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ADR011: Plugin Package Structure
Architecture Decision Record (ADR) for Plugin Package Structure

Context

A core feature of Backstage is the extensibility via plugins. The Backstage repository is open for contributions of plugins. Even most of the core features are implemented as plugins. A plugin consists of one or multiple packages in the plugins/ directory. Up till now, we have a simple conventions for naming plugin packages: Plugins are named x, with the option of having a related backend plugin called x-backend (where x is the plugin name, like catalog or techdocs). There is a need for sharing code between the frontend and backend of a plugin, between backend plugins, or components and hooks between different frontend plugins (some examples). This results in emerging plugin packages with shared code, like packages/catalog-client or packages/techdocs-common.

There is a common phrase in software development: Naming things is hard

To keep the contributed plugins consistent, this Architecture Decision Record provides rules for naming plugin packages.

Decision

We will place all plugin related code in the plugins/ directory. The packages/ directory is reserved for core package of Backstage.

We follow this structure for plugin packages (where x is the plugin name, for example catalog or techdocs):

  • x: Contains the main frontend code of the plugin.
  • x-backend: Contains the main backend code of the plugin.
  • x-backend-module-<name>: Contains optional modules related to the backend plugin.
  • x-react: Contains shared widgets, hooks and similar that both the plugin itself (x) and third-party frontend plugins can depend on.
  • x-node: Contains utilities for backends that both the plugin backend itself (x-backend) and third-party backend plugins can depend on.
  • x-common: An isomorphic package with platform agnostic models, clients, and utilities that all packages above or any third-party plugin package can depend on.

We prefix the package names with @backstage/plugin-.

This structure is based on a suggestion in issue #3655.

Consequences

We will actively migrate existing packages that are part of a plugin to the plugins/ folder. This affects packages like:

  • packages/techdocs-common which should be moved to plugins/techdocs-node and named @backstage/plugin-techdocs-node.
  • packages/catalog-client which will be part of a future plugins/catalog-common and named @backstage/plugin-catalog-common.
  • While the new location of packages/catalog-model should be plugins/catalog-common we might want to do an exception here, as it's a very central package.

We will actively migrate optional features of backend plugins into separate x-backend-module-<name> packages, for example the more specialized processors in the catalog backend.

The limited set of rules might not be sufficient in the future. If additional packages are required, we will revisit this decision and extend the pattern.

If possible, we will add tools, such as lint rules, to help enforce the package names and dependencies between them or CLI commands to generate these packages.

The distinction between core packages and plugins helps us to setup CODEOWNERS in the repository. We can set the code owners for the packages/ folder to the core team and create additional rules (like plugins/x*) for plugin maintainers.