I would like to understand how the indexing for the artifact repositories like Nexus and Artifactory works. What benefit does it provide? I mean -- how does it help and what is the logic that's used when resolving artifacts?
My understanding is that the Lucene indexes contain information concerning which artifacts are presents in a given proxied repository or group and that once these indexes have been downloaded, you can easily check if a remote repository contains the artifact you're looking for and you can try to resolve it from the repositories which have it. Is this the only use? Is the index also queried for local resolutions (because each repository does have an index)...? How does this actually work?
As Mark already said, Maven Index is all about searching (either server side, where search is exposed over UI, or using REST) or client side like for example M2E does (typical example is code completion in POM editor, where context hints uses index to provide you Gs, As and Vs while adding dependencies for example).
Nexus does NOT use index to fulfil it's main functionality: serving up artifacts and/or proxying them, while it DOES maintain the index on the fly. Again, indexes are not used in "resolution" or any other way, except for Search UI and downstream publishing reason (for clients like M2E is).
For example "client side" usage of Maven Indexer, you can look at the examples here.
HTH,
~t~