Redis系列（四）：数据结构String类型中基本操作命令和源码解析

1.介绍

string类型本质上是char[]数组的封装 

中文网：http://www.redis.cn/commands.html#string 

2.常用命令

set 命令

set命令的时间复杂度是O(1)

将键key设定为指定的“字符串”值。

如果 key 已经保存了一个值，那么这个操作会直接覆盖原来的值，并且忽略原始类型。

当set命令执行成功之后，之前设置的过期时间都将失效

SET key value [EX seconds] [PX milliseconds] [NX|XX]

EX seconds – 设置键key的过期时间，单位时秒

PX milliseconds – 设置键key的过期时间，单位时毫秒

NX – 只有键key不存在的时候才会设置key的值  

XX – 只有键key存在的时候才会设置key的值

127.0.0.1:> set myKey "Hello"
OK
127.0.0.1:> get myKey
"Hello"
127.0.0.1:> set userId ""
OK
127.0.0.1:> get userId
""
127.0.0.1:> object encoding userId
"int"
127.0.0.1:> object encoding myKey
"embstr"
127.0.0.1:> set myKey World NX
(nil)
127.0.0.1:> set myKey World XX
OK
127.0.0.1:>

NX ：应用场景分布式锁：通过myKey的赋值来判断是否获取到了一个分布式锁  如果OK说明获取到了锁 如果nil说明没有获取到了锁

如果存放到string中的value是int，那么在内部还是int ，可以从encoding

redisObject中有一个type属性和encoding属性

incr/incrby/decr/decrby命令

incr/decr 自增或者自减1

incrby/decrby:自增或者自减指定数

127.0.0.1:> INCR userId
(integer)
127.0.0.1:> INCR userId
(integer)
127.0.0.1:> get userId
""
127.0.0.1:> INCRBY userId
(integer)
127.0.0.1:> DECR userId
(integer)
127.0.0.1:> DECRBY userId
(integer) 

append命令

127.0.0.1:> APPEND myKey "!"
(integer)
127.0.0.1:> get myKey
"World!"
127.0.0.1:>

3.源码解析

redisCommand存放着所有的命令

    {"set",setCommand,-,
     "write use-memory @string",
     ,NULL,,,,,,},

 setCommand源码

setCommand传入一个client结构体 

最后调用setGenericCommand函数来处理set

/* SET key value [NX] [XX] [KEEPTTL] [EX <seconds>] [PX <milliseconds>] */
void setCommand(client *c) {
    int j;
    robj *expire = NULL;
    int unit = UNIT_SECONDS;
    int flags = OBJ_SET_NO_FLAGS;

    for (j = ; j < c->argc; j++) {
        char *a = c->argv[j]->ptr;
        robj *next = (j == c->argc-) ? NULL : c->argv[j+];

        if ((a[] == 'n' || a[] == 'N') &&
            (a[] == 'x' || a[] == 'X') && a[] == '\0' &&
            !(flags & OBJ_SET_XX))
        {
            flags |= OBJ_SET_NX;
        } else if ((a[] == 'x' || a[] == 'X') &&
                   (a[] == 'x' || a[] == 'X') && a[] == '\0' &&
                   !(flags & OBJ_SET_NX))
        {
            flags |= OBJ_SET_XX;
        } else if (!strcasecmp(c->argv[j]->ptr,"KEEPTTL") &&
                   !(flags & OBJ_SET_EX) && !(flags & OBJ_SET_PX))
        {
            flags |= OBJ_SET_KEEPTTL;
        } else if ((a[] == 'e' || a[] == 'E') &&
                   (a[] == 'x' || a[] == 'X') && a[] == '\0' &&
                   !(flags & OBJ_SET_KEEPTTL) &&
                   !(flags & OBJ_SET_PX) && next)
        {
            flags |= OBJ_SET_EX;
            unit = UNIT_SECONDS;
            expire = next;
            j++;
        } else if ((a[] == 'p' || a[] == 'P') &&
                   (a[] == 'x' || a[] == 'X') && a[] == '\0' &&
                   !(flags & OBJ_SET_KEEPTTL) &&
                   !(flags & OBJ_SET_EX) && next)
        {
            flags |= OBJ_SET_PX;
            unit = UNIT_MILLISECONDS;
            expire = next;
            j++;
        } else {
            addReply(c,shared.syntaxerr);
            return;
        }
    }

    c->argv[] = tryObjectEncoding(c->argv[]);
    setGenericCommand(c,flags,c->argv[],c->argv[],expire,unit,NULL,NULL);
}

 setGenericCommand源码

可以看出调用了genericSetKey函数

void setGenericCommand(client *c, int flags, robj *key, robj *val, robj *expire, int unit, robj *ok_reply, robj *abort_reply) {
    long long milliseconds = ; /* initialized to avoid any harmness warning */

    if (expire) {
        if (getLongLongFromObjectOrReply(c, expire, &milliseconds, NULL) != C_OK)
            return;
        if (milliseconds <= ) {
            addReplyErrorFormat(c,"invalid expire time in %s",c->cmd->name);
            return;
        }
        if (unit == UNIT_SECONDS) milliseconds *= ;
    }

    if ((flags & OBJ_SET_NX && lookupKeyWrite(c->db,key) != NULL) ||
        (flags & OBJ_SET_XX && lookupKeyWrite(c->db,key) == NULL))
    {
        addReply(c, abort_reply ? abort_reply : shared.null[c->resp]);
        return;
    }
    genericSetKey(c,c->db,key,val,flags & OBJ_SET_KEEPTTL,);
    server.dirty++;
    if (expire) setExpire(c,c->db,key,mstime()+milliseconds);
    notifyKeyspaceEvent(NOTIFY_STRING,"set",key,c->db->id);
    if (expire) notifyKeyspaceEvent(NOTIFY_GENERIC,
        "expire",key,c->db->id);
    addReply(c, ok_reply ? ok_reply : shared.ok);
}

 genericSetKey源码

void genericSetKey(client *c, redisDb *db, robj *key, robj *val, int keepttl, int signal) {
    if (lookupKeyWrite(db,key) == NULL) {
        dbAdd(db,key,val);
    } else {
        dbOverwrite(db,key,val);
    }
    incrRefCount(val);
    if (!keepttl) removeExpire(db,key);
    if (signal) signalModifiedKey(c,db,key);
}

 dbOverwrite源码

可以看出最后存入dict中

void dbOverwrite(redisDb *db, robj *key, robj *val) {
    dictEntry *de = dictFind(db->dict,key->ptr);

    serverAssertWithInfo(NULL,key,de != NULL);
    dictEntry auxentry = *de;
    robj *old = dictGetVal(de);
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        val->lru = old->lru;
    }
    dictSetVal(db->dict, de, val);

    if (server.lazyfree_lazy_server_del) {
        freeObjAsync(old);
        dictSetVal(db->dict, &auxentry, NULL);
    }

    dictFreeVal(db->dict, &auxentry);
}