redis的事件驅動
redis效能很好，而且是一個單執行緒的框架。得益於redis主要通過非同步IO， 多路複用的技術，使用反應堆（reactor）模式，把大量的io操作通過訊息驅動的方式單執行緒一條條處理，這樣可以很好的利用CPU資源。因為沒有同步呼叫，所以處理速度非常快。使得多個Client訪問redis-server時候，併發效能很高。 那麼具體redis是如何實現的呢？
1 redis的多路複用技術
redis是一個C/S架構的框架，所以支援多個Client通過網路來訪問Server端。redis-server為了同時支援多個client發來的資料庫操作請求，使用了IO多路複用技術。
在一個執行緒裡面，通過系統UNIX提供的系統API(select, poll, epoll等)，同時對n個檔案描述符fd（socket也可以抽象成為檔案描述符），進行讀寫偵聽，一旦系統偵聽的fd發生了 可讀/可寫事件的時候，通過系統API函式，可以獲取到對應的fd，對於對應的檔案事件進行分派，同時處理。
類似於一個老師（redis-server）一個人照看一個班n個學生（n個redis-cli的socket），一旦某個學生舉手（socket 檔案描述符發生可讀可寫事件），這個老師立馬處理這個學生的需求（檔案事件分發器），處理完了立馬回來，看著一個班的n個學生，看看是不是還有人舉手，周而復始的進行處理。
epoll， kqueue， select，evport 這幾種其實都是UNIX的多路複用介面，因為redis對於類unix作業系統的相容性其實做的比較好，所以redis對這幾種介面都是支援的。對應的程式碼實現分別是：ae_epoll.c, ae_kqueue.c, ae_select.c, ae_evport.c.
因為我使用的是Ubuntu作業系統，所以本文就使用epoll為例子，看下redis的epoll的事件驅動是如何實現的。
2 redis 的epoll原始碼分析
2.1 redis eventpoll 的啟動初始化
在redi-server啟動的時候，會走到initServer()函式中，這個函式是對redisServer server;這個全域性唯一變數的初始化，這個server的結構定義了整個server相關的所有資訊，具體結構非常複雜，這裡就按下不表，但是注意裡面有一個結構：
aeEventLoop *el; //這個就是redis的所有事件迴圈的註冊結構
複製程式碼/* 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;
複製程式碼
/* File event structure */
typedef struct aeFileEvent {
int mask; /* one of AE_(READABLE|WRITABLE|BARRIER) */
aeFileProc *rfileProc;
aeFileProc *wfileProc;
void *clientData;
} aeFileEvent;
複製程式碼從程式碼上不太能看清楚裡面的結構，看下圖：
具體的初始化函式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;
}
複製程式碼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;
}
複製程式碼接著在initServer函式中，redis會根據配置嘗試去偵聽埠：
/* 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);
複製程式碼在listenToPort函式中，redis會嘗試bind/listen多個ip，同時考慮了IPV4/IPV6兩種場景，原始碼如下：
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;
}
複製程式碼建立成功後，作為的server端的socket會做為檔案描述符被儲存在server的ipfd陣列中：
int ipfd[CONFIG_BINDADDR_MAX]; /* TCP socket file descriptors */
複製程式碼接著還是在initServer函式中，會為這幾個server socket的ipfd 建立事件註冊，原始碼如下：
/* 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.");
}
}
複製程式碼可以看出aeCreateFileEvent 這個函式會把檔案描述符server.ipfd[i]和事件AE_READABLE，以及回撥函式acceptTcpHandler做了關聯，也就是每當client發來tcp建鏈請求事件發生時，就觸發acceptTcpHandler函式。 下面看看這個函式到底是如何利用上面圖中的資料結構，把這幾樣東西結合在一起的。
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;
}
複製程式碼從上面的原始碼可以看出，這個函式主要做了兩件事，一個就是把事件，回撥函式儲存在eventLoop->events[fd]結構中。再然後就是呼叫了aeApiAddEvent，而這個函式其實就是epoll介面函式的一層封裝。具體實現如下：
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;
}
複製程式碼程式碼邏輯很清晰，其實核心就是呼叫了epoll介面中的epoll_ctl，把server socket的fd放到了epoll中進行monitor。
2.2 redis 服務的epoll迴圈呼叫
初始化完了後，redis就會進入迴圈狀態，程式碼如下：
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);
}
}
複製程式碼迴圈狀態會不停的去嘗試處理事件，也就是aeProcessEvents函式。這個函式會處理redis所有事件，包括檔案事件和定時器事件，對於檔案事件來說，核心程式碼如下：
/* Call the multiplexing API, will return only on timeout or when
* some event fires. */
numevents = aeApiPoll(eventLoop, tvp);//這裡會去當前的反應堆裡面看看有沒待處理的事件
/* 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++;
}
複製程式碼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;
}
複製程式碼每次迴圈都會呼叫aeApiPoll，而這個函式其實還是epoll介面函式的一層封裝，程式碼邏輯其實就是看看當前monitor的檔案描述符是否有事件可以觸發，如果有的話，就呼叫回撥函式進行處理。
2.3 redis 客戶端建立連線和處理流程
在2.1小節裡面已經提到了，對於server的socket 的檔案描述符和AE_READABLE事件，關聯了一個回撥函式acceptTcpHandler，這個函式就是當server 的socket可讀的時候，觸發的函式。
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--) {//因為可能同時有多個client發起連結
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);
}
}
複製程式碼可以看出來redis會用socket 的accept 函式去一個個的接受tcp的建鏈請求，然後轉交acceptCommonHandler函式處理。
#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;
}
...後面還有一些不影響主流程，所以暫時略過不表。
複製程式碼這裡會建立一個client的資料區，用來表示一個客戶端，具體的邏輯如下：
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;
}
複製程式碼createClient 這個函式其實做了兩件事
- 申請記憶體同時初始化了client結構體；
- 呼叫把客戶端的socket檔案描述符fd和AE_READABLE進行關聯,當客戶端發來訊息，呼叫
readQueryFromClient函式，放入了反應堆server.el中
if (aeCreateFileEvent(server.el,fd,AE_READABLE,
readQueryFromClient, c) == AE_ERR)
{
close(fd);
zfree(c);
return NULL;
}
複製程式碼而當redis-server 收到某個客戶端發來的資料庫操作請求時，就會觸發下面這個回撥函式，這個函式中會從socket中讀資料，並開始處理。
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);//此處呼叫socket介面函式從client socket讀取資料，然後進行處理
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);
}
複製程式碼在上面的函式中會分配一個最夠大的buffer，同時呼叫socket介面函式從client socket讀取資料，然後進行處理。最後交到processInputBufferAndReplicate(c); 這個函式裡面會進行redis 正常命令的解析和處理。
至此一個基本的啟動listen埠，然後提供服務，再到客戶端發來建鏈請求，然後發來資料庫操作業務訊息流程就全部串起來了。