api versioning / api version call

[tykling]

implement some sort of api versioning, and make an api call so clients can determine which API version a server supports

[rlindsgaard]

I suggest a naming scheme /api/v1/endpoint /api/v2/endpoint etc. Servers are obliged to support older api schemes. The client then negotiates by getting a 200 response from the server when visiting /api/v2 and downgrades to /api/v1 if necessary. We could also use /api/x.y/ if necessary.