Nine chapters have taught one style of API very thoroughly: addresses, verbs, status codes, JSON, keys, errors. That style is the one you will meet at almost every public counter, which is exactly why it was worth walking end to end. But it is not the only style in the world, and the day you open a documentation page written in a different one, the right feeling is recognition rather than alarm. This chapter names the four others you are most likely to meet, one page each, and then turns back to your own desk for the tools that put a friendly face on the requests you already speak.
Everything so far has been one dialect. It has a name that gets used loosely in job ads and meetings: REST, or more honestly "an API over HTTP with addresses for things and verbs for intentions." Tandem speaks it, most public partner APIs speak it, and the fluency you now have transfers across almost all of them without adjustment.
This chapter is an atlas, not a lab. Four other styles get one page apiece, and each page answers the same four questions: what is it, what problem was it built for, what does it cost, and how do I spot one in the wild. You will not practice them here, and you do not need to. Recognizing which species of API is in front of you — and knowing who to hand it to — is a genuinely professional skill on its own, and it is the one this chapter gives you.
One family tree, one branch walked deeply, the rest now recognizable
APIs
a promise between programs, written down
REST over HTTP
this book, end to end
the public counter, almost everywhere
GraphQL
one address, and you describe the answer you want
gRPC
compiled contracts and binary messages, machine to machine