Proxying requests to Upstream APIs

To proxy requests to an upstream API, you can simply use a combination of two FLAT Swagger enhancements:

• wildcard paths, and

• x-flat-proxy.

Imagine, the upstream API, you want to use, is located at https://upstream.api/api/docs and provides a route to get a document with a certain ID (/get-doc/{docid}). To set FLAT as a proxy to this API, the following swagger.yaml snippet will do the job:

…
basePath: /api
…
paths:
  /docs/**:
    x-flat-proxy:
      origin: https://upstream.api    # replaces the origin
…

Assuming a client origin of https://client.example.com, a client request to e.g. https://client.example.com/api/docs/get-doc/42 will be proxied to https://upstream.api/api/docs/get-doc/42.

Note, that only the origin is replaced, the path is not modified.

If the upstream API for docs is located at https://docs.upstream.api/v4 instead of https://upstream.api/api/docs, the path has to be adjusted, too:

…
basePath: /flat
…
paths:
  /docs/**:
    x-flat-proxy:
      origin: https://docs.upstream.api
      stripEndpoint: true             # strips /flat/docs from the path
      addPrefix: /v4                  # inserts /v4 before the stripped path
…

The client request will be proxied to https://docs.upstream.api/v4/get-doc/42.

Assuming you want to plug-in a different API (https://users.upstream/v3), just add the following:

…
  /users/**:
    x-flat-proxy:
      origin: https://users.upstream.api
      stripEndpoint: true             # strips /flat/users from the path
      addPrefix: /v3                  # inserts /v3 before the stripped path

A client request to https://client.example.com/flat/users/profile/4711 will be proxied to https://users.upstream.api/v3/profile/4711.