作者： HappyUncle 原文来源： https://tidb.net/blog/a26401be
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 for component 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
Waiting for 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 --host 127.0.0.1 --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 --pid 10115
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 -P 4000 --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
var gracefulCloseConnectionsTimeout = 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, it's safe to shutdown it.
if atomic.CompareAndSwapInt32(&cc.status, connStatusReading, connStatusShutdown) {
return true
}
// If the client connection status is dispatching, we can't 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。