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One of the things various people have asked MusicBrainz for time and time again has been genres. However, genres are hard to do right and they’re very much subjective—with MusicBrainz dealing almost exclusively with objective data. It’s been a recurring discussion on almost all of our summits, but a couple years ago (with some help from our friend Alastair Porter and his research colleagues at UPF), we finally came to a path forward—and recently Nicolás Tamargo (reosarevok) finally turned that path forward into code… which has now been released! You can see it in action on e.g., Nine Inch Nails’ Year Zero release group.
How does it work?
For now genres are exactly the same as (folksonomy) tags behind the scenes; some tags simply have become the chosen ones and are listed and presented as genres. The list of which tags are considered as genres is currently hardcoded, and no doubt it is missing a lot of our users’ favourite genres. We plan to expand the genre list based on your requests, so if you find a genre that is missing from it, request it by adding a style ticket with the “Genres” component.
As we mentioned above, genres are very subjective, so just like with folksonomy tags, you can upvote and downvote genres you agree or disagree with on any given entity, and you can also submit genre(s) for the entity that no one has added yet.
What about the API?
A bunch of the people asking for genres in MusicBrainz have been application developers, and this type of people are usually more interested in how to actually extract the genres from our data.
The method to request genres mirrors that of tags: you can use inc=genres to get all the genres everyone has proposed for the entity, or inc=user-genres to get all the genres you have proposed yourself (or both!). For the same release group as before, you’d want https://musicbrainz.org/ws/2/release-group/3bd76d40-7f0e-36b7-9348-91a33afee20e?inc=genres+user-genres for the XML API and https://musicbrainz.org/ws/2/release-group/3bd76d40-7f0e-36b7-9348-91a33afee20e?inc=genres+user-genres&fmt=json for the JSON API.
Since genres are tags, all the genres will continue to be served with inc=tags as before as well. As such, you can always use the tag endpoint if you would rather filter the tags by your own genre list rather than follow the MusicBrainz one, or if you want to also get other non-genre tags (maybe you want moods, or maybe you’re really interested in finding artists who perform hip hop music and were murdered – we won’t stop you!).
I use the database directly, not the API
You can parse the tag→genres from entities.json in the root of the “musicbrainz-server” repository which will give you a list of what we currently consider genres. Then you can simply compare any folksonomy tags from the %_tag tables.
Note about licensing
One thing to keep in mind for any data consumers out there is that, as per our data licensing, tags—and thus also genres—are not part of our “core (CC0-licensed) data”, but rather part of our “supplementary data” which is available under a Creative Commons Attribution-ShareAlike-NonCommercial license. Thus, if you wish to use our genre data for something commercial, you should get a commercial use license from the MetaBrainz Foundation. (Of course, if you’re going to provide a commercial product using data from MusicBrainz, you should always sign up as a supporter regardless. :)).
The future?
We are hoping to get a better coverage of genres (especially genres outside of the Western tradition, of which we have a very small amount right now) with your help! That applies both to expanding the genre list and actually applying genres to entities. For the latter, remember that everyone can downvote your genre suggestion if they don’t agree, so don’t think too much about “what genres does the world think apply to this artist/release/whatever”. Just add what you feel is right; if everyone does that we’ll get much better information. 
In the near future we’re hoping to move the genre list from the code to the database (which shouldn’t mean too much for most of you, other than less waiting between a new genre being proposed for the list and it being added, but is much better for future development). Also planned is a way to indicate that several tags are the same genre (so that if you tag something as “hiphop”, “hip hop” or “hip-hop” the system will understand that’s really all the same). Further down the line, who knows! We might eventually make genres into limited entities of a sort, in order to allow linking to, say, the appropriate Wikidata/Wikipedia pages. We might do some fun stuff. Time will tell!
