Merge pull request #2132 from spotify/freben/proxy-config

feat(proxy-backend): more proxy config tweaks and docs

app-config.yaml

@@ -14,21 +14,15 @@ backend:
     client: sqlite3
     connection: ':memory:'
 
+# See README.md in the proxy-backend plugin for information on the configuration format
 proxy:
-  '/circleci/api':
-    target: 'https://circleci.com/api/v1.1'
-    changeOrigin: true
-    pathRewrite:
-      '^/proxy/circleci/api/': '/'
+  '/circleci/api': https://circleci.com/api/v1.1
   '/jenkins/api':
-    target: 'http://localhost:8080'
-    changeOrigin: true
+    target: http://localhost:8080
     headers:
       Authorization:
         $secret:
           env: JENKINS_BASIC_AUTH_HEADER
-    pathRewrite:
-      '^/proxy/jenkins/api/': '/'
 
 organization:
   name: Spotify

docs/plugins/call-existing-api.md

@@ -71,11 +71,7 @@ Example:
 ```yaml
 # In app-config.yaml
 proxy:
-  '/frobs':
-    target: 'http://api.frobsco.com/v1'
-    changeOrigin: true
-    pathRewrite:
-      '^/proxy/frobs/': '/'
+  '/frobs': http://api.frobsco.com/v1
 ```
 
 ```ts
@@ -86,9 +82,8 @@ fetch(`${backendUrl}/proxy/frobs/list`)
   .then(payload => setFrobs(payload as Frob[]));
 ```
 
-The proxy is powered by the `http-proxy-middleware` package, and supports all of
-its
-[configuration options](https://github.com/chimurai/http-proxy-middleware#options).
+The proxy is powered by the `http-proxy-middleware` package. See
+[Proxying](proxying.md) for a full description of its configuration options.
 
 Internally at Spotify, the proxy option has been the overwhelmingly most popular
 choice for plugin makers. Since we have DNS based service discovery in place and

docs/plugins/proxying.md

@@ -3,4 +3,73 @@ id: proxying
 title: Proxying
 ---
 
-## TODO
+## 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](call-existing-api.md) 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`:
+
+```ts
+const proxyEnv = useHotMemoize(module, () => createEnv('proxy'));
+
+const service = createServiceBuilder(module)
+  .loadConfig(configReader)
+  /** ... other routers ... */
+  .addRouter('/proxy', await proxy(proxyEnv, '/proxy'));
+```
+
+## Configuration
+
+Configuration for the proxy plugin lives under a `proxy` root key of your
+`app-config.yaml` file.
+
+Example:
+
+```yaml
+# 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:
+        $secret:
+          env: EXAMPLE_AUTH_HEADER
+```
+
+Each key under the proxy configuration entry is a route to match, below the
+prefix that the proxy plugin is mounted on. It must start with a slash. For
+example, if the backend mounts the proxy plugin as `/proxy`, the above
+configuration will lead to the proxy acting on backend requests to
+`/proxy/simple-example/...` and `/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](https://www.npmjs.com/package/http-proxy-middleware).
+
+If the value is a string, it is assumed to correspond to:
+
+```yaml
+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
+  `'^/proxy/larger-example/v1/': '/'` is added. That means that a request to
+  `/proxy/larger-example/v1/some/path` will be translated to a request to
+  `http://larger.example.com:8080/svc.v1/some/path`.

packages/backend/src/index.ts

@@ -82,7 +82,7 @@ async function main() {
     .addRouter('/auth', await auth(authEnv))
     .addRouter('/identity', await identity(identityEnv))
     .addRouter('/techdocs', await techdocs(techdocsEnv))
-    .addRouter('/proxy', await proxy(proxyEnv))
+    .addRouter('/proxy', await proxy(proxyEnv, '/proxy'))
     .addRouter('/graphql', await graphql(graphqlEnv));
 
   await service.start().catch(err => {

packages/backend/src/plugins/proxy.ts

@@ -13,13 +13,14 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 // @ts-ignore
 import { createRouter } from '@backstage/plugin-proxy-backend';
 import { PluginEnvironment } from '../types';
 
-export default async function createPlugin({
-  logger,
-  config,
-}: PluginEnvironment) {
-  return await createRouter({ logger, config });
+export default async function createPlugin(
+  { logger, config }: PluginEnvironment,
+  pathPrefix: string,
+) {
+  return await createRouter({ logger, config, pathPrefix });
 }

packages/create-app/templates/default-app/packages/backend/src/index.ts

@@ -55,7 +55,7 @@ async function main() {
     .addRouter('/auth', await auth(authEnv))
     .addRouter('/identity', await identity(identityEnv))
     .addRouter('/techdocs', await techdocs(techdocsEnv))
-    .addRouter('/proxy', await proxy(proxyEnv));
+    .addRouter('/proxy', await proxy(proxyEnv, '/proxy'));
 
   await service.start().catch(err => {
     console.log(err);

packages/create-app/templates/default-app/packages/backend/src/plugins/proxy.ts

@@ -2,9 +2,9 @@
 import { createRouter } from '@backstage/plugin-proxy-backend';
 import { PluginEnvironment } from '../types';
 
-export default async function createPlugin({
-  logger,
-  config,
-}: PluginEnvironment) {
-  return await createRouter({ logger, config });
+export default async function createPlugin(
+  { logger, config }: PluginEnvironment,
+  pathPrefix: string,
+) {
+  return await createRouter({ logger, config, pathPrefix });
 }

plugins/proxy-backend/README.md

@@ -1,6 +1,7 @@
 # Proxy backend plugin
 
-This is the backend plugin that enables proxy definitions to be declared in and read from app-config.yaml.
+This is the backend plugin that enables proxy definitions to be declared in,
+and read from, `app-config.yaml`.
 
 Relies on the `http-proxy-middleware` package.
 
@@ -10,27 +11,22 @@ This backend plugin can be started in a standalone mode from directly in this pa
 with `yarn start`. However, it will have limited functionality and that process is
 most convenient when developing the plugin itself.
 
-To run it within the backend do:
-
-1. Register the router in `packages/backend/src/index.ts`:
-
-```ts
-const proxyEnv = useHotMemoize(module, () => createEnv('proxy'));
-
-const service = createServiceBuilder(module)
-  .loadConfig(configReader)
-  /** several different routers */
-  .addRouter('/', await proxy(proxyEnv));
-```
-
-2. Start the backend
+The proxy is already installed in the Backstage backend per default, so you can also
+start up the full example backend to experiment with the proxy.
 
 ```bash
 yarn workspace example-backend start
 ```
 
-This will launch the full example backend.
+## Configuration
+
+See [the proxy docs](https://backstage.io/docs/plugins/proxying).
 
 ## Links
 
+- [Call Existing API](https://backstage.io/docs/plugins/call-existing-api) helps the
+  decision process of what method of communication to use from a frontend plugin to
+  your API
+- [The proxy plugin documentation](https://backstage.io/docs/plugins/proxying) describes
+  configuration options and more
 - [http-proxy-middleware](https://www.npmjs.com/package/http-proxy-middleware)

plugins/proxy-backend/src/service/router.test.ts

@@ -26,6 +26,7 @@ describe('createRouter', () => {
     const router = await createRouter({
       config,
       logger,
+      pathPrefix: '/proxy',
     });
     expect(router).toBeDefined();
   });

plugins/proxy-backend/src/service/router.ts

@@ -17,21 +17,57 @@
 import { Config } from '@backstage/config';
 import express from 'express';
 import Router from 'express-promise-router';
-import createProxyMiddleware from 'http-proxy-middleware';
+import createProxyMiddleware, {
+  Config as ProxyConfig,
+  Proxy,
+} from 'http-proxy-middleware';
 import { Logger } from 'winston';
 
 export interface RouterOptions {
   logger: Logger;
   config: Config;
+  // The URL path prefix that the router itself is mounted as, commonly "/proxy"
+  pathPrefix: string;
+}
+
+// Creates a proxy middleware, possibly with defaults added on top of the
+// given config.
+function buildMiddleware(
+  pathPrefix: string,
+  route: string,
+  config: string | ProxyConfig,
+): Proxy {
+  const fullConfig =
+    typeof config === 'string' ? { target: config } : { ...config };
+
+  // Default is to do a path rewrite that strips out the proxy's path prefix
+  // and the rest of the route.
+  if (fullConfig.pathRewrite === undefined) {
+    const routeWithSlash = route.endsWith('/') ? route : `${route}/`;
+    fullConfig.pathRewrite = {
+      [`^${pathPrefix}${routeWithSlash}`]: '/',
+    };
+  }
+
+  // Default is to update the Host header to the target
+  if (fullConfig.changeOrigin === undefined) {
+    fullConfig.changeOrigin = true;
+  }
+
+  return createProxyMiddleware(fullConfig);
 }
 
 export async function createRouter(
   options: RouterOptions,
 ): Promise<express.Router> {
   const router = Router();
-  const proxyConfig = options.config.get('proxy') ?? {};
+
+  const proxyConfig = options.config.getOptional('proxy') ?? {};
   Object.entries(proxyConfig).forEach(([route, proxyRouteConfig]) => {
-    router.use(route, createProxyMiddleware(proxyRouteConfig));
+    router.use(
+      route,
+      buildMiddleware(options.pathPrefix, route, proxyRouteConfig),
+    );
   });
 
   return router;

plugins/proxy-backend/src/service/standaloneServer.ts

@@ -40,6 +40,7 @@ export async function startStandaloneServer(
   const router = await createRouter({
     config,
     logger,
+    pathPrefix: '/proxy',
   });
   const service = createServiceBuilder(module)
     .enableCors({ origin: 'http://localhost:3000' })