为什么ZooKeeper Server出现java.io.IOException: Len的错误日志

问题

在父目录中创建大量的znode之后，当ZooKeeper客户端尝试在单个请求中获取该父目录中的所有子节点时，将返回失败。

客户端日志，如下所示：

2017-07-11 13:17:19,610 [myid:] - WARN  [New I/O worker #3:ClientCnxnSocketNetty$ZKClientHandler@468] - Exception caught: [id: 0xb66cbb85, /10.18.97.97:49192 :> 10.18.97.97/10.18.97.97:2181] EXCEPTION: java.nio.channels.ClosedChannelException
java.nio.channels.ClosedChannelException
at org.jboss.netty.handler.ssl.SslHandler$6.run(SslHandler.java:1580)
at org.jboss.netty.channel.socket.ChannelRunnableWrapper.run(ChannelRunnableWrapper.java:40)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.executeInIoThread(AbstractNioWorker.java:71)
at org.jboss.netty.channel.socket.nio.NioWorker.executeInIoThread(NioWorker.java:36)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.executeInIoThread(AbstractNioWorker.java:57)
at org.jboss.netty.channel.socket.nio.NioWorker.executeInIoThread(NioWorker.java:36)
at org.jboss.netty.channel.socket.nio.AbstractNioChannelSink.execute(AbstractNioChannelSink.java:34)
at org.jboss.netty.handler.ssl.SslHandler.channelClosed(SslHandler.java:1566)
at org.jboss.netty.channel.Channels.fireChannelClosed(Channels.java:468
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.close(AbstractNioWorker.java:376)
at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:93)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109)
at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90)
at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

Leader节点的日志，如下所示：

2017-07-11 13:17:33,043 [myid:1] - WARN  [New I/O worker #7:NettyServerCnxn@445] - Closing connection to /10.18.101.110:39856
java.io.IOException: Len error 45
at org.apache.zookeeper.server.NettyServerCnxn.receiveMessage(NettyServerCnxn.java:438)
at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.processMessage(NettyServerCnxnFactory.java:267)
at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.messageReceived(NettyServerCnxnFactory.java:187)
at org.jboss.netty.channel.SimpleChannelHandler.handleUpstream(SimpleChannelHandler.java:88)
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268)
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255)
at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109)
at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90)
at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

回答

在单个父目录中创建大量的znode后，当客户端尝试在单个请求中获取所有子节点时，服务端将无法返回，因为结果将超出可存储在znode上的数据的最大长度。

为了避免这个问题，应该根据客户端应用的实际情况将“jute.maxbuffer”参数配置为一个更高的值。

“jute.maxbuffer”只能设置为Java系统属性，且没有zookeeper前缀。如果要将“jute.maxbuffer”的值设为X，在ZooKeeper客户端或服务端启动时传入以下系统属性：-Djute.maxbuffer=X。

例如，将参数值设置为4MB：-Djute.maxbuffer=0x400000。