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        在Redis中，MULTI和EXEC命令提供了一种机制来确保一系列命令以原子性的方式被执行。这意味着一旦这些命令被组合在一起并发送到服务器端，它们将作为一个整体被处理：要么全部成功执行，要么完全不执行。这一特性对于需要保证数据一致性的应用场景非常有用。
具体来说，当你向Redis客户端发送MULTI命令时，标志着开始一个事务块。从这时起，直到发出EXEC为止，所有发往该Redis连接的命令都不会立即执行，而是被暂时缓存起来。只有当EXEC命令被调用后，之前缓存的所有命令才会按照接收顺序一次性地在服务器端执行，并且整个过程是不可中断的。如果在这批命令执行过程中出现了错误（例如某个命令语法有误），那么除了导致错误的那个命令之外，其他命令仍然会被正常执行。
示例如下:

jedis.watch(key); // 在下一节中解析
// 开启批量执行
jedis.multi();
try{jedis.set(key,value);jedis.expire(expire,ttl);// 服务端开始执行jedis.exec();
catch(Exeception e) {// 放弃jedis.discard();
}

源码分析

multi命令

当我们调用multi命令时，会来到mutilCommand方法(multi.c)。

void multiCommand(client *c) {if (c->flags & CLIENT_MULTI) {addReplyError(c,"MULTI calls can not be nested");return;}// 给当前的客户端添加一个CLIENT_MULTI的标识c->flags |= CLIENT_MULTI;addReply(c,shared.ok);
}

该命令只是给当前客户端对象添加一个CLIENT_MULTI就直接返回了。

其他命令

而后，当我们调用set 和 expire 命令时，会经过以下的判断（server.c中方法processCommand）：

// 如果连接的客户端是已经调用了multi命令
/* Exec the command */
if (c->flags & CLIENT_MULTI &&c->cmd->proc != execCommand &&c->cmd->proc != discardCommand &&c->cmd->proc != multiCommand &&c->cmd->proc != watchCommand &&c->cmd->proc != quitCommand &&c->cmd->proc != resetCommand)
{// 将命令添加到队列queueMultiCommand(c);addReply(c, shared.queued);
}

会判断当前客户端是否有CLIENT_MULTI，并且不是相关的命令，则会调用queueMultiCommand方法将命令加入队列。

void queueMultiCommand(client *c) {multiCmd *mc;/* No sense to waste memory if the transaction is already aborted.* this is useful in case client sends these in a pipeline, or doesn't* bother to read previous responses and didn't notice the multi was already* aborted. */// 如果标识中有CLIENT_DIRTY_CAS，则不处理，说明watch的key已经被修改// 在下一节中说明if (c->flags & (CLIENT_DIRTY_CAS|CLIENT_DIRTY_EXEC))return;// 申请命令缓存// 进行扩容c->mstate.commands = zrealloc(c->mstate.commands,sizeof(multiCmd)*(c->mstate.count+1));// 获取当前的最后一个对象// 进行缓存redis命令&参数mc = c->mstate.commands+c->mstate.count;mc->cmd = c->cmd;mc->argc = c->argc;mc->argv = c->argv;mc->argv_len = c->argv_len;c->mstate.count++;c->mstate.cmd_flags |= c->cmd->flags;c->mstate.cmd_inv_flags |= ~c->cmd->flags;c->mstate.argv_len_sums += c->argv_len_sum + sizeof(robj*)*c->argc;/* Reset the client's args since we copied them into the mstate and shouldn't* reference them from c anymore. */c->argv = NULL;c->argc = 0;c->argv_len_sum = 0;c->argv_len = 0;
}

exec 命令

void execCommand(client *c) {int j;robj **orig_argv;int orig_argc, orig_argv_len;struct redisCommand *orig_cmd;// 如果没有搭配multi一起使用，报错if (!(c->flags & CLIENT_MULTI)) {addReplyError(c,"EXEC without MULTI");return;}// .. 省略部分代码// 标识正在exec命令中server.in_exec = 1;orig_argv = c->argv;orig_argv_len = c->argv_len;orig_argc = c->argc;orig_cmd = c->cmd;addReplyArrayLen(c,c->mstate.count);// 执行所有的命令for (j = 0; j < c->mstate.count; j++) {c->argc = c->mstate.commands[j].argc;c->argv = c->mstate.commands[j].argv;c->argv_len = c->mstate.commands[j].argv_len;c->cmd = c->realcmd = c->mstate.commands[j].cmd;/* ACL permissions are also checked at the time of execution in case* they were changed after the commands were queued. */int acl_errpos;int acl_retval = ACLCheckAllPerm(c,&acl_errpos);if (acl_retval != ACL_OK) {// 省略部分代码..} else {// 调用命令if (c->id == CLIENT_ID_AOF)call(c,CMD_CALL_NONE);elsecall(c,CMD_CALL_FULL);serverAssert((c->flags & CLIENT_BLOCKED) == 0);}/* Commands may alter argc/argv, restore mstate. */c->mstate.commands[j].argc = c->argc;c->mstate.commands[j].argv = c->argv;c->mstate.commands[j].cmd = c->cmd;}// restore old DENY_BLOCKING valueif (!(old_flags & CLIENT_DENY_BLOCKING))c->flags &= ~CLIENT_DENY_BLOCKING;c->argv = orig_argv;c->argv_len = orig_argv_len;c->argc = orig_argc;c->cmd = c->realcmd = orig_cmd;discardTransaction(c);server.in_exec = 0;
}

该命令就是依次遍历已经缓存的所有命令。

discard 命令

该命令放弃所有在multi之后的命令。来看下discardCommand

void discardCommand(client *c) {if (!(c->flags & CLIENT_MULTI)) {addReplyError(c,"DISCARD without MULTI");return;}// 放弃所有的命令discardTransaction(c);addReply(c,shared.ok);
}

discardTransaction主要处理该连接的命令缓存、重置连接状态等操作

void discardTransaction(client *c) {// 释放客户端的命令缓存状态freeClientMultiState(c);// 重置该连接的状态initClientMultiState(c);// 去除相关的标识c->flags &= ~(CLIENT_MULTI|CLIENT_DIRTY_CAS|CLIENT_DIRTY_EXEC);unwatchAllKeys(c);
}

freeClientMultiState释放客户端上的资源

void freeClientMultiState(client *c) {int j;// 依次释放资源for (j = 0; j < c->mstate.count; j++) {int i;multiCmd *mc = c->mstate.commands+j;//释放参数的引用for (i = 0; i < mc->argc; i++)decrRefCount(mc->argv[i]);zfree(mc->argv);}// 释放数组zfree(c->mstate.commands);
}