/**
* Copyright 2011 The Apache Software Foundation
*
* 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.hbase.zookeeper;
import java.io.IOException;
import java.lang.management.ManagementFactory;
import java.util.ArrayList;
import java.util.List;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.util.RetryCounter;
import org.apache.hadoop.hbase.util.RetryCounterFactory;
import org.apache.zookeeper.AsyncCallback;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.ZooKeeper.States;
import org.apache.zookeeper.data.ACL;
import org.apache.zookeeper.data.Stat;
/**
* A zookeeper that can handle 'recoverable' errors.
* To handle recoverable errors, developers need to realize that there are two
* classes of requests: idempotent and non-idempotent requests. Read requests
* and unconditional sets and deletes are examples of idempotent requests, they
* can be reissued with the same results.
* (Although, the delete may throw a NoNodeException on reissue its effect on
* the ZooKeeper state is the same.) Non-idempotent requests need special
* handling, application and library writers need to keep in mind that they may
* need to encode information in the data or name of znodes to detect
* retries. A simple example is a create that uses a sequence flag.
* If a process issues a create("/x-", ..., SEQUENCE) and gets a connection
* loss exception, that process will reissue another
* create("/x-", ..., SEQUENCE) and get back x-111. When the process does a
* getChildren("/"), it sees x-1,x-30,x-109,x-110,x-111, now it could be
* that x-109 was the result of the previous create, so the process actually
* owns both x-109 and x-111. An easy way around this is to use "x-process id-"
* when doing the create. If the process is using an id of 352, before reissuing
* the create it will do a getChildren("/") and see "x-222-1", "x-542-30",
* "x-352-109", x-333-110". The process will know that the original create
* succeeded an the znode it created is "x-352-109".
* @see "http://wiki.apache.org/hadoop/ZooKeeper/ErrorHandling"
*/
public class RecoverableZooKeeper {
private static final Log LOG = LogFactory.getLog(RecoverableZooKeeper.class);
// the actual ZooKeeper client instance
private ZooKeeper zk;
private final RetryCounterFactory retryCounterFactory;
// An identifier of this process in the cluster
private final String identifier;
private final byte[] id;
private int retryIntervalMillis;
// The metadata attached to each piece of data has the
// format:
// <magic> 1-byte constant
// <id length> 4-byte big-endian integer (length of next field)
// <id> identifier corresponding uniquely to this process
// It is prepended to the data supplied by the user.
// the magic number is to be backward compatible
private static final byte MAGIC =(byte) 0XFF;
private static final int MAGIC_SIZE = Bytes.SIZEOF_BYTE;
private static final int ID_LENGTH_OFFSET = MAGIC_SIZE;
private static final int ID_LENGTH_SIZE = Bytes.SIZEOF_INT;
public RecoverableZooKeeper(String quorumServers, int seesionTimeout,
Watcher watcher, int maxRetries, int retryIntervalMillis)
throws IOException {
this.zk = new ZooKeeper(quorumServers, seesionTimeout, watcher);
this.retryCounterFactory =
new RetryCounterFactory(maxRetries, retryIntervalMillis);
this.retryIntervalMillis = retryIntervalMillis;
// the identifier = processID@hostName
this.identifier = ManagementFactory.getRuntimeMXBean().getName();
LOG.info("The identifier of this process is " + identifier);
this.id = Bytes.toBytes(identifier);
}
/**
* delete is an idempotent operation. Retry before throwing exception.
* This function will not throw NoNodeException if the path does not
* exist.
*/
public void delete(String path, int version)
throws InterruptedException, KeeperException {
RetryCounter retryCounter = retryCounterFactory.create();
boolean isRetry = false; // False for first attempt, true for all retries.
while (true) {
try {
zk.delete(path, version);
return;
} catch (KeeperException e) {
switch (e.code()) {
case NONODE:
if (isRetry) {
LOG.info("Node " + path + " already deleted. Assuming that a " +
"previous attempt succeeded.");
return;
}
LOG.warn("Node " + path + " already deleted, and this is not a " +
"retry");
throw e;
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "delete");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
isRetry = true;
}
}
/**
* exists is an idempotent operation. Retry before throwing exception
* @return A Stat instance
*/
public Stat exists(String path, Watcher watcher)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
return zk.exists(path, watcher);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "exists");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* exists is an idempotent operation. Retry before throwing exception
* @return A Stat instance
*/
public Stat exists(String path, boolean watch)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
return zk.exists(path, watch);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "exists");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
private void retryOrThrow(RetryCounter retryCounter, KeeperException e,
String opName) throws KeeperException {
LOG.warn("Possibly transient ZooKeeper exception: " + e);
if (!retryCounter.shouldRetry()) {
LOG.error("ZooKeeper " + opName + " failed after "
+ retryCounter.getMaxRetries() + " retries");
throw e;
}
}
/**
* getChildren is an idempotent operation. Retry before throwing exception
* @return List of children znodes
*/
public List<String> getChildren(String path, Watcher watcher)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
return zk.getChildren(path, watcher);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "getChildren");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* getChildren is an idempotent operation. Retry before throwing exception
* @return List of children znodes
*/
public List<String> getChildren(String path, boolean watch)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
return zk.getChildren(path, watch);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "getChildren");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* getData is an idempotent operation. Retry before throwing exception
* @return Data
*/
public byte[] getData(String path, Watcher watcher, Stat stat)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
byte[] revData = zk.getData(path, watcher, stat);
return this.removeMetaData(revData);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "getData");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* getData is an idemnpotent operation. Retry before throwing exception
* @return Data
*/
public byte[] getData(String path, boolean watch, Stat stat)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
while (true) {
try {
byte[] revData = zk.getData(path, watch, stat);
return this.removeMetaData(revData);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "getData");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* setData is NOT an idempotent operation. Retry may cause BadVersion Exception
* Adding an identifier field into the data to check whether
* badversion is caused by the result of previous correctly setData
* @return Stat instance
*/
public Stat setData(String path, byte[] data, int version)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
byte[] newData = appendMetaData(data);
while (true) {
try {
return zk.setData(path, newData, version);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "setData");
break;
case BADVERSION:
// try to verify whether the previous setData success or not
try{
Stat stat = new Stat();
byte[] revData = zk.getData(path, false, stat);
int idLength = Bytes.toInt(revData, ID_LENGTH_SIZE);
int dataLength = revData.length-ID_LENGTH_SIZE-idLength;
int dataOffset = ID_LENGTH_SIZE+idLength;
if(Bytes.compareTo(revData, ID_LENGTH_SIZE, id.length,
revData, dataOffset, dataLength) == 0) {
// the bad version is caused by previous successful setData
return stat;
}
} catch(KeeperException keeperException){
// the ZK is not reliable at this moment. just throwing exception
throw keeperException;
}
// throw other exceptions and verified bad version exceptions
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
/**
* <p>
* NONSEQUENTIAL create is idempotent operation.
* Retry before throwing exceptions.
* But this function will not throw the NodeExist exception back to the
* application.
* </p>
* <p>
* But SEQUENTIAL is NOT idempotent operation. It is necessary to add
* identifier to the path to verify, whether the previous one is successful
* or not.
* </p>
*
* @return Path
*/
public String create(String path, byte[] data, List<ACL> acl,
CreateMode createMode)
throws KeeperException, InterruptedException {
byte[] newData = appendMetaData(data);
switch (createMode) {
case EPHEMERAL:
case PERSISTENT:
return createNonSequential(path, newData, acl, createMode);
case EPHEMERAL_SEQUENTIAL:
case PERSISTENT_SEQUENTIAL:
return createSequential(path, newData, acl, createMode);
default:
throw new IllegalArgumentException("Unrecognized CreateMode: " +
createMode);
}
}
private String createNonSequential(String path, byte[] data, List<ACL> acl,
CreateMode createMode) throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
boolean isRetry = false; // False for first attempt, true for all retries.
while (true) {
try {
return zk.create(path, data, acl, createMode);
} catch (KeeperException e) {
switch (e.code()) {
case NODEEXISTS:
if (isRetry) {
// If the connection was lost, there is still a possibility that
// we have successfully created the node at our previous attempt,
// so we read the node and compare.
byte[] currentData = zk.getData(path, false, null);
if (currentData != null &&
Bytes.compareTo(currentData, data) == 0) {
// We successfully created a non-sequential node
return path;
}
LOG.error("Node " + path + " already exists with " +
Bytes.toStringBinary(currentData) + ", could not write " +
Bytes.toStringBinary(data));
throw e;
}
LOG.error("Node " + path + " already exists and this is not a " +
"retry");
throw e;
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "create");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
isRetry = true;
}
}
private String createSequential(String path, byte[] data,
List<ACL> acl, CreateMode createMode)
throws KeeperException, InterruptedException {
RetryCounter retryCounter = retryCounterFactory.create();
boolean first = true;
String newPath = path+this.identifier;
while (true) {
try {
if (!first) {
// Check if we succeeded on a previous attempt
String previousResult = findPreviousSequentialNode(newPath);
if (previousResult != null) {
return previousResult;
}
}
first = false;
return zk.create(newPath, data, acl, createMode);
} catch (KeeperException e) {
switch (e.code()) {
case CONNECTIONLOSS:
case OPERATIONTIMEOUT:
retryOrThrow(retryCounter, e, "create");
break;
default:
throw e;
}
}
retryCounter.sleepUntilNextRetry();
retryCounter.useRetry();
}
}
private String findPreviousSequentialNode(String path)
throws KeeperException, InterruptedException {
int lastSlashIdx = path.lastIndexOf('/');
assert(lastSlashIdx != -1);
String parent = path.substring(0, lastSlashIdx);
String nodePrefix = path.substring(lastSlashIdx+1);
List<String> nodes = zk.getChildren(parent, false);
List<String> matching = filterByPrefix(nodes, nodePrefix);
for (String node : matching) {
String nodePath = parent + "/" + node;
Stat stat = zk.exists(nodePath, false);
if (stat != null) {
return nodePath;
}
}
return null;
}
public byte[] removeMetaData(byte[] data) {
if(data == null || data.length == 0) {
return data;
}
// check the magic data; to be backward compatible
byte magic = data[0];
if(magic != MAGIC) {
return data;
}
int idLength = Bytes.toInt(data, ID_LENGTH_OFFSET);
int dataLength = data.length-MAGIC_SIZE-ID_LENGTH_SIZE-idLength;
int dataOffset = MAGIC_SIZE+ID_LENGTH_SIZE+idLength;
byte[] newData = new byte[dataLength];
System.arraycopy(data, dataOffset, newData, 0, dataLength);
return newData;
}
private byte[] appendMetaData(byte[] data) {
if(data == null || data.length == 0){
return data;
}
byte[] newData = new byte[MAGIC_SIZE+ID_LENGTH_SIZE+id.length+data.length];
int pos = 0;
pos = Bytes.putByte(newData, pos, MAGIC);
pos = Bytes.putInt(newData, pos, id.length);
pos = Bytes.putBytes(newData, pos, id, 0, id.length);
pos = Bytes.putBytes(newData, pos, data, 0, data.length);
return newData;
}
public long getSessionId() {
return zk.getSessionId();
}
public void close() throws InterruptedException {
zk.close();
}
public States getState() {
return zk.getState();
}
public ZooKeeper getZooKeeper() {
return zk;
}
public byte[] getSessionPasswd() {
return zk.getSessionPasswd();
}
public void sync(String path, AsyncCallback.VoidCallback cb, Object ctx) {
this.zk.sync(path, null, null);
}
/**
* Filters the given node list by the given prefixes.
* This method is all-inclusive--if any element in the node list starts
* with any of the given prefixes, then it is included in the result.
*
* @param nodes the nodes to filter
* @param prefixes the prefixes to include in the result
* @return list of every element that starts with one of the prefixes
*/
private static List<String> filterByPrefix(List<String> nodes,
String... prefixes) {
List<String> lockChildren = new ArrayList<String>();
for (String child : nodes){
for (String prefix : prefixes){
if (child.startsWith(prefix)){
lockChildren.add(child);
break;
}
}
}
return lockChildren;
}
}