cosmos-api | applications built on the Cosmos SDK | SDK library

hm is short hermes, a protocol used internally between servers at Spotify. It is basically zeromq with a protobuf envelope with some defined headers.

So, kind of like HTTP define verbs and structure on-top of TCP, Hermes define verbs and structure on-top of zeromq. It is used for HTTP-like Request/Response as well as Publish/Subscribe. For instance, in the example you found, a client request data about an album and waits for a response. Another example could be a client subscribing to events about a playlist. The moment someone publishes a change to the playlist, the client will know.

It gets extra complicated in the example you found. It seems to be a javascript snippet found in the Spotify web player. Javascript in the browser can of course not talk this protocol, so because you see the uri there, it means there is some kind of tunneling going on as well.

In one sense, it can do more than HTTP, but in another sense, it is much simpler because of the limited use. It was built many years ago, before HTTP/2 and grpc. It is still used heavily at Spotify.

I found a thesis work and an industry article that mentioned hermes in more detail.

http://www.diva-portal.org/smash/get/diva2:706244/FULLTEXT01.pdf https://www.csc.kth.se/~gkreitz/spotifypubsub/spotifypubsub.pdf