详解 gRPC 客户端长连接机制实现
本文作者：
熊喵君，原文链接：https://pandaychen.github.io/2020/09/01/GRPC-CLIENT-CONN-LASTING/
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0x00 前言
HTTP2 是一个全双工的流式协议, 服务端也可以主动 ping 客户端, 且服务端还会有一些检测连接可用性和控制客户端 ping 包频率的配置。gRPC 就是采用 HTTP2 来作为其基础通信模式的，所以默认的 gRPC 客户端都是长连接。 有这么一种场景，需要客户端和服务端保持持久的长连接，即无论服务端、客户端异常断开或重启，长连接都要具备重试保活（当然前提是两方重启都成功）的需求。在 gRPC 中，对于已经建立的长连接，服务端异常重启之后，客户端一般会收到如下错误：
rpc error: code = Unavailable desc = transport is closing
大部分的 gRPC 客户端封装都没有很好的处理这类 case，参见 Warden 关于 Server 端服务重启后 Client 连接断开之后的重试问题[1]，对于这种错误，推荐有两种处理方法：
- 重试：在客户端调用失败时，选择以指数退避（Exponential Backoff ）来优雅进行重试
- 增加 keepalive 的保活策略
- 增加重连（auto reconnect）策略
这篇文章就来分析下如何实现这样的客户端保活（keepalive）逻辑。提到保活机制，我们先看下 gRPC 的 keepalive 机制[2]。 0x01 HTTP2 的 GOAWAY 帧 HTTP2 使用 GOAWAY 帧信号来控制连接关闭，GOAWAY 用于启动连接关闭或发出严重错误状态信号。GOAWAY 语义为允许端点正常停止接受新的流，同时仍然完成对先前建立的流的处理，当 client 收到这个包之后就会主动关闭连接。下次需要发送数据时，就会重新建立连接。GOAWAY 是实现 grpc.gracefulStop 机制的重要保证。
gRPC 客户端 keepalive
gRPC 客户端提供 keepalive 配置如下：
var kacp = keepalive.ClientParameters{
Time: 10 * time.Second, // send pings every 10 seconds if there is no activity
Timeout: time.Second, // wait 1 second for ping ack before considering the connection dead
PermitWithoutStream: true, // send pings even without active streams
}
//Dial 中传入 keepalive 配置
conn, err := grpc.Dial(*addr, grpc.WithInsecure(), grpc.WithKeepaliveParams(kacp))
keepalive.ClientParameters 参数的含义如下:
- Time：如果没有 activity， 则每隔 10s 发送一个 ping 包
- Timeout：如果 ping ack 1s 之内未返回则认为连接已断开
- PermitWithoutStream：如果没有 active 的 stream， 是否允许发送 ping 联想到，在项目中
ssh客户端[3] 和 mysql 客户端中都有着类似的实现，即单独开启协程来实现 keepalive：如下面的代码（以 ssh 为例）：
go func() {
t := time.NewTicker(2 * time.Second)
defer t.Stop()
for range t.C {
_, _, err := client.Conn.SendRequest("[email protected]", true, nil)
if err != nil {
return
}
}
}()
gPRC 的实现
在 grpc-go 的 newHTTP2Client[4] 方法中，有下面的逻辑：即在新建一个 HTTP2Client 的时候会启动一个 goroutine 来处理 keepalive
// newHTTP2Client constructs a connected ClientTransport to addr based on HTTP2
// and starts to receive messages on it. Non-nil error returns if construction
// fails.
func newHTTP2Client(connectCtx, ctx context.Context, addr resolver.Address, opts ConnectOptions, onPrefaceReceipt func(), onGoAway func(GoAwayReason), onClose func()) (_ *http2Client, err error) {
...
if t.keepaliveEnabled {
t.kpDormancyCond = sync.NewCond(&t.mu)
go t.keepalive()
}
...
}
接下来，看下 keepalive 方法[5] 的实现：
func (t *http2Client) keepalive() {
p := &ping{data: [8]byte{}} //ping 的内容
timer := time.NewTimer(t.kp.Time) // 启动一个定时器, 触发时间为配置的 Time 值
//for loop
for {
select {
// 定时器触发
case <-timer.C:
if atomic.CompareAndSwapUint32(&t.activity, 1, 0) {
timer.Reset(t.kp.Time)
continue
}
// Check if keepalive should go dormant.
t.mu.Lock()
if len(t.activeStreams) < 1 && !t.kp.PermitWithoutStream {
// Make awakenKeepalive writable.
<-t.awakenKeepalive
t.mu.Unlock()
select {
case <-t.awakenKeepalive:
// If the control gets here a ping has been sent
// need to reset the timer with keepalive.Timeout.
case <-t.ctx.Done():
return
}
} else {
t.mu.Unlock()
if channelz.IsOn() {
atomic.AddInt64(&t.czData.kpCount, 1)
}
// Send ping.
t.controlBuf.put(p)
}
// By the time control gets here a ping has been sent one way or the other.
timer.Reset(t.kp.Timeout)
select {
case <-timer.C:
if atomic.CompareAndSwapUint32(&t.activity, 1, 0) {
timer.Reset(t.kp.Time)
continue
}
t.Close()
return
case <-t.ctx.Done():
if !timer.Stop() {
<-timer.C
}
return
}
// 上层通知 context 结束
case <-t.ctx.Done():
if !timer.Stop() {
// 返回 false，表示 timer 未被销毁
<-timer.C
}
return
}
}
}
从客户端的 keepalive 实现中梳理下执行逻辑：
- 填充 ping 包内容, 为 [8]byte{}，创建定时器, 触发时间为用户配置中的 Time
- 循环处理，select 的两大分支，一为定时器触发后执行的逻辑，另一分支为 t.ctx.Done()，即 keepalive 的上层应用调用了 cancel 结束 context 子树
- 核心逻辑在定时器触发的过程中
gRPC 服务端的 keepalive
gRPC 的服务端主要有两块逻辑： 接收并相应客户端的 ping 包 单独启动 goroutine 探测客户端是否存活 gRPC 服务端提供 keepalive 配置，分为两部分 keepalive.EnforcementPolicy 和 keepalive.ServerParameters，如下：
var kaep = keepalive.EnforcementPolicy{
MinTime: 5 * time.Second, // If a client pings more than once every 5 seconds, terminate the connection
PermitWithoutStream: true, // Allow pings even when there are no active streams
}
var kasp = keepalive.ServerParameters{
MaxConnectionIdle: 15 * time.Second, // If a client is idle for 15 seconds, send a GOAWAY
MaxConnectionAge: 30 * time.Second, // If any connection is alive for more than 30 seconds, send a GOAWAY
MaxConnectionAgeGrace: 5 * time.Second, // Allow 5 seconds for pending RPCs to complete before forcibly closing connections
Time: 5 * time.Second, // Ping the client if it is idle for 5 seconds to ensure the connection is still active
Timeout: 1 * time.Second, // Wait 1 second for the ping ack before assuming the connection is dead
}
func main(){
...
s := grpc.NewServer(grpc.KeepaliveEnforcementPolicy(kaep), grpc.KeepaliveParams(kasp))
...
}
keepalive.EnforcementPolicy：
- MinTime：如果客户端两次 ping 的间隔小于 5s，则关闭连接
- PermitWithoutStream：即使没有 active stream, 也允许 ping keepalive.ServerParameters：
- MaxConnectionIdle：如果一个 client 空闲超过 15s, 发送一个 GOAWAY, 为了防止同一时间发送大量 GOAWAY, 会在 15s 时间间隔上下浮动 15*10%, 即 15+1.5 或者 15-1.5
- MaxConnectionAge：如果任意连接存活时间超过 30s, 发送一个 GOAWAY
- MaxConnectionAgeGrace：在强制关闭连接之间, 允许有 5s 的时间完成 pending 的 rpc 请求
- Time：如果一个 client 空闲超过 5s, 则发送一个 ping 请求
- Timeout：如果 ping 请求 1s 内未收到回复, 则认为该连接已断开
gRPC 的实现
服务端处理客户端的 ping 包的 response 的逻辑在 handlePing 方法[6] 中。handlePing 方法会判断是否违反两条 policy, 如果违反则将 pingStrikes++, 当违反次数大于 maxPingStrikes(2) 时, 打印一条错误日志并且发送一个 goAway 包，断开这个连接，具体实现如下：
func (t *http2Server) handlePing(f *http2.PingFrame) {
if f.IsAck() {
if f.Data == goAwayPing.data && t.drainChan != nil {
close(t.drainChan)
return
}
// Maybe it's a BDP ping.
if t.bdpEst != nil {
t.bdpEst.calculate(f.Data)
}
return
}
pingAck := &ping{ack: true}
copy(pingAck.data[:], f.Data[:])
t.controlBuf.put(pingAck)
now := time.Now()
defer func() {
t.lastPingAt = now
}()
// A reset ping strikes means that we don't need to check for policy
// violation for this ping and the pingStrikes counter should be set
// to 0.
if atomic.CompareAndSwapUint32(&t.resetPingStrikes, 1, 0) {
t.pingStrikes = 0
return
}
t.mu.Lock()
ns := len(t.activeStreams)
t.mu.Unlock()
if ns < 1 && !t.kep.PermitWithoutStream {
// Keepalive shouldn't be active thus, this new ping should
// have come after at least defaultPingTimeout.
if t.lastPingAt.Add(defaultPingTimeout).After(now) {
t.pingStrikes++
}
} else {
// Check if keepalive policy is respected.
if t.lastPingAt.Add(t.kep.MinTime).After(now) {
t.pingStrikes++
}
}
if t.pingStrikes > maxPingStrikes {
// Send goaway and close the connection.
if logger.V(logLevel) {
logger.Errorf("transport: Got too many pings from the client, closing the connection.")
}
t.controlBuf.put(&goAway{code: http2.ErrCodeEnhanceYourCalm, debugData: []byte("too_many_pings"), closeConn: true})
}
}
注意，对 pingStrikes 累加的逻辑：
- t.lastPingAt.Add(defaultPingTimeout).After(now)：
- t.lastPingAt.Add(t.kep.MinTime).After(now)：
func (t *http2Server) handlePing(f *http2.PingFrame) {
...
if ns < 1 && !t.kep.PermitWithoutStream {
// Keepalive shouldn't be active thus, this new ping should
// have come after at least defaultPingTimeout.
if t.lastPingAt.Add(defaultPingTimeout).After(now) {
t.pingStrikes++
}
} else {
// Check if keepalive policy is respected.
if t.lastPingAt.Add(t.kep.MinTime).After(now) {
t.pingStrikes++
}
}
if t.pingStrikes > maxPingStrikes {
// Send goaway and close the connection.
errorf("transport: Got too many pings from the client, closing the connection.")
t.controlBuf.put(&goAway{code: http2.ErrCodeEnhanceYourCalm, debugData: []byte("too_many_pings"), closeConn: true})
}
}
keepalive 相关代码
gRPC 服务端新建一个 HTTP2 server 的时候会启动一个单独的 goroutine 处理 keepalive 逻辑，newHTTP2Server 方法[7]：
func newHTTP2Server(conn net.Conn, config *ServerConfig) (_ ServerTransport, err error) {
...
go t.keepalive()
...
}
简单分析下 keepalive 的实现，核心逻辑是启动 3 个定时器，分别为 maxIdle、maxAge 和 keepAlive，然后在 for select 中处理相关定时器触发事件：
- maxIdle 逻辑：判断 client 空闲时间是否超出配置的时间, 如果超时, 则调用 t.drain, 该方法会发送一个 GOAWAY 包 maxAge 逻辑：触发之后首先调用 t.drain 发送 GOAWAY 包, 接着重置定时器, 时间设置为 MaxConnectionAgeGrace, 再次触发后调用 t.Close() 直接关闭（有些 graceful 的意味）
- keepalive 逻辑：首先判断 activity 是否为 1, 如果不是则置 pingSent 为 true, 并且发送 ping 包, 接着重置定时器时间为 Timeout, 再次触发后如果 activity 不为 1（即未收到 ping 的回复） 并且 pingSent 为 true, 则调用 t.Close() 关闭连接
func (t *http2Server) keepalive() {
p := &ping{}
var pingSent bool
maxIdle := time.NewTimer(t.kp.MaxConnectionIdle)
maxAge := time.NewTimer(t.kp.MaxConnectionAge)
keepalive := time.NewTimer(t.kp.Time)
// NOTE: All exit paths of this function should reset their
// respective timers. A failure to do so will cause the
// following clean-up to deadlock and eventually leak.
defer func() {
// 退出前，完成定时器的回收工作
if !maxIdle.Stop() {
<-maxIdle.C
}
if !maxAge.Stop() {
<-maxAge.C
}
if !keepalive.Stop() {
<-keepalive.C
}
}()
for {
select {
case <-maxIdle.C:
t.mu.Lock()
idle := t.idle
if idle.IsZero() { // The connection is non-idle.
t.mu.Unlock()
maxIdle.Reset(t.kp.MaxConnectionIdle)
continue
}
val := t.kp.MaxConnectionIdle - time.Since(idle)
t.mu.Unlock()
if val <= 0 {
// The connection has been idle for a duration of keepalive.MaxConnectionIdle or more.
// Gracefully close the connection.
t.drain(http2.ErrCodeNo, []byte{})
// Resetting the timer so that the clean-up doesn't deadlock.
maxIdle.Reset(infinity)
return
}
maxIdle.Reset(val)
case <-maxAge.C:
t.drain(http2.ErrCodeNo, []byte{})
maxAge.Reset(t.kp.MaxConnectionAgeGrace)
select {
case <-maxAge.C:
// Close the connection after grace period.
t.Close()
// Resetting the timer so that the clean-up doesn't deadlock.
maxAge.Reset(infinity)
case <-t.ctx.Done():
}
return
case <-keepalive.C:
if atomic.CompareAndSwapUint32(&t.activity, 1, 0) {
pingSent = false
keepalive.Reset(t.kp.Time)
continue
}
if pingSent {
t.Close()
// Resetting the timer so that the clean-up doesn't deadlock.
keepalive.Reset(infinity)
return
}
pingSent = true
if channelz.IsOn() {
atomic.AddInt64(&t.czData.kpCount, 1)
}
t.controlBuf.put(p)
keepalive.Reset(t.kp.Timeout)
case <-t.ctx.Done():
return
}
}
}
实现健壮的长连接客户端
参考资料 [1] Warden 关于 Server 端服务重启后 Client 连接断开之后的重试问题: https://github.com/go-kratos/kratos/issues/177
[2] keepalive 机制: https://github.com/grpc/grpc/blob/master/doc/keepalive.md
[3] ssh 客户端: https://pandaychen.github.io/2019/10/20/HOW-TO-BUILD-A-SSHD-WITH-GOLANG/#客户端-keepalive-机制
[4] newHTTP2Client: https://github.com/grpc/grpc-go/blob/master/internal/transport/http2_client.go#L166
[5] keepalive 方法: https://github.com/grpc/grpc-go/blob/master/internal/transport/http2_client.go#L1350
[6] handlePing 方法: https://github.com/grpc/grpc-go/blob/master/internal/transport/http2_server.go#L693
[7] newHTTP2Server 方法: https://github.com/grpc/grpc-go/blob/master/internal/transport/http2_server.go#L129
[8] 服务端: https://github.com/grpc/grpc-go/blob/master/examples/features/keepalive/server/main.go
[9] 客户端: https://github.com/grpc/grpc-go/blob/master/examples/features/keepalive/client/main.go
[10] GRPC 开箱手册: https://juejin.im/post/6844904096474857485
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