpha
> -- A
> - bravo (has extra R repository like your cloudera, contains Ac)
> -- A
> - charlie
> -- A
>
> For release version assuming empty local repo.
> - Build order this time alpha, bravo, charlie
> - Alpha needs A, and Aa is downloaded as it's in central repo and that
> was 1st repo in the order list
> - Bravo needs A, it sees Aa in local repo and uses it
> - Charlie needs A, it sees Aa in local repo and uses it
>
> Same as before but maven believe build order is bravo, alpha, charlie,
> i.e. you've changed an internal dependency or reorder you modules
> section.
> - Bravo needs A, and Ac is downloaded as it's in R repo and that was
> 1st repo in the order list, which i think it's pom repo's then parent
> pom's then settings file.
> - Alpha needs A, it sees Ac in local repo and uses it
> - Charlie needs A, it sees Ac in local repo and uses it
>
> Next is if A is a -SNAPSHOT dependency assuming empty local repo, Aa
> is v 1-1, Ab is v1-1 and Ac is v1-1
> - Build order this time alpha, bravo, charlie
> - Alpha needs A, and Aa is downloaded as it's in central repo and that
> was 1st repo in the order list, but it has a cache of Ab existing
> - Bravo needs A, it sees it doesn't have a cache of A from R repo, so
> looks up A from R repo, and if Ac in R repo is newer it is downloaded
> and used. in this case Aa is newer so Aa is used.
> - Charlie needs A, it sees all repo's have caches of A, and just uses
> the A in the local repo, in this case Aa is used.
>
> Next is if A is a -SNAPSHOT dependency assuming empty local repo, Aa
> is v 1-1, Ab is v1-1 and Ac is v1-2
> - Build order this time alpha, bravo, charlie
> - Alpha needs A, and Aa is downloaded as it's in central repo and that
> was 1st repo in the order list, but it has a cache of Ab existing
> - Bravo needs A, it sees it doesn't have a cache of A from R repo, so
> looks up A from R repo, and if Ac in R repo is newer it is downloaded
> and used. in this case Ac is newer so Ac is used.
> - Charlie needs A, it sees all repo's have caches of A, and just uses
> the A in the local repo, in this case Ac is used.
>
>
>
> As you can see it can get very messy when -SNAPSHOT's are being used
> and different pom's have repo. This is my hands-on experience of trial
> and error, debugging and troubleshooting of build processes.
>
> Hope it was helpful.
>
> John
>
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