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Proxying

Overview

The Backstage backend comes packaged with a basic HTTP proxy, that can aid in reaching backend service APIs from frontend plugin code. See Call Existing API for a description of when the proxy can be the best choice for communicating with an API.

Getting Started

The plugin is already added to a default Backstage project.

In packages/backend/src/index.ts:

const proxyEnv = useHotMemoize(module, () => createEnv('proxy'));

const service = createServiceBuilder(module)
.loadConfig(configReader)
/** ... other routers ... */
.addRouter('/proxy', await proxy(proxyEnv));

Configuration

Configuration for the proxy plugin lives under a proxy root key of your app-config.yaml file.

Example:

# in app-config.yaml
proxy:
/simple-example: http://simple.example.com:8080
'/larger-example/v1':
target: http://larger.example.com:8080/svc.v1
headers:
Authorization: ${EXAMPLE_AUTH_HEADER}
# ...or interpolating a value into part of a string,
# Authorization: Bearer ${EXAMPLE_AUTH_TOKEN}

Each key under the proxy configuration entry is a route to match, below the prefix that the proxy plugin is mounted on. If it does not start with a slash, one will be prefixed automatically. For example, if the backend mounts the proxy plugin as /proxy, the above configuration will lead to the proxy acting on backend requests to /api/proxy/simple-example/... and /api/proxy/larger-example/v1/....

The value inside each route is either a simple URL string, or an object on the format accepted by http-proxy-middleware.

If the value is a string, it is assumed to correspond to:

target: <the string>
changeOrigin: true
pathRewrite:
'^<url prefix><the string>/': '/'

When the target is an object, it is given verbatim to http-proxy-middleware except with the following caveats for convenience:

  • If changeOrigin is not specified, it is set to true. This is the most commonly useful value.
  • If pathRewrite is not specified, it is set to a single rewrite that removes the entire prefix and route. In the above example, a rewrite of '^/api/proxy/larger-example/v1/': '/' is added. That means that a request to /api/proxy/larger-example/v1/some/path will be translated to a request to http://larger.example.com:8080/svc.v1/some/path.

There are also additional settings:

  • allowedMethods: Limit the forwarded HTTP methods. For example allowedMethods: ['GET'] enforces read-only access.
  • allowedHeaders: A list of headers that should be forwarded to and received from the target.

By default, the proxy will only forward safe HTTP request headers to the target. Those are based on the headers that are considered safe for CORS and includes headers like content-type or last-modified, as well as all headers that are set by the proxy. If the proxy should forward other headers like authorization, this must be enabled by the allowedHeaders config, for example allowedHeaders: ['Authorization']. This should help to not accidentally forward confidential headers (cookie, X-Auth-Request-User) to third-parties.

The same logic applies to headers that are sent from the target back to the frontend.