熊猫TV的礼物系统使用了golang的 mongo库 mgo，中间踩了一些坑，总结下避免大家再踩坑
golang的mgo库说明里是说明了开启连接复用的，但观察实验发现，这并没有根本实现连接的控制，连接复用其实仅在当前操作 (session.Close 之前 )生效，最终还是需要程序员自行去限制连接才行。
废话不多说，开始上代码
GlobalMgoSession, err := mgo.Dial(host)
func (m *MongoBaseDao) Get(tablename string, id string, result interface{}) interface{} {
session := GlobalMgoSession.Clone()
defer session.Close()
collection := session.DB(globalMgoDbName).C(tablename)
err := collection.FindId(bson.ObjectIdHex(id)).One(result)
if err != nil {
logkit.Logger.Error("mongo_base method:Get " + err.Error())
}
return result
}
golang main入口启动时，我们会创建一个全局session，然后每次使用时clone session的信息和连接，用于本次请求，使用后调用session.Close() 释放连接。
// Clone works just like Copy, but also reuses the same socket as the original
// session, in case it had already reserved one due to its consistency
// guarantees. This behavior ensures that writes performed in the old session
// are necessarily observed when using the new session, as long as it was a
// strong or monotonic session. That said, it also means that long operations
// may cause other goroutines using the original session to wait.
func (s *Session) Clone() *Session {
s.m.Lock()
scopy := copySession(s, true)
s.m.Unlock()
return scopy
}
// Close terminates the session. It's a runtime error to use a session
// after it has been closed.
func (s *Session) Close() {
s.m.Lock()
if s.cluster_ != nil {
debugf("Closing session %p", s)
s.unsetSocket() //释放当前线程占用的socket 置为nil
s.cluster_.Release()
s.cluster_ = nil
}
s.m.Unlock()
}
Clone的方法注释里说明会重用原始session的socket连接，但是并发请求一大，其他协程来不及释放连接，当前协程会怎么办？
func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {
// Read-only lock to check for previously reserved socket.
s.m.RLock()
// If there is a slave socket reserved and its use is acceptable, take it as long
// as there isn't a master socket which would be preferred by the read preference mode.
if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
socket := s.slaveSocket
socket.Acquire()
s.m.RUnlock()
logkit.Logger.Info("sgp_test 1 acquireSocket slave is ok!")
return socket, nil
}
if s.masterSocket != nil {
socket := s.masterSocket
socket.Acquire()
s.m.RUnlock()
logkit.Logger.Info("sgp_test 1 acquireSocket master is ok!")
return socket, nil
}
s.m.RUnlock()
// No go. We may have to request a new socket and change the session,
// so try again but with an exclusive lock now.
s.m.Lock()
defer s.m.Unlock()
if s.slaveSocket != nil && s.slaveOk && slaveOk && (s.masterSocket == nil || s.consistency != PrimaryPreferred && s.consistency != Monotonic) {
s.slaveSocket.Acquire()
logkit.Logger.Info("sgp_test 2 acquireSocket slave is ok!")
return s.slaveSocket, nil
}
if s.masterSocket != nil {
s.masterSocket.Acquire()
logkit.Logger.Info("sgp_test 2 acquireSocket master is ok!")
return s.masterSocket, nil
}
// Still not good. We need a new socket.
sock, err := s.cluster().AcquireSocket(s.consistency, slaveOk && s.slaveOk, s.syncTimeout, s.sockTimeout, s.queryConfig.op.serverTags, s.poolLimit)
......
logkit.Logger.Info("sgp_test 3 acquireSocket cluster AcquireSocket is ok!")
return sock, nil
}
在源码中加debug，结果日志说明一切：
Mar 25 09:46:40 dev02.com[12607]: [info] sgp_test 1 acquireSocket master is ok! Mar 25 09:46:40 dev02.com[12607]: [info] sgp_test 1 acquireSocket master is ok! Mar 25 09:46:41 dev02.com[12607]: [info] sgp_test 1 acquireSocket slave is ok! Mar 25 09:46:41 dev02.com[12607]: [info] sgp_test 3 acquireSocket cluster AcquireSocket is ok! Mar 25 09:46:41 dev02.com[12607]: [info] sgp_test 3 acquireSocket cluster AcquireSocket is ok! Mar 25 09:46:41 dev02.com[12607]: [info] sgp_test 3 acquireSocket cluster AcquireSocket is ok!
不断的创建连接 AcquireSocket
$ netstat -nat|grep -i 27017|wc -l
400
如果每个session 不调用close，会达到恐怖的4096,并堵死其他请求，所以clone或copy session时一定要defer close掉
启用maxPoolLimit 参数则会限制总连接大小，连接到限制则当前协程会sleep等待 直到可以创建连接，高并发时锁有问题，会导致多创建几个连接
src/gopkg.in/mgo.v2/cluster.go
s, abended, err := server.AcquireSocket(poolLimit, socketTimeout)
if err == errPoolLimit {
if !warnedLimit {
warnedLimit = true
logkit.Logger.Error("sgp_test WARNING: Per-server connection limit reached. " + err.Error())
log("WARNING: Per-server connection limit reached.")
}
time.Sleep(100 * time.Millisecond)
continue
}
session.go:
// SetPoolLimit sets the maximum number of sockets in use in a single server
// before this session will block waiting for a socket to be available.
// The default limit is 4096.
//
// This limit must be set to cover more than any expected workload of the
// application. It is a bad practice and an unsupported use case to use the
// database driver to define the concurrency limit of an application. Prevent
// such concurrency "at the door" instead, by properly restricting the amount
// of used resources and number of goroutines before they are created.
func (s *Session) SetPoolLimit(limit int) {
s.m.Lock()
s.poolLimit = limit
s.m.Unlock()
}
连接池设置方法：
1、配置中 增加
[host]:[port]?maxPoolSize=10
2、代码中 ：
dao.GlobalMgoSession.SetPoolLimit(10)
再做压测：
$ netstat -nat|grep -i 27017|wc -l
15
结论：
每次clone session之后，操作结束时如果调用 session.Close 则会unset Socket ，置nil， 所以socket复用，仅在当前session范围内生效，所以非全局的session无法共用，每个协程请求到来都会创建socket连接，直到达到最大值4096，而mongo的连接数上限一般也就是1万，也就是一个端口你只能启动一个进程保证连接不被撑爆，过多的连接数客户端效率不高，server端更会耗费内存和CPU，所以需要启用自定义连接池 , 启用连接池也需要注意如果有pooMaxLimit个协程执行过长或者死循环不释放socket连接，也会悲剧。
mgo并没有从底层实现socket的预先创建和整个生命周期的连接池复用，需要自行优化。
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