笃行杂记之Zookeeper SessionTimeOut分析

摘要：前言本文为笃行日常工作记录，烂笔头系列。最终通过分析源码了解到最终的确定是一个协商的过程，而不是简单的配置生效。根据客户端上报的和服务端自身的。如果上报的小于则设置为如果上报的大于则设置为如果介于两则之间，则以上报的时间为准。

本文为笃行日常工作记录，烂笔头系列。

源码前面，了无秘密 — by 侯杰

近期的一个C++项目里使用了Zookeeper做服务发现，期间遇到了SessionTimeOut问题的困扰，明明通过zookeeper c client设置了超时时间，但无效。

请原谅我一开始对zookeeper不熟悉。最终通过分析源码了解到SessionTimeOut最终的确定是一个协商的过程，而不是简单的配置生效。

在这里记录下Session超时时间的有关分析,基于zookeeper 3.4.8

项目中使用的是 C client,通过zookeer_init 创建zk session，调用了zookeeper_init其实就开始了建立链接到ZK集群的过程，这里设置的recv_timeout 为客户端所期望的session超时时间，单位为毫秒。

ZOOAPI zhandle_t *zookeeper_init(const char *host, watcher_fn fn,
  int recv_timeout, const clientid_t *clientid, void *context, int flags);

连接成功之后客户端发起握手协议,可以看到之前设置的recv_timeout随握手协议一起发送给服务端,zookeeper.c#L1485。

static int prime_connection(zhandle_t *zh)
{
    int rc;
    /*this is the size of buffer to serialize req into*/
    char buffer_req[HANDSHAKE_REQ_SIZE];
    int len = sizeof(buffer_req);
    int hlen = 0;
    struct connect_req req;
    req.protocolVersion = 0;
    req.sessionId = zh->seen_rw_server_before ? zh->client_id.client_id : 0;
    req.passwd_len = sizeof(req.passwd);
    memcpy(req.passwd, zh->client_id.passwd, sizeof(zh->client_id.passwd));
    req.timeOut = zh->recv_timeout; <-这里设置timeOut
    req.lastZxidSeen = zh->last_zxid;
    req.readOnly = zh->allow_read_only;
    hlen = htonl(len);
    /* We are running fast and loose here, but this string should fit in the initial buffer! */
    rc=zookeeper_send(zh->fd, &hlen, sizeof(len));
    serialize_prime_connect(&req, buffer_req);
    rc=rc<0 ? rc : zookeeper_send(zh->fd, buffer_req, len);
    if (rc<0) {
        return handle_socket_error_msg(zh, __LINE__, ZCONNECTIONLOSS,
                "failed to send a handshake packet: %s", strerror(errno));
    }

再来看看处理握手协议Resp的逻辑 zookeeper.c L1767

static int check_events(zhandle_t *zh, int events)
{
    if (zh->fd == -1)
        return ZINVALIDSTATE;
  ……
  ……
  ……
   deserialize_prime_response(&zh->primer_storage, zh->primer_buffer.buffer);
                /* We are processing the primer_buffer, so we need to finish
                 * the connection handshake */
                oldid = zh->client_id.client_id;
                newid = zh->primer_storage.sessionId;
                if (oldid != 0 && oldid != newid) {
                    zh->state = ZOO_EXPIRED_SESSION_STATE;
                    errno = ESTALE;
                    return handle_socket_error_msg(zh,__LINE__,ZSESSIONEXPIRED,
                            "sessionId=%#llx has expired.",oldid);
                } else {
                    zh->recv_timeout = zh->primer_storage.timeOut; //设置为Resp的Timeout
                    zh->client_id.client_id = newid;
                 
}

至此可以发现，最终客户端的SessionTimeOut时间实际是经过服务端下发之后的，并不一定是最先设置的。

来看看服务端握手的处理逻辑ZooKeeperServer.java#L876。

public void processConnectRequest(ServerCnxn cnxn, ByteBuffer incomingBuffer) throws IOException {
        BinaryInputArchive bia = BinaryInputArchive.getArchive(new ByteBufferInputStream(incomingBuffer));
        ConnectRequest connReq = new ConnectRequest();
        connReq.deserialize(bia, "connect");
……
……
……
  //根据客户端上报的timeout和服务端自身的minSessionTimeOut。
  //如果上报的timeout小于minSessionTimeOut则 设置timeout为minSessionTimeOut.
  //如果上报的timeout大于maxSessionTimeOut则 设置timeout为maxSessionTimeOut.
  //如果介于两则之间，则以上报的时间为准。
        int sessionTimeout = connReq.getTimeOut();
        byte passwd[] = connReq.getPasswd();
        int minSessionTimeout = getMinSessionTimeout();
        if (sessionTimeout < minSessionTimeout) {
            sessionTimeout = minSessionTimeout;
        }
        int maxSessionTimeout = getMaxSessionTimeout();
        if (sessionTimeout > maxSessionTimeout) {
            sessionTimeout = maxSessionTimeout;
        }
        cnxn.setSessionTimeout(sessionTimeout);
……
……
……
    }

可以一句话概括，客户端上报的期望timeout一定要在服务端设置的上下界之间，如果越过边界，则以边界为准。

继续看ZooKeeperServer.java#L104和ZooKeeperServer.java#L791

    public static final int DEFAULT_TICK_TIME = 3000;
    protected int tickTime = DEFAULT_TICK_TIME;
    /** value of -1 indicates unset, use default */
    protected int minSessionTimeout = -1;
    /** value of -1 indicates unset, use default */
    protected int maxSessionTimeout = -1;
    protected SessionTracker sessionTracker;

tickTime为3000毫秒，minSessionTimeOut和maxSessionTimeOut缺省值为-1

 public int getTickTime() {
        return tickTime;
    }

    public void setTickTime(int tickTime) {
        LOG.info("tickTime set to " + tickTime);
        this.tickTime = tickTime;
    }

    public int getMinSessionTimeout() {
        return minSessionTimeout == -1 ? tickTime * 2 : minSessionTimeout;
      //如果minSessionTimeOut为缺省值这设置minSessionTimeOut为2倍tickTime
    }

    public void setMinSessionTimeout(int min) {
        LOG.info("minSessionTimeout set to " + min);
        this.minSessionTimeout = min;
    }

    public int getMaxSessionTimeout() {
        return maxSessionTimeout == -1 ? tickTime * 20 : maxSessionTimeout;
      //如果maxSessionTimeout为缺省值则设置maxSessionTimeout为20倍tickTime
    }

    public void setMaxSessionTimeout(int max) {
        LOG.info("maxSessionTimeout set to " + max);
        this.maxSessionTimeout = max;
    }

可以知道minSessionTimeOut和maxSessionTimeOut在缺省的时候则跟tickTime有关，分别为2和20倍tickTime，继续分析。ZooKeeperServer.java#L160

  public ZooKeeperServer(FileTxnSnapLog txnLogFactory, int tickTime,
            int minSessionTimeout, int maxSessionTimeout,
            DataTreeBuilder treeBuilder, ZKDatabase zkDb) {
        serverStats = new ServerStats(this);
        this.txnLogFactory = txnLogFactory;
        this.zkDb = zkDb;
        this.tickTime = tickTime;
        this.minSessionTimeout = minSessionTimeout;
        this.maxSessionTimeout = maxSessionTimeout;
        
        LOG.info("Created server with tickTime " + tickTime
                + " minSessionTimeout " + getMinSessionTimeout()
                + " maxSessionTimeout " + getMaxSessionTimeout()
                + " datadir " + txnLogFactory.getDataDir()
                + " snapdir " + txnLogFactory.getSnapDir());
    }

tickTime、minSessionTimeOut、maxSessionTimeOut实际构造函数传入，当然还有一个无参构造函数以及一些setter和getter可以设置这几个参数。

继续分析ZooKeeperServerMain.java#L94

 public void runFromConfig(ServerConfig config) throws IOException {
        LOG.info("Starting server");
        FileTxnSnapLog txnLog = null;
        try {
            // Note that this thread isn"t going to be doing anything else,
            // so rather than spawning another thread, we will just call
            // run() in this thread.
            // create a file logger url from the command line args
            ZooKeeperServer zkServer = new ZooKeeperServer();

            txnLog = new FileTxnSnapLog(new File(config.dataLogDir), new File(
                    config.dataDir));
            zkServer.setTxnLogFactory(txnLog);
            zkServer.setTickTime(config.tickTime);
          //我们可以发现实际运行的几个参数除了默认值以外，可以通过配置文件来配置生效。
            zkServer.setMinSessionTimeout(config.minSessionTimeout);
            zkServer.setMaxSessionTimeout(config.maxSessionTimeout);
            cnxnFactory = ServerCnxnFactory.createFactory();
            cnxnFactory.configure(config.getClientPortAddress(),
                    config.getMaxClientCnxns());
            cnxnFactory.startup(zkServer);
            cnxnFactory.join();
            if (zkServer.isRunning()) {
                zkServer.shutdown();
            }
        } catch (InterruptedException e) {
            // warn, but generally this is ok
            LOG.warn("Server interrupted", e);
        } finally {
            if (txnLog != null) {
                txnLog.close();
            }
        }
    }

到此问题就明了了，我们可以通过配置来修改SessionTimeOut，默认配置文件只配置了tickTime，如下。

# The number of milliseconds of each tick
tickTime=2000
# The number of ticks that the initial 
# synchronization phase can take
initLimit=10
# The number of ticks that can pass between 
# sending a request and getting an acknowledgement
syncLimit=5
# the directory where the snapshot is stored.
# do not use /tmp for storage, /tmp here is just 
# example sakes.
dataDir=/tmp/zookeeper
# the port at which the clients will connect
clientPort=2181
# the maximum number of client connections.
# increase this if you need to handle more clients
#maxClientCnxns=60
#
# Be sure to read the maintenance section of the 
# administrator guide before turning on autopurge.
#
# http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_maintenance
#
# The number of snapshots to retain in dataDir
#autopurge.snapRetainCount=3
# Purge task interval in hours
# Set to "0" to disable auto purge feature
#autopurge.purgeInterval=1

经过源码分析，得出SessionTimeOut的协商如下：

情况1: 配置文件配置了maxSessionTimeOut和minSessionTimeOut

最终SessionTimeOut,必须在minSessionTimeOut和maxSessionTimeOut区间里，如果跨越上下界，则以跨越的上届或下界为准。

情况2:配置文件没有配置maxSessionTimeOut和minSessionTimeOut

maxSessionTimeout没配置则 maxSessionTimeOut设置为 20 * tickTime

minSessionTimeOut没配置则 minSessionTimeOut设置为 2 * tickTime

也就是默认情况下, SessionTimeOut的合法范围为 4秒~40秒，默认配置中tickTime为2秒。

如果tickTime也没配置，那么tickTime缺省为3秒。

遇到问题从源码分析一定是最好的，能使得理解更深入记忆更深刻。

最后 ending...如有不足请指点，亦可留言或联系 fobcrackgp@163.com.

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