TiDB数据库的优雅关闭技巧

背景

今天使用tiup做实验的事后，将tidb节点从2个缩到1个，发现tiup返回成功但是tidb-server进程还在。

这就引发的我的好奇心，why？

实验复现

启动集群

#( 07/31/23@ 8:32下午 )( happy@ZBMAC-f298743e3 ):~/docker/tiup/tiproxy tiup playground v6.4.0 --db 2 --kv 1 --pd 1 --tiflash 0 --without-monitor --db.config tidb.toml tiup is checking updates forcomponent playground... Starting component `playground`: /Users/happy/.tiup/components/playground/v1.12.5/tiup-playground v6.4.0 --db 2 --kv 1 --pd 1 --tiflash 0--without-monitor --db.config tidb.toml Start pd instance:v6.4.0 Start tikv instance:v6.4.0 Start tidb instance:v6.4.0 Start tidb instance:v6.4.0 Waitingfor tidb instances ready 127.0.0.1:4000 ... Done 127.0.0.1:4001 ... Done TiDB Playground Cluster is started, enjoy! Connect TiDB: mysql --comments --host 127.0.0.1 --port 4000-u root Connect TiDB: mysql --comments --host127.0.0.1 --port 4001 -u root TiDB Dashboard: http://127.0.0.1:2379/dashboard

查看节点信息

#( 07/31/23@ 8:32下午 )( happy@ZBMAC-f298743e3 ):~ tiup playground display tiup is checking updates for component playground ... Starting component `playground`: /Users/happy/.tiup/components/playground/v1.12.5/tiup-playground display Pid Role Uptime --- ---- ------ 10113 pd 49.376485092s 10114 tikv 49.32262974s 10115 tidb 49.283144092s 10116 tidb 49.245069308s

缩掉一个tidb节点

#( 07/31/23@ 8:34下午 )( happy@ZBMAC-f298743e3 ):~tiup playground scale-in --pid10115 tiup is checking updates for component playground ... Starting component `playground`: /Users/happy/.tiup/components/playground/v1.12.5/tiup-playground scale-in --pid 10115 scale in tidb success

这里可以看到已经返回了 scale in tidb success

查看进程

#( 07/31/23@ 8:34下午 )( happy@ZBMAC-f298743e3 ):~ ps -ef | grep 10115 502 11371 99718 0 8:34下午 ttys001 0:00.00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn --exclude-dir=.idea --exclude-dir=.tox 10115 502 10115 10111 0 8:32下午 ttys005 0:04.29 /Users/happy/.tiup/components/tidb/v6.4.0/tidb-server -P4000 --store=tikv --host=127.0.0.1 --status=10080 --path=127.0.0.1:2379 --log-file=/Users/happy/.tiup/data/TlaeoSj/tidb-0/tidb.log --config=/Users/happy/.tiup/data/TlaeoSj/tidb-0/tidb.toml

进程还是存在

分析

于是查看了 v6.4.0 的 tidb-server 代码。首先想到去main函数看下close的流程

main

func main() { //.. signal.SetupSignalHandler(func(graceful bool) { svr.Close() cleanup(svr, storage, dom, graceful) cpuprofile.StopCPUProfiler() close(exited) }) // ... }

在这里发现两个重要的逻辑 svr.Close()，cleanup(svr, storage, dom, graceful)

svr.Close()

// Close closes the server. func (s *Server) Close() { s.startShutdown() s.rwlock.Lock() // prevent new connections defer s.rwlock.Unlock() if s.listener != nil { err := s.listener.Close() terror.Log(errors.Trace(err)) s.listener = nil } if s.socket != nil { err := s.socket.Close() terror.Log(errors.Trace(err)) s.socket = nil } if s.statusServer != nil { err := s.statusServer.Close() terror.Log(errors.Trace(err)) s.statusServer = nil } if s.grpcServer != nil { s.grpcServer.Stop() s.grpcServer = nil } if s.autoIDService != nil { s.autoIDService.Close() } if s.authTokenCancelFunc != nil { s.authTokenCancelFunc() } s.wg.Wait() metrics.ServerEventCounter.WithLabelValues(metrics.EventClose).Inc() } func (s *Server) startShutdown() { s.rwlock.RLock() logutil.BgLogger().Info("setting tidb-server to report unhealthy (shutting-down)") s.inShutdownMode = true s.rwlock.RUnlock() // give the load balancer a chance to receive a few unhealthy health reports // before acquiring the s.rwlock and blocking connections. waitTime := time.Duration(s.cfg.GracefulWaitBeforeShutdown) * time.Second if waitTime > 0 { logutil.BgLogger().Info("waiting for stray connections before starting shutdown process", zap.Duration("waitTime", waitTime)) time.Sleep(waitTime) } }

从上面的逻辑可以看到，close的时候先startShutdown再进行资源回收。而在执行startShutdown的时候，居然有个time.Sleep(waitTime)。

然后研究下 graceful-wait-before-shutdown  参数，发现参数是0，不是此处导致的。

在 TiDB 等待服务器关闭期间，HTTP 状态会显示失败，使得负载均衡器可以重新路由流量 默认值：0 指定关闭服务器时 TiDB 等待的秒数，使得客户端有时间断开连接。

cleanup()

在 cleanup 中看到了 GracefulDown 和 TryGracefulDown 两个方法

func cleanup(svr *server.Server, storage kv.Storage, dom *domain.Domain, graceful bool) { if graceful { done := make(chan struct{}) svr.GracefulDown(context.Background(), done) } else { svr.TryGracefulDown() } plugin.Shutdown(context.Background()) closeDomainAndStorage(storage, dom) disk.CleanUp() topsql.Close() }

TryGracefulDown

研究发现使用 SIGHUP 终止进程时使用 TryGracefulDown 方法，其他时候使用 GracefulDown。对比 TryGracefulDown 和 GracefulDown 实现， TryGracefulDown 只是多个15s的超时处理，底层逻辑还是 GracefulDown

vargracefulCloseConnectionsTimeout= 15 * time.Second // TryGracefulDown will try to gracefully close all connection first with timeout. if timeout, will close all connection directly. func (s *Server) TryGracefulDown() { ctx, cancel := context.WithTimeout(context.Background(), gracefulCloseConnectionsTimeout) defer cancel() done := make(chan struct{}) go func() { s.GracefulDown(ctx, done) }() select { case <-ctx.Done(): s.KillAllConnections() case <-done: return } }

GracefulDown

下面是 GracefulDown 实现，原来在这里会间隔1s，一直判断客户端连接是否存在，如果不存在才退出。

// GracefulDown waits all clients to close. func (s *Server) GracefulDown(ctx context.Context, done chan struct{}) { logutil.Logger(ctx).Info("[server] graceful shutdown.") metrics.ServerEventCounter.WithLabelValues(metrics.EventGracefulDown).Inc() count := s.ConnectionCount() for i := 0; count > 0; i++ { s.kickIdleConnection() count = s.ConnectionCount() if count == 0 { break } // Print information for every 30s. if i%30 == 0 { logutil.Logger(ctx).Info("graceful shutdown...", zap.Int("conn count", count)) } ticker := time.After(time.Second) select { case <-ctx.Done(): return case <-ticker: } } close(done) }

ConnectionCount

判断连接个数的逻辑也很简单，就是对算下 s.clients 的 length

// ConnectionCount gets current connection count. func (s *Server) ConnectionCount() int { s.rwlock.RLock() cnt := len(s.clients) s.rwlock.RUnlock() return cnt }

其中还有一个奇怪的函数 kickIdleConnection，这个是做什么的？

kickIdleConnection

看逻辑是收集可以被close的会话然后close掉。

func (s *Server) kickIdleConnection() { var conns []*clientConn s.rwlock.RLock() for _, cc := range s.clients { if cc.ShutdownOrNotify() { // Shutdowned conn will be closed by us, and notified conn will exist themselves. conns = append(conns, cc) } } s.rwlock.RUnlock() for _, cc := range conns { err := cc.Close() if err != nil { logutil.BgLogger().Error("close connection", zap.Error(err)) } } }

那么什么样的会话可以被close呢？

ShutdownOrNotify

有三类：

client 状态处于 ServerStatusInTrans；

状态处于 connStatusReading

以及处于 connStatusDispatching 在 clientConn.Run 方法中被回收

// ShutdownOrNotify will Shutdown this client connection, or do its best to notify. func (cc *clientConn) ShutdownOrNotify() bool { if (cc.ctx.Status() & mysql.ServerStatusInTrans) > 0 { return false } // If the client connection status is reading, its safe to shutdown it. if atomic.CompareAndSwapInt32(&cc.status, connStatusReading, connStatusShutdown) { return true } // If the client connection status is dispatching, we cant shutdown it immediately, // so set the status to WaitShutdown as a notification, the loop in clientConn.Run // will detect it and then exit. atomic.StoreInt32(&cc.status,connStatusWaitShutdown) return false } const ( connStatusDispatching int32 = iota connStatusReading connStatusShutdown // Closed by server. connStatusWaitShutdown // Notified by server to close. )

破案

通过上面的分析，我们注意到了处于 ServerStatusInTrans 状态的连接不会被关闭，然后连接该节点执行show processlist发现的确有个处于事务中的会话

mysql> show processlist; +---------------------+------+-----------------+------+---------+------+----------------------------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +---------------------+------+-----------------+------+---------+------+----------------------------+------------------+ | 7794237818187809175 | root | 127.0.0.1:61293 | a | Query | 0 | in transaction; autocommit | show processlist | +---------------------+------+-----------------+------+---------+------+----------------------------+------------------+ 1 row in set (0.00 sec)

平时mysql使用的多，mysql在关闭的时候不管会话处于什么阶段，不管不顾直接停服，而tidb的这样处理着实让我想不到。

总结

本文简短的分析了下 tidb 进程关闭的处理流程，最终定位到进程没有及时关闭的原因。

对比于mysql的停服行为，让我们对tidb的处理方式有了不一样的理解。

对于 “graceful-wait-before-shutdown 参数”、“停服时等待事务结束的逻辑”的确需要在实践中才能积累。