Modularize the concept of "servers" (including the Admin)

[jesstelford]

From https://github.com/keystonejs/keystone-5/pull/960#issuecomment-480101846

Goals

1️⃣ Maintain separation between index.js & server.js ("Custom Server").
2️⃣ You'd setup UI's/Servers in one or the other of index.js or server.js. Ie; If you have a Custom Server, you should not export servers: from index.js (technically you could, it just wouldn't do anything).
3️⃣ Reframe the Admin UI from being "something special" to "Just another frontend"
4️⃣ All the UI's / Servers follow a common API (very similar to how we already do fields!)
5️⃣ The custom server is still un-opinionated, and you can really do whatever you want there.
6️⃣ All the servers need a build step for preparing for a production deploy
7️⃣ Without a Custom Server, the CLI would become just index.servers.forEach(server => app.use(server.middleware()))

Example

index.js

const { Keystone } = require('@keystone-alpha/keystone');
const AdminUI = require('@keystone-alpha/server-admin');
const NextUI = require('@keystone-alpha/server-next');
const StaticUI = require('@keystone-alpha/server-static');

const keystone = new Keystone(/* ... */);

const admin = new AdminUI({
  path: '/admin',
  keystone,
  session: authStrategy.sessionHandler(),
});

const next = new NextUI({
  path: '/'
  session: authStrategy.sessionHandler(),
  nextConfig: { /* ... */ },
});

const staticFiles = new StaticUI({
  path: `${__dirname}/static`,
  route: '/public',
});

module.exports = {
  keystone,
  servers: [
    staticFiles,
    admin,
    next
  ],
};

- OR -

server.js

const keystone = require('@keystone-alpha/core');
const path = require('path');
const PORT = process.env.PORT || 3000;
const AdminUI = require('@keystone-alpha/server-admin');
const NextUI = require('@keystone-alpha/server-next');

const { keystone } = require('./index');

keystone.prepare().then(() => {
  const app = express();

  const admin = new AdminUI({
    path: '/admin',
    keystone,
    session: authStrategy.sessionHandler(),
  });

  const next = new NextUI({
    path: '/'
    session: authStrategy.sessionHandler(),
    nextConfig: { /* ... */ },
  });

  app.use(admin.middleware());
  app.use(next.middleware());

  app.listen(3000);
}).catch(error => {
  console.error(error);
  process.exit(1);
});

@keystone-alpha/server-next

const next = require('next');
module.exports = class {
  constructor(nextConfig) {
    this.config = nextConfig;
  }

  middleware(server) {
    const nextApp = next(nextConfig);
    server.use(nextApp.requestHandler());
  }

  build() {
    this.next.build({ out: this.config.distDir });
  }
}

@keystone-alpha/server-static

module.exports = class {
  constructor(config) {
    this.config = config;
  }

  middleware(server) {
    server.use(express.static(this.config.route, this.config.path));
  }

  build() {
    // noop
  }
}

@keystone-alpha/server-create-react-app

module.exports = class {
  constructor(craConfig) {
    /* ... */
  }

  middleware(server) {
    server.use(express.static(this.config.path, this.config.craOutDir));
  }

  build() {
    createReactApp.build({ out: this.config.craOutDir });
  }
}

[jesstelford]

Done in #1109