Using a Maven mirror to replace Maven Central can speed up the Impala build substantially. However, the artifacts that are present in the toolchain s3 bucket are unlikely to be able to resolved by the mirror, because they are not in Maven Central or other repositories. If the Maven mirror has a long list of source repositories, a miss can be expensive, because it may try each of the mirror's source repositories. It would be useful to exclude the s3 bucket Maven repositories from the mirroring. For example, this settings.xml would do that:
It mirrors everything that is not local and not from impala.cdp.repo (which points to an S3 bucket).
Unfortunately, this rule doesn't work. Everything still tries the mirror. Maven is trying repositories in the order that they are specified in the pom.xml, and it sees cdh.rcs.releases.repo before it sees impala.cdp.repo ( https://github.com/apache/impala/blob/master/java/pom.xml#L150 ). It also sees multiple banned repos (i.e. repos where both snapshots and releases are disabled). Based on my testing, seeing the cdh.rcs.releases.repo causes it to try the mirror, because it matches the mirrorOf conditions. It seems like the banned repositories may also a problem, depending on how smart Maven is.
Reordering the repositories can fix these semantics. If the impala.cdp.repo comes first (along with the impala.toolchain.kudu.repo), then anything that matches that would avoid hitting the mirror. Specifically, it seems like the best ordering would be impala.toolchain.kudu.repo (a local filesystem repo), impala.cdp.repo (an s3 repo), then the normal server repos, and lastly the banned repositories.