using-backstage-backend-system

Backstage Backend System Patterns

When to use this skill

• Defining a new backend plugin (src/plugin.ts)
• Defining a backend module (src/module.ts) that extends another plugin
• Choosing between a full plugin vs a module
• Exposing extension points so other modules can extend your plugin

Choose the right pattern

Two invocation modes

• Standalone: editing an existing backend plugin/module.
• Post-scaffold (called by creating-backstage-plugin for backend-plugin, backend-plugin-module, catalog-provider-module, scaffolder-backend-module): wire up the canonical structure (src/plugin.ts or src/module.ts, src/index.ts re-export).

Common gotchas

• All wiring imports come from @backstage/backend-plugin-api, not @backstage/core-plugin-api (which is frontend-only).
• The register(env) callback receives an environment object with env.registerInit({ deps, async init }) and env.registerExtensionPoint(point, impl). deps is a map of service refs the init function needs; init receives the resolved instances.
• registerInit may be called only once per plugin/module. registerExtensionPoint must be called before registerInit.
• Plugin-scoped vs root-scoped services: most refs (logger, httpRouter, database, scheduler, auth, httpAuth, userInfo, cache, discovery, urlReader, lifecycle, permissions) are plugin-scoped — each plugin gets its own instance with plugin-aware behavior (e.g. logger pre-tagged with pluginId, database giving a per-plugin schema). Six refs (rootConfig, rootLogger, rootLifecycle, rootHealth, rootHttpRouter, rootInstanceMetadata) are root-scoped — single shared instance. Both can be depended on from a plugin's register block; the distinction matters for behavior, not for injection syntax.
• A plugin's pluginId and a module's moduleId must match the pattern [a-z][a-z0-9-]* (letters, digits, dashes; starts with a letter).
• Modules depend on the extension points exported by the target plugin's library package (e.g. @backstage/plugin-catalog-node), not on the plugin's main package.