[RFC] Introduce express middleware extension point

CODEOWNERS

@@ -21,6 +21,7 @@ packages/cli/* @raymondfeng @bajtos
 packages/context/* @bajtos @raymondfeng
 packages/core/* @bajtos @raymondfeng
 packages/eslint-config/* @raymondfeng
+packages/express-middleware/* @raymondfeng
 packages/metadata/* @raymondfeng
 packages/model-api-builder/* @bajtos @nabdelgadir
 packages/openapi-spec-builder/* @bajtos @raymondfeng

docs/site/MONOREPO.md

@@ -32,6 +32,7 @@ The [loopback-next](https://github.com/strongloop/loopback-next) repository uses
 | [example-todo](https://github.com/strongloop/loopback-next/tree/master/examples/todo)                                        | @loopback/example-todo                | A basic tutorial for getting started with Loopback 4                                                                                                                                                                                                                                             |
 | [http-caching-proxy](https://github.com/strongloop/loopback-next/tree/master/packages/http-caching-proxy)                    | @loopback/http-caching-proxy          | A caching HTTP proxy for integration tests. NOT SUITABLE FOR PRODUCTION USE!                                                                                                                                                                                                                     |
 | [http-server](https://github.com/strongloop/loopback-next/tree/master/packages/http-server)                                  | @loopback/http-server                 | A wrapper for creating HTTP/HTTPS servers                                                                                                                                                                                                                                                        |
+| [express-middleware](https://github.com/strongloop/loopback-next/tree/master/packages/express-middleware)                    | @loopback/express-middleware          | Extensions to manage Express middleware                                                                                                                                                                                                                                                          |
 | [metadata](https://github.com/strongloop/loopback-next/tree/master/packages/metadata)                                        | @loopback/metadata                    | Utilities to help developers implement TypeScript decorators, define/merge metadata, and inspect metadata                                                                                                                                                                                        |
 | [model-api-builder](https://github.com/strongloop/loopback-next/tree/master/packages/model-api-builder)                      | @loopback/model-api-builder           | Types and helpers for packages contributing Model API builders.                                                                                                                                                                                                                                  |
 | [openapi-spec-builder](https://github.com/strongloop/loopback-next/tree/master/packages/openapi-spec-builder)                | @loopback/openapi-spec-builder        | Builders to create OpenAPI (Swagger) specification documents in tests                                                                                                                                                                                                                            |

docs/site/Reserved-binding-keys.md

@@ -43,3 +43,4 @@ the prefix. Example: `@loopback/authentication` component uses the prefix
 - [RestBindings](https://loopback.io/doc/en/lb4/apidocs.rest.restbindings.html)
 - [RestBindings.Http](https://loopback.io/doc/en/lb4/apidocs.rest.http.html)
 - [RestBindings.SequenceActions](https://loopback.io/doc/en/lb4/apidocs.rest.sequenceactions.html)
+- [ExpressBindings](https://loopback.io/doc/en/lb4/apidocs.express-middleware.expressbindings.html)

packages/express-middleware/LICENSE

@@ -0,0 +1,25 @@
+Copyright (c) IBM Corp. 2017,2019. All Rights Reserved.
+Node module: @loopback/express-middleware
+This project is licensed under the MIT License, full text below.
+
+--------
+
+MIT license
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

packages/express-middleware/README.md

@@ -0,0 +1,128 @@
+# @loopback/express-middleware
+
+[Express middleware](https://expressjs.com/en/guide/using-middleware.html) are
+key building blocks for Express applications. Simple APIs are provided by
+Express to set up middleware. It works for applications where the middleware
+chain registration can be centralized, such as:
+
+```js
+app.use(middlewareHandler1);
+app.use(middlewareHandler2);
+// ...
+app.use(middlewareHandlerN);
+```
+
+The middleware handlers will be executed by Express in the order of `app.use()`
+is called. There are obvious limitations of this approach if your application
+allows multiple modules to contribute middleware.
+
+1. It's hard to align middleware in the right order
+2. It's hard to separate configuration of middleware
+
+This module introduces a simple extension that utilizes LoopBack 4's inversion
+of controller (Ioc) dependency injection (DI) container offered by
+`@loopback/context`.
+
+## Overview
+
+We introduce `MiddlewareRegistry` to manage middleware registration and provide
+a handler function for Express applications.
+
+1. `MiddlewareRegistry` allows us to bind middleware handlers with optional
+   settings such as `path` and `phase` as tags. Each binding is tagged with
+   `middleware` so that it can be discovered.
+
+2. `MiddlewareRegistry` sorts the middleware entries using `phase`. The order of
+   phases can be configured.
+
+3. `MiddlewareRegistry` exposes `requestHandler` function which is an Express
+   middleware handler function that can be mounted to Express applications.
+
+4. `MiddlewareRegistry` observes binding events to keep track of latest list of
+   middleware and rebuilds the chain if necessary.
+
+## Installation
+
+To use this package, you'll need to install `@loopback/express-middleware`.
+
+```sh
+npm i @loopback/express-middleware
+```
+
+## Basic Use
+
+### Register middleware handler
+
+Handler functions compatible with Express middleware can be registered as
+follows:
+
+```ts
+const middlewareRegistry = new MiddlewareRegistry();
+middlewareRegistry.middleware(cors(), {phase: 'cors'});
+const helloRoute: RequestHandler = (req, res, next) => {
+  res.setHeader('content-type', 'text/plain');
+  res.send('Hello world!');
+};
+middlewareRegistry.middleware(helloRoute, {phase: 'route', path: '/api'});
+```
+
+Please note we allow `phase` to be set for a given middleware.
+
+### Register middleware provider class
+
+To leverage dependency injection, middleware can be wrapped as a provider class.
+For example:
+
+```ts
+@middleware({phase: 'log', name: 'logger'})
+class LogMiddlewareProvider implements Provider<RequestHandler> {
+  constructor(
+    @config()
+    private options: Record<string, string> = {},
+  ) {}
+
+  value(): RequestHandler {
+    /** We use wrap existing express middleware here too
+     * such as:
+     * const m = require('existingExpressMiddleware');
+     * return m(this.options);
+     */
+    return (req, res, next) => {
+      recordReq(req, this.options.name || 'logger');
+      next();
+    };
+  }
+}
+```
+
+Then it can be configured and registered to the express context:
+
+```ts
+middlewareRegistry.middlewareProvider(LogMiddlewareProvider);
+middlewareRegistry.configure('middleware.logger').to({name: 'my-logger'});
+```
+
+### Mount to Express application
+
+```ts
+const expressApp = express();
+expressApp.use(middlewareRegistry.requestHandler);
+```
+
+## Contributions
+
+- [Guidelines](https://github.com/strongloop/loopback-next/blob/master/docs/CONTRIBUTING.md)
+- [Join the team](https://github.com/strongloop/loopback-next/issues/110)
+
+## Tests
+
+Run `npm test` from the root folder.
+
+## Contributors
+
+See
+[all contributors](https://github.com/strongloop/loopback-next/graphs/contributors).
+
+## License
+
+MIT

packages/express-middleware/index.d.ts

@@ -0,0 +1,6 @@
+// Copyright IBM Corp. 2019. All Rights Reserved.
+// Node module: @loopback/express-middleware
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+export * from './dist';

packages/express-middleware/index.js

@@ -0,0 +1,6 @@
+// Copyright IBM Corp. 2019. All Rights Reserved.
+// Node module: @loopback/express-middleware
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+module.exports = require('./dist');

packages/express-middleware/index.ts

@@ -0,0 +1,8 @@
+// Copyright IBM Corp. 2019. All Rights Reserved.
+// Node module: @loopback/express-middleware
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+// DO NOT EDIT THIS FILE
+// Add any additional (re)exports to src/index.ts instead.
+export * from './src';