HDFS-12975. [SBN read] Changes to the NameNode to support reads from standby. Contributed by Chao Sun.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/common/HdfsServerConstants.java

@@ -157,7 +157,9 @@ enum StartupOption{
     // only used for StorageDirectory.analyzeStorage() in hot swap drive scenario.
     // TODO refactor StorageDirectory.analyzeStorage() so that we can do away with
     // this in StartupOption.
-    HOTSWAP("-hotswap");
+    HOTSWAP("-hotswap"),
+    // Startup the namenode in observer mode.
+    OBSERVER("-observer");
 
     private static final Pattern ENUM_WITH_ROLLING_UPGRADE_OPTION = Pattern.compile(
         "(\\w+)\\((\\w+)\\)");

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java

@@ -542,7 +542,8 @@ private void logAuditEvent(boolean succeeded,
   private final ReentrantLock cpLock;
 
   /**
-   * Used when this NN is in standby state to read from the shared edit log.
+   * Used when this NN is in standby or observer state to read from the
+   * shared edit log.
    */
   private EditLogTailer editLogTailer = null;
 
@@ -1380,24 +1381,25 @@ void stopActiveServices() {
   }
   
   /**
-   * Start services required in standby state 
+   * Start services required in standby or observer state
    * 
    * @throws IOException
    */
-  void startStandbyServices(final Configuration conf) throws IOException {
+  void startStandbyServices(final Configuration conf, boolean isObserver)
-    LOG.info("Starting services required for standby state");
+      throws IOException {
+    LOG.info("Starting services required for " +
+        (isObserver ? "observer" : "standby") + " state");
     if (!getFSImage().editLog.isOpenForRead()) {
       // During startup, we're already open for read.
       getFSImage().editLog.initSharedJournalsForRead();
     }
-    
     blockManager.setPostponeBlocksFromFuture(true);
 
     // Disable quota checks while in standby.
     dir.disableQuotaChecks();
     editLogTailer = new EditLogTailer(this, conf);
     editLogTailer.start();
-    if (standbyShouldCheckpoint) {
+    if (!isObserver && standbyShouldCheckpoint) {
       standbyCheckpointer = new StandbyCheckpointer(conf, this);
       standbyCheckpointer.start();
     }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNode.java

@@ -365,6 +365,7 @@ public long getProtocolVersion(String protocol,
       LoggerFactory.getLogger("BlockStateChange");
   public static final HAState ACTIVE_STATE = new ActiveState();
   public static final HAState STANDBY_STATE = new StandbyState();
+  public static final HAState OBSERVER_STATE = new StandbyState(true);
 
   private static final String NAMENODE_HTRACE_PREFIX = "namenode.htrace.";
 
@@ -987,6 +988,8 @@ protected HAState createHAState(StartupOption startOpt) {
     if (!haEnabled || startOpt == StartupOption.UPGRADE
         || startOpt == StartupOption.UPGRADEONLY) {
       return ACTIVE_STATE;
+    } else if (startOpt == StartupOption.OBSERVER) {
+      return OBSERVER_STATE;
     } else {
       return STANDBY_STATE;
     }
@@ -1481,6 +1484,8 @@ static StartupOption parseArguments(String args[]) {
         startOpt = StartupOption.BACKUP;
       } else if (StartupOption.CHECKPOINT.getName().equalsIgnoreCase(cmd)) {
         startOpt = StartupOption.CHECKPOINT;
+      } else if (StartupOption.OBSERVER.getName().equalsIgnoreCase(cmd)) {
+        startOpt = StartupOption.OBSERVER;
       } else if (StartupOption.UPGRADE.getName().equalsIgnoreCase(cmd)
           || StartupOption.UPGRADEONLY.getName().equalsIgnoreCase(cmd)) {
         startOpt = StartupOption.UPGRADE.getName().equalsIgnoreCase(cmd) ? 
@@ -1794,6 +1799,11 @@ synchronized void transitionToActive()
     if (!haEnabled) {
       throw new ServiceFailedException("HA for namenode is not enabled");
     }
+    if (state == OBSERVER_STATE) {
+      // TODO: we may need to remove this when enabling failover for observer
+      throw new ServiceFailedException(
+          "Cannot transition from Observer to Active");
+    }
     state.setState(haContext, ACTIVE_STATE);
   }
   
@@ -1803,6 +1813,11 @@ synchronized void transitionToStandby()
     if (!haEnabled) {
       throw new ServiceFailedException("HA for namenode is not enabled");
     }
+    if (state == OBSERVER_STATE) {
+      // TODO: we may need to remove this when enabling failover for observer
+      throw new ServiceFailedException(
+          "Cannot transition from Observer to Standby");
+    }
     state.setState(haContext, STANDBY_STATE);
   }
 
@@ -1857,6 +1872,7 @@ public String getNNRole() {
 
   @Override // NameNodeStatusMXBean
   public String getState() {
+    // TODO: maybe we should return a different result for observer namenode?
     String servStateStr = "";
     HAServiceState servState = getServiceState();
     if (null != servState) {
@@ -1957,7 +1973,8 @@ public void stopActiveServices() throws IOException {
     @Override
     public void startStandbyServices() throws IOException {
       try {
-        namesystem.startStandbyServices(getConf());
+        namesystem.startStandbyServices(getConf(),
+            state == NameNode.OBSERVER_STATE);
       } catch (Throwable t) {
         doImmediateShutdown(t);
       }
@@ -2004,6 +2021,9 @@ public void checkOperation(final OperationCategory op)
     
     @Override
     public boolean allowStaleReads() {
+      if (state == OBSERVER_STATE) {
+        return true;
+      }
       return allowStaleStandbyReads;
     }
 
@@ -2017,6 +2037,10 @@ public boolean isActiveState() {
     return (state.equals(ACTIVE_STATE));
   }
 
+  public boolean isObserverState() {
+    return state.equals(OBSERVER_STATE);
+  }
+
   /**
    * Returns whether the NameNode is completely started
    */

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNodeRpcServer.java

@@ -1575,7 +1575,7 @@ public Boolean call() throws IOException {
 
     if (nn.getFSImage().isUpgradeFinalized() &&
         !namesystem.isRollingUpgrade() &&
-        !nn.isStandbyState() &&
+        nn.isActiveState() &&
         noStaleStorages) {
       return new FinalizeCommand(poolId);
     }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/StandbyState.java

@@ -39,8 +39,15 @@
  */
 @InterfaceAudience.Private
 public class StandbyState extends HAState {
+  private final boolean isObserver;
+
   public StandbyState() {
+    this(false);
+  }
+
+  public StandbyState(boolean isObserver) {
     super(HAServiceState.STANDBY);
+    this.isObserver = isObserver;
   }
 
   @Override
@@ -49,6 +56,11 @@ public void setState(HAContext context, HAState s) throws ServiceFailedException
       setStateInternal(context, s);
       return;
     }
+    if (isObserver && s == NameNode.STANDBY_STATE) {
+      // To guard against the exception in the following super call.
+      // The other case, standby -> observer, should not happen.
+      return;
+    }
     super.setState(context, s);
   }
 
@@ -92,5 +104,10 @@ public void checkOperation(HAContext context, OperationCategory op)
   public boolean shouldPopulateReplQueues() {
     return false;
   }
+
+  @Override
+  public String toString() {
+    return isObserver ? "observer" : "standby";
+  }
 }