private CompletableFuture<FlushRegionResponse> flush(ServerName serverName, RegionInfo regionInfo,
    byte[] columnFamily, boolean writeFlushWALMarker) {
    return this.<FlushRegionResponse> newAdminCaller().serverName(serverName)
      .action((controller, stub) -> this.<FlushRegionRequest, FlushRegionResponse,
        FlushRegionResponse> adminCall(controller, stub,
          RequestConverter.buildFlushRegionRequest(regionInfo.getRegionName(), columnFamily,
            writeFlushWALMarker),
          (s, c, req, done) -> s.flushRegion(c, req, done), resp -> resp))
      .call();
  }
This method is for internal use only, where we need the response of the flush.
<p/>
As it exposes the protobuf message, please do <strong>NOT</strong> try to expose it as a public
API.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/RawAsyncHBaseAdmin.java