/**
* 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
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* 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.hdfs.server.namenode.ha;
import java.io.IOException;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.ha.ServiceFailedException;
import org.apache.hadoop.hdfs.server.namenode.NameNode.OperationCategory;
import org.apache.hadoop.ipc.StandbyException;
/**
* Context that is to be used by {@link HAState} for getting/setting the
* current state and performing required operations.
*/
@InterfaceAudience.Private
public interface HAContext {
/** Set the state of the context to given {@code state} */
public void setState(HAState state);
/** Get the state from the context */
public HAState getState();
/** Start the services required in active state */
public void startActiveServices() throws IOException;
/** Stop the services when exiting active state */
public void stopActiveServices() throws IOException;
/** Start the services required in standby state */
public void startStandbyServices() throws IOException;
/** Prepare to exit the standby state */
public void prepareToStopStandbyServices() throws ServiceFailedException;
/** Stop the services when exiting standby state */
public void stopStandbyServices() throws IOException;
/**
* Take a write-lock on the underlying namesystem
* so that no concurrent state transitions or edits
* can be made.
*/
void writeLock();
/**
* Unlock the lock taken by {@link #writeLock()}
*/
void writeUnlock();
/**
* Verify that the given operation category is allowed in the current state.
* This is to allow NN implementations (eg BackupNode) to override it with
* node-specific handling.
*
* If the operation which is being checked will be taking the FSNS lock, it's
* advisable to check the operation category both immediately before and after
* taking the lock. This is because clients rely on the StandbyException
* thrown by this method in order to trigger client failover, and if a client
* first tries to contact the Standby NN, it could block for a long time if
* the Standby is holding the lock for a while, e.g. when performing a
* checkpoint. See HDFS-4591 for more details.
*/
void checkOperation(OperationCategory op) throws StandbyException;
/**
* @return true if the node should allow stale reads (ie reads
* while the namespace is not up to date)
*/
boolean allowStaleReads();
}