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docs/api/backend: updates to fit current system and reuse existing docs

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Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
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Patrik Oldsberg
2022-10-23 18:32:58 +02:00
parent 6a8f7e5a73
commit a83eb55d1c
1 changed files with 57 additions and 45 deletions
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docs/api/backend.md
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@@ -1,38 +1,43 @@
---
id: backend
title: Backend
description: About Backend
title: New Backend System
description: Details of the upcoming backend system
---
## Backend System
> **DISCLAMER: The new backend system is under active development and is not considered stable**
**DISCLAMER: The new backend system is under active development and is not considered stable**
## Overview
This is an example of how you create, start and add existing plugins to your backend.
```ts
import { createBackend } from '@backstage/backend-defaults';
import { catalogPlugin } from '@backstage/plugin-catalog-backend';
// Create your backend instance
const backend = createBackend();
// backend.add(catalogPlugin());
// Install all desired features
backend.add(catalogPlugin());
// Start up the backend
await backend.start();
```
### Overview
## Backend Services
The default backend provides several _services_ out of the box which includes access to config, logging, scheduling and more.
Service are declared using their _serviceRef_ in the `deps` section of plugin or module requiring them and are then available in the `init` method of the plugin or module.
The default backend provides several _services_ out of the box which includes access to configuration, logging, databases and more.
Services are declared using their _serviceRef_ in the `deps` section of plugin or module requiring them and are then available in the `init` method of the plugin or module.
### Service Refs
### Service References
A serviceRef is a named reference to an interface which are later used to resolve the concrete service implementation. Conceptually this is very similar to `ApiRef`s in the frontend.
A `ServiceRef` is a named reference to an interface which are later used to resolve the concrete service implementation. Conceptually this is very similar to `ApiRef`s in the frontend.
Services is what provides common utilities that previously resided in the `PluginEnvironment` such as Config, Logging and Database.
On startup the backend will make sure that the services are initialized before being passed to the plugin/module that depend on them.
ServiceRefs contain a scope which is used to determine if the serviceFactory creating the service will create a new instance scoped per plugin/module or if it will be shared. `plugin` scoped services will be created once per plugin/module and `root` scoped services will be created once per backend instance.
#### Defining a ServiceRef
#### Defining a Service
```ts
import {
@@ -61,51 +66,59 @@ export const exampleServiceRef = createServiceRef<ExampleApi>({
},
// Logger is available directly in the factory as it's a root scoped service and will be created once per backend instance.
async factory({ logger }) {
// plugin is available as it's a plugin scoped service and will be created once per plugin.
return async ({ plugin }) => {
// This block will be executed once per plugin depending on this serviceRef
logger.info(`Creating example service for for plugin ${plugin.id}`);
return new ExampleImpl({logger});
// This block will be executed once for every plugin that depends on this service
logger.info('Initializing example service plugin instance');
return new ExampleImpl({ logger });
};
},
}),
}),
});
```
### Overriding services
### Overriding Services
In this example replace the default log implementation with a custom logger.
In this example replace the default root logger service implementation with a custom one that streams logs to GCP. The `rootLoggerServiceRef` has a `'root'` scope, meaning there are no plugin-specific instances of this service.
```ts
import {
createServiceFactory,
loggerServiceRef,
rootLoggerServiceRef,
LoggerService,
} from '@backstage/backend-plugin-api';
export const gcpLoggerFactory = createServiceFactory({
service: loggerServiceRef,
deps: {},
async factory({}) {
return async ({}) => {
// This custom implementation conform with the type of the loggerServiceRef
// This custom implementation would typically live separately from
// the backend setup code, either nearby such as in
// packages/backend/src/services/logger/GoogleCloudLogger.ts
// Or you can let it live in its own library package.
class GoogleCloudLogger implements LoggerService {
static factory = createServiceFactory({
service: rootLoggerServiceRef,
deps: {},
async factory() {
return new GoogleCloudLogger();
};
},
});
},
});
// custom implementation here ...
}
// packages/backend/src/index.ts
const backend = createBackend({
services: [
// supplies additional/replacement services to the backend
gcpLoggerFactory,
// supplies additional or replacement services to the backend
GoogleCloudLogger.factory(),
],
})
});
```
## Writing Plugins
```ts
import { configServiceRef, createBackendPlugin } from '@backstage/backend-plugin-api';
import {
configServiceRef,
createBackendPlugin,
} from '@backstage/backend-plugin-api';
// export type ExamplePluginOptions = { exampleOption: boolean };
export const examplePlugin = createBackendPlugin({
@@ -134,6 +147,7 @@ backend.add(examplePlugin());
// Options can be passed to the plugin
// backend.add(examplePlugin({ exampleOption: true}));
```
## Writing Modules
Some facts about modules
@@ -142,9 +156,9 @@ Some facts about modules
- A module can only extend one plugin but can interact with multiple `ExtensionPoint`s registered by that plugin.
- A module is always initialized before the plugin it extends.
A module depend on the extensionPoint exported by the plugins library package(eg `catalog-node`, `scaffolder-backend`) and does not directly declare a dependency on the plugin package itself.
A module depend on the `ExtensionPoint`s exported by the target plugin's library package, for example `@backstage/plugin-catalog-node`, and does not directly declare a dependency on the plugin package itself.
Here's an example on how to create a module that adds a new processor using the `catalogProcessingExtensionPoint`
Here's an example on how to create a module that adds a new processor using the `catalogProcessingExtensionPoint`:
```ts
import { createBackendModule } from '@backstage/backend-plugin-api';
@@ -177,7 +191,7 @@ Modules depend on extension points just as a regular dependency but specifying i
import { createExtensionPoint } from '@backstage/backend-plugin-api';
export interface ScaffolderActionsExtensionPoint {
addAction(action: ScaffolderAction): void;
addAction(action: ScaffolderAction): void;
}
export const ScaffolderActionsExtensionPoint =
@@ -190,8 +204,7 @@ export const ScaffolderActionsExtensionPoint =
Extension points are registered by a plugin and extended by modules.
### Testing
## Testing
Utilities for testing backend plugins and modules are available in `@backstage/backend-test-utils`.
@@ -206,18 +219,17 @@ describe('Example', () => {
// plugins and modules for testing
features: [testModule()],
});
// assertions
// assertions
});
});
```
## Package structure
The package relationship between plugins, modules and extension are illustrated in the following diagram.
A detailed explanation of the package architecture can be found in the [Backstage Architecture Overview](../overview/architecture-overview.md#package-architecture). The most important packages to consider for this system are `backend`, `plugin-<pluginId>-backend`, `plugin-<pluginId>-node`, and `plugin-<pluginId>-backend-module-<moduleId>`.
Taken with an artificial foobar backend plugin.
- `plugin-foobar-backend` houses the plugin and registers the extension points into the backend system.
- `plugin-foobar-common` houses the shared types including the Extension Point registered by the backend.
- `plugin-foobar-XYZ-module` houses the modules that extend the foobar backend with extension points imported from `plugin-foobar-common`
- `plugin-<pluginId>-backend` houses the implementation of the plugins themselves.
- `plugin-<pluginId>-node` houses the extension points and any other utilities that modules or other plugins might need.
- `plugin-<pluginId>-backend-module-<moduleId>` houses the modules that extend the plugins via the extension points.
- `backend` is the backend itself that wires everything together to something that you can deploy.