环境
Hadoop版本：Apache Hadoop 2.6.3ZooKeeper版本：ZooKeeper 3.4.10- 两个
ResourceManager节点：主节点RM01，从节点RM02
背景
最近有一次我们的Hadoop监控平台发出ResourceManager（以下简称RM）崩溃的报警信息，于是我们分析了RM的日志和ZooKeeper服务端（以下简称ZK）的日志，异常日志信息分别如下：
RM日志
RM01的异常日志如下：
2019-03-03 02:33:01,826 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Retrying operation on ZK. Retry no. 999
2019-03-03 02:33:02,088 INFO org.apache.zookeeper.ClientCnxn: Opening socket connection to server 10.204.245.44/10.204.245.44:5181. Will not attempt to authenticate using SASL (unknown error)
2019-03-03 02:33:02,089 INFO org.apache.zookeeper.ClientCnxn: Socket connection established to 10.204.245.44/10.204.245.44:5181, initiating session
2019-03-03 02:33:02,090 INFO org.apache.zookeeper.ClientCnxn: Session establishment complete on server 10.204.245.44/10.204.245.44:5181, sessionid = 0x1690d678e9448da, negotiated timeout = 60000
2019-03-03 02:33:02,101 WARN org.apache.zookeeper.ClientCnxn: Session 0x1690d678e9448da for server 10.204.245.44/10.204.245.44:5181, unexpected error, closing socket connection and attempting reconnect
java.io.IOException: Broken pipe
at sun.nio.ch.FileDispatcherImpl.write0(Native Method)
at sun.nio.ch.SocketDispatcher.write(SocketDispatcher.java:47)
at sun.nio.ch.IOUtil.writeFromNativeBuffer(IOUtil.java:93)
at sun.nio.ch.IOUtil.write(IOUtil.java:65)
at sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:471)
at org.apache.zookeeper.ClientCnxnSocketNIO.doIO(ClientCnxnSocketNIO.java:117)
at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:366)
at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1081)
2019-03-03 02:33:02,201 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Exception while executing a ZK operation.
org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /bi-rmstore/ZKRMStateRoot/RMAppRoot/application_1551528254601_0725/appattempt_1551528254601_0725_000001
at org.apache.zookeeper.KeeperException.create(KeeperException.java:99)
at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1045)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1073)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1058)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithCheck(ZKRMStateStore.java:1128)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithRetries(ZKRMStateStore.java:1161)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.existsWithRetries(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.updateApplicationAttemptStateInternal(ZKRMStateStore.java:712)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:243)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:226)
at org.apache.hadoop.yarn.state.StateMachineFactory$SingleInternalArc.doTransition(StateMachineFactory.java:362)
at org.apache.hadoop.yarn.state.StateMachineFactory.doTransition(StateMachineFactory.java:302)
at org.apache.hadoop.yarn.state.StateMachineFactory.access$300(StateMachineFactory.java:46)
at org.apache.hadoop.yarn.state.StateMachineFactory$InternalStateMachine.doTransition(StateMachineFactory.java:448)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore.handleStoreEvent(RMStateStore.java:812)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:872)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:867)
at org.apache.hadoop.yarn.event.AsyncDispatcher.dispatch(AsyncDispatcher.java:182)
at org.apache.hadoop.yarn.event.AsyncDispatcher$1.run(AsyncDispatcher.java:109)
at java.lang.Thread.run(Thread.java:745)
2019-03-03 02:33:02,201 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Maxed out ZK retries. Giving up!
2019-03-03 02:33:02,201 ERROR org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore: Error updating appAttempt: appattempt_1551528254601_0725_000001
org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /bi-rmstore/ZKRMStateRoot/RMAppRoot/application_1551528254601_0725/appattempt_1551528254601_0725_000001
at org.apache.zookeeper.KeeperException.create(KeeperException.java:99)
at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1045)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1073)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1058)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithCheck(ZKRMStateStore.java:1128)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithRetries(ZKRMStateStore.java:1161)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.existsWithRetries(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.updateApplicationAttemptStateInternal(ZKRMStateStore.java:712)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:243)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:226)
at org.apache.hadoop.yarn.state.StateMachineFactory$SingleInternalArc.doTransition(StateMachineFactory.java:362)
at org.apache.hadoop.yarn.state.StateMachineFactory.doTransition(StateMachineFactory.java:302)
at org.apache.hadoop.yarn.state.StateMachineFactory.access$300(StateMachineFactory.java:46)
at org.apache.hadoop.yarn.state.StateMachineFactory$InternalStateMachine.doTransition(StateMachineFactory.java:448)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore.handleStoreEvent(RMStateStore.java:812)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:872)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:867)
at org.apache.hadoop.yarn.event.AsyncDispatcher.dispatch(AsyncDispatcher.java:182)
at org.apache.hadoop.yarn.event.AsyncDispatcher$1.run(AsyncDispatcher.java:109)
at java.lang.Thread.run(Thread.java:745)
2019-03-03 02:33:02,201 ERROR org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore: State store operation failed
org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /bi-rmstore/ZKRMStateRoot/RMAppRoot/application_1551528254601_0725/appattempt_1551528254601_0725_000001
at org.apache.zookeeper.KeeperException.create(KeeperException.java:99)
at org.apache.zookeeper.KeeperException.create(KeeperException.java:51)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1045)
at org.apache.zookeeper.ZooKeeper.exists(ZooKeeper.java:1073)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1058)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$9.run(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithCheck(ZKRMStateStore.java:1128)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore$ZKAction.runWithRetries(ZKRMStateStore.java:1161)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.existsWithRetries(ZKRMStateStore.java:1055)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore.updateApplicationAttemptStateInternal(ZKRMStateStore.java:712)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:243)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$UpdateAppAttemptTransition.transition(RMStateStore.java:226)
at org.apache.hadoop.yarn.state.StateMachineFactory$SingleInternalArc.doTransition(StateMachineFactory.java:362)
at org.apache.hadoop.yarn.state.StateMachineFactory.doTransition(StateMachineFactory.java:302)
at org.apache.hadoop.yarn.state.StateMachineFactory.access$300(StateMachineFactory.java:46)
at org.apache.hadoop.yarn.state.StateMachineFactory$InternalStateMachine.doTransition(StateMachineFactory.java:448)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore.handleStoreEvent(RMStateStore.java:812)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:872)
at org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore$ForwardingEventHandler.handle(RMStateStore.java:867)
at org.apache.hadoop.yarn.event.AsyncDispatcher.dispatch(AsyncDispatcher.java:182)
at org.apache.hadoop.yarn.event.AsyncDispatcher$1.run(AsyncDispatcher.java:109)
at java.lang.Thread.run(Thread.java:745)
2019-03-03 02:33:03,074 INFO org.apache.zookeeper.ZooKeeper: Session: 0x1690d678e9448da closed
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.event.AsyncDispatcher: AsyncDispatcher is draining to stop, igonring any new events.
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.server.resourcemanager.rmapp.attempt.RMAppAttemptImpl: appattempt_1551528254601_0723_000001 State change from RUNNING to FINAL_SAVING
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Ignore watcher event type: None with state:Disconnected for path:null from old session
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Ignore watcher event type: None with state:SyncConnected for path:null from old session
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Ignore watcher event type: None with state:Disconnected for path:null from old session
2019-03-03 02:33:03,074 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Ignore watcher event type: None with state:SyncConnected for path:null from old session
2019-03-03 02:33:03,074 WARN org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore: State-store fenced ! Transitioning RM to standby
RM02的异常日志如下：
2019-03-05 12:10:24,634 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Retrying operation on ZK. Retry no. 124
2019-03-05 12:10:25,447 INFO org.apache.zookeeper.ClientCnxn: Opening socket connection to server 10.204.245.45/10.204.245.45:5181. Will not attempt to authenticate using SASL (unknown error)
2019-03-05 12:10:25,447 INFO org.apache.zookeeper.ClientCnxn: Socket connection established to 10.204.245.45/10.204.245.45:5181, initiating session
2019-03-05 12:10:25,449 INFO org.apache.zookeeper.ClientCnxn: Session establishment complete on server 10.204.245.45/10.204.245.45:5181, sessionid = 0x1693f9adb5401ae, negotiated timeout = 60000
2019-03-05 12:10:26,704 WARN org.apache.zookeeper.ClientCnxn: Session 0x1693f9adb5401ae for server 10.204.245.45/10.204.245.45:5181, unexpected error, closing socket connection and attempting reconnect
java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:3236)
at java.io.ByteArrayOutputStream.grow(ByteArrayOutputStream.java:118)
at java.io.ByteArrayOutputStream.ensureCapacity(ByteArrayOutputStream.java:93)
at java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:135)
at java.io.DataOutputStream.writeInt(DataOutputStream.java:197)
at org.apache.jute.BinaryOutputArchive.writeInt(BinaryOutputArchive.java:55)
at org.apache.zookeeper.proto.SetDataRequest.serialize(SetDataRequest.java:59)
at org.apache.zookeeper.MultiTransactionRecord.serialize(MultiTransactionRecord.java:77)
at org.apache.zookeeper.ClientCnxn$Packet.createBB(ClientCnxn.java:292)
at org.apache.zookeeper.ClientCnxnSocketNIO.doIO(ClientCnxnSocketNIO.java:115)
at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:366)
at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1081)
以上是两个RM节点的日志，可以忽略时间点，3月3号和3月5号发生异常的轨迹类似，异常发生的过程如下：
1、我们在yarn-site.xml配置了yarn.resourcemanager.store.class这个属性的值为org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore，也就是说，RM会将它的状态信息保存到ZK中，与此属性相关的，还有如下两个重要属性：
yarn.resourcemanager.zk-num-retries：表示RM与ZK的连接丢失后，尝试与ZK建立连接的次数，我们线上默认是1000yarn.resourcemanager.zk-retry-interval-ms：尝试连接ZK的时间间隔，我们线上默认是1000毫秒，也就是1秒
基于以上配置，在RM与ZK连接失败后，每隔1秒RM就会尝试与ZK建立连接，直到达到最大重试次数，因此，从第一段日志我们可以看出如下日志轨迹：
（1）主节点RM01上的RM向ZK的/bi-rmstore节点中写数据发生异常，返回连接丢失错误码KeeperErrorCode=ConnectionLoss：
2019-03-03 02:33:02,201 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Exception while executing a ZK operation.
org.apache.zookeeper.KeeperException$ConnectionLossException: KeeperErrorCode = ConnectionLoss for /bi-rmstore/ZKRMStateRoot/RMAppRoot/application_1551528254601_0725/appattempt_1551528254601_0725_000001
（2）RM接收到连接丢失的错误码，按照重试逻辑进行重试，一直重试到999次，按照重试间隔时间，这期间花了至少999秒，也就是至少15分钟：
2019-03-03 02:33:01,826 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Retrying operation on ZK. Retry no. 999
（3）重试到第999次，还是连接丢失，那么，进行第1000次重试，但是，这个时候已经达到最大重试次数限制了，于是RM就打印达到最大重试次数、保存状态失败的信息：
2019-03-03 02:33:02,201 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Maxed out ZK retries. Giving up!
2019-03-03 02:33:02,201 ERROR org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore: State store operation failed
（4）主节点的RM发现自己怎么向ZK中写数据都写不进去，于是就标识自己出了问题，接着开始故障转移，将请求转移到RM02服务器，也就是从节点上，此时RM发生了主从切换：
2019-03-03 02:33:03,074 WARN org.apache.hadoop.yarn.server.resourcemanager.recovery.RMStateStore: State-store fenced ! Transitioning RM to standby
（5）因为发生了主从切换，从节点RM02此时是主节点。它切换完成后，也立即执行RM01上没有完成的操作，也就是往ZK写数据，但是也写不进去，最后重试到124次的时候，发生了JVM堆内存溢出异常：
2019-03-05 12:10:24,634 INFO org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore: Retrying operation on ZK. Retry no. 124
java.lang.OutOfMemoryError: Java heap space
堆内存溢出异常发生的原因一般是JVM的堆内存区域中有很多存活的对象，然后一个新对象申请不到所需的堆内存时，就会发生此类异常。
（6）此时，RM02内存溢出了，但是它自己进程没有退出，也没有故障转移，受影响的是90个NodeManager，它们发现连接不到RM了，就自己挂掉了，于是整个集群挂了。在整个故障发生过程中，我们可能会有一个疑问——为什么RM向ZK写数据写不进去？这个问题要结合ZK日志来分析，先抓住一个点，那就是发生内存溢出异常的代码位置：
java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:3236)
at java.io.ByteArrayOutputStream.grow(ByteArrayOutputStream.java:118)
at java.io.ByteArrayOutputStream.ensureCapacity(ByteArrayOutputStream.java:93)
at java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:135)
at java.io.DataOutputStream.writeInt(DataOutputStream.java:197)
at org.apache.jute.BinaryOutputArchive.writeInt(BinaryOutputArchive.java:55)
at org.apache.zookeeper.proto.SetDataRequest.serialize(SetDataRequest.java:59)
at org.apache.zookeeper.MultiTransactionRecord.serialize(MultiTransactionRecord.java:77)
at org.apache.zookeeper.ClientCnxn$Packet.createBB(ClientCnxn.java:292)
at org.apache.zookeeper.ClientCnxnSocketNIO.doIO(ClientCnxnSocketNIO.java:115)
at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:366)
at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1081)
在这里，重点关注org.apache.zookeeper.proto.SetDataRequest.serialize这个位置的代码，经过查看源码可以知道，这是ZK客户端往服务端写数据时，执行的正常序列化操作，也可以从serialize这个方法名字看出来该方法的作用，关键是写了多大的数据给ZK。接下来我们来看看ZK日志。
ZK日志
ZK日志如下：
2019-03-05 12:59:46,158 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:NIOServerCnxnFactory@197] - Accepted socket connection from /10.216.2.25:41586
2019-03-05 12:59:46,160 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:ZooKeeperServer@966] - Client attempting to renew session 0x2690d678e98ae8b at /10.216.2.25:41586
2019-03-05 12:59:46,160 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:Learner@108] - Revalidating client: 0x2690d678e98ae8b
2019-03-05 12:59:46,161 [myid:1] - INFO [QuorumPeer[myid=1]/0:0:0:0:0:0:0:0:5181:ZooKeeperServer@712] - Established session 0x2690d678e98ae8b with negotiated timeout 60000 for client /10.216.2.25:41586
2019-03-05 12:59:46,167 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:ZooKeeperServer@997] - got auth packet /10.216.2.25:41586
2019-03-05 12:59:46,167 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:ZooKeeperServer@1031] - auth success /10.216.2.25:41586
2019-03-05 12:59:46,177 [myid:1] - WARN [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:NIOServerCnxn@380] - Exception causing close of session 0x2690d678e98ae8b due to java.io.IOException: Len error 1788046
2019-03-05 12:59:46,177 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:NIOServerCnxn@1091] - Closed socket connection for client /10.216.2.25:41586 which had sessionid 0x2690d678e98ae8b
在ZK日志中，我们可以看到ZK Server接收到来自10.216.2.25:41586（RM01）进程的连接请求，最后发生了异常关闭了session，重点关注如下日志片段：
2019-03-05 12:59:46,177 [myid:1] - WARN [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:NIOServerCnxn@380] - Exception causing close of session 0x2690d678e98ae8b due to java.io.IOException: Len error 1788046
2019-03-05 12:59:46,177 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:5181:NIOServerCnxn@1091] - Closed socket connection for client /10.216.2.25:41586 which had sessionid 0x2690d678e98ae8b
以上日志显示ZK接收到一个1788046字节（1.7MB）的数据，接着发生了异常，最后关闭了session。在ZK的配置中，有一个jute.maxbuffer参数，这个参数是个全局参数，定义了每个ZNode节点所能存储的最大数据量，以字节为单位，默认为1048576字节，也就是说，默认情况下，ZK规定它的每个ZNode只能存储1兆数据。在这里，YARN集群的RM节点给ZK发送了一个1.7MB的数据，ZK判断这个数据超过了它所限制的最大数据量，因此就抛出了Len error 1788046的异常，最后直接关闭了session，给客户端返回了KeeperErrorCode=ConnectionLoss连接丢失错误码。
在这里，我们基本上清楚了两个RM为什么向ZK写数据写不进去了。还有一个问题，为什么会发生JVM内存溢出呢？这个问题的原因不在于ZK写数据异常，在于以下代码，接下来进行代码剖析。
YARN读写ZK代码
以下代码摘自org.apache.hadoop.yarn.server.resourcemanager.recovery.ZKRMStateStore类的保存状态和重试逻辑部分，具体细节见注释。先看下保存应用状态的逻辑：
@Override
public synchronized void updateApplicationStateInternal(ApplicationId appId,
ApplicationStateData appStateDataPB) throws Exception {
String nodeUpdatePath = getNodePath(rmAppRoot, appId.toString());
if (LOG.isDebugEnabled()) {
LOG.debug("Storing final state info for app: " + appId + " at: "
+ nodeUpdatePath);
}
// RM状态字节数组
byte[] appStateData = appStateDataPB.getProto().toByteArray();
// 调用重试逻辑写数据
if (existsWithRetries(nodeUpdatePath, true) != null) {
setDataWithRetries(nodeUpdatePath, appStateData, -1);
} else {
createWithRetries(nodeUpdatePath, appStateData, zkAcl,
CreateMode.PERSISTENT);
LOG.debug(appId + " znode didn't exist. Created a new znode to"
+ " update the application state.");
}
}
该段代码最终会调用以下重试逻辑：
T runWithRetries() throws Exception {
int retry = 0; // 保存重试次数
while (true) {
try {
return runWithCheck();
} catch (KeeperException.NoAuthException nae) {
if (HAUtil.isHAEnabled(getConfig())) {
// NoAuthException possibly means that this store is fenced due to
// another RM becoming active. Even if not,
// it is safer to assume we have been fenced
throw new StoreFencedException();
}
} catch (KeeperException ke) {
if (ke.code() == Code.NODEEXISTS) {
LOG.info("znode already exists!");
return null;
}
LOG.info("Exception while executing a ZK operation.", ke);
retry++; // 累计重试次数
/**
* 在未达到最大重试次数的条件的下，根据ZK Server返回的错误码执行不同的重试策略
*/
if (shouldRetry(ke.code()) && retry < numRetries) { // 如果ZK Server返回的错误码为连接丢失、连接超时，则直接进行重试
LOG.info("Retrying operation on ZK. Retry no. " + retry);
Thread.sleep(zkRetryInterval);
continue;
}
if (shouldRetryWithNewConnection(ke.code()) && retry < numRetries) { // 如果ZK Server返回的错误码为会话丢失、会话转移，则创建连接后进行重试
LOG.info("Retrying operation on ZK with new Connection. " +
"Retry no. " + retry);
Thread.sleep(zkRetryInterval);
createConnection();
syncInternal(ke.getPath());
continue;
}
LOG.info("Maxed out ZK retries. Giving up!"); // 达到最大重试次数，放弃重试，抛出异常
throw ke;
}
}
}
以上代码就是RM读写ZK失败时的重试逻辑，其中zkRetryInterval和numRetries两个变量的值来源于RM日志部分所列的两个重试配置。
问题原因
总的看来，JVM内存溢出异常发生的轨迹是这样的，RM往ZK写数据，ZK判断这个数据超过了最大数据量限制，然后就关闭了RM与ZK之间的连接。RM捕捉到ZK返回的连接丢失状态码，接着进行重试，重试机制是这样的：1秒重试一次，一直重试到1000次，最后搞的RM节点内存溢出了。观察数据发现，内存溢出时YRAN队列里堆积了200多个需要执行的任务，并且有几十个在执行，RM上的JVM堆内存消耗平时消耗4、5G，堆最大值6G。因此内存溢出的原因就是重试次数太多、太频繁，创建了太多对象，导致内存溢出的。
ZK数据量限制是诱因，重试机制引起的程序循环调用是根源。因为程序运行过程中，抛出了异常，然后在抛出异常的位置执行重试逻辑，重试间隔时间短、频率高，这个时候，方法栈内存和方法在执行过程中引用的堆内存对象也不会被释放，这也间接说明：
解决方案
在Hadoop YARN jira YARN-2368中发现，之前也有人碰到过类似的问题，官方也给出了相关的解决方案，但是Hadoop版本之间有差异，以Apache Hadoop版本为准：
1、Hadoop 2.9.0以后：
该版本在yarn-site.xml中增加了yarn.resourcemanager.zk-max-znode-size.bytes参数，该参数定义了ZK的ZNode节点所能存储的最大数据量，以字节为单位，默认是1048576字节，也就是1MB。
2、Hadoop 2.9.0以前：
通过在/etc/hadoop/conf/yarn-env.sh设置-Djute.maxbuffer=<bytes>参数解决。ZK在创建客户端连接时，会自动读取jute.maxbuffer的值。在yarn-env.sh末尾增加配置如下，这里我设置该值为3145728字节，也就是3MB:
YARN_OPTS="$YARN_OPTS -Djute.maxbuffer=3145728"
无论采用以上哪一种解决方案，都必须使ZK客户端配置的值与ZK服务端的jute.maxbuffer参数值保持一致，如果要修改，需要同时修改ZK服务端的jute.maxbuffer参数，否则在ZK读写时会发生异常。配置不一致时发生异常如下：
-
ZK客户端的jute.maxbuffer参数值大于ZK服务端的jute.maxbuffer参数值：客户端向服务端写入的数据量超过服务端配置的值时抛出java.io.IOException: Len error异常
-
ZK客户端的jute.maxbuffer参数值小于ZK服务端的jute.maxbuffer参数值：客户端从服务端读取的数据量超过客户端配置的值时抛出java.io.IOException: Unreasonable length异常
相关思考
ZK是一套高吞吐量的系统，为了提高系统的读取速度，ZK不允许从文件中读取需要的数据，而是直接从内存中查找。
换句话说，ZK集群中每一台服务器都包含全量的数据，并且这些数据都会加载到内存中，同时ZNode的数据不支持Append操作，全部都是Replace。
所以从上面分析可以看出，如果ZNode过大，那么读写某一个ZNode将造成不确定的延时，同时ZNode过大，将过快地耗尽ZK服务器的内存，这也是为什么ZK不适合存储大量数据的原因。
我们说性能，可以从两个方面去考虑：写入的性能与读取的性能。
由于ZK的写入首先需要通过Leader，然后这个写入的消息需要传播到半数以上的Follower通过才能完成整个写入，所以整个集群写入的性能无法通过增加服务器的数量达到目的，相反，整个集群中Follower数量越多，整个集群写入的性能越差。
ZK集群中的每一台服务器都可以提供数据的读取服务，所以整个集群中服务器的数量越多，读取的性能就越好，但是Follower增加又会降低整个集群的写入性能。
- 在生产环境下配置，
jute.maxbuffer参数的值的设置需要考虑哪些因素
首先，要考虑依赖ZK的服务数据量多大的问题。ZK作为一个分布式协调服务，它是要和其他服务整合才能发挥作用的。这里以YARN为例，YARN使用ZK做主备节点之间的故障转移，并且将RM的状态信息保存到ZK中。因此这里设置该参数的时候，要考虑这些数据信息有多大，故障转移节点保存的数据量很少，无非是主备节点的IP、host等信息。状态信息数据量相对来说比较大，RM将调度过程中产生的应用状态信息都保存到了ZK中，统计发现，我们现在出问题的YARN集群节点共有90个，高峰期任务数是节点数的3倍，每天调度几千个任务，任务量不是特别大，状态数据信息大部分在几百K到1M之间，偶尔会产生2M的状态数据信息，因此我们将该参数的值调整为3M。
其次，需要考虑改变该值之后，对其他依赖ZK的服务有没有影响。生产环境中，依赖ZK服务的服务有HBase、JStorm和Hadoop，理论上，这些服务都要修改各自客户端的该参数值，这是一个很繁琐的工作。目前从每天监控结果来看，其他服务保存在ZK中的数据，没有超过500K的，因此不需要改变客户端的该参数值。
最后，还要考虑读写场景和频率，因为数据量太大的话，读写还很多，可能会造成ZK服务端同步数据慢。
不会。在一定条件下，RM会删除保存在ZK中的applicationId对应的状态数据，ZKRMStateStore中对应的逻辑如下：
public synchronized void removeApplicationStateInternal(ApplicationState appState)
throws Exception {
String appId = appState.getAppId().toString(); // 获取应用ID
String appIdRemovePath = getNodePath(rmAppRoot, appId); // 根据应用ID获取对应的路径
ArrayList<Op> opList = new ArrayList<Op>();
// 执行状态信息删除操作
for (ApplicationAttemptId attemptId : appState.attempts.keySet()) {
String attemptRemovePath = getNodePath(appIdRemovePath, attemptId.toString());
opList.add(Op.delete(attemptRemovePath, -1));
}
opList.add(Op.delete(appIdRemovePath, -1));
if (LOG.isDebugEnabled()) {
LOG.debug("Removing info for app: " + appId + " at: " + appIdRemovePath
+ " and its attempts.");
}
doMultiWithRetries(opList);
}
这是一个额外的话题，引入Protocal Buffer的一个最大好处——满足向后兼容性。具体原因参考<<YARN/MRv2 RPC框架深入剖析—引入Protocal Buffer的好处>>这篇文章。
参考资料
李亚坤：Hadoop YARN在字节跳动的实践——推荐，总结了头条在YARN使用和改造方面的经验
ResourceManager failed when ZKRMStateStore tries to update znode data larger than 1MB
RM状态存储与还原机制详解
Remove jute.maxbuffer limit packetLen in client side
YARN源码分析(三)-----ResourceManager HA之应用状态存储与恢复
源码走读-Yarn-ResourceManager01-基础概念
Zookeeper在RM HA的应用
YARN/MRv2 RPC框架深入剖析—引入Protocal Buffer的好处
Zookeeper 服务器端和客户端扩大节点数据1M大小限制
ZOOKEEPER jute.maxbuffer参数解惑
