0.前言
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
1.zookeeper client SessionTimeOut
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。
2.Zookeeper Server SessionTimeOut
2.1协商客户端上报的SessionTimeOut
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。
2.2 服务端MinSessionTimeOut和MaxSessionTimeOut的确定
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=13.总结
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.
