[App Configuration] - Head request support (#36959)

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there are more than one possible design, why was the one in this PR
chosen?


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request PRs)_

### Checklists
- [ ] Added impacted package name to the issue description
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Author: linglingye001

sdk/appconfiguration/app-configuration/assets.json

@@ -2,5 +2,5 @@
   "AssetsRepo": "Azure/azure-sdk-assets",
   "AssetsRepoPrefixPath": "js",
   "TagPrefix": "js/appconfiguration/app-configuration",
-  "Tag": "js/appconfiguration/app-configuration_b28373e403"
+  "Tag": "js/appconfiguration/app-configuration_0282ec10b3"
 }

sdk/appconfiguration/app-configuration/review/app-configuration-node.api.md

@@ -31,6 +31,7 @@ export class AppConfigurationClient {
     archiveSnapshot(name: string, options?: UpdateSnapshotOptions): Promise<UpdateSnapshotResponse>;
     beginCreateSnapshot(snapshot: SnapshotInfo, options?: CreateSnapshotOptions): Promise<SimplePollerLike<OperationState<CreateSnapshotResponse>, CreateSnapshotResponse>>;
     beginCreateSnapshotAndWait(snapshot: SnapshotInfo, options?: CreateSnapshotOptions): Promise<CreateSnapshotResponse>;
+    checkConfigurationSettings(options?: CheckConfigurationSettingsOptions): PagedAsyncIterableIterator<ConfigurationSetting, ListConfigurationSettingPage, PageSettings>;
     deleteConfigurationSetting(id: ConfigurationSettingId, options?: DeleteConfigurationSettingOptions): Promise<DeleteConfigurationSettingResponse>;
     getConfigurationSetting(id: ConfigurationSettingId, options?: GetConfigurationSettingOptions): Promise<GetConfigurationSettingResponse>;
     getSnapshot(name: string, options?: GetSnapshotOptions): Promise<GetSnapshotResponse>;
@@ -51,6 +52,11 @@ export interface AppConfigurationClientOptions extends CommonClientOptions {
     audience?: string;
 }
 
+// @public
+export interface CheckConfigurationSettingsOptions extends OperationOptions, ListSettingsOptions {
+    pageEtags?: string[];
+}
+
 // @public
 export type ConfigurationSetting<T extends string | FeatureFlagValue | SecretReferenceValue | SnapshotReferenceValue = string> = ConfigurationSettingParam<T> & {
     isReadOnly: boolean;

sdk/appconfiguration/app-configuration/src/appConfigurationClient.ts

@@ -9,6 +9,7 @@ import {
   type AddConfigurationSettingParam,
   type AddConfigurationSettingResponse,
   type AppConfigurationClientOptions,
+  type CheckConfigurationSettingsOptions,
   type ConfigurationSetting,
   type ConfigurationSettingId,
   type CreateSnapshotOptions,
@@ -44,9 +45,11 @@ import type {
   AppConfigurationGetKeyValuesHeaders,
   AppConfigurationGetRevisionsHeaders,
   AppConfigurationGetSnapshotsHeaders,
+  AppConfigurationCheckKeyValuesHeaders,
   GetKeyValuesResponse,
   GetRevisionsResponse,
   GetSnapshotsResponse,
+  CheckKeyValuesResponse,
   ConfigurationSnapshot,
   GetLabelsResponse,
   AppConfigurationGetLabelsHeaders,
@@ -447,6 +450,81 @@ export class AppConfigurationClient {
     return getPagedAsyncIterator(pagedResult);
   }
 
+  /**
+   * Checks settings from the Azure App Configuration service using a HEAD request, returning only headers without the response body.
+   * This is useful for efficiently checking if settings have changed by comparing ETags.
+   *
+   * Example code:
+   * ```ts snippet:CheckConfigurationSettings
+   * import { DefaultAzureCredential } from "@azure/identity";
+   * import { AppConfigurationClient } from "@azure/app-configuration";
+   *
+   * // The endpoint for your App Configuration resource
+   * const endpoint = "https://example.azconfig.io";
+   * const credential = new DefaultAzureCredential();
+   * const client = new AppConfigurationClient(endpoint, credential);
+   *
+   * const pageIterator = client.checkConfigurationSettings({ keyFilter: "MyKey" }).byPage();
+   * ```
+   * @param options - Optional parameters for the request.
+   */
+  checkConfigurationSettings(
+    options: CheckConfigurationSettingsOptions = {},
+  ): PagedAsyncIterableIterator<ConfigurationSetting, ListConfigurationSettingPage, PageSettings> {
+    const pageEtags = options.pageEtags ? [...options.pageEtags] : undefined;
+    delete options.pageEtags;
+    const pagedResult: PagedResult<ListConfigurationSettingPage, PageSettings, string | undefined> =
+      {
+        firstPageLink: undefined,
+        getPage: async (pageLink: string | undefined) => {
+          const etag = pageEtags?.shift();
+          try {
+            const response = await this.checkConfigurationSettingsRequest(
+              { ...options, etag },
+              pageLink,
+            );
+            const link = response._response?.headers?.get("link");
+            const continuationToken = link ? extractAfterTokenFromLinkHeader(link) : undefined;
+            const currentResponse: ListConfigurationSettingPage = {
+              ...response,
+              etag: response._response?.headers?.get("etag"),
+              items: [],
+              continuationToken: continuationToken,
+              _response: response._response,
+            };
+            return {
+              page: currentResponse,
+              nextPageLink: currentResponse.continuationToken,
+            };
+          } catch (error) {
+            const err = error as RestError;
+
+            const link = err.response?.headers?.get("link");
+            const continuationToken = link ? extractAfterTokenFromLinkHeader(link) : undefined;
+
+            if (err.statusCode === 304) {
+              err.message = `Status 304: No updates for this page`;
+              logger.info(
+                `[checkConfigurationSettings] No updates for this page. The current etag for the page is ${etag}`,
+              );
+              return {
+                page: {
+                  items: [],
+                  etag,
+                  _response: { ...err.response, status: 304 },
+                } as unknown as ListConfigurationSettingPage,
+                nextPageLink: continuationToken,
+              };
+            }
+
+            throw err;
+          }
+        },
+        toElements: (page) => page.items,
+      };
+    return getPagedAsyncIterator(pagedResult);
+  }
+
   /**
    * Lists settings from the Azure App Configuration service for snapshots based on name, optionally
    * filtered by key names, labels and accept datetime.
@@ -580,6 +658,28 @@ export class AppConfigurationClient {
     );
   }
 
+  private async checkConfigurationSettingsRequest(
+    options: SendConfigurationSettingsOptions & PageSettings = {},
+    pageLink: string | undefined,
+  ): Promise<CheckKeyValuesResponse & HttpResponseField<AppConfigurationCheckKeyValuesHeaders>> {
+    return tracingClient.withSpan(
+      "AppConfigurationClient.checkConfigurationSettings",
+      options,
+      async (updatedOptions) => {
+        const response = await this.client.checkKeyValues({
+          ...updatedOptions,
+          ...formatAcceptDateTime(options),
+          ...formatConfigurationSettingsFiltersAndSelect(options),
+          ...checkAndFormatIfAndIfNoneMatch({ etag: options.etag }, { onlyIfChanged: true }),
+          after: pageLink,
+        });
+
+        return response as CheckKeyValuesResponse &
+          HttpResponseField<AppConfigurationCheckKeyValuesHeaders>;
+      },
+    );
+  }
+
   /**
    * Lists revisions of a set of keys, optionally filtered by key names,
    * labels and accept datetime.

sdk/appconfiguration/app-configuration/src/models.ts

@@ -360,6 +360,16 @@ export interface ListConfigurationSettingsOptions extends OperationOptions, List
   pageEtags?: string[];
 }
 
+/**
+ * Options for checkConfigurationSettings that allow for filtering based on keys, labels and other fields.
+ */
+export interface CheckConfigurationSettingsOptions extends OperationOptions, ListSettingsOptions {
+  /**
+   * Etags list for page
+   */
+  pageEtags?: string[];
+}
+
 /**
  * Options for listLabels
  */

sdk/appconfiguration/app-configuration/test/public/index.spec.ts

@@ -1151,6 +1151,171 @@ describe("AppConfigurationClient", () => {
     });
   });
 
+  describe("checkConfigurationSettings", () => {
+    it("returns empty items with valid response structure", async () => {
+      const key = recorder.variable(
+        "checkConfigSetting-emptyItems",
+        `checkConfigSetting-emptyItems${Math.floor(Math.random() * 100000)}`,
+      );
+
+      await client.addConfigurationSetting({
+        key,
+        value: "[A] production value",
+      });
+
+      try {
+        const pageIterator = client.checkConfigurationSettings({ keyFilter: key }).byPage();
+
+        const firstPage = await pageIterator.next();
+        assert.isFalse(firstPage.done);
+        assert.isDefined(firstPage.value);
+        assert.equal(firstPage.value.items.length, 0, "items should be empty for HEAD request");
+        assert.isDefined(firstPage.value.etag, "etag should be present");
+        assert.equal(firstPage.value._response.status, 200);
+        assert.isDefined(firstPage.value._response.headers.get("x-ms-date"));
+      } finally {
+        await deleteKeyCompletely([key], client);
+      }
+    });
+
+    it("returns 304 when using valid etag and no changes occurred", async () => {
+      const key = recorder.variable(
+        "checkConfigSetting-304",
+        `checkConfigSetting-304${Math.floor(Math.random() * 100000)}`,
+      );
+
+      await client.addConfigurationSetting({
+        key,
+        value: "[A] production value",
+      });
+
+      try {
+        // First call to get the etag
+        const pageIterator1 = client.checkConfigurationSettings({ keyFilter: key }).byPage();
+        const firstPage1 = await pageIterator1.next();
+        const etag = firstPage1.value.etag;
+
+        assert.isDefined(etag);
+        const etags: string[] = [etag!];
+
+        // Second call with the same etag - should return 304
+        const pageIterator2 = client
+          .checkConfigurationSettings({ keyFilter: key, pageEtags: etags })
+          .byPage();
+        const firstPage2 = await pageIterator2.next();
+
+        assert.isFalse(firstPage2.done);
+        assert.equal(firstPage2.value.items.length, 0);
+        assert.equal(firstPage2.value._response.status, 304, "should return 304 Not Modified");
+        assert.equal(firstPage2.value.etag, etag, "etag should be the same");
+        assert.isDefined(firstPage2.value._response.headers.get("x-ms-date"));
+      } finally {
+        await deleteKeyCompletely([key], client);
+      }
+    });
+
+    it("returns 200 when using etag but changes were made", async () => {
+      const key = recorder.variable(
+        "checkConfigSetting-200",
+        `checkConfigSetting-200${Math.floor(Math.random() * 100000)}`,
+      );
+
+      await client.addConfigurationSetting({
+        key,
+        value: "[A] production value",
+      });
+
+      try {
+        // First call to get the etag
+        const pageIterator1 = client.checkConfigurationSettings({ keyFilter: key }).byPage();
+        const firstPage1 = await pageIterator1.next();
+        const etag = firstPage1.value.etag;
+
+        assert.isDefined(etag);
+        const etags: string[] = [etag!];
+
+        // Make a change
+        await client.setConfigurationSetting({
+          key,
+          value: "[A] modified value",
+        });
+
+        // Second call with the old etag - should return 200 because content changed
+        const pageIterator2 = client
+          .checkConfigurationSettings({ keyFilter: key, pageEtags: etags })
+          .byPage();
+        const firstPage2 = await pageIterator2.next();
+
+        assert.isFalse(firstPage2.done);
+        assert.equal(firstPage2.value.items.length, 0);
+        assert.equal(firstPage2.value._response.status, 200, "should return 200 with changes");
+        assert.notEqual(firstPage2.value.etag, etag, "etag should be different");
+      } finally {
+        await deleteKeyCompletely([key], client);
+      }
+    });
+
+    // Skip in live mode to avoid throttling (429) when creating 100+ settings
+    it("returns different etags for different pages", { skip: isLiveMode() }, async () => {
+      const key = recorder.variable(
+        "checkConfigSetting-multiPage",
+        `checkConfigSetting-multiPage${Math.floor(Math.random() * 100000)}`,
+      );
+
+      // Create 101 settings to ensure we have at least 2 pages (page size is 100)
+      const expectedNumberOfLabels = 101;
+
+      let addSettingPromises = [];
+      for (let i = 0; i < expectedNumberOfLabels; i++) {
+        addSettingPromises.push(
+          client.addConfigurationSetting({
+            key,
+            value: `value for ${i}`,
+            label: i.toString(),
+          }),
+        );
+
+        if (i !== 0 && i % 10 === 0) {
+          await Promise.all(addSettingPromises);
+          addSettingPromises = [];
+        }
+      }
+      await Promise.all(addSettingPromises);
+
+      try {
+        const pageIterator = client.checkConfigurationSettings({ keyFilter: key }).byPage();
+
+        // Get first page
+        const firstPage = await pageIterator.next();
+        assert.isFalse(firstPage.done);
+        assert.isDefined(firstPage.value.etag, "first page etag should be present");
+        assert.equal(firstPage.value.items.length, 0, "items should be empty for HEAD request");
+        assert.equal(firstPage.value._response.status, 200);
+        const firstPageEtag = firstPage.value.etag;
+
+        // Get second page
+        const secondPage = await pageIterator.next();
+        assert.isFalse(secondPage.done);
+        assert.isDefined(secondPage.value.etag, "second page etag should be present");
+        assert.equal(secondPage.value.items.length, 0, "items should be empty for HEAD request");
+        assert.equal(secondPage.value._response.status, 200);
+        const secondPageEtag = secondPage.value.etag;
+
+        // Verify that each page has a different etag
+        assert.notEqual(
+          firstPageEtag,
+          secondPageEtag,
+          "different pages should have different etags",
+        );
+      } finally {
+        // Clean up all created settings
+        for (let i = 0; i < expectedNumberOfLabels; i++) {
+          await client.deleteConfigurationSetting({ key, label: i.toString() });
+        }
+      }
+    });
+  });
+
   describe("listConfigSettings", () => {
     let key1: string;
     let key2: string;
@@ -1589,7 +1754,7 @@ describe("AppConfigurationClient", () => {
     });
 
     // Skipping all "accepts operation options flaky tests" https://github.com/Azure/azure-sdk-for-js/issues/26447
-    it.skip("accepts  operation options", async () => {
+    it.skip("accepts operation options", async () => {
       const key = recorder.variable(
         `setConfigTestNA`,
         `setConfigTestNA${Math.floor(Math.random() * 1000)}`,

sdk/appconfiguration/app-configuration/test/snippets.spec.ts

@@ -223,6 +223,15 @@ describe("snippets", () => {
     const allSettingsWithLabel = client.listConfigurationSettings({ labelFilter: "MyLabel" });
   });
 
+  it("CheckConfigurationSettings", async () => {
+    // The endpoint for your App Configuration resource
+    const endpoint = "https://example.azconfig.io";
+    const credential = new DefaultAzureCredential();
+    const client = new AppConfigurationClient(endpoint, credential);
+    // @ts-preserve-whitespace
+    const pageIterator = client.checkConfigurationSettings({ keyFilter: "MyKey" }).byPage();
+  });
+
   it("ListConfigurationSettingsForSnashots", async () => {
     // The endpoint for your App Configuration resource
     const endpoint = "https://example.azconfig.io";