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篤行雜記之Zookeeper SessionTimeOut分析
0.前言
本文為篤行日常工作記錄，爛筆頭係列。
源碼前麵，了無秘密 — by 侯傑
近期的一個C++項目裏使用了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。
在這裏記錄下Session超時時間的有關分析,基於zookeeper 3.4.8
1.zookeeper client SessionTimeOut
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，這裏設置的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);
連接成功之後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; <-這裏設置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後的，並不一定是最先設置的。
2.Zookeeper Server SessionTimeOut
2.1協商客戶端上報的SessionTimeOut
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小於minSessionTimeOut則 設置timeout為minSessionTimeOut.
//如果上報的timeout大於maxSessionTimeOut則 設置timeout為maxSessionTimeOut.
//如果介於兩則之間，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。
2.2 服務端MinSessionTimeOut和MaxSessionTimeOut的確定
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，默認配置文件隻配置了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.
#
# https://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
3.總結
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區間裏，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的合法範圍為 4秒~40秒，默認配置中tickTime為2秒。
如果tickTime也沒配置，那麼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。
最後 ending...如有不足請指點，亦可留言或聯係 fobcrackgp@163.com.
最後更新：2017-07-06 00:02:10