Deprecate synced flush (#50835)

A normal flush has the same effect as a synced flush on Elasticsearch 
7.6 or later. It's deprecated in 7.6 and will be removed in 8.0.

Relates #50776

client/rest-high-level/src/main/java/org/elasticsearch/client/IndicesClient.java

@@ -601,7 +601,10 @@ public Cancellable flushAsync(FlushRequest flushRequest, RequestOptions options,
      * @param options the request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
      * @return the response
      * @throws IOException in case there is a problem sending the request or parsing back the response
+     * @deprecated synced flush is deprecated and will be removed in 8.0.
+     * Use {@link #flush(FlushRequest, RequestOptions)} instead.
      */
+    @Deprecated
     public SyncedFlushResponse flushSynced(SyncedFlushRequest syncedFlushRequest, RequestOptions options) throws IOException {
         return restHighLevelClient.performRequestAndParseEntity(syncedFlushRequest, IndicesRequestConverters::flushSynced, options,
                 SyncedFlushResponse::fromXContent, emptySet());
@@ -615,7 +618,10 @@ public SyncedFlushResponse flushSynced(SyncedFlushRequest syncedFlushRequest, Re
      * @param options the request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
      * @param listener the listener to be notified upon request completion
      * @return cancellable that may be used to cancel the request
+     * @deprecated synced flush is deprecated and will be removed in 8.0.
+     * Use {@link #flushAsync(FlushRequest, RequestOptions, ActionListener)} instead.
      */
+    @Deprecated
     public Cancellable flushSyncedAsync(SyncedFlushRequest syncedFlushRequest, RequestOptions options,
                                         ActionListener<SyncedFlushResponse> listener) {
         return restHighLevelClient.performRequestAsyncAndParseEntity(syncedFlushRequest, IndicesRequestConverters::flushSynced, options,

client/rest-high-level/src/test/java/org/elasticsearch/client/IndicesClientIT.java

@@ -100,6 +100,7 @@
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
+import org.elasticsearch.indices.flush.SyncedFlushService;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.rest.action.admin.indices.RestCreateIndexAction;
 import org.elasticsearch.rest.action.admin.indices.RestGetFieldMappingAction;
@@ -982,7 +983,8 @@ public void testSyncedFlush() throws IOException {
             createIndex(index, settings);
             SyncedFlushRequest syncedFlushRequest = new SyncedFlushRequest(index);
             SyncedFlushResponse flushResponse =
-                    execute(syncedFlushRequest, highLevelClient().indices()::flushSynced, highLevelClient().indices()::flushSyncedAsync);
+                    execute(syncedFlushRequest, highLevelClient().indices()::flushSynced, highLevelClient().indices()::flushSyncedAsync,
+                        expectWarnings(SyncedFlushService.SYNCED_FLUSH_DEPRECATION_MESSAGE));
             assertThat(flushResponse.totalShards(), equalTo(1));
             assertThat(flushResponse.successfulShards(), equalTo(1));
             assertThat(flushResponse.failedShards(), equalTo(0));
@@ -997,7 +999,8 @@ public void testSyncedFlush() throws IOException {
                     execute(
                         syncedFlushRequest,
                         highLevelClient().indices()::flushSynced,
-                        highLevelClient().indices()::flushSyncedAsync
+                        highLevelClient().indices()::flushSyncedAsync,
+                        expectWarnings(SyncedFlushService.SYNCED_FLUSH_DEPRECATION_MESSAGE)
                     )
             );
             assertEquals(RestStatus.NOT_FOUND, exception.status());

client/rest-high-level/src/test/java/org/elasticsearch/client/RestHighLevelClientTests.java

@@ -839,7 +839,8 @@ public void testApiNamingConventions() throws Exception {
         // looking like it doesn't have a valid implementatation when it does.
         apiUnsupported.remove("indices.get_template");
 
-
+        // Synced flush is deprecated
+        apiUnsupported.remove("indices.flush_synced");
 
         for (Map.Entry<String, Set<Method>> entry : methods.entrySet()) {
             String apiName = entry.getKey();

client/rest-high-level/src/test/java/org/elasticsearch/client/documentation/IndicesClientDocumentationIT.java

@@ -1019,7 +1019,9 @@ public void testSyncedFlushIndex() throws Exception {
             // end::flush-synced-request-indicesOptions
 
             // tag::flush-synced-execute
-            SyncedFlushResponse flushSyncedResponse = client.indices().flushSynced(request, RequestOptions.DEFAULT);
+            SyncedFlushResponse flushSyncedResponse = client.indices().flushSynced(request, expectWarnings(
+                "Synced flush is deprecated and will be removed in 8.0. Use flush at _/flush or /{index}/_flush instead."
+            ));
             // end::flush-synced-execute
 
             // tag::flush-synced-response
@@ -1063,7 +1065,9 @@ public void onFailure(Exception e) {
             listener = new LatchedActionListener<>(listener, latch);
 
             // tag::flush-synced-execute-async
-            client.indices().flushSyncedAsync(request, RequestOptions.DEFAULT, listener); // <1>
+            client.indices().flushSyncedAsync(request, expectWarnings(
+                "Synced flush is deprecated and will be removed in 8.0. Use flush at _/flush or /{index}/_flush instead."
+            ), listener); // <1>
             // end::flush-synced-execute-async
 
             assertTrue(latch.await(30L, TimeUnit.SECONDS));

docs/reference/indices/synced-flush.asciidoc

@@ -4,6 +4,10 @@
 <titleabbrev>Synced flush</titleabbrev>
 ++++
 
+deprecated::[7.6, synced-flush is deprecated and will be removed in 8.0.
+Use <<indices-flush,flush>> instead. A <<indices-flush,flush>> has the
+same effect as a synced flush on Elasticsearch 7.6 or later]
+
 Performs a synced flush on one or more indices.
 
 [source,console]

docs/reference/upgrade/synced-flush.asciidoc

@@ -3,8 +3,12 @@
 --------------------------------------------------
 POST _flush/synced
 --------------------------------------------------
+// TEST[skip: will fail as synced flush is deprecated]
 
 When you perform a synced flush, check the response to make sure there are
 no failures. Synced flush operations that fail due to pending indexing
 operations are listed in the response body, although the request itself
 still returns a 200 OK status. If there are failures, reissue the request.
+
+Note that synced flush is deprecated and will be removed in 8.0. A flush
+has the same effect as a synced flush on Elasticsearch 7.6 or later.

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -745,7 +745,7 @@ public void testRecovery() throws Exception {
                 // We had a bug before where we failed to perform peer recovery with sync_id from 5.x to 6.x.
                 // We added this synced flush so we can exercise different paths of recovery code.
                 try {
-                    client().performRequest(new Request("POST", index + "/_flush/synced"));
+                    performSyncedFlush(index);
                 } catch (ResponseException ignored) {
                     // synced flush is optional here
                 }

qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/RecoveryIT.java

@@ -591,7 +591,7 @@ private void syncedFlush(String index) throws Exception {
         // A synced-flush request considers the global checkpoint sync as an going operation because it acquires a shard permit.
         assertBusy(() -> {
             try {
-                Response resp = client().performRequest(new Request("POST", index + "/_flush/synced"));
+                Response resp = performSyncedFlush(index);
                 Map<String, Object> result = ObjectPath.createFromResponse(resp).evaluate("_shards");
                 assertThat(result.get("failed"), equalTo(0));
             } catch (ResponseException ex) {

rest-api-spec/src/main/resources/rest-api-spec/api/indices.flush_synced.json

@@ -2,7 +2,7 @@
   "indices.flush_synced":{
     "documentation":{
       "url":"https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush-api.html",
-      "description":"Performs a synced flush operation on one or more indices."
+      "description":"Performs a synced flush operation on one or more indices. Synced flush is deprecated and will be removed in 8.0. Use flush instead"
     },
     "stability":"stable",
     "url":{

rest-api-spec/src/main/resources/rest-api-spec/test/cat.shards/10_basic.yml

@@ -88,54 +88,6 @@
   - match:
       $body: |
                /^$/
-
-  - do:
-      indices.create:
-        index: sync_id_test
-        body:
-          settings:
-            number_of_shards: 5
-            number_of_replicas: 0
-
-  - do:
-      indices.flush_synced:
-        index: sync_id_test
-
-  - is_false: _shards.failed
-
-  - do:
-      cat.shards:
-        index: sync_id_test
-        h: index,state,sync_id
-# 20 chars for sync ids with 5.x which uses time-based uuids and 22 with 6.x which uses random uuids
-  - match:
-      $body: |
-               /^(sync_id_test\s+STARTED\s+[A-Za-z0-9_\-]{20,22}\n){5}$/
-
-  - do:
-      indices.delete:
-        index: sync_id_test
-
-  - do:
-      indices.create:
-        index: sync_id_no_flush_test
-        body:
-          settings:
-            number_of_shards: 5
-            number_of_replicas: 0
-
-  - do:
-      cat.shards:
-        index: sync_id_no_flush_test
-        h: index,state,sync_id
-  - match:
-      $body: |
-               /^(sync_id_no_flush_test\s+STARTED\s+\n){5}$/
-
-  - do:
-      indices.delete:
-        index: sync_id_no_flush_test
-
   - do:
       indices.create:
         index: index1

rest-api-spec/src/main/resources/rest-api-spec/test/indices.flush/10_basic.yml

@@ -1,26 +1,32 @@
 ---
 "Index synced flush rest test":
-    - do:
-       indices.create:
-          index: testing
-          body:
-            settings:
-              index:
-                number_of_replicas: 0
+  - skip:
+      version: " - 7.5.99"
+      reason: "synced flush is deprecated in 7.6"
+      features: "warnings"
+  - do:
+      indices.create:
+        index: testing
+        body:
+          settings:
+            index:
+              number_of_replicas: 0
 
-    - do:
-       cluster.health:
-          wait_for_status: green
-    - do:
-       indices.flush_synced:
-          index: testing
+  - do:
+      cluster.health:
+        wait_for_status: green
+  - do:
+      warnings:
+        - Synced flush is deprecated and will be removed in 8.0. Use flush at _/flush or /{index}/_flush instead.
+      indices.flush_synced:
+        index: testing
 
-    - is_false: _shards.failed
+  - is_false: _shards.failed
 
-    - do:
-       indices.stats: {level: shards}
+  - do:
+      indices.stats: {level: shards}
 
-    - is_true: indices.testing.shards.0.0.commit.user_data.sync_id
+  - is_true: indices.testing.shards.0.0.commit.user_data.sync_id
 
 ---
 "Flush stats":

server/src/main/java/org/elasticsearch/indices/flush/SyncedFlushService.java

@@ -41,6 +41,7 @@
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
 import org.elasticsearch.common.util.concurrent.CountDown;
 import org.elasticsearch.index.Index;
@@ -71,11 +72,17 @@
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentMap;
+import java.util.stream.StreamSupport;
 
 public class SyncedFlushService implements IndexEventListener {
 
     private static final Logger logger = LogManager.getLogger(SyncedFlushService.class);
 
+    private static final DeprecationLogger DEPRECATION_LOGGER = new DeprecationLogger(logger);
+
+    public static final String SYNCED_FLUSH_DEPRECATION_MESSAGE =
+        "Synced flush is deprecated and will be removed in 8.0. Use flush at _/flush or /{index}/_flush instead.";
+
     private static final String PRE_SYNCED_FLUSH_ACTION_NAME = "internal:indices/flush/synced/pre";
     private static final String SYNCED_FLUSH_ACTION_NAME = "internal:indices/flush/synced/sync";
     private static final String IN_FLIGHT_OPS_ACTION_NAME = "internal:indices/flush/synced/in_flight";
@@ -130,6 +137,9 @@ public void attemptSyncedFlush(final String[] aliasesOrIndices,
                                    IndicesOptions indicesOptions,
                                    final ActionListener<SyncedFlushResponse> listener) {
         final ClusterState state = clusterService.state();
+        if (StreamSupport.stream(state.nodes().spliterator(), false).allMatch(n -> n.getVersion().onOrAfter(Version.V_7_6_0))) {
+            DEPRECATION_LOGGER.deprecatedAndMaybeLog("synced_flush", SYNCED_FLUSH_DEPRECATION_MESSAGE);
+        }
         final Index[] concreteIndices = indexNameExpressionResolver.concreteIndices(state, indicesOptions, aliasesOrIndices);
         final Map<String, List<ShardsSyncedFlushResult>> results = ConcurrentCollections.newConcurrentMap();
         int numberOfShards = 0;

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -54,6 +54,7 @@
 import org.elasticsearch.core.internal.io.IOUtils;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.seqno.ReplicationTracker;
+import org.elasticsearch.indices.flush.SyncedFlushService;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.snapshots.SnapshotState;
 import org.elasticsearch.test.ESTestCase;
@@ -1222,4 +1223,19 @@ protected static Version minimumNodeVersion() throws IOException {
         assertNotNull(minVersion);
         return minVersion;
     }
+
+    protected static Response performSyncedFlush(String indexName) throws IOException {
+        final Request request = new Request("POST", indexName + "/_flush/synced");
+        final List<String> expectedWarnings = Collections.singletonList(SyncedFlushService.SYNCED_FLUSH_DEPRECATION_MESSAGE);
+        if (nodeVersions.stream().allMatch(version -> version.onOrAfter(Version.V_7_6_0))) {
+            final Builder options = RequestOptions.DEFAULT.toBuilder();
+            options.setWarningsHandler(warnings -> warnings.equals(expectedWarnings) == false);
+            request.setOptions(options);
+        } else if (nodeVersions.stream().anyMatch(version -> version.onOrAfter(Version.V_7_6_0))) {
+            final Builder options = RequestOptions.DEFAULT.toBuilder();
+            options.setWarningsHandler(warnings -> warnings.isEmpty() == false && warnings.equals(expectedWarnings) == false);
+            request.setOptions(options);
+        }
+        return client().performRequest(request);
+    }
 }