redis代码解析-事务

Posted 2020-10-12

redis 的事务相关的几个命令分别为 watch multi exec.

 

watch 可以监控一个变量在事务开始执行之前是否有被修改.使用方式为: WATCH key [key ...]

    在redis内部的实现是每个db有一个名为watched_keys的dict,这个dict的key为监控的key,value为所有监控该key的client组成的链表.

 

    所有的涉及到修改数据库的命令执行成功之后都会执行signalModifiedKey->touchWatchedKey,将修改的db中的watched_keys里面记录的所有监控了本次修改的key的client设置c->flags |= CLIENT_DIRTY_CAS;

 

multi 表示开始一次事务,使用方式为: MULTI

实现为只是对client标记上 c->flags |= CLIENT_MULTI;它会先检查一下是否已经标记了 CLIENT_MULTI,如果标记了本次multi命令会失败,因为redis不允许事务嵌套.

 

在multi开始之后所有的命令(除了multi,exec,discard,watch)都会进入multicommand queue.

我们知道一条命令进入server的流程为,

第一步,底层的io多路复用处理器会上报client可读事件,调用注册的处理可读事件的函数readQueryFromClient将命令读入client的querybuf,

第二步,进入到processInputBuffer处理读入的buffer,分解出argv,argc;

第三步,再进入processCommand,先根据argv[1]在命令表的dict中找到对应的command,找到command后会检查本次的command是否合法(名字是否合法,参数个数是否合法),如果command不合法在返回错误的同时会检查是否处于事务状态中,如果命令

不合法CLIENT_DIRTY_EXEC标志将导致事务执行失败.

/* Flag the transacation as DIRTY_EXEC so that EXEC will fail.
 * Should be called every time there is an error while queueing a command. */
void flagTransaction(client *c) {
    if (c->flags & CLIENT_MULTI)
        c->flags |= CLIENT_DIRTY_EXEC;
}

然后(略过一些与事务无关的步骤),检查c->flags & CLIENT_MULTI,来决定是否直接执行命令还是将命令进入queueMultiCommand

第四步,在queueMultiCommand中,将本次命令command,argc,argv加入到c->mstate.commands队列中.

/* Client MULTI/EXEC state */
typedef struct multiCmd {
    robj **argv;
    int argc;
    struct redisCommand *cmd;
} multiCmd;

typedef struct multiState {
    multiCmd *commands;     /* Array of MULTI commands */
    int count;              /* Total number of MULTI commands */
    int minreplicas;        /* MINREPLICAS for synchronous replication */
    time_t minreplicas_timeout; /* MINREPLICAS timeout as unixtime. */
} multiState;

typedef struct client {
 ...
 multiState mstate;      /* MULTI/EXEC state */
 ...
} client;

 

exec 命令开始执行事务队列中的全部命令,使用方式为:EXEC

exec会检查当前是否处于事务状态

if (!(c->flags & CLIENT_MULTI)) {
        addReplyError(c,"EXEC without MULTI");
        return;
    }

然后检查该条命令来自的client监控的key是否有被touch

/* Check if we need to abort the EXEC because:
     * 1) Some WATCHed key was touched.
     * 2) There was a previous error while queueing commands.
     * A failed EXEC in the first case returns a multi bulk nil object
     * (technically it is not an error but a special behavior), while
     * in the second an EXECABORT error is returned. */
    if (c->flags & (CLIENT_DIRTY_CAS|CLIENT_DIRTY_EXEC)) {
        addReply(c, c->flags & CLIENT_DIRTY_EXEC ? shared.execaborterr :
                                                  shared.nullmultibulk);
        discardTransaction(c);
        goto handle_monitor;
    }

最后是遍历c->mstate.commands执行所有事务命令队列中的命令

/* Exec all the queued commands */
    unwatchAllKeys(c); /* Unwatch ASAP otherwise we‘ll waste CPU cycles */
    orig_argv = c->argv;
    orig_argc = c->argc;
    orig_cmd = c->cmd;
    addReplyMultiBulkLen(c,c->mstate.count);
    for (j = 0; j < c->mstate.count; j++) {
        c->argc = c->mstate.commands[j].argc;
        c->argv = c->mstate.commands[j].argv;
        c->cmd = c->mstate.commands[j].cmd;

        /* Propagate a MULTI request once we encounter the first command which
         * is not readonly nor an administrative one.
         * This way we‘ll deliver the MULTI/..../EXEC block as a whole and
         * both the AOF and the replication link will have the same consistency
         * and atomicity guarantees. */
        if (!must_propagate && !(c->cmd->flags & (CMD_READONLY|CMD_ADMIN))) {
            execCommandPropagateMulti(c);
            must_propagate = 1;
        }

        call(c,CMD_CALL_FULL);

        /* 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;
    }
    c->argv = orig_argv;
    c->argc = orig_argc;
    c->cmd = orig_cmd;
    discardTransaction(c);