golang mgo的mongo连接池设置：必须手动加上maxPoolSize
本司礼物系统使用了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，连接复用仅在有空闲连接时生效，高并发时无可用连接会不断创建新连接，所以最终还是需要程序员自行去限制最大连接才行。
废话不多说，开始上代码
GlobalMgoSession, err := mgo.Dial(host) func (m *MongoBaseDao) Get(tablename string,>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 1acquireSocket 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 2acquireSocket slave is ok!") return s.slaveSocket, nil } if s.masterSocket != nil { s.masterSocket.Acquire() logkit.Logger.Info("sgp_test 2acquireSocket 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.pandatv.com bikini: sgp_test 1acquireSocket master is ok!Mar 25 09:46:40 dev02.pandatv.com bikini: sgp_test 1acquireSocket master is ok!Mar 25 09:46:41 dev02.pandatv.com bikini: sgp_test 1 acquireSocket slave is ok!Mar 25 09:46:41 dev02.pandatv.com bikini: sgp_test 3 acquireSocket cluster AcquireSocket is ok!Mar 25 09:46:41 dev02.pandatv.com bikini: sgp_test 3 acquireSocket cluster AcquireSocket is ok!Mar 25 09:46:41 dev02.pandatv.com bikini: 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
:?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，socket refer数减少，如果不设置上限，每个协程请求到来发现无空闲连接就会创建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一两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底层socket连接池只在maxPooMaxLimit 范围内实现复用，需要自行优化。
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