[GitHub] [hadoop] bharatviswa504 commented on a change in pull request #810: HDDS-1512. Implement DoubleBuffer in OzoneManager.

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bharatviswa504 commented on a change in pull request #810: HDDS-1512. Implement DoubleBuffer in OzoneManager.
URL: https://github.com/apache/hadoop/pull/810#discussion_r284787939
 
 

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 File path: hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/ratis/OzoneManagerDoubleBuffer.java
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+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.
+ */
+
+package org.apache.hadoop.ozone.om.ratis;
+
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicLong;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+import org.apache.hadoop.ozone.om.OMMetadataManager;
+import org.apache.hadoop.ozone.om.ratis.helpers.DoubleBufferEntry;
+import org.apache.hadoop.ozone.om.response.OMClientResponse;
+import org.apache.hadoop.util.Daemon;
+import org.apache.hadoop.util.ExitUtil;
+import org.apache.hadoop.utils.db.BatchOperation;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * A double-buffer for OM requests.
+ */
+public class OzoneManagerDoubleBuffer {
+
+  private static final Logger LOG =
+      LoggerFactory.getLogger(OzoneManagerDoubleBuffer.class.getName());
+
+  private TransactionBuffer currentBuffer;
+  private TransactionBuffer readyBuffer;
+  private Daemon daemon;
+  private volatile boolean syncToDB;
+  private final OMMetadataManager omMetadataManager;
+  private AtomicLong flushedTransactionCount = new AtomicLong(0);
+  private AtomicLong flushIterations = new AtomicLong(0);
+
+  public OzoneManagerDoubleBuffer(OMMetadataManager omMetadataManager) {
+    this.currentBuffer = new TransactionBuffer();
+    this.readyBuffer = new TransactionBuffer();
+    this.omMetadataManager = omMetadataManager;
+
+    // Daemon thread which runs in back ground and flushes transactions to DB.
+    daemon = new Daemon(this::flushTransactions);
+    daemon.start();
+
+  }
+
+  /**
+   * Runs in a background thread and batches the transaction in currentBuffer
+   * and commit to DB.
+   */
+  private void flushTransactions() {
+    while(true) {
+      if (canFlush()) {
+        syncToDB = true;
+        setReadyBuffer();
+        final BatchOperation batchOperation = omMetadataManager.getStore()
+            .initBatchOperation();
+
+        readyBuffer.iterator().forEachRemaining((entry) -> {
+          try {
+            entry.getResponse().addToRocksDBBatch(omMetadataManager,
+                batchOperation);
+          } catch (IOException ex) {
+            // During Adding to RocksDB batch entry got an exception.
+            // We should terminate the OM.
+            String message = "During flush to DB encountered error " +
+                ex.getMessage();
+            ExitUtil.terminate(1, message);
+          }
+        });
+
+        try {
+          omMetadataManager.getStore().commitBatchOperation(batchOperation);
+        } catch (IOException ex) {
+          // During flush to rocksdb got an exception.
+          // We should terminate the OM.
+          String message = "During flush to DB encountered error " +
+              ex.getMessage();
+          ExitUtil.terminate(1, message);
+        }
+
+        int flushedTransactionsSize = readyBuffer.size();
+        flushedTransactionCount.addAndGet(flushedTransactionsSize);
+        flushIterations.incrementAndGet();
+
+        LOG.info("Sync Iteration {} flushed transactions in this iteration{}",
+            flushIterations.get(), flushedTransactionsSize);
+        readyBuffer.clear();
+        syncToDB = false;
+        // TODO: update the last updated index in OzoneManagerStateMachine.
+      }
+    }
+  }
+
+  /**
+   * Returns the flushed transaction count to OM DB.
+   * @return flushedTransactionCount
+   */
+  public long getFlushedTransactionCount() {
+    return flushedTransactionCount.get();
+  }
+
+  /**
+   * Returns total number of flush iterations run by sync thread.
+   * @return flushIterations
+   */
+  public long getFlushIterations() {
+    return flushIterations.get();
+  }
+
+  /**
+   * Add OmResponseBufferEntry to buffer.
+   * @param response
+   * @param transactionIndex
+   */
+  public synchronized void add(OMClientResponse response,
+      long transactionIndex) {
+    currentBuffer.add(new DoubleBufferEntry<>(transactionIndex, response));
+  }
+
+  /**
+   * Check can we flush transactions or not.
+   *
+   * @return boolean
+   */
+  @SuppressFBWarnings(value="IS2_INCONSISTENT_SYNC", justification =
+      "Just checking size here, so we don't need synchronize here")
+  private boolean canFlush() {
+    return !syncToDB && currentBuffer.size() > 0;
+  }
+
+  /**
+   * Prepares the readyBuffer which is used by sync thread to flush
+   * transactions to OM DB.
+   */
+  private synchronized void setReadyBuffer() {
+    TransactionBuffer temp = currentBuffer;
+    currentBuffer = readyBuffer;
+    readyBuffer = temp;
+  }
+
+  /**
+   * TransactionBuffer which holds queue of
+   * {@link org.apache.hadoop.ozone.om.ratis.helpers.DoubleBufferEntry}.
+   */
+  static class TransactionBuffer {
+
+    private Queue<DoubleBufferEntry<OMClientResponse>> queue;
+
+    TransactionBuffer() {
+      // Taken unbounded queue, if sync thread is taking too long time, we
+      // might end up taking huge memory to add entries to the buffer.
+      // TODO: We can avoid this using unbounded queue and use queue with
+      // capcity, if queue is full we can wait for sync to be completed to
+      // add entries. But in this also we might block rpc handlers, as we
+      // clear entries after sync. Or we can come up with a good approach to
+      // solve this.
+      queue = new ConcurrentLinkedQueue<>();
 
 Review comment:
   So your suggestion is in add check if it is full have a flush() and then add to buffer?

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