redis的事件驱动
redis性能很好，而且是一个单线程的框架。得益于redis主要通过异步IO， 多路复用的技术，使用反应堆（reactor）模式，把大量的io操作通过消息驱动的方式单线程一条条处理，这样可以很好的利用CPU资源。因为没有同步调用，所以处理速度非常快。使得多个Client访问redis-server时候，并发性能很高。 那么具体redis是如何实现的呢？
1 redis的多路复用技术
redis是一个C/S架构的框架，所以支持多个Client通过网络来访问Server端。redis-server为了同时支持多个client发来的数据库操作请求，使用了IO多路复用技术。
在一个线程里面，通过系统UNIX提供的系统API(select, poll, epoll等)，同时对n个文件描述符fd（socket也可以抽象成为文件描述符），进行读写侦听，一旦系统侦听的fd发生了 可读/可写事件的时候，通过系统API函数，可以获取到对应的fd，对于对应的文件事件进行分派，同时处理。
类似于一个老师（redis-server）一个人照看一个班n个学生（n个redis-cli的socket），一旦某个学生举手（socket 文件描述符发生可读可写事件），这个老师立马处理这个学生的需求（文件事件分发器），处理完了立马回来，看着一个班的n个学生，看看是不是还有人举手，周而复始的进行处理。
epoll， kqueue， select，evport 这几种其实都是UNIX的多路复用接口，因为redis对于类unix操作系统的兼容性其实做的比较好，所以redis对这几种接口都是支持的。对应的代码实现分别是：ae_epoll.c, ae_kqueue.c, ae_select.c, ae_evport.c.
因为我使用的是Ubuntu操作系统，所以本文就使用epoll为例子，看下redis的epoll的事件驱动是如何实现的。
2 redis 的epoll源码分析
2.1 redis eventpoll 的启动初始化
在redi-server启动的时候，会走到initServer()函数中，这个函数是对redisServer server;这个全局唯一变量的初始化，这个server的结构定义了整个server相关的所有信息，具体结构非常复杂，这里就按下不表，但是注意里面有一个结构：
aeEventLoop *el; //这个就是redis的所有事件循环的注册结构
/* State of an event based program */
typedef struct aeEventLoop {
int maxfd; /* highest file descriptor currently registered */
int setsize; /* max number of file descriptors tracked */
long long timeEventNextId;
time_t lastTime; /* Used to detect system clock skew */
aeFileEvent *events; /* Registered events */
aeFiredEvent *fired; /* Fired events */
aeTimeEvent *timeEventHead;
int stop;
void *apidata; /* This is used for polling API specific data */
aeBeforeSleepProc *beforesleep;
aeBeforeSleepProc *aftersleep;
} aeEventLoop;
/* File event structure */
typedef struct aeFileEvent {
int mask; /* one of AE_(READABLE|WRITABLE|BARRIER) */
aeFileProc *rfileProc;
aeFileProc *wfileProc;
void *clientData;
} aeFileEvent;
从代码上不太能看清楚里面的结构，看下图：
具体的初始化函数aeCreateEventLoop如下：
aeEventLoop *aeCreateEventLoop(int setsize) {
aeEventLoop *eventLoop;
int i;
if ((eventLoop = zmalloc(sizeof(*eventLoop))) == NULL) goto err;
eventLoop->events = zmalloc(sizeof(aeFileEvent)*setsize);
eventLoop->fired = zmalloc(sizeof(aeFiredEvent)*setsize);
if (eventLoop->events == NULL || eventLoop->fired == NULL) goto err;
eventLoop->setsize = setsize;
eventLoop->lastTime = time(NULL);
eventLoop->timeEventHead = NULL;
eventLoop->timeEventNextId = 0;
eventLoop->stop = 0;
eventLoop->maxfd = -1;
eventLoop->beforesleep = NULL;
eventLoop->aftersleep = NULL;
if (aeApiCreate(eventLoop) == -1) goto err; //主要是初始化eventLoop->apidata
// Events with mask == AE_NONE are not set.
//So let's initialize the vector with it.
for (i = 0; i < setsize; i++)
eventLoop->events[i].mask = AE_NONE;
return eventLoop;
err:
if (eventLoop) {
zfree(eventLoop->events);
zfree(eventLoop->fired);
zfree(eventLoop);
}
return NULL;
}
aeApiCreate
static int aeApiCreate(aeEventLoop *eventLoop) {
aeApiState *state = zmalloc(sizeof(aeApiState));
if (!state) return -1;
state->events = zmalloc(sizeof(struct epoll_event)*eventLoop->setsize);
if (!state->events) {
zfree(state);
return -1;
}
state->epfd = epoll_create(1024); /* 1024 is just a hint for the kernel */
if (state->epfd == -1) {
zfree(state->events);
zfree(state);
return -1;
}
eventLoop->apidata = state;
return 0;
}
接着在initServer函数中，redis会根据配置尝试去侦听端口：
/* Open the TCP listening socket for the user commands. */
if (server.port != 0 &&
listenToPort(server.port,server.ipfd,&server.ipfd_count) == C_ERR)
exit(1);
在listenToPort函数中，redis会尝试bind/listen多个ip，同时考虑了IPV4/IPV6两种场景，源码如下：
int listenToPort(int port, int *fds, int *count) {
int j;
/* Force binding of 0.0.0.0 if no bind address is specified, always
* entering the loop if j == 0. */
if (server.bindaddr_count == 0) server.bindaddr[0] = NULL;
for (j = 0; j < server.bindaddr_count || j == 0; j++) {
if (server.bindaddr[j] == NULL) {
int unsupported = 0;
/* Bind * for both IPv6 and IPv4, we enter here only if
* server.bindaddr_count == 0. */
fds[*count] = anetTcp6Server(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
} else if (errno == EAFNOSUPPORT) {
unsupported++;
serverLog(LL_WARNING,"Not listening to IPv6: unsupproted");
}
if (*count == 1 || unsupported) {
/* Bind the IPv4 address as well. */
fds[*count] = anetTcpServer(server.neterr,port,NULL,
server.tcp_backlog);
if (fds[*count] != ANET_ERR) {
anetNonBlock(NULL,fds[*count]);
(*count)++;
} else if (errno == EAFNOSUPPORT) {
unsupported++;
serverLog(LL_WARNING,"Not listening to IPv4: unsupproted");
}
}
/* Exit the loop if we were able to bind * on IPv4 and IPv6,
* otherwise fds[*count] will be ANET_ERR and we'll print an
* error and return to the caller with an error. */
if (*count + unsupported == 2) break;
} else if (strchr(server.bindaddr[j],':')) {
/* Bind IPv6 address. */
fds[*count] = anetTcp6Server(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
} else {
/* Bind IPv4 address. */
fds[*count] = anetTcpServer(server.neterr,port,server.bindaddr[j],
server.tcp_backlog);
}
if (fds[*count] == ANET_ERR) {
serverLog(LL_WARNING,
"Creating Server TCP listening socket %s:%d: %s",
server.bindaddr[j] ? server.bindaddr[j] : "*",
port, server.neterr);
return C_ERR;
}
anetNonBlock(NULL,fds[*count]);
(*count)++;
}
return C_OK;
}
创建成功后，作为的server端的socket会做为文件描述符被存储在server的ipfd数组中：
int ipfd[CONFIG_BINDADDR_MAX]; /* TCP socket file descriptors */
接着还是在initServer函数中，会为这几个server socket的ipfd 创建事件注册，源码如下：
/* Create an event handler for accepting new connections in TCP and Unix
* domain sockets. */
for (j = 0; j < server.ipfd_count; j++) {
if (aeCreateFileEvent(server.el, server.ipfd[j], AE_READABLE,
acceptTcpHandler,NULL) == AE_ERR)
{
serverPanic(
"Unrecoverable error creating server.ipfd file event.");
}
}
可以看出aeCreateFileEvent 这个函数会把文件描述符server.ipfd[i]和事件AE_READABLE，以及回调函数acceptTcpHandler做了关联，也就是每当client发来tcp建链请求事件发生时，就触发acceptTcpHandler函数。 下面看看这个函数到底是如何利用上面图中的数据结构，把这几样东西结合在一起的。
int aeCreateFileEvent(aeEventLoop *eventLoop, int fd, int mask,
aeFileProc *proc, void *clientData)
{
if (fd >= eventLoop->setsize) {
errno = ERANGE;
return AE_ERR;
}
aeFileEvent *fe = &eventLoop->events[fd];
if (aeApiAddEvent(eventLoop, fd, mask) == -1)
return AE_ERR;
fe->mask |= mask;
if (mask & AE_READABLE) fe->rfileProc = proc;
if (mask & AE_WRITABLE) fe->wfileProc = proc;
fe->clientData = clientData;
if (fd > eventLoop->maxfd)
eventLoop->maxfd = fd;
return AE_OK;
}
从上面的源码可以看出，这个函数主要做了两件事，一个就是把事件，回调函数保存在eventLoop->events[fd]结构中。再然后就是调用了aeApiAddEvent，而这个函数其实就是epoll接口函数的一层封装。具体实现如下：
static int aeApiAddEvent(aeEventLoop *eventLoop, int fd, int mask) {
aeApiState *state = eventLoop->apidata;
struct epoll_event ee = {0}; /* avoid valgrind warning */
/* If the fd was already monitored for some event, we need a MOD
* operation. Otherwise we need an ADD operation. */
int op = eventLoop->events[fd].mask == AE_NONE ?
EPOLL_CTL_ADD : EPOLL_CTL_MOD;
ee.events = 0;
mask |= eventLoop->events[fd].mask; /* Merge old events */
if (mask & AE_READABLE) ee.events |= EPOLLIN;
if (mask & AE_WRITABLE) ee.events |= EPOLLOUT;
ee.data.fd = fd;
if (epoll_ctl(state->epfd,op,fd,&ee) == -1) return -1;
return 0;
}
代码逻辑很清晰，其实核心就是调用了epoll接口中的epoll_ctl，把server socket的fd放到了epoll中进行monitor。
2.2 redis 服务的epoll循环调用
初始化完了后，redis就会进入循环状态，代码如下：
void aeMain(aeEventLoop *eventLoop) {
eventLoop->stop = 0;
while (!eventLoop->stop) {
if (eventLoop->beforesleep != NULL)
eventLoop->beforesleep(eventLoop);
aeProcessEvents(eventLoop, AE_ALL_EVENTS|AE_CALL_AFTER_SLEEP);
}
}
循环状态会不停的去尝试处理事件，也就是aeProcessEvents函数。这个函数会处理redis所有事件，包括文件事件和定时器事件，对于文件事件来说，核心代码如下：
/* Call the multiplexing API, will return only on timeout or when
* some event fires. */
numevents = aeApiPoll(eventLoop, tvp);//这里会去当前的反应堆里面看看有没待处理的事件
/* After sleep callback. */
if (eventLoop->aftersleep != NULL && flags & AE_CALL_AFTER_SLEEP)
eventLoop->aftersleep(eventLoop);
for (j = 0; j < numevents; j++) {
aeFileEvent *fe = &eventLoop->events[eventLoop->fired[j].fd];
int mask = eventLoop->fired[j].mask;
int fd = eventLoop->fired[j].fd;
int fired = 0; /* Number of events fired for current fd. */
/* Normally we execute the readable event first, and the writable
* event laster. This is useful as sometimes we may be able
* to serve the reply of a query immediately after processing the
* query.
*
* However if AE_BARRIER is set in the mask, our application is
* asking us to do the reverse: never fire the writable event
* after the readable. In such a case, we invert the calls.
* This is useful when, for instance, we want to do things
* in the beforeSleep() hook, like fsynching a file to disk,
* before replying to a client. */
int invert = fe->mask & AE_BARRIER;
/* Note the fe->mask & mask & ... code: maybe an already
* processed event removed an element that fired and we still
* didnt processed, so we check if the event is still valid.
*
* Fire the readable event if the call sequence is not
* inverted. */
if (!invert && fe->mask & mask & AE_READABLE) {
fe->rfileProc(eventLoop,fd,fe->clientData,mask);
fired++;
}
/* Fire the writable event. */
if (fe->mask & mask & AE_WRITABLE) {
if (!fired || fe->wfileProc != fe->rfileProc) {
fe->wfileProc(eventLoop,fd,fe->clientData,mask);
fired++;
}
}
/* If we have to invert the call, fire the readable event now
* after the writable one. */
if (invert && fe->mask & mask & AE_READABLE) {
if (!fired || fe->wfileProc != fe->rfileProc) {
fe->rfileProc(eventLoop,fd,fe->clientData,mask);
fired++;
}
}
processed++;
}
static int aeApiPoll(aeEventLoop *eventLoop, struct timeval *tvp) {
aeApiState *state = eventLoop->apidata;
int retval, numevents = 0;
retval = epoll_wait(state->epfd,state->events,eventLoop->setsize,
tvp ? (tvp->tv_sec*1000 + tvp->tv_usec/1000) : -1);
if (retval > 0) {
int j;
numevents = retval;
for (j = 0; j < numevents; j++) {
int mask = 0;
struct epoll_event *e = state->events+j;
if (e->events & EPOLLIN) mask |= AE_READABLE;
if (e->events & EPOLLOUT) mask |= AE_WRITABLE;
if (e->events & EPOLLERR) mask |= AE_WRITABLE;
if (e->events & EPOLLHUP) mask |= AE_WRITABLE;
eventLoop->fired[j].fd = e->data.fd;
eventLoop->fired[j].mask = mask;
}
}
return numevents;
}
每次循环都会调用aeApiPoll，而这个函数其实还是epoll接口函数的一层封装，代码逻辑其实就是看看当前monitor的文件描述符是否有事件可以触发，如果有的话，就调用回调函数进行处理。
2.3 redis 客户端建立连接和处理流程
在2.1小节里面已经提到了，对于server的socket 的文件描述符和AE_READABLE事件，关联了一个回调函数acceptTcpHandler，这个函数就是当server 的socket可读的时候，触发的函数。
void acceptTcpHandler(aeEventLoop *el, int fd, void *privdata, int mask) {
int cport, cfd, max = MAX_ACCEPTS_PER_CALL;
char cip[NET_IP_STR_LEN];
UNUSED(el);
UNUSED(mask);
UNUSED(privdata);
while(max--) {//因为可能同时有多个client发起链接
cfd = anetTcpAccept(server.neterr, fd, cip, sizeof(cip), &cport);
if (cfd == ANET_ERR) {
if (errno != EWOULDBLOCK)
serverLog(LL_WARNING,
"Accepting client connection: %s", server.neterr);
return;
}
serverLog(LL_VERBOSE,"Accepted %s:%d", cip, cport);
acceptCommonHandler(cfd,0,cip);
}
}
可以看出来redis会用socket 的accept 函数去一个个的接受tcp的建链请求，然后转交acceptCommonHandler函数处理。
#define MAX_ACCEPTS_PER_CALL 1000
static void acceptCommonHandler(int fd, int flags, char *ip) {
client *c;
if ((c = createClient(fd)) == NULL) {
serverLog(LL_WARNING,
"Error registering fd event for the new client: %s (fd=%d)",
strerror(errno),fd);
close(fd); /* May be already closed, just ignore errors */
return;
}
...后面还有一些不影响主流程，所以暂时略过不表。
这里会创建一个client的数据区，用来表示一个客户端，具体的逻辑如下：
client *createClient(int fd) {
client *c = zmalloc(sizeof(client));
/* passing -1 as fd it is possible to create a non connected client.
* This is useful since all the commands needs to be executed
* in the context of a client. When commands are executed in other
* contexts (for instance a Lua script) we need a non connected client. */
if (fd != -1) {
anetNonBlock(NULL,fd);
anetEnableTcpNoDelay(NULL,fd);
if (server.tcpkeepalive)
anetKeepAlive(NULL,fd,server.tcpkeepalive);
if (aeCreateFileEvent(server.el,fd,AE_READABLE,
readQueryFromClient, c) == AE_ERR)
{
close(fd);
zfree(c);
return NULL;
}
}
selectDb(c,0);
uint64_t client_id;
atomicGetIncr(server.next_client_id,client_id,1);
c->id = client_id;
c->fd = fd;
c->name = NULL;
c->bufpos = 0;
c->qb_pos = 0;
c->querybuf = sdsempty();
c->pending_querybuf = sdsempty();
c->querybuf_peak = 0;
c->reqtype = 0;
c->argc = 0;
c->argv = NULL;
c->cmd = c->lastcmd = NULL;
c->multibulklen = 0;
c->bulklen = -1;
c->sentlen = 0;
c->flags = 0;
c->ctime = c->lastinteraction = server.unixtime;
c->authenticated = 0;
c->replstate = REPL_STATE_NONE;
c->repl_put_online_on_ack = 0;
c->reploff = 0;
c->read_reploff = 0;
c->repl_ack_off = 0;
c->repl_ack_time = 0;
c->slave_listening_port = 0;
c->slave_ip[0] = '\0';
c->slave_capa = SLAVE_CAPA_NONE;
c->reply = listCreate();
c->reply_bytes = 0;
c->obuf_soft_limit_reached_time = 0;
listSetFreeMethod(c->reply,freeClientReplyValue);
listSetDupMethod(c->reply,dupClientReplyValue);
c->btype = BLOCKED_NONE;
c->bpop.timeout = 0;
c->bpop.keys = dictCreate(&objectKeyHeapPointerValueDictType,NULL);
c->bpop.target = NULL;
c->bpop.xread_group = NULL;
c->bpop.xread_consumer = NULL;
c->bpop.xread_group_noack = 0;
c->bpop.numreplicas = 0;
c->bpop.reploffset = 0;
c->woff = 0;
c->watched_keys = listCreate();
c->pubsub_channels = dictCreate(&objectKeyPointerValueDictType,NULL);
c->pubsub_patterns = listCreate();
c->peerid = NULL;
c->client_list_node = NULL;
listSetFreeMethod(c->pubsub_patterns,decrRefCountVoid);
listSetMatchMethod(c->pubsub_patterns,listMatchObjects);
if (fd != -1) linkClient(c);
initClientMultiState(c);
return c;
}
createClient 这个函数其实做了两件事
- 申请内存同时初始化了client结构体；
- 调用把客户端的socket文件描述符fd和AE_READABLE进行关联,当客户端发来消息，调用
readQueryFromClient函数，放入了反应堆server.el中
if (aeCreateFileEvent(server.el,fd,AE_READABLE,
readQueryFromClient, c) == AE_ERR)
{
close(fd);
zfree(c);
return NULL;
}
而当redis-server 收到某个客户端发来的数据库操作请求时，就会触发下面这个回调函数，这个函数中会从socket中读数据，并开始处理。
void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) {
client *c = (client*) privdata;
int nread, readlen;
size_t qblen;
UNUSED(el);
UNUSED(mask);
readlen = PROTO_IOBUF_LEN;
/* If this is a multi bulk request, and we are processing a bulk reply
* that is large enough, try to maximize the probability that the query
* buffer contains exactly the SDS string representing the object, even
* at the risk of requiring more read(2) calls. This way the function
* processMultiBulkBuffer() can avoid copying buffers to create the
* Redis Object representing the argument. */
if (c->reqtype == PROTO_REQ_MULTIBULK && c->multibulklen && c->bulklen != -1
&& c->bulklen >= PROTO_MBULK_BIG_ARG)
{
ssize_t remaining = (size_t)(c->bulklen+2)-sdslen(c->querybuf);
/* Note that the 'remaining' variable may be zero in some edge case,
* for example once we resume a blocked client after CLIENT PAUSE. */
if (remaining > 0 && remaining < readlen) readlen = remaining;
}
qblen = sdslen(c->querybuf);
if (c->querybuf_peak < qblen) c->querybuf_peak = qblen;
c->querybuf = sdsMakeRoomFor(c->querybuf, readlen);
nread = read(fd, c->querybuf+qblen, readlen);//此处调用socket接口函数从client socket读取数据，然后进行处理
if (nread == -1) {
if (errno == EAGAIN) {
return;
} else {
serverLog(LL_VERBOSE, "Reading from client: %s",strerror(errno));
freeClient(c);
return;
}
} else if (nread == 0) {
serverLog(LL_VERBOSE, "Client closed connection");
freeClient(c);
return;
} else if (c->flags & CLIENT_MASTER) {
/* Append the query buffer to the pending (not applied) buffer
* of the master. We'll use this buffer later in order to have a
* copy of the string applied by the last command executed. */
c->pending_querybuf = sdscatlen(c->pending_querybuf,
c->querybuf+qblen,nread);
}
sdsIncrLen(c->querybuf,nread);
c->lastinteraction = server.unixtime;
if (c->flags & CLIENT_MASTER) c->read_reploff += nread;
server.stat_net_input_bytes += nread;
if (sdslen(c->querybuf) > server.client_max_querybuf_len) {
sds ci = catClientInfoString(sdsempty(),c), bytes = sdsempty();
bytes = sdscatrepr(bytes,c->querybuf,64);
serverLog(LL_WARNING,"Closing client that reached max query buffer length: %s (qbuf initial bytes: %s)", ci, bytes);
sdsfree(ci);
sdsfree(bytes);
freeClient(c);
return;
}
/* Time to process the buffer. If the client is a master we need to
* compute the difference between the applied offset before and after
* processing the buffer, to understand how much of the replication stream
* was actually applied to the master state: this quantity, and its
* corresponding part of the replication stream, will be propagated to
* the sub-slaves and to the replication backlog. */
processInputBufferAndReplicate(c);
}
在上面的函数中会分配一个最够大的buffer，同时调用socket接口函数从client socket读取数据，然后进行处理。最后交到processInputBufferAndReplicate(c); 这个函数里面会进行redis 正常命令的解析和处理。
至此一个基本的启动listen端口，然后提供服务，再到客户端发来建链请求，然后发来数据库操作业务消息流程就全部串起来了。
