Redis：set/get 命令解析

Posted 2021-03-14 等你归去来

　　经过前两篇的介绍，我们对整个redis的动作流程已经有比较清晰的认识。

　　接下来就是到具体的命令处理方式的理解了，想来我们用这些工具的意义也是在此。虽然没有人觉得，一个set/get方法会有难度，但是我们毕竟不是很清楚，否则也不至于在谈到深处就懵逼了。

　　我觉得本文的一个重要意义就是: 让set/get还原成它本来样子，和写"hello world"一样简单。

框架性质的东西，我们前面已经讲解，就直接进入主题： set/get 的操作。

　　set/get 对应的两个处理函数 (redisCommand) 定义是这样的: 

    // rF 代表 getCommand 是只读命令，又快又准，时间复杂度 O(1)或者O(log(n))
    // wm 代表 setCommand 是个写命令,当心空间问题
    {"get",getCommand,2,"rF",0,NULL,1,1,1,0,0}
    {"set",setCommand,-3,"wm",0,NULL,1,1,1,0,0}

　　所以，我们只要理解了， setCommand,getCommand 之后，就可以完全自信的说，set/get 就是和 "hello world" 一样简单了。

 

零、hash 算法 

---

　　很显然，kv型的存储一定是hash相关算法的实现。那么redis中如何使用这个hash算法的呢？

　　redis 中许多不同场景的hash算法，其原型是在 dictType 中定义的。

typedef struct dictType {
    // hash 算法原型
    unsigned int (*hashFunction)(const void *key);
    void *(*keyDup)(void *privdata, const void *key);
    void *(*valDup)(void *privdata, const void *obj);
    int (*keyCompare)(void *privdata, const void *key1, const void *key2);
    void (*keyDestructor)(void *privdata, void *key);
    void (*valDestructor)(void *privdata, void *obj);
} dictType;

　　针对大部分场景，我们的key一般都是 string 类型的，但是还是会稍微有不一样的。这里我们就两个场景来说明下：

1. 命令集构建的hash算法

　　即是 server.commands 中的key的hash算法，这里元素是有限的。其定义如下:

/* Command table. sds string -> command struct pointer. */
dictType commandTableDictType = {
    // 即 dictSdsCaseHash 是 command 的hash算法实现
    dictSdsCaseHash,            /* hash function */
    NULL,                       /* key dup */
    NULL,                       /* val dup */
    dictSdsKeyCaseCompare,      /* key compare */
    dictSdsDestructor,          /* key destructor */
    NULL                        /* val destructor */
};
// server.c, 不区分大小写的key hash
unsigned int dictSdsCaseHash(const void *key) {
    return dictGenCaseHashFunction((unsigned char*)key, sdslen((char*)key));
}
// dict.c, 不区分大小写的key-hash算法: hash * 33 + c
/* And a case insensitive hash function (based on djb hash) */
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
    // static uint32_t dict_hash_function_seed = 5381;
    unsigned int hash = (unsigned int)dict_hash_function_seed;

    while (len--)
        hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */
    return hash;
}

　　

2. 针对普通kv查询的hash算法

　　整个nosql就是kv的增删改查，所以这是个重要的算法。

/* Db->dict, keys are sds strings, vals are Redis objects. */
dictType dbDictType = {
    // k 的hash算法实现
    dictSdsHash,                /* hash function */
    NULL,                       /* key dup */
    NULL,                       /* val dup */
    dictSdsKeyCompare,          /* key compare */
    dictSdsDestructor,          /* key destructor */
    dictObjectDestructor   /* val destructor */
};
// server.c, 调用 dict 的实现
unsigned int dictSdsHash(const void *key) {
    return dictGenHashFunction((unsigned char*)key, sdslen((char*)key));
}
// dict.c, 数据key的hash算法，或者通用的 string 的hashCode 算法
/* MurmurHash2, by Austin Appleby
 * Note - This code makes a few assumptions about how your machine behaves -
 * 1. We can read a 4-byte value from any address without crashing
 * 2. sizeof(int) == 4
 *
 * And it has a few limitations -
 *
 * 1. It will not work incrementally.
 * 2. It will not produce the same results on little-endian and big-endian
 *    machines.
 */
unsigned int dictGenHashFunction(const void *key, int len) {
    /* \'m\' and \'r\' are mixing constants generated offline.
     They\'re not really \'magic\', they just happen to work well.  */
    uint32_t seed = dict_hash_function_seed;
    const uint32_t m = 0x5bd1e995;
    const int r = 24;

    /* Initialize the hash to a \'random\' value */
    uint32_t h = seed ^ len;

    /* Mix 4 bytes at a time into the hash */
    const unsigned char *data = (const unsigned char *)key;
    // 核心算法: step1. *m, ^k>>r, *m, *m, ^k, 每4位做一次运算
    while(len >= 4) {
        uint32_t k = *(uint32_t*)data;

        k *= m;
        k ^= k >> r;
        k *= m;

        h *= m;
        h ^= k;

        data += 4;
        len -= 4;
    }

    /* Handle the last few bytes of the input array  */
    // step2. 倒数第三位 ^<<16, 第二位 ^<<8, 第一位 ^, 然后 *m
    switch(len) {
        case 3: h ^= data[2] << 16;
        case 2: h ^= data[1] << 8;
        case 1: h ^= data[0]; h *= m;
    };

    /* Do a few final mixes of the hash to ensure the last few
     * bytes are well-incorporated. */
    // step3. 再混合 ^>>13, *m, ^>>15
    h ^= h >> 13;
    h *= m;
    h ^= h >> 15;

    return (unsigned int)h;
}

　　可以看到，针对普通的字符串的hash可是要复杂许多呢，因为这里数据远比 command 的数据多，情况更复杂，这样的算法唯一的目标就是尽量避免hash冲突。(虽然不知道为啥这么干，但它就是牛逼)

　　redis中还有其他的hash算法，比如dictObjHash,dictEncObjHash, 后续有接触我们再聊。

 

接下来，我们正式来看看 set/get 到底如何？

 

一、getCommand 解析

---

　　很显然，get 会是个最简单的命令，自然要检软柿子捏了。

// t_string.c
void getCommand(client *c) {
    getGenericCommand(c);
}
int getGenericCommand(client *c) {
    robj *o;
    // 如果在kv里找不到，则直接响应空，shared.nullbulk 作为全局常量的优势就体现出来了
    // shared.nullbulk = createObject(OBJ_STRING,sdsnew("$-1\\r\\n"));
    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.nullbulk)) == NULL)
        return C_OK;
    // 找到对应的数据，但是类型不匹配，说明不能使用 get 命令，响应错误信息
    // shared.wrongtypeerr = "-WRONGTYPE Operation against a key holding the wrong kind of value\\r\\n"
    if (o->type != OBJ_STRING) {
        addReply(c,shared.wrongtypeerr);
        return C_ERR;
    } else {
        // 正常情况则直接响应结果即可
        addReplyBulk(c,o);
        return C_OK;
    }
}

　　整个处理流程果然是异常简单，感觉人生已经达到了巅峰！但是，我们还没有看到关键，那就是查找 key 的过程。我们通过之前的介绍，知道有个叫做 redisDb 的东西，看起来它是负责所有的数据管理。它应该不会因为简单而不存储某些数据吧。

// db.c, 查找某个key对应的元素或者直接响应客户端
robj *lookupKeyReadOrReply(client *c, robj *key, robj *reply) {
    // 使用 c->db 对应的数据库进行查询，所以要求客户端必须针对某db进行操作，且不能跨库操作是原理决定
    robj *o = lookupKeyRead(c->db, key);
    // 如果没有查到数据就直接使用默认的 reply, 响应客户端了
    if (!o) addReply(c,reply);
    return o;
}
// db.c, 读取key 对应值
robj *lookupKeyRead(redisDb *db, robj *key) {
    robj *val;
    // 检查过期情况，如果过期，则不用再查了
    if (expireIfNeeded(db,key) == 1) {
        /* Key expired. If we are in the context of a master, expireIfNeeded()
         * returns 0 only when the key does not exist at all, so it\'s save
         * to return NULL ASAP. */
        if (server.masterhost == NULL) return NULL;

        /* However if we are in the context of a slave, expireIfNeeded() will
         * not really try to expire the key, it only returns information
         * about the "logical" status of the key: key expiring is up to the
         * master in order to have a consistent view of master\'s data set.
         *
         * However, if the command caller is not the master, and as additional
         * safety measure, the command invoked is a read-only command, we can
         * safely return NULL here, and provide a more consistent behavior
         * to clients accessign expired values in a read-only fashion, that
         * will say the key as non exisitng.
         *
         * Notably this covers GETs when slaves are used to scale reads. */
        if (server.current_client &&
            server.current_client != server.master &&
            server.current_client->cmd &&
            server.current_client->cmd->flags & CMD_READONLY)
        {
            return NULL;
        }
    }
    // 然后从db中查找对应的key值，其实就是一个 hash 查找
    // 缓存命中统计
    val = lookupKey(db,key);
    if (val == NULL)
        server.stat_keyspace_misses++;
    else
        server.stat_keyspace_hits++;
    return val;
}
// db.c, 过期的处理有点复杂，我们稍后再看，先看 db 的查找key过程
robj *lookupKey(redisDb *db, robj *key) {
    // 直接在 db->dict 中进行hash查找即可，前面已经介绍完成，关键优化点在增量rehash
    dictEntry *de = dictFind(db->dict,key->ptr);
    if (de) {
        robj *val = dictGetVal(de);

        /* Update the access time for the ageing algorithm.
         * Don\'t do it if we have a saving child, as this will trigger
         * a copy on write madness. */
        if (server.rdb_child_pid == -1 && server.aof_child_pid == -1)
            val->lru = LRU_CLOCK();
        return val;
    } else {
        return NULL;
    }
}

// db.c, 接下来看下，检查过期情况
int expireIfNeeded(redisDb *db, robj *key) {
    mstime_t when = getExpire(db,key);
    mstime_t now;

    if (when < 0) return 0; /* No expire for this key */

    /* Don\'t expire anything while loading. It will be done later. */
    if (server.loading) return 0;

    /* If we are in the context of a Lua script, we claim that time is
     * blocked to when the Lua script started. This way a key can expire
     * only the first time it is accessed and not in the middle of the
     * script execution, making propagation to slaves / AOF consistent.
     * See issue #1525 on Github for more information. */
    now = server.lua_caller ? server.lua_time_start : mstime();

    /* If we are running in the context of a slave, return ASAP:
     * the slave key expiration is controlled by the master that will
     * send us synthesized DEL operations for expired keys.
     *
     * Still we try to return the right information to the caller,
     * that is, 0 if we think the key should be still valid, 1 if
     * we think the key is expired at this time. */
    if (server.masterhost != NULL) return now > when;

    /* Return when this key has not expired */
    // 如果还没到期就直接返回
    if (now <= when) return 0;

    /* Delete the key */
    server.stat_expiredkeys++;
    // key过期，是一个写动作，需要传播到 AOF 或者 slaves...
    propagateExpire(db,key,server.lazyfree_lazy_expire);
    // pub/sub 监控通知
    notifyKeyspaceEvent(NOTIFY_EXPIRED,
        "expired",key,db->id);
    // 同步删除或者异步删除, 稍后讨论
    return server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :
                                         dbSyncDelete(db,key);
}

/* Return the expire time of the specified key, or -1 if no expire
 * is associated with this key (i.e. the key is non volatile) */
long long getExpire(redisDb *db, robj *key) {
    dictEntry *de;
    /* No expire? return ASAP */
    // 查找过期队列, 数据量小
    if (dictSize(db->expires) == 0 ||
       (de = dictFind(db->expires,key->ptr)) == NULL) return -1;

    /* The entry was found in the expire dict, this means it should also
     * be present in the main dict (safety check). */
    serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
    // 返回到期时间戳, union 的应用
    return dictGetSignedIntegerVal(de);
}

// 删除过期数据key的两种方式，同步+异步
// db.c, 同步删除, 删除 expires 队列和 dict 数据
/* Delete a key, value, and associated expiration entry if any, from the DB */
int dbSyncDelete(redisDb *db, robj *key) {
    /* Deleting an entry from the expires dict will not free the sds of
     * the key, because it is shared with the main dictionary. */
    if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);
    if (dictDelete(db->dict,key->ptr) == DICT_OK) {
        if (server.cluster_enabled) slotToKeyDel(key);
        return 1;
    } else {
        return 0;
    }
}
// lazyfree.c, 异步删除过期数据, 一看就很复杂
/* Delete a key, value, and associated expiration entry if any, from the DB.
 * If there are enough allocations to free the value object may be put into
 * a lazy free list instead of being freed synchronously. The lazy free list
 * will be reclaimed in a different bio.c thread. */
#define LAZYFREE_THRESHOLD 64
int dbAsyncDelete(redisDb *db, robj *key) {
    /* Deleting an entry from the expires dict will not free the sds of
     * the key, because it is shared with the main dictionary. */
    if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);

    /* If the value is composed of a few allocations, to free in a lazy way
     * is actually just slower... So under a certain limit we just free
     * the object synchronously. */
    dictEntry *de = dictFind(db->dict,key->ptr);
    if (de) {
        robj *val = dictGetVal(de);
        // 判断删除的数据的影响范围，与 数据类型有关，string为1，hash/set则计算count，list计算length
        size_t free_effort = lazyfreeGetFreeEffort(val);

        /* If releasing the object is too much work, let\'s put it into the
         * lazy free list. */
        if (free_effort > LAZYFREE_THRESHOLD) {
            // 将相关的数据放入队列中，后台任务慢慢删除
            atomicIncr(lazyfree_objects,1,&lazyfree_objects_mutex);
            bioCreateBackgroundJob(BIO_LAZY_FREE,val,NULL,NULL);
            // 自身则立即设置为 NULL
            dictSetVal(db->dict,de,NULL);
        }
    }

    /* Release the key-val pair, or just the key if we set the val
     * field to NULL in order to lazy free it later. */
    if (dictDelete(db->dict,key->ptr) == DICT_OK) {
        if (server.cluster_enabled) slotToKeyDel(key);
        return 1;
    } else {
        return 0;
    }
}

　　怎么样？是不是有一首歌叫凉凉~

　　可以说，get操作本身是相当简单的，在无hash冲突前提下，O(1)的复杂度搞定。然而它还要处理过期的数据问题，就不那么简单了。

　　我们用一个时序图整体体会下get的流程:

 

 

二、setCommand 解析

---

　　setCommand 是个写操作，就不是 get 那么简单了。

// t_string.c, set 的所有用法都统一 setCommand, 多个参数共同解析为 flags
/* SET key value [NX] [XX] [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 = 3; j < c->argc; j++) {
        char *a = c->argv[j]->ptr;
        robj *next = (j == c->argc-1) ? NULL : c->argv[j+1];
        // NX 与 XX 互斥
        if ((a[0] == \'n\' || a[0] == \'N\') &&
            (a[1] == \'x\' || a[1] == \'X\') && a[2] == \'\\0\' &&
            !(flags & OBJ_SET_XX))
        {
            flags |= OBJ_SET_NX;
        } else if ((a[0] == \'x\' || a[0] == \'X\') &&
                   (a[1] == \'x\' || a[1] == \'X\') && a[2] == \'\\0\' &&
                   !(flags & OBJ_SET_NX))
        {
            flags |= OBJ_SET_XX;
        } 
        // PX 与 EX 互斥
        else if ((a[0] == \'e\' || a[0] == \'E\') &&
                   (a[1] == \'x\' || a[1] == \'X\') && a[2] == \'\\0\' &&
                   !(flags & OBJ_SET_PX) && next)
        {
            flags |= OBJ_SET_EX;
            unit = UNIT_SECONDS;
            expire = next;
            j++;
        } else if ((a[0] == \'p\' || a[0] == \'P\') &&
                   (a[1] == \'x\' || a[1] == \'X\') && a[2] == \'\\0\' &&
                   !(flags & OBJ_SET_EX) && next)
        {
            flags |= OBJ_SET_PX;
            unit = UNIT_MILLISECONDS;
            expire = next;
            j++;
        } else {
            addReply(c,shared.syntaxerr);
            return;
        }
    }
    // 尝试压缩 value 值以节省空间 (原始命令: set key value)
    c->argv[2] = tryObjectEncoding(c->argv[2]);
    setGenericCommand(c,flags,c->argv[1],c->argv[2],expire,unit,NULL,NULL);
}
// object.c, 压缩字符串
/* Try to encode a string object in order to save space */
robj *tryObjectEncoding(robj *o) {
    long value;
    sds s = o->ptr;
    size_t len;

    /* Make sure this is a string object, the only type we encode
     * in this function. Other types use encoded memory efficient
     * representations but are handled by the commands implementing
     * the type. */
    serverAssertWithInfo(NULL,o,o->type == OBJ_STRING);

    /* We try some specialized encoding only for objects that are
     * RAW or EMBSTR encoded, in other words objects that are still
     * in represented by an actually array of chars. */
    if (!sdsEncodedObject(o)) return o;

    /* It\'s not safe to encode shared objects: shared objects can be shared
     * everywhere in the "object space" of Redis and may end in places where
     * they are not handled. We handle them only as values in the keyspace. */
     if (o->refcount > 1) return o;

    /* Check if we can represent this string as a long integer.
     * Note that we are sure that a string larger than 21 chars is not
     * representable as a 32 nor 64 bit integer. */
    len = sdslen(s);
    // 针对小于21个字符串的字符，尝试转为 long 型
    if (len <= 21 && string2l(s,len,&value)) {
        /* This object is encodable as a long. Try to use a shared object.
         * Note that we avoid using shared integers when maxmemory is used
         * because every object needs to have a private LRU field for the LRU
         * algorithm to work well. */
        if ((server.maxmemory == 0 ||
             (server.maxmemory_policy != MAXMEMORY_VOLATILE_LRU &&
              server.maxmemory_policy != MAXMEMORY_ALLKEYS_LRU)) &&
            value >= 0 &&
            value < OBJ_SHARED_INTEGERS)
        {
            decrRefCount(o);
            incrRefCount(shared.integers[value]);
            return shared.integers[value];
        } else {
            if (o->encoding == OBJ_ENCODING_RAW) sdsfree(o->ptr);
            o->encoding = OBJ_ENCODING_INT;
            o->ptr = (void*) value;
            return o;
        }
    }

    /* If the string is small and is still RAW encoded,
     * try the EMBSTR encoding which is more efficient.
     * In this representation the object and the SDS string are allocated
     * in the same chunk of memory to save space and cache misses. */
    // 44
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT) {
        robj *emb;

        if (o->encoding == OBJ_ENCODING_EMBSTR) return o;
        // 使用 EMBSTR 编码转换，实际就是同一个 s 返回
        emb = createEmbeddedStringObject(s,sdslen(s));
        decrRefCount(o);
        return emb;
    }

    /* We can\'t encode the object...
     *
     * Do the last try, and at least optimize the SDS string inside
     * the string object to require little space, in case there
     * is more than 10% of free space at the end of the SDS string.
     *
     * We do that only for relatively large strings as this branch
     * is only entered if the length of the string is greater than
     * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
    if (o->encoding == OBJ_ENCODING_RAW &&
        sdsavail(s) > len/10)
    {
        o->ptr = sdsRemoveFreeSpace(o->ptr);
    }

    /* Return the original object. */
    return o;
}
// sds.c, 去除无用空间占用
/* Reallocate the sds string so that it has no free space at the end. The
 * contained string remains not altered, but next concatenation operations
 * will require a reallocation.
 *
 * After the call, the passed sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
sds sdsRemoveFreeSpace(sds s) {
    void *sh, *newsh;
    // s[-1] 指针不越界, 它是在新建一个 sds 对象时，在该指针前一位写入的值，确定sds类型
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;
    size_t len = sdslen(s);
    // sdsHdrSize: sds头部大小
    sh = (char*)s-sdsHdrSize(oldtype);

    type = sdsReqType(len);
    hdrlen = sdsHdrSize(type);
    if (oldtype==type) {
        newsh = s_realloc(sh, hdrlen+len+1);
        if (newsh == NULL) return NULL;
        s = (char*)newsh+hdrlen;
    } else {
        newsh = s_malloc(hdrlen+len+1);
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1);
        s_free(sh);
        s = (char*)newsh+hdrlen;
        s[-1] = type;
        sdssetlen(s, len);
    }
    sdssetalloc(s, len);
    return s;
}

// t_string.c, expire 为超时时间设置
void setGenericCommand(client *c, int flags, robj *key, robj *val, robj *expire, int unit, robj *ok_reply, robj *abort_reply) {
    long long milliseconds = 0; /* initialized to avoid any harmness warning */

    if (expire) {
        // 解析 expire 到 milliseconds 中
        if (getLongLongFromObjectOrReply(c, expire, &milliseconds, NULL) != C_OK)
            return;
        if (milliseconds <= 0) {
            addReplyErrorFormat(c,"invalid expire time in %s",c->cmd->name);
            return;
        }
        if (unit == UNIT_SECONDS) milliseconds *= 1000;
    }
    // 语法限制检测, NX 要求不存在, XX 要求存在
    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.nullbulk);
        return;
    }
    // 切实存储 kv
    setKey(c->db,key,val);
    server.dirty++;
    // 设置超时
    if (expire) setExpire(c->db,key,mstime()+milliseconds);
    // 通知pub/sub变更
    notifyKeyspaceEvent(NOTIFY_STRING,"set",key,c->db->id);
    // 通知expire事件
    if (expire) notifyKeyspaceEvent(NOTIFY_GENERIC,
        "expire",key,c->db->id);
    addReply(c, ok_reply ? ok_reply : shared.ok);
}
// object.c, 
int getLongLongFromObjectOrReply(client *c, robj *o, long long *target, const char *msg) {
    long long value;
    if (getLongLongFromObject(o, &value) != C_OK) {
        if (msg != NULL) {
            addReplyError(c,(char*)msg);
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