认识 Redis client-output-buffer-limit 参数与源码分析

概述

Redis 的 ​​client-output-buffer-limit​​ 可以用来强制断开无法足够快从 redis 服务器端读取数据的客户端。
保护机制规则如下:

  1. [hard limit] 大小限制,当某一客户端缓冲区超过设定值后,直接关闭连接。
  2. [soft limit] 持续时间限制,当某一客户端缓冲区持续一段时间占用过大空间时关闭连接。

该参数一般用在以下几类客户端中:

  • 普通 client,包括 monitor
  • 主从同步时的 slave client
  • Pub/Sub 模式中的 client

配置介绍与分析

该参数的配置语法:

client-output-buffer-limit <class> <hard limit> <soft limit> <soft seconds>

配置实例:

# 普通client buffer限制 
client-output-buffer-limit normal 0 0 0
# slave client buffer限制
client-output-buffer-limit slave 256mb 64mb 60
# pubsub client buffer限制
client-output-buffer-limit pubsub 32mb 8mb 60
  • client-output-buffer-limit normal 0 0 0

将 hard limit 和 soft limit 同时设置为 0,则表示关闭该限制。

  • client-output-buffer-limit slave 256mb 64mb 60

该配置表示,对于 slave 客户端来说,如果 output-buffer 占用内存达到 256M 或者超过 64M 的时间达到 60s,则关闭客户端连接。

  • client-output-buffer-limit pubsub 32mb 8mb 60

该配置表示,对于 Pub/Sub 客户端来说,若 output-buffer 占用内存达到 32M 或者超过 8M 的时间达到 60s,则关闭客户端连接。

概括说明: 一般情况下,对于普通客户端,client-output-buffer 是不设限制的,因为 server 只会在 client 请求数据的时候才会发送,不会产生积压。
而在 server 主动发送,client 来处理的场景下,这种一般都是异步处理的,会划出一个缓冲区来“暂存”未处理的数据,若 server 发送数据比 client 处理数据快时,就会发生缓冲区积压。对于用作 Pub/Sub 和 slave 的客户端,server 会主动把数据推送给他们,故需要设置 client-output-buffer 的限制。

示例分析

下面我们以主从同步时的 slave 客户端,来具体分析下。
在 redis 在主从同步时,master 会为 slave 创建一个输出缓冲区。在 master 保存 rdb,将 rdb 文件传输给 slave,slave 加载 rdb 完成之前,master 会将接收到的所有写命令,写入到内存中的这个输出缓冲区去。
若 rdb 的保存,传输,加载耗时过长,或者在此期间的写命令过多,则可能会造成超过缓冲区限制,造成 master 和 slave 的连接断开。此时则需要适当调整下 ​​client-output-buffer-limit slave​​配置。

源码浅析-主从同步时 output buffer 使用

基于 redis5.0 版本源码

redis server 通过 addReply 将数据发送给客户端,以下源码见 ​​https://github.com/redis/redis/blob/5.0/src/networking.c#L190-L211​

/* Add the object 'obj' string representation to the client output buffer. */
void addReply(client *c, robj *obj) {
if (prepareClientToWrite(c) != C_OK) return;

if (sdsEncodedObject(obj)) {
if (_addReplyToBuffer(c,obj->ptr,sdslen(obj->ptr)) != C_OK)
_addReplyStringToList(c,obj->ptr,sdslen(obj->ptr));
} else if (obj->encoding == OBJ_ENCODING_INT) {
char buf[32];
size_t len = ll2string(buf,sizeof(buf),(long)obj->ptr);
if (_addReplyToBuffer(c,buf,len) != C_OK)
_addReplyStringToList(c,buf,len);
} else {
serverPanic("Wrong obj->encoding in addReply()");
}
}

在函数的开头,会通过​​prepareClientToWrite(c)​​判断是否需要将数据写入客户端的 output buffer 中。我们看下什么条件下数据会被写入客户端的 output buffer 中,即返回 ​​C_OK​​。

/* This function is called every time we are going to transmit new data
* to the client. The behavior is the following:
*
* If the client should receive new data (normal clients will) the function
* returns C_OK, and make sure to install the write handler in our event
* loop so that when the socket is writable new data gets written.
*
* If the client should not receive new data, because it is a fake client
* (used to load AOF in memory), a master or because the setup of the write
* handler failed, the function returns C_ERR.
*
* The function may return C_OK without actually installing the write
* event handler in the following cases:
*
* 1) The event handler should already be installed since the output buffer
* already contains something.
* 2) The client is a slave but not yet online, so we want to just accumulate
* writes in the buffer but not actually sending them yet.
*
* Typically gets called every time a reply is built, before adding more
* data to the clients output buffers. If the function returns C_ERR no
* data should be appended to the output buffers. */
int prepareClientToWrite(client *c) {
/* If it's the Lua client we always return ok without installing any
* handler since there is no socket at all. */
if (c->flags & (CLIENT_LUA|CLIENT_MODULE)) return C_OK;

/* CLIENT REPLY OFF / SKIP handling: don't send replies. */
if (c->flags & (CLIENT_REPLY_OFF|CLIENT_REPLY_SKIP)) return C_ERR;

/* Masters don't receive replies, unless CLIENT_MASTER_FORCE_REPLY flag
* is set. */
if ((c->flags & CLIENT_MASTER) &&
!(c->flags & CLIENT_MASTER_FORCE_REPLY)) return C_ERR;

if (c->fd <= 0) return C_ERR; /* Fake client for AOF loading. */

/* Schedule the client to write the output buffers to the socket, unless
* it should already be setup to do so (it has already pending data). */
if (!clientHasPendingReplies(c)) clientInstallWriteHandler(c);

/* Authorize the caller to queue in the output buffer of this client. */
return C_OK;
}

/* Return true if the specified client has pending reply buffers to write to
* the socket. */
int clientHasPendingReplies(client *c) {
return c->bufpos || listLength(c->reply);
}

void clientInstallWriteHandler(client *c) {
/* Schedule the client to write the output buffers to the socket only
* if not already done and, for slaves, if the slave can actually receive
* writes at this stage. */
if (!(c->flags & CLIENT_PENDING_WRITE) &&
(c->replstate == REPL_STATE_NONE ||
(c->replstate == SLAVE_STATE_ONLINE && !c->repl_put_online_on_ack)))
{
c->flags |= CLIENT_PENDING_WRITE;
listAddNodeHead(server.clients_pending_write,c);
}
}

由于函数默认返回​​C_OK​​,我们只需要看哪几类情况返回的不是​​C_OK​​,即​​C_ERR​​,数据就不会被写入到客户端的 output buffer 中。
返回​​C_ERR​​的情况:

  • 客户端是个 fake client(用于加载 AOF 文件)
  • 客户端是一个 master
  • slave 的状态为 SLAVE_STATE_ONLINE 且其回调函数失败((c->replstate == SLAVE_STATE_ONLINE && !c->repl_put_online_on_ack)),或slave 的状态为 REPL_STATE_NONE

If the client should not receive new data, because it is a fake client (used to load AOF in memory), a master or because the setup of the write handler failed, the function returns C_ERR.

在 master 保存和发送 rdb 文件时,slave 的状态是以下几种,所以在这期间的写命令都会保存在 slave 的 output buffer。由于没有设置回调函数,数据并不会发送到 slave 上,仅存储在 master 为 slave 创建的 output buffer 内。

#define SLAVE_STATE_WAIT_BGSAVE_START 6 /* We need to produce a new RDB file. */
#define SLAVE_STATE_WAIT_BGSAVE_END 7 /* Waiting RDB file creation to finish. */
#define SLAVE_STATE_SEND_BULK 8 /* Sending RDB file to slave. */

那么何时才会从 output buffer 中“刷入”slave 呢?直到 master 将 rdb 文件完全发送给 slave 后,master 会在 ​​sendBulkToSlave​​函数中进行相关操作。以下源码见:​​https://github.com/redis/redis/blob/5.0/src/replication.c#L876-L930​

void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) {
// 此处省略部分源码

// rdb 文件已完全发送给 slave
if (slave->repldboff == slave->repldbsize) {
close(slave->repldbfd);
slave->repldbfd = -1;
aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE);
putSlaveOnline(slave);
}
}

void putSlaveOnline(client *slave) {
slave->replstate = SLAVE_STATE_ONLINE;
slave->repl_put_online_on_ack = 0;
slave->repl_ack_time = server.unixtime; /* Prevent false timeout. */
if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE,
sendReplyToClient, slave) == AE_ERR) {
serverLog(LL_WARNING,"Unable to register writable event for replica bulk transfer: %s", strerror(errno));
freeClient(slave);
return;
}
refreshGoodSlavesCount();
serverLog(LL_NOTICE,"Synchronization with replica %s succeeded",
replicationGetSlaveName(slave));
}

此处会将 slave 状态改为 ​​SLAVE_STATE_ONLINE​​,并将​​repl_put_online_on_ack​​置为0,(有没有很熟悉,对了,就是上面​​clientInstallWriteHandler​​中判断的内容)。同时也会设置回调函数​​sendReplyToClient​​,将此前 master 为 slave 创建的 output buffer 中的写操作全部发送到 slave 上。同时 slave 状态的变更,会使得后续 master 上的写操作可以正常的 push 到 slave 上了(直接,无需走 output buffer)。

总结

参考内容

  1. ​https://www.cnblogs.com/wangcp-2014/p/15505180.html​
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