Redis启动分析
文件入口：server.c##main
配置初始化
这一步表示Redis服务器基本数据结构和各种参数的初始化。在Redis源码中，Redis服务器是用一个叫做redisServer的struct来表达的，里面定义了Redis服务器赖以运行的各种参数，比如监听的端口号和文件描述符、当前连接的各个client端、Redis命令表(command table)配置、持久化相关的各种参数，以及事件循环结构。
void initServerConfig(void) {
int j;
...
/* Command table -- we initiialize it here as it is part of the
* initial configuration, since command names may be changed via
* redis.conf using the rename-command directive. */
server.commands = dictCreate(&commandTableDictType,NULL);
server.orig_commands = dictCreate(&commandTableDictType,NULL);
populateCommandTable();
server.delCommand = lookupCommandByCString("del");
server.multiCommand = lookupCommandByCString("multi");
server.lpushCommand = lookupCommandByCString("lpush");
server.lpopCommand = lookupCommandByCString("lpop");
server.rpopCommand = lookupCommandByCString("rpop");
server.zpopminCommand = lookupCommandByCString("zpopmin");
server.zpopmaxCommand = lookupCommandByCString("zpopmax");
server.sremCommand = lookupCommandByCString("srem");
server.execCommand = lookupCommandByCString("exec");
server.expireCommand = lookupCommandByCString("expire");
server.pexpireCommand = lookupCommandByCString("pexpire");
server.xclaimCommand = lookupCommandByCString("xclaim");
server.xgroupCommand = lookupCommandByCString("xgroup");
server.rpoplpushCommand = lookupCommandByCString("rpoplpush");
/* Debugging */
server.assert_failed = "";
server.assert_file = "";
server.assert_line = 0;
server.bug_report_start = 0;
server.watchdog_period = 0;
/* By default we want scripts to be always replicated by effects
* (single commands executed by the script), and not by sending the
* script to the slave / AOF. This is the new way starting from
* Redis 5. However it is possible to revert it via redis.conf. */
server.lua_always_replicate_commands = 1;
initConfigValues();
} Redis服务器在运行时就是由这个redisServer类型的全局变量来表示的（变量名就叫server），这一步的初始化主要就是对于这个全局变量进行初始化。
在整个初始化过程中，有一个需要特别关注的函数：populateCommandTable。它初始化了Redis命令表，通过它可以由任意一个Redis命令的名字查找该命令的配置信息（比如该命令接收的命令参数个数、执行函数入口等）。
void populateCommandTable(void) {
int j;
int numcommands = sizeof(redisCommandTable)/sizeof(struct redisCommand);
for (j = 0; j < numcommands; j++) {
struct redisCommand *c = redisCommandTable+j;
int retval1, retval2;
/* Translate the command string flags description into an actual
* set of flags. */
if (populateCommandTableParseFlags(c,c->sflags) == C_ERR)
serverPanic("Unsupported command flag");
c->id = ACLGetCommandID(c->name); /* Assign the ID used for ACL. */
retval1 = dictAdd(server.commands, sdsnew(c->name), c);
/* Populate an additional dictionary that will be unaffected
* by rename-command statements in redis.conf. */
retval2 = dictAdd(server.orig_commands, sdsnew(c->name), c);
serverAssert(retval1 == DICT_OK && retval2 == DICT_OK);
}
}redis的命令是硬编码的，我们可以进入redisCommandTable看到如下：
struct redisCommand redisCommandTable[] = {
{"module",moduleCommand,-2,
"admin no-script",
0,NULL,0,0,0,0,0,0},
{"get",getCommand,2,
"read-only fast @string",
0,NULL,1,1,1,0,0,0},
/* Note that we can't flag set as fast, since it may perform an
* implicit DEL of a large key. */
{"set",setCommand,-3,
"write use-memory @string",
0,NULL,1,1,1,0,0,0},
...
}表里包含了命令以及命令对应的函数，其中，每个命令的结构如下：
struct redisCommand {
char *name;
redisCommandProc *proc;
int arity;
char *sflags; /* Flags as string representation, one char per flag. */
uint64_t flags; /* The actual flags, obtained from the 'sflags' field. */
/* Use a function to determine keys arguments in a command line.
* Used for Redis Cluster redirect. */
redisGetKeysProc *getkeys_proc;
/* What keys should be loaded in background when calling this command? */
int firstkey; /* The first argument that's a key (0 = no keys) */
int lastkey; /* The last argument that's a key */
int keystep; /* The step between first and last key */
long long microseconds, calls;
int id; /* Command ID. This is a progressive ID starting from 0 that
is assigned at runtime, and is used in order to check
ACLs. A connection is able to execute a given command if
the user associated to the connection has this command
bit set in the bitmap of allowed commands. */
};读取配置文件
回到初始化server结构体代码中，我们可以看到：在对全局的redisServer结构进行了初始化之后，还需要从配置文件（redis.conf）中加载配置。这个过程可能覆盖掉之前初始化过的redisServer结构中的某些参数。换句话说，就是先经过一轮初始化，保证Redis的各个内部数据结构以及参数都有缺省值，然后再从配置文件中加载自定义的配置。
void initConfigValues() {
for (standardConfig *config = configs; config->name != NULL; config++) {
config->interface.init(config->data);
}
}创建事件循环
在Redis中，事件循环是用一个叫aeEventLoop的struct来表示的。「创建事件循环」这一步主要就是创建一个aeEventLoop结构，并存储到server全局变量（即前面提到的redisServer类型的结构）中。另外，事件循环的执行依赖系统底层的I/O多路复用机制(I/O multiplexing)，比如Linux系统上的epoll机制。因此，这一步也包含对于底层I/O多路复用机制的初始化（调用系统API）。
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;
eventLoop->flags = 0;
if (aeApiCreate(eventLoop) == -1) goto err;
/* 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;
}开启socket监听
服务器程序需要监听才能收到请求。根据配置，这一步可能会打开两种监听：对于TCP连接的监听和对于Unix domain socket的监听。Unix domain socket是一种高效的进程间通信(IPC)机制，在POSIX规范中也有明确的定义，用于在同一台主机上的两个不同进程之间进行通信，比使用TCP协议性能更高（因为省去了协议栈的开销）。当使用Redis客户端连接同一台机器上的Redis服务器时，可以选择使用「Unix domain socket」进行连接。但不管是哪一种监听，程序都会获得文件描述符，并存储到server全局变量中。对于TCP的监听来说，由于监听的IP地址和端口可以绑定多个，因此获得的用于监听TCP连接的文件描述符也可以包含多个。后面，程序就可以拿这一步获得的文件描述符去注册I/O事件回调了。
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: unsupported");
}
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: unsupported");
}
}
/* 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,
"Could not create server TCP listening socket %s:%d: %s",
server.bindaddr[j] ? server.bindaddr[j] : "*",
port, server.neterr);
if (errno == ENOPROTOOPT || errno == EPROTONOSUPPORT ||
errno == ESOCKTNOSUPPORT || errno == EPFNOSUPPORT ||
errno == EAFNOSUPPORT || errno == EADDRNOTAVAIL)
continue;
return C_ERR;
}
anetNonBlock(NULL,fds[*count]);
(*count)++;
}
return C_OK;
}注册timer事件回调
Redis作为一个单线程(single-threaded)的程序，它如果想调度一些异步执行的任务，比如周期性地执行过期key的回收动作，除了依赖事件循环机制，没有其它的办法。这一步就是向前面刚刚创建好的事件循环中注册一个timer事件，并配置成可以周期性地执行一个回调函数：serverCron。由于Redis只有一个主线程，因此这个函数周期性的执行也是在这个线程内，它由事件循环来驱动（即在合适的时机调用），但不影响同一个线程上其它逻辑的执行（相当于按时间分片了）。serverCron函数到底做了什么呢？实际上，它除了周期性地执行过期key的回收动作，还执行了很多其它任务，比如主从重连、Cluster节点间的重连、BGSAVE和AOF rewrite的触发执行，等等
long long aeCreateTimeEvent(aeEventLoop *eventLoop, long long milliseconds,
aeTimeProc *proc, void *clientData,
aeEventFinalizerProc *finalizerProc)
{
long long id = eventLoop->timeEventNextId++;
aeTimeEvent *te;
te = zmalloc(sizeof(*te));
if (te == NULL) return AE_ERR;
te->id = id;
aeAddMillisecondsToNow(milliseconds,&te->when_sec,&te->when_ms);
te->timeProc = proc;
te->finalizerProc = finalizerProc;
te->clientData = clientData;
te->prev = NULL;
te->next = eventLoop->timeEventHead;
if (te->next)
te->next->prev = te;
eventLoop->timeEventHead = te;
return id;
}注册I/O事件回调
Redis服务端最主要的工作就是监听I/O事件，从中分析出来自客户端的命令请求，执行命令，然后返回响应结果。对于I/O事件的监听，自然也是依赖事件循环。前面提到过，Redis可以打开两种监听：对于TCP连接的监听和对于Unix domain socket的监听。因此，这里就包含对于这两种I/O事件的回调的注册，两个回调函数分别是acceptTcpHandler和acceptUnixHandler。对于来自Redis客户端的请求的处理，就会走到这两个函数中去。另外，其实Redis在这里还会注册一个I/O事件，用于通过管道(pipe)机制与module进行双向通信。
/* 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.");
}
}
for (j = 0; j < server.tlsfd_count; j++) {
if (aeCreateFileEvent(server.el, server.tlsfd[j], AE_READABLE,
acceptTLSHandler,NULL) == AE_ERR)
{
serverPanic(
"Unrecoverable error creating server.tlsfd file event.");
}
}
if (server.sofd > 0 && aeCreateFileEvent(server.el,server.sofd,AE_READABLE,
acceptUnixHandler,NULL) == AE_ERR) serverPanic("Unrecoverable error creating server.sofd file event.");
/* Register a readable event for the pipe used to awake the event loop
* when a blocked client in a module needs attention. */
if (aeCreateFileEvent(server.el, server.module_blocked_pipe[0], AE_READABLE,
moduleBlockedClientPipeReadable,NULL) == AE_ERR) {
serverPanic(
"Error registering the readable event for the module "
"blocked clients subsystem.");
}接下来就是InitServerLast方法：
void InitServerLast() {
bioInit();
initThreadedIO();
set_jemalloc_bg_thread(server.jemalloc_bg_thread);
server.initial_memory_usage = zmalloc_used_memory();
}初始化后台线程
Redis会创建一些额外的线程，在后台运行，专门用于处理一些耗时的并且可以被延迟执行的任务（一般是一些清理工作）。在Redis里面这些后台线程被称为bio(Background I/O service)。它们负责的任务包括：可以延迟执行的文件关闭操作(比如unlink命令的执行)，AOF的持久化写库操作(即fsync调用，但注意只有可以被延迟执行的fsync操作才在后台线程执行)，还有一些大key的清除操作(比如flushdb async命令的执行)。可见bio这个名字有点名不副实，它做的事情不一定跟I/O有关。对于这些后台线程，我们可能还会产生一个疑问：前面的初始化过程，已经注册了一个timer事件回调，即serverCron函数，按说后台线程执行的这些任务似乎也可以放在serverCron中去执行。因为serverCron函数也是可以用来执行后台任务的。实际上这样做是不行的。前面我们已经提到过，serverCron由事件循环来驱动，执行还是在Redis主线程上，相当于和主线程上执行的其它操作（主要是对于命令请求的执行）按时间进行分片了。这样的话，serverCron里面就不能执行过于耗时的操作，否则它就会影响Redis执行命令的响应时间。因此，对于耗时的、并且可以被延迟执行的任务，就只能放到单独的线程中去执行了。
void bioInit(void) {
pthread_attr_t attr;
pthread_t thread;
size_t stacksize;
int j;
/* Initialization of state vars and objects */
for (j = 0; j < BIO_NUM_OPS; j++) {
pthread_mutex_init(&bio_mutex[j],NULL);
pthread_cond_init(&bio_newjob_cond[j],NULL);
pthread_cond_init(&bio_step_cond[j],NULL);
bio_jobs[j] = listCreate();
bio_pending[j] = 0;
}
/* Set the stack size as by default it may be small in some system */
pthread_attr_init(&attr);
pthread_attr_getstacksize(&attr,&stacksize);
if (!stacksize) stacksize = 1; /* The world is full of Solaris Fixes */
while (stacksize < REDIS_THREAD_STACK_SIZE) stacksize *= 2;
pthread_attr_setstacksize(&attr, stacksize);
/* Ready to spawn our threads. We use the single argument the thread
* function accepts in order to pass the job ID the thread is
* responsible of. */
for (j = 0; j < BIO_NUM_OPS; j++) {
void *arg = (void*)(unsigned long) j;
if (pthread_create(&thread,&attr,bioProcessBackgroundJobs,arg) != 0) {
serverLog(LL_WARNING,"Fatal: Can't initialize Background Jobs.");
exit(1);
}
bio_threads[j] = thread;
}
}初始化主线程IO
前面创建好了事件循环的结构，但还没有真正进入循环的逻辑。过了这一步，事件循环就运行起来，驱动前面注册的timer事件回调和I/O事件回调不断执行。
void initThreadedIO(void) {
io_threads_active = 0; /* We start with threads not active. */
/* Don't spawn any thread if the user selected a single thread:
* we'll handle I/O directly from the main thread. */
if (server.io_threads_num == 1) return;
if (server.io_threads_num > IO_THREADS_MAX_NUM) {
serverLog(LL_WARNING,"Fatal: too many I/O threads configured. "
"The maximum number is %d.", IO_THREADS_MAX_NUM);
exit(1);
}
/* Spawn and initialize the I/O threads. */
for (int i = 0; i < server.io_threads_num; i++) {
/* Things we do for all the threads including the main thread. */
io_threads_list[i] = listCreate();
if (i == 0) continue; /* Thread 0 is the main thread. */
/* Things we do only for the additional threads. */
pthread_t tid;
pthread_mutex_init(&io_threads_mutex[i],NULL);
io_threads_pending[i] = 0;
pthread_mutex_lock(&io_threads_mutex[i]); /* Thread will be stopped. */
if (pthread_create(&tid,NULL,IOThreadMain,(void*)(long)i) != 0) {
serverLog(LL_WARNING,"Fatal: Can't initialize IO thread.");
exit(1);
}
io_threads[i] = tid;
}
}还原数据库
初始化完服务器的状态后，服务器已经处于一个可启动状态，因为redis有持久化特性，服务器还需要加载相应的文件来还原之前数据库的数据。判断Redis当前开启了哪种模式，如果是AOF，则通过AOF还原数据库的数据，否则，载入RDB文件，通过RDB文件还原数据库的数据。
void loadDataFromDisk(void) {
long long start = ustime();
if (server.aof_state == AOF_ON) {
if (loadAppendOnlyFile(server.aof_filename) == C_OK)
serverLog(LL_NOTICE,"DB loaded from append only file: %.3f seconds",(float)(ustime()-start)/1000000);
} else {
rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
if (rdbLoad(server.rdb_filename,&rsi,RDBFLAGS_NONE) == C_OK) {
serverLog(LL_NOTICE,"DB loaded from disk: %.3f seconds",
(float)(ustime()-start)/1000000);
/* Restore the replication ID / offset from the RDB file. */
if ((server.masterhost ||
(server.cluster_enabled &&
nodeIsSlave(server.cluster->myself))) &&
rsi.repl_id_is_set &&
rsi.repl_offset != -1 &&
/* Note that older implementations may save a repl_stream_db
* of -1 inside the RDB file in a wrong way, see more
* information in function rdbPopulateSaveInfo. */
rsi.repl_stream_db != -1)
{
memcpy(server.replid,rsi.repl_id,sizeof(server.replid));
server.master_repl_offset = rsi.repl_offset;
/* If we are a slave, create a cached master from this
* information, in order to allow partial resynchronizations
* with masters. */
replicationCacheMasterUsingMyself();
selectDb(server.cached_master,rsi.repl_stream_db);
}
} else if (errno != ENOENT) {
serverLog(LL_WARNING,"Fatal error loading the DB: %s. Exiting.",strerror(errno));
exit(1);
}
}
}启动事件监听
main函数会设置beforeSleep和afterSleep回调函数，然后调用aeMain函数启动事件循环器，开始监听事件。aeMain函数是一个死循环，不断的监听新请求的到来。
aeSetBeforeSleepProc(server.el,beforeSleep);
aeSetAfterSleepProc(server.el,afterSleep);
aeMain(server.el);
aeDeleteEventLoop(server.el);