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Subject [GitHub] [hadoop] bharatviswa504 commented on a change in pull request #810: HDDS-1512. Implement DoubleBuffer in OzoneManager.
Date Fri, 17 May 2019 22:58:00 GMT
bharatviswa504 commented on a change in pull request #810: HDDS-1512. Implement DoubleBuffer
in OzoneManager.
URL: https://github.com/apache/hadoop/pull/810#discussion_r285311016
 
 

 ##########
 File path: hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/ratis/OzoneManagerDoubleBuffer.java
 ##########
 @@ -0,0 +1,212 @@
+/**
+ * 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:
   > We can setReadyBuffer if we check the buffer if full, so this will not block requests
long time. Then background will flush ready buffer. Only when the ready buffer is flushing
meanwhile current buffer is full, the request will be blocked.
   
   One disadvantage I see is if flush is taking a too long time, and currentBuffer can grow
very high and may cause OOM. So, if we use a fixed size buffer we might not see but we will
block further transactions. 
   

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