feat: Implement CatalogEntityClient, mainly to get Template entity from template name

plugins/scaffolder-backend/src/lib/catalog/CatalogEntityClient.ts

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import fetch from 'node-fetch';
+import { TemplateEntityV1alpha1 } from '@backstage/catalog-model';
+import {
+  ConflictError,
+  NotFoundError,
+  PluginEndpointDiscovery,
+} from '@backstage/backend-common';
+
+/**
+ * A catalog client tailored for reading out entity data from the catalog.
+ */
+export class CatalogEntityClient {
+  private readonly discovery: PluginEndpointDiscovery;
+
+  constructor(options: { discovery: PluginEndpointDiscovery }) {
+    this.discovery = options.discovery;
+  }
+
+  /**
+   * Looks up a single template using a template name.
+   *
+   * Throws a NotFoundError or ConflictError if 0 or multiple templates are found.
+   */
+  async findTemplate(templateName: string): Promise<TemplateEntityV1alpha1> {
+    const params = new URLSearchParams();
+    params.append('kind', 'Template');
+
+    params.append('metadata.name', templateName);
+
+    const baseUrl = await this.discovery.getBaseUrl('catalog');
+    const response = await fetch(`${baseUrl}/entities?${params}`);
+
+    if (!response.ok) {
+      const text = await response.text();
+      throw new Error(
+        `Request failed with ${response.status} ${response.statusText}, ${text}`,
+      );
+    }
+
+    const templates: TemplateEntityV1alpha1[] = await response.json();
+
+    if (templates.length !== 1) {
+      if (templates.length > 1) {
+        throw new ConflictError(
+          'Templates lookup resulted in multiple matches',
+        );
+      } else {
+        throw new NotFoundError('Template not found');
+      }
+    }
+
+    return templates[0];
+  }
+}

plugins/scaffolder-backend/src/lib/catalog/index.ts

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+export { CatalogEntityClient } from './CatalogEntityClient';