摘要：前言本文為篤行日常工作記錄，爛筆頭系列。最終通過分析源碼了解到最終的確定是一個協商的過程，而不是簡單的配置生效。根據客戶端上報的和服務端自身的。如果上報的小于則設置為如果上報的大于則設置為如果介于兩則之間，則以上報的時間為準。

本文為篤行日常工作記錄，爛筆頭系列。

源碼前面，了無秘密 — by 侯杰

近期的一個C++項目里使用了Zookeeper做服務發現，期間遇到了SessionTimeOut問題的困擾，明明通過zookeeper c client設置了超時時間，但無效。

請原諒我一開始對zookeeper不熟悉。最終通過分析源碼了解到SessionTimeOut最終的確定是一個協商的過程，而不是簡單的配置生效。

在這里記錄下Session超時時間的有關分析,基于zookeeper 3.4.8

項目中使用的是 C client,通過zookeer_init 創建zk session，調用了zookeeper_init其實就開始了建立鏈接到ZK集群的過程，這里設置的recv_timeout 為客戶端所期望的session超時時間，單位為毫秒。

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

連接成功之后客戶端發起握手協議,可以看到之前設置的recv_timeout隨握手協議一起發送給服務端,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; <-這里設置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));
    }

再來看看處理握手協議Resp的邏輯 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; //設置為Resp的Timeout
                    zh->client_id.client_id = newid;
                 
}

至此可以發現，最終客戶端的SessionTimeOut時間實際是經過服務端下發之后的，并不一定是最先設置的。

來看看服務端握手的處理邏輯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");
……
……
……
  //根據客戶端上報的timeout和服務端自身的minSessionTimeOut。
  //如果上報的timeout小于minSessionTimeOut則 設置timeout為minSessionTimeOut.
  //如果上報的timeout大于maxSessionTimeOut則 設置timeout為maxSessionTimeOut.
  //如果介于兩則之間，則以上報的時間為準。
        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);
……
……
……
    }

可以一句話概括，客戶端上報的期望timeout一定要在服務端設置的上下界之間，如果越過邊界，則以邊界為準。

繼續看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為3000毫秒，minSessionTimeOut和maxSessionTimeOut缺省值為-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為缺省值這設置minSessionTimeOut為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為缺省值則設置maxSessionTimeout為20倍tickTime
    }

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

可以知道minSessionTimeOut和maxSessionTimeOut在缺省的時候則跟tickTime有關，分別為2和20倍tickTime，繼續分析。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實際構造函數傳入，當然還有一個無參構造函數以及一些setter和getter可以設置這幾個參數。

繼續分析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);
          //我們可以發現實際運行的幾個參數除了默認值以外，可以通過配置文件來配置生效。
            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();
            }
        }
    }

到此問題就明了了，我們可以通過配置來修改SessionTimeOut，默認配置文件只配置了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

經過源碼分析，得出SessionTimeOut的協商如下：

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

最終SessionTimeOut,必須在minSessionTimeOut和maxSessionTimeOut區間里，如果跨越上下界，則以跨越的上屆或下界為準。

情況2:配置文件沒有配置maxSessionTimeOut和minSessionTimeOut

maxSessionTimeout沒配置則 maxSessionTimeOut設置為 20 * tickTime

minSessionTimeOut沒配置則 minSessionTimeOut設置為 2 * tickTime

也就是默認情況下, SessionTimeOut的合法范圍為 4秒~40秒，默認配置中tickTime為2秒。

如果tickTime也沒配置，那么tickTime缺省為3秒。

遇到問題從源碼分析一定是最好的，能使得理解更深入記憶更深刻。

最后 ending...如有不足請指點，亦可留言或聯系 fobcrackgp@163.com.

本文為篤行原創文章首發于大題小作,永久鏈接:篤行雜記之Zookeeper SessionTimeOut分析

https://www.ifobnn.com/zookeepersessiontimeout.html