如何深入理解Redis事务
本篇内容主要讲解“如何深入理解redis事务”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“如何深入理解Redis事务”吧!
成都创新互联是一家专注于网站设计制作、网站设计与策划设计,枣强网站建设哪家好?成都创新互联做网站,专注于网站建设十余年,网设计领域的专业建站公司;建站业务涵盖:枣强等地区。枣强做网站价格咨询:18982081108
定制制作可以根据自己的需求进行定制，成都做网站、成都网站建设、成都外贸网站建设构思过程中功能建设理应排到主要部位公司成都做网站、成都网站建设、成都外贸网站建设的运用实际效果公司网站制作网站建立与制做的实际意义
Redis可以看成NOSQL类型的数据库系统, Redis也提供了事务, 但是和传统的关系型数据库的事务既有相似性, 也存在区别.因为Redis的架构基于操作系统的多路复用的IO接口,主处理流程是一个单线程,因此对于一个完整的命令, 其处理都是原子性的, 但是如果需要将多个命令作为一个不可分割的处理序列, 就需要使用事务.
Redis事务有如下一些特点:
事务中的命令序列执行的时候是原子性的,也就是说,其不会被其他客户端的命令中断. 这和传统的数据库的事务的属性是类似的.
尽管Redis事务中的命令序列是原子执行的, 但是事务中的命令序列执行可以部分成功,这种情况下,Redis事务不会执行回滚操作. 这和传统关系型数据库的事务是有区别的.
尽管Redis有RDB和AOF两种数据持久化机制, 但是其设计目标是高效率的cache系统. Redis事务只保证将其命令序列中的操作结果提交到内存中,不保证持久化到磁盘文件. 更进一步的, Redis事务和RDB持久化机制没有任何关系, 因为RDB机制是对内存数据结构的全量的快照.由于AOF机制是一种增量持久化,所以事务中的命令序列会提交到AOF的缓存中.但是AOF机制将其缓存写入磁盘文件是由其配置的实现策略决定的,和Redis事务没有关系.
Redis事务API
从宏观上来讲, Redis事务开始后, 会缓存后续的操作命令及其操作数据,当事务提交时,原子性的执行缓存的命令序列.
从版本2.2开始,Redis提供了一种乐观的锁机制, 配合这种机制,Redis事务提交时, 变成了事务的条件执行. 具体的说,如果乐观锁失败了,事务提交时, 丢弃事务中的命令序列,如果乐观锁成功了, 事务提交时,才会执行其命令序列.当然,也可以不使用乐观锁机制, 在事务提交时, 无条件执行事务的命令序列.
Redis事务涉及到MULTI, EXEC, DISCARD, WATCH和UNWATCH这五个命令:
事务开始的命令是MULTI, 该命令返回OK提示信息. Redis不支持事务嵌套,执行多次MULTI命令和执行一次是相同的效果.嵌套执行MULTI命令时,Redis只是返回错误提示信息.
EXEC是事务的提交命令,事务中的命令序列将被执行(或者不被执行,比如乐观锁失败等).该命令将返回响应数组,其内容对应事务中的命令执行结果.
WATCH命令是开始执行乐观锁,该命令的参数是key(可以有多个), Redis将执行WATCH命令的客户端对象和key进行关联,如果其他客户端修改了这些key,则执行WATCH命令的客户端将被设置乐观锁失败的标志.该命令必须在事务开始前执行,即在执行MULTI命令前执行WATCH命令,否则执行无效,并返回错误提示信息.
UNWATCH命令将取消当前客户端对象的乐观锁key,该客户端对象的事务提交将变成无条件执行.
DISCARD命令将结束事务,并且会丢弃全部的命令序列.
需要注意的是,EXEC命令和DISCARD命令结束事务时,会调用UNWATCH命令,取消该客户端对象上所有的乐观锁key.
无条件提交
如果不使用乐观锁, 则事务为无条件提交.下面是一个事务执行的例子:
multi +OK incr key1 +QUEUED set key2 val2 +QUEUED exec *2 :1 +OK
当客户端开始事务后, 后续发送的命令将被Redis缓存起来,Redis向客户端返回响应提示字符串QUEUED.当执行EXEC提交事务时,缓存的命令依次被执行,返回命令序列的执行结果.
事务的错误处理
事务提交命令EXEC有可能会失败, 有三种类型的失败场景:
在事务提交之前,客户端执行的命令缓存失败.比如命令的语法错误(命令参数个数错误, 不支持的命令等等).如果发生这种类型的错误,Redis将向客户端返回包含错误提示信息的响应.
事务提交时,之前缓存的命令有可能执行失败.
由于乐观锁失败,事务提交时,将丢弃之前缓存的所有命令序列.
当发生第一种失败的情况下,客户端在执行事务提交命令EXEC时,将丢弃事务中所有的命令序列.下面是一个例子:
multi +OK incr num1 num2 -ERR wrong number of arguments for 'incr' command set key1 val1 +QUEUED exec -EXECABORT Transaction discarded because of previous errors.
命令incr num1 num2并没有缓存成功, 因为incr命令只允许有一个参数,是个语法错误的命令.Redis无法成功缓存该命令,向客户端发送错误提示响应.接下来的set key1 val1命令缓存成功.最后执行事务提交的时候,因为发生过命令缓存失败,所以事务中的所有命令序列被丢弃.
如果事务中的所有命令序列都缓存成功,在提交事务的时候,缓存的命令中仍可能执行失败.但Redis不会对事务做任何回滚补救操作.下面是一个这样的例子:
multi +OK set key1 val1 +QUEUED lpop key1 +QUEUED incr num1 +QUEUED exec *3 +OK -WRONGTYPE Operation against a key holding the wrong kind of value :1
所有的命令序列都缓存成功,但是在提交事务的时候,命令set key1 val1和incr num1执行成功了,Redis保存了其执行结果,但是命令lpop key1执行失败了.
乐观锁机制
Redis事务和乐观锁一起使用时,事务将成为有条件提交.
关于乐观锁,需要注意的是:
WATCH命令必须在MULTI命令之前执行. WATCH命令可以执行多次.
WATCH命令可以指定乐观锁的多个key,如果在事务过程中,任何一个key被其他客户端改变,则当前客户端的乐观锁失败,事务提交时,将丢弃所有命令序列.
多个客户端的WATCH命令可以指定相同的key.
WATCH命令指定乐观锁后,可以接着执行MULTI命令进入事务上下文,也可以在WATCH命令和MULTI命令之间执行其他命令. 具体使用方式取决于场景需求,不在事务中的命令将立即被执行.
如果WATCH命令指定的乐观锁的key,被当前客户端改变,在事务提交时,乐观锁不会失败.
如果WATCH命令指定的乐观锁的key具有超时属性,并且该key在WATCH命令执行后, 在事务提交命令EXEC执行前超时, 则乐观锁不会失败.如果该key被其他客户端对象修改,则乐观锁失败.
一个执行乐观锁机制的事务例子:
rpush list v1 v2 v3 :3 watch list +OK multi +OK lpop list +QUEUED exec *1 $2 v1
下面是另一个例子,乐观锁被当前客户端改变, 事务提交成功:
watch num +OK multi +OK incr num +QUEUED exec *1 :2
Redis事务和乐观锁配合使用时, 可以构造实现单个Redis命令不能完成的更复杂的逻辑.
Redis事务的源码实现机制
首先,事务开始的MULTI命令执行的函数为multiCommand, 其实现为(multi.c):
void multiCommand(redisClient *c) { if (c->flags & REDIS_MULTI) { addReplyError(c,"MULTI calls can not be nested"); return; } c->flags |= REDIS_MULTI; addReply(c,shared.ok); }该命令只是在当前客户端对象上加上REDIS_MULTI标志, 表示该客户端进入了事务上下文.
客户端进入事务上下文后,后续执行的命令将被缓存. 函数processCommand是Redis处理客户端命令的入口函数, 其实现为(redis.c):
int processCommand(redisClient *c) { /* The QUIT command is handled separately. Normal command procs will * go through checking for replication and QUIT will cause trouble * when FORCE_REPLICATION is enabled and would be implemented in * a regular command proc. */ if (!strcasecmp(c->argv[0]->ptr,"quit")) { addReply(c,shared.ok); c->flags |= REDIS_CLOSE_AFTER_REPLY; return REDIS_ERR; } /* Now lookup the command and check ASAP about trivial error conditions * such as wrong arity, bad command name and so forth. */ c->ccmd = c->lastcmd = lookupCommand(c->argv[0]->ptr); if (!c->cmd) { flagTransaction(c); addReplyErrorFormat(c,"unknown command '%s'", (char*)c->argv[0]->ptr); return REDIS_OK; } else if ((c->cmd->arity > 0 && c->cmd->arity != c->argc) || (c->argc < -c->cmd->arity)) { flagTransaction(c); addReplyErrorFormat(c,"wrong number of arguments for '%s' command", c->cmd->name); return REDIS_OK; } /* Check if the user is authenticated */ if (server.requirepass && !c->authenticated && c->cmd->proc != authCommand) { flagTransaction(c); addReply(c,shared.noautherr); return REDIS_OK; } /* Handle the maxmemory directive. * * First we try to free some memory if possible (if there are volatile * keys in the dataset). If there are not the only thing we can do * is returning an error. */ if (server.maxmemory) { int retval = freeMemoryIfNeeded(); /* freeMemoryIfNeeded may flush slave output buffers. This may result * into a slave, that may be the active client, to be freed. */ if (server.current_client == NULL) return REDIS_ERR; /* It was impossible to free enough memory, and the command the client * is trying to execute is denied during OOM conditions? Error. */ if ((c->cmd->flags & REDIS_CMD_DENYOOM) && retval == REDIS_ERR) { flagTransaction(c); addReply(c, shared.oomerr); return REDIS_OK; } } /* Don't accept write commands if there are problems persisting on disk * and if this is a master instance. */ if (((server.stop_writes_on_bgsave_err && server.saveparamslen > 0 && server.lastbgsave_status == REDIS_ERR) || server.aof_last_write_status == REDIS_ERR) && server.masterhost == NULL && (c->cmd->flags & REDIS_CMD_WRITE || c->cmd->proc == pingCommand)) { flagTransaction(c); if (server.aof_last_write_status == REDIS_OK) addReply(c, shared.bgsaveerr); else addReplySds(c, sdscatprintf(sdsempty(), "-MISCONF Errors writing to the AOF file: %s\r\n", strerror(server.aof_last_write_errno))); return REDIS_OK; } /* Don't accept write commands if there are not enough good slaves and * user configured the min-slaves-to-write option. */ if (server.masterhost == NULL && server.repl_min_slaves_to_write && server.repl_min_slaves_max_lag && c->cmd->flags & REDIS_CMD_WRITE && server.repl_good_slaves_count < server.repl_min_slaves_to_write) { flagTransaction(c); addReply(c, shared.noreplicaserr); return REDIS_OK; } /* Don't accept write commands if this is a read only slave. But * accept write commands if this is our master. */ if (server.masterhost && server.repl_slave_ro && !(c->flags & REDIS_MASTER) && c->cmd->flags & REDIS_CMD_WRITE) { addReply(c, shared.roslaveerr); return REDIS_OK; } /* Only allow SUBSCRIBE and UNSUBSCRIBE in the context of Pub/Sub */ if (c->flags & REDIS_PUBSUB && c->cmd->proc != pingCommand && c->cmd->proc != subscribeCommand && c->cmd->proc != unsubscribeCommand && c->cmd->proc != psubscribeCommand && c->cmd->proc != punsubscribeCommand) { addReplyError(c,"only (P)SUBSCRIBE / (P)UNSUBSCRIBE / QUIT allowed in this context"); return REDIS_OK; } /* Only allow INFO and SLAVEOF when slave-serve-stale-data is no and * we are a slave with a broken link with master. */ if (server.masterhost && server.repl_state != REDIS_REPL_CONNECTED && server.repl_serve_stale_data == 0 && !(c->cmd->flags & REDIS_CMD_STALE)) { flagTransaction(c); addReply(c, shared.masterdownerr); return REDIS_OK; } /* Loading DB? Return an error if the command has not the * REDIS_CMD_LOADING flag. */ if (server.loading && !(c->cmd->flags & REDIS_CMD_LOADING)) { addReply(c, shared.loadingerr); return REDIS_OK; } /* Lua script too slow? Only allow a limited number of commands. */ if (server.lua_timedout && c->cmd->proc != authCommand && c->cmd->proc != replconfCommand && !(c->cmd->proc == shutdownCommand && c->argc == 2 && tolower(((char*)c->argv[1]->ptr)[0]) == 'n') && !(c->cmd->proc == scriptCommand && c->argc == 2 && tolower(((char*)c->argv[1]->ptr)[0]) == 'k')) { flagTransaction(c); addReply(c, shared.slowscripterr); return REDIS_OK; } /* Exec the command */ if (c->flags & REDIS_MULTI && c->cmd->proc != execCommand && c->cmd->proc != discardCommand && c->cmd->proc != multiCommand && c->cmd->proc != watchCommand) { queueMultiCommand(c); addReply(c,shared.queued); } else { call(c,REDIS_CALL_FULL); if (listLength(server.ready_keys)) handleClientsBlockedOnLists(); } return REDIS_OK; }Line145:151当客户端处于事务上下文时, 如果接收的是非事务命令(MULTI, EXEC, WATCH, DISCARD), 则调用queueMultiCommand将命令缓存起来,然后向客户端发送成功响应.
在函数processCommand中, 在缓存命令之前, 如果检查到客户端发送的命令不存在,或者命令参数个数不正确等情况, 会调用函数flagTransaction标命令缓存失败.也就是说,函数processCommand中, 所有调用函数flagTransaction的条件分支,都是返回失败响应.
缓存命令的函数queueMultiCommand的实现为(multi.c):
/* Add a new command into the MULTI commands queue */ void queueMultiCommand(redisClient *c) { multiCmd *mc; int j; c->mstate.commands = zrealloc(c->mstate.commands, sizeof(multiCmd)*(c->mstate.count+1)); mc = c->mstate.commands+c->mstate.count; mc->ccmd = c->cmd; mc->argc = c->argc; mc->argv = zmalloc(sizeof(robj*)*c->argc); memcpy(mc->argv,c->argv,sizeof(robj*)*c->argc); for (j = 0; j < c->argc; j++) incrRefCount(mc->argv[j]); c->mstate.count++; }在事务上下文中, 使用multiCmd结构来缓存命令, 该结构定义为(redis.h):
/* Client MULTI/EXEC state */ typedef struct multiCmd { robj **argv; int argc; struct redisCommand *cmd; } multiCmd;其中argv字段指向命令的参数内存地址,argc为命令参数个数, cmd为命令描述结构, 包括名字和函数指针等.
命令参数的内存空间已经使用动态分配记录于客户端对象的argv字段了, multiCmd结构的argv字段指向客户端对象redisClient的argv即可.
无法缓存命令时, 调用函数flagTransaction,该函数的实现为(multi.c):
/* 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(redisClient *c) { if (c->flags & REDIS_MULTI) c->flags |= REDIS_DIRTY_EXEC; }该函数在客户端对象中设置REDIS_DIRTY_EXEC标志, 如果设置了这个标志, 事务提交时, 命令序列将被丢弃.
最后,在事务提交时, 函数processCommand中将调用call(c,REDIS_CALL_FULL);, 其实现为(redis.c):
/* Call() is the core of Redis execution of a command */ void call(redisClient *c, int flags) { long long dirty, start, duration; int cclient_old_flags = c->flags; /* Sent the command to clients in MONITOR mode, only if the commands are * not generated from reading an AOF. */ if (listLength(server.monitors) && !server.loading && !(c->cmd->flags & (REDIS_CMD_SKIP_MONITOR|REDIS_CMD_ADMIN))) { replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc); } /* Call the command. */ c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL); redisOpArrayInit(&server.also_propagate); dirty = server.dirty; start = ustime(); c->cmd->proc(c); duration = ustime()-start; dirty = server.dirty-dirty; if (dirty < 0) dirty = 0; /* When EVAL is called loading the AOF we don't want commands called * from Lua to go into the slowlog or to populate statistics. */ if (server.loading && c->flags & REDIS_LUA_CLIENT) flags &= ~(REDIS_CALL_SLOWLOG | REDIS_CALL_STATS); /* If the caller is Lua, we want to force the EVAL caller to propagate * the script if the command flag or client flag are forcing the * propagation. */ if (c->flags & REDIS_LUA_CLIENT && server.lua_caller) { if (c->flags & REDIS_FORCE_REPL) server.lua_caller->flags |= REDIS_FORCE_REPL; if (c->flags & REDIS_FORCE_AOF) server.lua_caller->flags |= REDIS_FORCE_AOF; } /* Log the command into the Slow log if needed, and populate the * per-command statistics that we show in INFO commandstats. */ if (flags & REDIS_CALL_SLOWLOG && c->cmd->proc != execCommand) { char *latency_event = (c->cmd->flags & REDIS_CMD_FAST) ? "fast-command" : "command"; latencyAddSampleIfNeeded(latency_event,duration/1000); slowlogPushEntryIfNeeded(c->argv,c->argc,duration); } if (flags & REDIS_CALL_STATS) { c->cmd->microseconds += duration; c->cmd->calls++; } /* Propagate the command into the AOF and replication link */ if (flags & REDIS_CALL_PROPAGATE) { int flags = REDIS_PROPAGATE_NONE; if (c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL; if (c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF; if (dirty) flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF); if (flags != REDIS_PROPAGATE_NONE) propagate(c->cmd,c->db->id,c->argv,c->argc,flags); } /* Restore the old FORCE_AOF/REPL flags, since call can be executed * recursively. */ c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL); c->flags |= client_old_flags & (REDIS_FORCE_AOF|REDIS_FORCE_REPL); /* Handle the alsoPropagate() API to handle commands that want to propagate * multiple separated commands. */ if (server.also_propagate.numops) { int j; redisOp *rop; for (j = 0; j < server.also_propagate.numops; j++) { rop = &server.also_propagate.ops[j]; propagate(rop->cmd, rop->dbid, rop->argv, rop->argc, rop->target); } redisOpArrayFree(&server.also_propagate); } server.stat_numcommands++; }在函数call中通过执行c->cmd->proc(c);调用具体的命令函数.事务提交命令EXEC对应的执行函数为execCommand, 其实现为(multi.c):
void execCommand(redisClient *c) { int j; robj **orig_argv; int orig_argc; struct redisCommand *orig_cmd; int must_propagate = 0; /* Need to propagate MULTI/EXEC to AOF / slaves? */ if (!(c->flags & REDIS_MULTI)) { addReplyError(c,"EXEC without MULTI"); return; } /* 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 & (REDIS_DIRTY_CAS|REDIS_DIRTY_EXEC)) { addReply(c, c->flags & REDIS_DIRTY_EXEC ? shared.execaborterr : shared.nullmultibulk); discardTransaction(c); goto handle_monitor; } /* 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->ccmd = c->mstate.commands[j].cmd; /* Propagate a MULTI request once we encounter the first write op. * 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 & REDIS_CMD_READONLY)) { execCommandPropagateMulti(c); must_propagate = 1; } call(c,REDIS_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); /* Make sure the EXEC command will be propagated as well if MULTI * was already propagated. */ if (must_propagate) server.dirty++; handle_monitor: /* Send EXEC to clients waiting data from MONITOR. We do it here * since the natural order of commands execution is actually: * MUTLI, EXEC, ... commands inside transaction ... * Instead EXEC is flagged as REDIS_CMD_SKIP_MONITOR in the command * table, and we do it here with correct ordering. */ if (listLength(server.monitors) && !server.loading) replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc); }LINE8:11检查EXEC命令和MULTI命令是否配对使用, 单独执行EXEC命令是没有意义的.
LINE19:24检查客户端对象是否具有REDIS_DIRTY_CAS或者REDIS_DIRTY_EXEC标志, 如果存在,则调用函数discardTransaction丢弃命令序列, 向客户端返回失败响应.
如果没有检查到任何错误,则先执行unwatchAllKeys(c);取消该客户端上所有的乐观锁key.
LINE32:52依次执行缓存的命令序列,这里有两点需要注意的是:
事务可能需要同步到AOF缓存或者replica备份节点中.如果事务中的命令序列都是读操作, 则没有必要向AOF和replica进行同步.如果事务的命令序列中包含写命令,则MULTI, EXEC和相关的写命令会向AOF和replica进行同步.根据LINE41:44的条件判断,执行execCommandPropagateMulti(c);保证MULTI命令同步, LINE59检查EXEC命令是否需要同步, 即MULTI命令和EXEC命令必须保证配对同步.EXEC命令的同步执行在函数的call中LINE62propagate(c->cmd,c->db->id,c->argv,c->argc,flags);, 具体的写入命令由各自的执行函数负责同步.
这里执行命令序列时, 通过执行call(c,REDIS_CALL_FULL);所以call函数是递归调用.
所以,综上所述, Redis事务其本质就是,以不可中断的方式依次执行缓存的命令序列,将结果保存到内存cache中.
事务提交时, 丢弃命令序列会调用函数discardTransaction, 其实现为(multi.c):
void discardTransaction(redisClient *c) { freeClientMultiState(c); initClientMultiState(c); c->flags &= ~(REDIS_MULTI|REDIS_DIRTY_CAS|REDIS_DIRTY_EXEC); unwatchAllKeys(c); }该函数调用freeClientMultiState释放multiCmd对象内存.调用initClientMultiState复位客户端对象的缓存命令管理结构.调用unwatchAllKeys取消该客户端的乐观锁.
WATCH命令执行乐观锁, 其对应的执行函数为watchCommand, 其实现为(multi.c):
void watchCommand(redisClient *c) { int j; if (c->flags & REDIS_MULTI) { addReplyError(c,"WATCH inside MULTI is not allowed"); return; } for (j = 1; j < c->argc; j++) watchForKey(c,c->argv[j]); addReply(c,shared.ok); }进而调用函数watchForKey, 其实现为(multi.c):
/* Watch for the specified key */ void watchForKey(redisClient *c, robj *key) { list *clients = NULL; listIter li; listNode *ln; watchedKey *wk; /* Check if we are already watching for this key */ listRewind(c->watched_keys,&li); while((ln = listNext(&li))) { wk = listNodeValue(ln); if (wk->db == c->db && equalStringObjects(key,wk->key)) return; /* Key already watched */ } /* This key is not already watched in this DB. Let's add it */ clients = dictFetchValue(c->db->watched_keys,key); if (!clients) { clients = listCreate(); dictAdd(c->db->watched_keys,key,clients); incrRefCount(key); } listAddNodeTail(clients,c); /* Add the new key to the list of keys watched by this client */ wk = zmalloc(sizeof(*wk)); wk->keykey = key; wk->db = c->db; incrRefCount(key); listAddNodeTail(c->watched_keys,wk); }关于乐观锁的key, 既保存于其客户端对象的watched_keys链表中, 也保存于全局数据库对象的watched_keys哈希表中.
LINE10:14检查客户端对象的链表中是否已经存在该key, 如果已经存在, 则直接返回.LINE16在全局数据库中返回该key对应的客户端对象链表, 如果链表不存在, 说明其他客户端没有使用该key作为乐观锁, 如果链表存在, 说明其他客户端已经使用该key作为乐观锁. LINE22将当前客户端对象记录于该key对应的链表中. LINE28将该key记录于当前客户端的key链表中.
当前客户端执行乐观锁以后, 其他客户端的写入命令可能修改该key值.所有具有写操作属性的命令都会执行函数signalModifiedKey, 其实现为(db.c):
void signalModifiedKey(redisDb *db, robj *key) { touchWatchedKey(db,key); }函数touchWatchedKey的实现为(multi.c):
/* "Touch" a key, so that if this key is being WATCHed by some client the * next EXEC will fail. */ void touchWatchedKey(redisDb *db, robj *key) { list *clients; listIter li; listNode *ln; if (dictSize(db->watched_keys) == 0) return; clients = dictFetchValue(db->watched_keys, key); if (!clients) return; /* Mark all the clients watching this key as REDIS_DIRTY_CAS */ /* Check if we are already watching for this key */ listRewind(clients,&li); while((ln = listNext(&li))) { redisClient *c = listNodeValue(ln); c->flags |= REDIS_DIRTY_CAS; } }语句if (dictSize(db->watched_keys) == 0) return;检查全局数据库中的哈希表watched_keys是否为空, 如果为空,说明没有任何客户端执行WATCH命令, 直接返回.如果该哈希表不为空, 取回该key对应的客户端链表结构,并把该链表中的每个客户端对象设置REDIS_DIRTY_CAS标志. 前面在EXEC的执行命令中,进行过条件判断, 如果客户端对象具有这个标志, 则丢弃事务中的命令序列.
在执行EXEC, DISCARD, UNWATCH命令以及在客户端结束连接的时候,都会取消乐观锁, 最终都会执行函数unwatchAllKeys, 其实现为(multi.c):
/* Unwatch all the keys watched by this client. To clean the EXEC dirty * flag is up to the caller. */ void unwatchAllKeys(redisClient *c) { listIter li; listNode *ln; if (listLength(c->watched_keys) == 0) return; listRewind(c->watched_keys,&li); while((ln = listNext(&li))) { list *clients; watchedKey *wk; /* Lookup the watched key -> clients list and remove the client * from the list */ wk = listNodeValue(ln); clients = dictFetchValue(wk->db->watched_keys, wk->key); redisAssertWithInfo(c,NULL,clients != NULL); listDelNode(clients,listSearchKey(clients,c)); /* Kill the entry at all if this was the only client */ if (listLength(clients) == 0) dictDelete(wk->db->watched_keys, wk->key); /* Remove this watched key from the client->watched list */ listDelNode(c->watched_keys,ln); decrRefCount(wk->key); zfree(wk); } }语句if (listLength(c->watched_keys) == 0) return;判断如果当前客户端对象的watched_keys链表为空,说明当前客户端没有执行WATCH命令,直接返回.如果该链表非空, 则依次遍历该链表中的key, 并从该链表中删除key, 同时,获得全局数据库中的哈希表watched_keys中该key对应的客户端链表, 删除当前客户端对象.
到此，相信大家对“如何深入理解Redis事务”有了更深的了解，不妨来实际操作一番吧！这里是创新互联网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
当前文章：如何深入理解Redis事务
分享链接：http://pwwzsj.com/article/iijpsi.html
