golang mgo的mongo链接池设置：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，总结下避免你们再踩坑

golang的mgo库说明里是说明了开启链接复用的，但观察实验发现，这并无根本实现链接的控制，链接复用仅在有空闲链接时生效，高并发时无可用链接会不断建立新链接，因此最终仍是须要程序员自行去限制最大链接才行。

废话很少说，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，咱们会建立一个全局session，而后每次使用时clone session的信息和链接，用于本次请求，使用后调用session.Close() 释放链接。

// 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链接，可是并发请求一大，其余协程来不及释放链接，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.pandatv.com bikini[12607]:  [info] sgp_test 1  acquireSocket master is ok! Mar 25 09:46:40 dev02.pandatv.com bikini[12607]:  [info] sgp_test 1  acquireSocket master is ok! Mar 25 09:46:41 dev02.pandatv.com bikini[12607]:  [info] sgp_test 1 acquireSocket slave is ok! Mar 25 09:46:41 dev02.pandatv.com bikini[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok! Mar 25 09:46:41 dev02.pandatv.com bikini[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok! Mar 25 09:46:41 dev02.pandatv.com bikini[12607]:  [info] sgp_test 3   acquireSocket cluster AcquireSocket is ok!

不断的建立链接  AcquireSocket

 $  netstat -nat|grep -i 27017|wc -l

400

若是每一个session 不调用close，会达到恐怖的4096,并堵死其余请求，因此clone或copy session时必定要defer close掉

启用maxPoolLimit 参数则会限制总链接大小，链接到限制则当前协程会sleep等待  直到能够建立链接，高并发时锁有问题，会致使多建立几个链接

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()    }

链接池设置方法：

一、配置中 增长 

[host]:[port]?maxPoolSize=10

二、代码中 ：

dao.GlobalMgoSession.SetPoolLimit(10)

再作压测：

 $  netstat -nat|grep -i 27017|wc -l

15

 

 

结论：

每次clone session以后，操做结束时若是调用 session.Close 则会unset Socket  ，socket refer数减小，若是不设置上限，每一个协程请求到来发现无空闲链接就会建立socket链接，直到达到最大值4096，而mongo的链接数上限通常也就是1万，也就是一个端口你只能启动一两个进程保证链接不被撑爆，过多的链接数客户端效率不高，server端更会耗费内存和CPU，因此须要启用自定义链接池 , 启用链接池也须要注意若是有pooMaxLimit个协程执行过长或者死循环不释放socket链接，也会悲剧。

mgo底层socket链接池只在maxPooMaxLimit 范围内实现复用，须要自行优化。