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Migrating your Backend to the New Backend System

DISCLAIMER: The new backend system is in alpha, and still under active development. As such, it is not considered stable, and it is not recommended to migrate production backends to the new backend system until it has a stable release.

Overview​

This section describes how to migrate an existing Backstage backend service package (typically in packages/backend) to use the new backend system.

One of the main benefits of the new backend system is that it abstracts away the way that plugins and their dependencies are wired up, leading to a significantly simplified backend package that rarely if ever needs to change when plugins or their dependencies evolve. You generally don't have to convert all of your internal plugins and support classes themselves to the backend system first - the migration here will mostly deal with wiring and using compatibility wrappers where possible in the backend package itself. We hope that you will find that you end up with a much smaller, easier to understand, and easier to maintain package as a result of these steps, and then being able to migrate plugins as a separate endeavour later.

Overall Structure​

Your typical backend package has a few overall component parts:

  • An index.ts file that houses all of the creation and wiring together of all of the plugins and their dependencies
  • A types.ts file that defines the "environment", i.e. the various dependencies that get created by the backend and passed down into each plugin
  • A plugins folder which has one file for each plugin, e.g. plugins/catalog.ts

The index file has this overall shape:

import todo from './plugins/todo'; // repeated for N plugins

function makeCreateEnv(config: Config) {
  return (plugin: string): PluginEnvironment => {
    // ... build per-plugin environment
  };
}

async function main() {
  // ... early init
  const createEnv = makeCreateEnv(config);
  const todoEnv = useHotMemoize(module, () => createEnv('todo')); // repeated for N plugins
  const apiRouter = Router();
  apiRouter.use('/todo', await todo(todoEnv)); // repeated for N plugins
  // ... wire up and start http server
}

module.hot?.accept();
main().catch(...);

Migrating the Index File​

This migration will try to leave the plugins folder unchanged initially, first focusing on removing the environment type and reducing the index file to its bare minimum. Then as a later step, we can reduce the plugins folder bit by bit, replacing those files generally with one-liners in the index file instead.

Let's start by establishing the basis of your new index file. You may want to comment out its old contents, or renaming the old file to index.backup.ts for reference and making a new blank one to work on - whichever works best for you. These are our new blank contents in the index file:

packages/backend/src/index.ts
import { createBackend } from '@backstage/backend-defaults';

const backend = createBackend();
backend.start();

Note that the environment builder and the main dance are entirely gone.

We'll also want to add some backend system packages as dependencies. Run the following command:

# from the repository root
yarn add --cwd packages/backend @backstage/backend-defaults @backstage/backend-plugin-api

You should now be able to start this up with the familiar yarn workspace backend start command locally and seeing some logs scroll by. But it'll just be a blank service with no real features added. So let's stop it with Ctrl+C and reintroduce some plugins into the mix.

packages/backend/src/index.ts
import { createBackend } from '@backstage/backend-defaults';
import { legacyPlugin } from '@backstage/backend-common';

const backend = createBackend();
backend.add(legacyPlugin('todo', import('./plugins/todo')));
backend.start();

The todo plugin used above is just an example and you may not have it enabled in your own backend. Feel free to change it to some other plugin that you actually have in your plugins folder, for example backend.add(legacyPlugin('catalog', import('./plugins/catalog'))).

The legacyPlugin helper makes it easy to bridge the gap between the old-style plugin files and the new backend system. It ensures that the dependencies that you used to have to declare by hand in your env are gathered behind the scenes, then passes them into the relevant createPlugin export function, and makes sure that the route handler it returns is passed into the HTTP router with the given prefix.

Handling Custom Environments​

In the simple case, what we did above is sufficient, TypeScript is happy, and the backend runs with the new feature. If they do, feel free to skip this entire section, and deleting types.ts.

Sometimes though, type errors can be reported on the newly added line, saying that parts of the PluginEnvironment type do not match. This happens when the environment was changed from the defaults, perhaps with your own custom additions. If this is the case in your installation, you still aren't out of luck - you can build a customized legacyPlugin function.

packages/backend/src/index.ts
import { createBackend } from '@backstage/backend-defaults';
import { legacyPlugin } from '@backstage/backend-common';
import {
  makeLegacyPlugin,
  loggerToWinstonLogger,
} from '@backstage/backend-common';
import { coreServices } from '@backstage/backend-plugin-api';

const legacyPlugin = makeLegacyPlugin(
  {
    cache: coreServices.cache,
    config: coreServices.config,
    database: coreServices.database,
    discovery: coreServices.discovery,
    logger: coreServices.logger,
    permissions: coreServices.permissions,
    scheduler: coreServices.scheduler,
    tokenManager: coreServices.tokenManager,
    reader: coreServices.urlReader,
    identity: coreServices.identity,
    // ... and your own additions
  },
  {
    logger: log => loggerToWinstonLogger(log),
  },
);

const backend = createBackend();
backend.add(legacyPlugin('todo', import('./plugins/todo')));
backend.start();

The first argument to makeLegacyPlugin is the mapping from environment keys to references to actual backend system services. The second argument allows you to "tweak" the types of those services to something more fitting to your env. For example, you'll see that the logger service API type was changed from the raw Winston logger of old, to a different, custom API, so we use a helper function to transform that particular one.

To make additions as mentioned above to the environment, you will start to get into the weeds of how the backend system wiring works. You'll need to have a service reference and a service factory that performs the actual creation of your service. Please see the services article to learn how to create a service ref and its default factory. You can place that code directly in the index file for now if you want, or near the actual implementation class in question.

In this example, we'll assume that your added environment field is named example, and the created ref is named exampleServiceRef.

packages/backend/src/index.ts
import { exampleServiceRef } from '<somewhere>'; // if the definition is elsewhere

const legacyPlugin = makeLegacyPlugin(
  {
    // ... the above core services still go here
    example: exampleServiceRef,
  },
  {
    logger: log => loggerToWinstonLogger(log),
  },
);

After this, your backend will know how to instantiate your thing on demand and place it in the legacy plugin environment.

NOTE: If you happen to be dealing with a service ref that does NOT have a default implementation, but rather has a separate service factory, then you will also need to import that factory and pass it to the services array argument of createBackend.

Cleaning Up the Plugins Folder​

For plugins that are private and your own, you can follow a dedicated migration guide as you see fit, at a later time.

For third party backend plugins, in particular the larger core plugins that are maintained by the Backstage maintainers, you may find that they have already been migrated to the new backend system. This section describes some specific such migrations you can make.

NOTE: For each of these, note that your backend still needs to have a dependency (e.g. in packages/backend/package.json) to those plugin packages, and they still need to be configured properly in your app-config. Those mechanisms still work just the same as they used to in the old backend system.

The App Plugin​

The app backend plugin that serves the frontend from the backend can trivially be used in its new form.

packages/backend/src/index.ts
import { appPlugin } from '@backstage/plugin-app-backend';

const backend = createBackend();
backend.add(appPlugin({ appPackageName: 'app' }));

This is an example of how options can be passed into some backend plugins. The app plugin specifically needs to know the name of the package that holds the frontend code. This is the "name" field in that package's package.json, typically found in your packages/app folder. By default it's just plain "app".

You should be able to delete the plugins/app.ts file at this point.

The Catalog Plugin​

A basic installation of the catalog plugin looks as follows.

packages/backend/src/index.ts
import { catalogPlugin } from '@backstage/plugin-catalog-backend';
import { catalogModuleTemplateKind } from '@backstage/plugin-scaffolder-backend';

const backend = createBackend();
backend.add(catalogPlugin());
backend.add(catalogModuleTemplateKind());

Note that this also installs a module from the scaffolder, namely the one which enables the use of the Template kind. In the unlikely event that you do not use templates at all, you can remove those lines.

If you have other customizations made to plugins/catalog.ts, such as adding custom processors or entity providers, read on. Otherwise, you should be able to just delete that file at this point.

You will use the extension points mechanism to extend or tweak the functionality of the plugin. To do that, you'll make your own bespoke module which depends on the appropriate extension point and interacts with it.

packages/backend/src/index.ts
import { catalogProcessingExtensionPoint } from '@backstage/plugin-catalog-node';
import { createBackendModule } from '@backstage/backend-plugin-api';

const catalogModuleCustomExtensions = createBackendModule({
  pluginId: 'catalog', // name of the plugin that the module is targeting
  moduleId: 'customExtensions',
  register(env) {
    env.registerInit({
      deps: {
        catalog: catalogProcessingExtensionPoint,
        // ... and other dependencies as needed
      },
      init({ catalog /* ..., other dependencies */ }) {
        // Here you have the opportunity to interact with the extension
        // point before the plugin itself gets instantiated
        catalog.addEntityProvider(new MyEntityProvider()); // just an example
        catalog.addProcessor(new MyProcessor()); // just an example
      },
    });
  },
});

const backend = createBackend();
backend.add(catalogPlugin());
backend.add(catalogModuleTemplateKind());
backend.add(catalogModuleCustomExtensions());

This also requires that you have a dependency on the corresponding node package, if you didn't already have one.

# from the repository root
yarn add --cwd packages/backend @backstage/plugin-catalog-node

Here we've placed the module directly in the backend index file just to get going easily, but feel free to move it out to where it fits best. As you migrate your entire plugin flora to the new backend system, you will probably make more and more of these modules as "first class" things, living right next to the implementations that they represent, and being exported from there.

The Events Plugin​

A basic installation of the events plugin looks as follows.

packages/backend/src/index.ts
import { eventsPlugin } from '@backstage/plugin-events-backend';

const backend = createBackend();
backend.add(eventsPlugin());

If you have other customizations made to plugins/events.ts, such as adding custom subscribers, read on. Otherwise, you should be able to just delete that file at this point.

You will use the extension points mechanism to extend or tweak the functionality of the plugin. To do that, you'll make your own bespoke module which depends on the appropriate extension point and interacts with it.

packages/backend/src/index.ts
import { eventsExtensionPoint } from '@backstage/plugin-events-node';
import { createBackendModule } from '@backstage/backend-plugin-api';

const eventsModuleCustomExtensions = createBackendModule({
  pluginId: 'events', // name of the plugin that the module is targeting
  moduleId: 'customExtensions',
  register(env) {
    env.registerInit({
      deps: {
        events: eventsExtensionPoint,
        // ... and other dependencies as needed
      },
      init({ events /* ..., other dependencies */ }) {
        // Here you have the opportunity to interact with the extension
        // point before the plugin itself gets instantiated
        events.addSubscribers(new MySubscriber()); // just an example
      },
    });
  },
});

const backend = createBackend();
backend.add(eventsPlugin());
backend.add(eventsModuleCustomExtensions());

This also requires that you have a dependency on the corresponding node package, if you didn't already have one.

# from the repository root
yarn add --cwd packages/backend @backstage/plugin-events-node

Here we've placed the module directly in the backend index file just to get going easily, but feel free to move it out to where it fits best. As you migrate your entire plugin flora to the new backend system, you will probably make more and more of these modules as "first class" things, living right next to the implementations that they represent, and being exported from there.

The Scaffolder Plugin​

A basic installation of the scaffolder plugin looks as follows.

packages/backend/src/index.ts
import { scaffolderPlugin } from '@backstage/plugin-scaffolder-backend';

const backend = createBackend();
backend.add(scaffolderPlugin());

If you have other customizations made to plugins/scaffolder.ts, such as adding custom actions, read on. Otherwise, you should be able to just delete that file at this point.

You will use the extension points mechanism to extend or tweak the functionality of the plugin. To do that, you'll make your own bespoke module which depends on the appropriate extension point and interacts with it.

packages/backend/src/index.ts
import { scaffolderActionsExtensionPoint } from '@backstage/plugin-scaffolder-node';
import { createBackendModule } from '@backstage/backend-plugin-api';

const scaffolderModuleCustomExtensions = createBackendModule({
  pluginId: 'scaffolder', // name of the plugin that the module is targeting
  moduleId: 'customExtensions',
  register(env) {
    env.registerInit({
      deps: {
        scaffolder: scaffolderActionsExtensionPoint,
        // ... and other dependencies as needed
      },
      init({ scaffolder /* ..., other dependencies */ }) {
        // Here you have the opportunity to interact with the extension
        // point before the plugin itself gets instantiated
        scaffolder.addActions(new MyAction()); // just an example
      },
    });
  },
});

const backend = createBackend();
backend.add(scaffolderPlugin());
backend.add(scaffolderModuleCustomExtensions());

This also requires that you have a dependency on the corresponding node package, if you didn't already have one.

# from the repository root
yarn add --cwd packages/backend @backstage/plugin-scaffolder-node

Here we've placed the module directly in the backend index file just to get going easily, but feel free to move it out to where it fits best. As you migrate your entire plugin flora to the new backend system, you will probably make more and more of these modules as "first class" things, living right next to the implementations that they represent, and being exported from there.