backend-common: Move PluginEndpointDiscovery type to backend-plugin-api

Signed-off-by: Johan Haals <johan.haals@gmail.com>

packages/backend-plugin-api/src/services/definitions/discoveryServiceRef.ts

@@ -15,10 +15,54 @@
  */
 
 import { createServiceRef } from '../system/types';
-import { PluginEndpointDiscovery } from '@backstage/backend-common';
 
-/** @public */
-export type DiscoveryService = PluginEndpointDiscovery;
+/**
+ * The DiscoveryService is used to provide a mechanism for backend
+ * plugins to discover the endpoints for itself or other backend plugins.
+ *
+ * The purpose of the discovery API is to allow for many different deployment
+ * setups and routing methods through a central configuration, instead
+ * of letting each individual plugin manage that configuration.
+ *
+ * Implementations of the discovery API can be as simple as a URL pattern
+ * using the pluginId, but could also have overrides for individual plugins,
+ * or query a separate discovery service.
+ *
+ * @public
+ */
+export type DiscoveryService = {
+  /**
+   * Returns the internal HTTP base URL for a given plugin, without a trailing slash.
+   *
+   * The returned URL should point to an internal endpoint for the plugin, with
+   * the shortest route possible. The URL should be used for service-to-service
+   * communication within a Backstage backend deployment.
+   *
+   * This method must always be called just before making a request, as opposed to
+   * fetching the URL when constructing an API client. That is to ensure that more
+   * flexible routing patterns can be supported.
+   *
+   * For example, asking for the URL for `catalog` may return something
+   * like `http://10.1.2.3/api/catalog`
+   */
+  getBaseUrl(pluginId: string): Promise<string>;
+
+  /**
+   * Returns the external HTTP base backend URL for a given plugin, without a trailing slash.
+   *
+   * The returned URL should point to an external endpoint for the plugin, such that
+   * it is reachable from the Backstage frontend and other external services. The returned
+   * URL should be usable for example as a callback / webhook URL.
+   *
+   * The returned URL should be stable and in general not change unless other static
+   * or external configuration is changed. Changes should not come as a surprise
+   * to an operator of the Backstage backend.
+   *
+   * For example, asking for the URL for `catalog` may return something
+   * like `https://backstage.example.com/api/catalog`
+   */
+  getExternalBaseUrl(pluginId: string): Promise<string>;
+};
 
 /**
  * @public