Backend plugins

This page describes the process of creating and managing backend plugins in your Backstage repository.

Creating a Backend Plugin

A new, bare-bones backend plugin package can be created by issuing the following command in your Backstage repository root:

yarn new --select backend-plugin

Please also see the --help flag for the new command for some further options that are available, notably the --scope and --no-private flags that control naming and publishing of the newly created package. Your repo root package.json will probably also have some default values already set up for these.

You will be asked to supply a name for the plugin. This is an identifier that will be part of the NPM package name, so make it short and containing only lowercase characters separated by dashes, for example carmen, if it's a package that adds an integration with a system named Carmen, for example. The full NPM package name would then be something like @internal/plugin-carmen-backend, depending on the other flags passed to the new command, and your settings for the new command in your root package.json.

Creating the plugin will take a little while, so be patient. It will helpfully run the initial installation and build commands, so that your package is ready to be hacked on! It will be located in a new folder in your plugins directory, in this example plugins/carmen-backend.

For simple development purposes, a backend plugin can actually be started in a standalone mode. You can do a first-light test of your service:

cd plugins/carmen-backend
yarn start

This will think for a bit, and then say Listening on :7007. In a different terminal window, now run

curl localhost:7007/carmen/health

This should return {"status":"ok"}. Success! Press Ctrl + c to stop it again.

Developing your Backend Plugin

A freshly created backend plugin does basically nothing, in terms of the overall app. It has a small set of basic dependencies and exposes an Express router in src/service/router.ts. This is where you will start adding routes and connecting those to actual underlying functionality. But nothing in your Backstage application / backend exposes it.

To actually attach and run the plugin router, you will make some modifications to your backend.

# From your Backstage root directory
yarn add --cwd packages/backend @internal/plugin-carmen-backend@^0.1.0 # Change this to match the plugin's package.json

Create a new file named packages/backend/src/plugins/carmen.ts, and add the following to it

import { createRouter } from '@internal/plugin-carmen-backend';
import { Router } from 'express';
import { PluginEnvironment } from '../types';

export default async function createPlugin(
  env: PluginEnvironment,
): Promise<Router> {
  // Here is where you will add all of the required initialization code that
  // your backend plugin needs to be able to start!

  // The env contains a lot of goodies, but our router currently only
  // needs a logger
  return await createRouter({
    logger: env.logger,
  });
}

And finally, wire this into the overall backend router. Edit packages/backend/src/index.ts:

import carmen from './plugins/carmen';
// ...
async function main() {
  // ...
  const carmenEnv = useHotMemoize(module, () => createEnv('carmen'));
  apiRouter.use('/carmen', await carmen(carmenEnv));

After you start the backend (e.g. using yarn start-backend from the repo root), you should be able to fetch data from it.

# Note the extra /api here
curl localhost:7007/api/carmen/health

This should return {"status":"ok"} like before. Success!

Making Use of a Database

The Backstage backend comes with a builtin facility for SQL database access. Most plugins that have persistence needs will choose to make use of this facility, so that Backstage operators can manage database needs uniformly.

As part of the environment object that is passed to your createPlugin function, there is a database field. You can use that to get a Knex connection object.

// in packages/backend/src/plugins/carmen.ts
export default async function createPlugin(
  env: PluginEnvironment,
): Promise<Router> {
  const db: Knex<any, unknown[]> = await env.database.getClient();

  // You will then pass this client into your actual plugin implementation
  // code, maybe similar to the following:
  const model = new CarmenDatabaseModel(db);
  return await createRouter({
    model: model,
    logger: env.logger,
  });
}

You may note that the getClient call has no parameters. This is because all plugin database needs are configured under the backend.database config key of your app-config.yaml. The framework may even make sure behind the scenes that the logical database is created automatically if it doesn't exist, based on rules that the Backstage operator decides on.

The framework does not handle database schema migrations for you, however. The builtin plugins in the main repo have chosen to use the Knex library to manage schema migrations as well, but you can do so in any manner that you see fit.

See the Knex library documentation for examples and details on how to write schema migrations and perform SQL queries against your database..

Making Use of the User's Identity

The Backstage backend comes with a facility for retrieving the identity of the logged in user.

As part of the environment object that is passed to your createPlugin function, there is a identity field. You can use that to get an identity from the request.

// in packages/backend/src/plugins/carmen.ts
export default async function createPlugin(
  env: PluginEnvironment,
): Promise<Router> {
  return await createRouter({
    model: model,
    logger: env.logger,
    identity: env.identity,
  });
}

The plugin can then extract the identity from the request.

export async function createRouter(
  options: RouterOptions,
): Promise<express.Router> {
  const router = Router();
  const { identity } = options;

  router.post('/example', async (req, res) => {
    const userIdentity = await identity.getIdentity({ request: req });
    ...
  });