Goodbye and thank you synced flush! (#50882)

Synced flush was a brilliant idea. It supports instant recoveries with a 
quite small implementation. However, with the presence of sequence
numbers and retention leases, it is no longer needed. This change
removes it from 8.0.

Relates #5077

Author: dnhatn

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

@@ -27,7 +27,6 @@
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.admin.indices.flush.FlushRequest;
 import org.elasticsearch.action.admin.indices.flush.FlushResponse;
-import org.elasticsearch.action.admin.indices.flush.SyncedFlushRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeResponse;
 import org.elasticsearch.action.admin.indices.open.OpenIndexRequest;
@@ -415,41 +414,6 @@ public Cancellable flushAsync(FlushRequest flushRequest, RequestOptions options,
                 FlushResponse::fromXContent, listener, emptySet());
     }
 
-    /**
-     * Initiate a synced flush manually using the synced flush API.
-     * See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush-api.html">
-     *     Synced flush API on elastic.co</a>
-     * @param syncedFlushRequest the request
-     * @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());
-    }
-
-    /**
-     * Asynchronously initiate a synced flush manually using the synced flush API.
-     * See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush-api.html">
-     *     Synced flush API on elastic.co</a>
-     * @param syncedFlushRequest the request
-     * @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,
-                SyncedFlushResponse::fromXContent, listener, emptySet());
-    }
-
     /**
      * Retrieve the settings of one or more indices.
      * See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-get-settings.html">

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

@@ -29,7 +29,6 @@
 import org.elasticsearch.action.admin.indices.cache.clear.ClearIndicesCacheRequest;
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.admin.indices.flush.FlushRequest;
-import org.elasticsearch.action.admin.indices.flush.SyncedFlushRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.action.admin.indices.open.OpenIndexRequest;
 import org.elasticsearch.action.admin.indices.refresh.RefreshRequest;
@@ -194,15 +193,6 @@ static Request flush(FlushRequest flushRequest) {
         return request;
     }
 
-    static Request flushSynced(SyncedFlushRequest syncedFlushRequest) {
-        String[] indices = syncedFlushRequest.indices() == null ? Strings.EMPTY_ARRAY : syncedFlushRequest.indices();
-        Request request = new Request(HttpPost.METHOD_NAME, RequestConverters.endpoint(indices, "_flush/synced"));
-        RequestConverters.Params parameters = new RequestConverters.Params();
-        parameters.withIndicesOptions(syncedFlushRequest.indicesOptions());
-        request.addParameters(parameters.asMap());
-        return request;
-    }
-
     static Request forceMerge(ForceMergeRequest forceMergeRequest) {
         String[] indices = forceMergeRequest.indices() == null ? Strings.EMPTY_ARRAY : forceMergeRequest.indices();
         Request request = new Request(HttpPost.METHOD_NAME, RequestConverters.endpoint(indices, "_forcemerge"));