作者：陈科

联系方式：chenke1818@gmail.com

转载请说明出处：http://www.dumpcache.com/wiki/doku.php?id=about_redis_6

上一章我们在分析keys相关命令的时候发现，Redis最终对数据的存储都是基于redisDb的实现，所以我们这章先来分析下redisDb的实现,同时把keys相关命令的最后两个SORT和SCAN的实现分析一下。

redisDb最初的初始化是在initServer的时候:

server.db = zmalloc(sizeof(redisDb)*server.dbnum);

dbnum默认为16：

server.dbnum = REDIS_DEFAULT_DBNUM;

#define REDIS_DEFAULT_DBNUM     16

我们再来看下redisDb的结构：

typedef struct redisDb {
    dict *dict;                 /* The keyspace for this DB */
    dict *expires;              /* Timeout of keys with a timeout set */
    dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP) */
    dict *ready_keys;           /* Blocked keys that received a PUSH */
    dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
    int id;
    long long avg_ttl;          /* Average TTL, just for stats */
} redisDb;

redis的注释写的还是蛮清楚的，我们可以发现，redisDb其实就是对dict字典的封装。

dict *dict存储了key-value的映射。

dict *expires存储了key和过期时间的映射。

至于dict的实现，这里先不展开，我们只要知道它其实就是一个类似hashmap的实现即可。

命令格式为：

SORT key [BY pattern] [LIMIT offset count] [GET pattern [GET pattern …]] [ASC | DESC] [ALPHA] [STORE destination]

具体玩法可以参考：

http://redis.io/commands/sort

我们重点来分析源码：

/* The SORT command is the most complex command in Redis. Warning: this code
 * is optimized for speed and a bit less for readability */
void sortCommand(redisClient *c) {
    list *operations;
    unsigned int outputlen = 0;
    int desc = 0, alpha = 0;
    long limit_start = 0, limit_count = -1, start, end;
    int j, dontsort = 0, vectorlen;
    int getop = 0; /* GET operation counter */
    int int_convertion_error = 0;
    robj *sortval, *sortby = NULL, *storekey = NULL;
    redisSortObject *vector; /* Resulting vector to sort */

    /* Lookup the key to sort. It must be of the right types */
    sortval = lookupKeyRead(c->db,c->argv[1]);
    if (sortval && sortval->type != REDIS_SET &&
                   sortval->type != REDIS_LIST &&
                   sortval->type != REDIS_ZSET)
    {
        addReply(c,shared.wrongtypeerr);
        return;
    }

    /* Create a list of operations to perform for every sorted element.
     * Operations can be GET/DEL/INCR/DECR */
    operations = listCreate();
    listSetFreeMethod(operations,zfree);
    j = 2; /* options start at argv[2] */

    /* Now we need to protect sortval incrementing its count, in the future
     * SORT may have options able to overwrite/delete keys during the sorting
     * and the sorted key itself may get destroyed */
    if (sortval)
        incrRefCount(sortval);
    else
        sortval = createListObject();

    /* The SORT command has an SQL-alike syntax, parse it */
    while(j < c->argc) {
        int leftargs = c->argc-j-1;
        if (!strcasecmp(c->argv[j]->ptr,"asc")) {
            desc = 0;
        } else if (!strcasecmp(c->argv[j]->ptr,"desc")) {
            desc = 1;
        } else if (!strcasecmp(c->argv[j]->ptr,"alpha")) {
            alpha = 1;
        } else if (!strcasecmp(c->argv[j]->ptr,"limit") && leftargs >= 2) {
            if ((getLongFromObjectOrReply(c, c->argv[j+1], &limit_start, NULL)
                 != REDIS_OK) ||
                (getLongFromObjectOrReply(c, c->argv[j+2], &limit_count, NULL)
                 != REDIS_OK))
            {
                decrRefCount(sortval);
                listRelease(operations);
                return;
            }
            j+=2;
        } else if (!strcasecmp(c->argv[j]->ptr,"store") && leftargs >= 1) {
            storekey = c->argv[j+1];
            j++;
        } else if (!strcasecmp(c->argv[j]->ptr,"by") && leftargs >= 1) {
            sortby = c->argv[j+1];
            /* If the BY pattern does not contain '*', i.e. it is constant,
             * we don't need to sort nor to lookup the weight keys. */
            if (strchr(c->argv[j+1]->ptr,'*') == NULL) dontsort = 1;
            j++;
        } else if (!strcasecmp(c->argv[j]->ptr,"get") && leftargs >= 1) {
            listAddNodeTail(operations,createSortOperation(
                REDIS_SORT_GET,c->argv[j+1]));
            getop++;
            j++;
        } else {
            decrRefCount(sortval);
            listRelease(operations);
            addReply(c,shared.syntaxerr);
            return;
        }
        j++;
    }

    /* When sorting a set with no sort specified, we must sort the output
     * so the result is consistent across scripting and replication.
     *
     * The other types (list, sorted set) will retain their native order
     * even if no sort order is requested, so they remain stable across
     * scripting and replication. */
    if (dontsort &&
        sortval->type == REDIS_SET &&
        (storekey || c->flags & REDIS_LUA_CLIENT))
    {
        /* Force ALPHA sorting */
        dontsort = 0;
        alpha = 1;
        sortby = NULL;
    }

    /* Destructively convert encoded sorted sets for SORT. */
    if (sortval->type == REDIS_ZSET)
        zsetConvert(sortval, REDIS_ENCODING_SKIPLIST);

    /* Objtain the length of the object to sort. */
    switch(sortval->type) {
    case REDIS_LIST: vectorlen = listTypeLength(sortval); break;
    case REDIS_SET: vectorlen =  setTypeSize(sortval); break;
    case REDIS_ZSET: vectorlen = dictSize(((zset*)sortval->ptr)->dict); break;
    default: vectorlen = 0; redisPanic("Bad SORT type"); /* Avoid GCC warning */
    }

    /* Perform LIMIT start,count sanity checking. */
    start = (limit_start < 0) ? 0 : limit_start;
    end = (limit_count < 0) ? vectorlen-1 : start+limit_count-1;
    if (start >= vectorlen) {
        start = vectorlen-1;
        end = vectorlen-2;
    }
    if (end >= vectorlen) end = vectorlen-1;

    /* Optimization:
     *
     * 1) if the object to sort is a sorted set.
     * 2) There is nothing to sort as dontsort is true (BY <constant string>).
     * 3) We have a LIMIT option that actually reduces the number of elements
     *    to fetch.
     *
     * In this case to load all the objects in the vector is a huge waste of
     * resources. We just allocate a vector that is big enough for the selected
     * range length, and make sure to load just this part in the vector. */
    if (sortval->type == REDIS_ZSET &&
        dontsort &&
        (start != 0 || end != vectorlen-1))
    {
        vectorlen = end-start+1;
    }

    /* Load the sorting vector with all the objects to sort */
    vector = zmalloc(sizeof(redisSortObject)*vectorlen);
    j = 0;

    if (sortval->type == REDIS_LIST) {
        listTypeIterator *li = listTypeInitIterator(sortval,0,REDIS_TAIL);
        listTypeEntry entry;
        while(listTypeNext(li,&entry)) {
            vector[j].obj = listTypeGet(&entry);
            vector[j].u.score = 0;
            vector[j].u.cmpobj = NULL;
            j++;
        }
        listTypeReleaseIterator(li);
    } else if (sortval->type == REDIS_SET) {
        setTypeIterator *si = setTypeInitIterator(sortval);
        robj *ele;
        while((ele = setTypeNextObject(si)) != NULL) {
            vector[j].obj = ele;
            vector[j].u.score = 0;
            vector[j].u.cmpobj = NULL;
            j++;
        }
        setTypeReleaseIterator(si);
    } else if (sortval->type == REDIS_ZSET && dontsort) {
        /* Special handling for a sorted set, if 'dontsort' is true.
         * This makes sure we return elements in the sorted set original
         * ordering, accordingly to DESC / ASC options.
         *
         * Note that in this case we also handle LIMIT here in a direct
         * way, just getting the required range, as an optimization. */

        zset *zs = sortval->ptr;
        zskiplist *zsl = zs->zsl;
        zskiplistNode *ln;
        robj *ele;
        int rangelen = vectorlen;

        /* Check if starting point is trivial, before doing log(N) lookup. */
        if (desc) {
            long zsetlen = dictSize(((zset*)sortval->ptr)->dict);

            ln = zsl->tail;
            if (start > 0)
                ln = zslGetElementByRank(zsl,zsetlen-start);
        } else {
            ln = zsl->header->level[0].forward;
            if (start > 0)
                ln = zslGetElementByRank(zsl,start+1);
        }

        while(rangelen--) {
            redisAssertWithInfo(c,sortval,ln != NULL);
            ele = ln->obj;
            vector[j].obj = ele;
            vector[j].u.score = 0;
            vector[j].u.cmpobj = NULL;
            j++;
            ln = desc ? ln->backward : ln->level[0].forward;
        }
        /* The code producing the output does not know that in the case of
         * sorted set, 'dontsort', and LIMIT, we are able to get just the
         * range, already sorted, so we need to adjust "start" and "end"
         * to make sure start is set to 0. */
        end -= start;
        start = 0;
    } else if (sortval->type == REDIS_ZSET) {
        dict *set = ((zset*)sortval->ptr)->dict;
        dictIterator *di;
        dictEntry *setele;
        di = dictGetIterator(set);
        while((setele = dictNext(di)) != NULL) {
            vector[j].obj = dictGetKey(setele);
            vector[j].u.score = 0;
            vector[j].u.cmpobj = NULL;
            j++;
        }
        dictReleaseIterator(di);
    } else {
        redisPanic("Unknown type");
    }
    redisAssertWithInfo(c,sortval,j == vectorlen);

    /* Now it's time to load the right scores in the sorting vector */
    if (dontsort == 0) {
        for (j = 0; j < vectorlen; j++) {
            robj *byval;
            if (sortby) {
                /* lookup value to sort by */
                byval = lookupKeyByPattern(c->db,sortby,vector[j].obj);
                if (!byval) continue;
            } else {
                /* use object itself to sort by */
                byval = vector[j].obj;
            }

            if (alpha) {
                if (sortby) vector[j].u.cmpobj = getDecodedObject(byval);
            } else {
                if (byval->encoding == REDIS_ENCODING_RAW) {
                    char *eptr;

                    vector[j].u.score = strtod(byval->ptr,&eptr);
                    if (eptr[0] != '\0' || errno == ERANGE ||
                        isnan(vector[j].u.score))
                    {
                        int_convertion_error = 1;
                    }
                } else if (byval->encoding == REDIS_ENCODING_INT) {
                    /* Don't need to decode the object if it's
                     * integer-encoded (the only encoding supported) so
                     * far. We can just cast it */
                    vector[j].u.score = (long)byval->ptr;
                } else {
                    redisAssertWithInfo(c,sortval,1 != 1);
                }
            }

            /* when the object was retrieved using lookupKeyByPattern,
             * its refcount needs to be decreased. */
            if (sortby) {
                decrRefCount(byval);
            }
        }
    }

    if (dontsort == 0) {
        server.sort_desc = desc;
        server.sort_alpha = alpha;
        server.sort_bypattern = sortby ? 1 : 0;
        server.sort_store = storekey ? 1 : 0;
        if (sortby && (start != 0 || end != vectorlen-1))
            pqsort(vector,vectorlen,sizeof(redisSortObject),sortCompare, start,end);
        else
            qsort(vector,vectorlen,sizeof(redisSortObject),sortCompare);
    }

    /* Send command output to the output buffer, performing the specified
     * GET/DEL/INCR/DECR operations if any. */
    outputlen = getop ? getop*(end-start+1) : end-start+1;
    if (int_convertion_error) {
        addReplyError(c,"One or more scores can't be converted into double");
    } else if (storekey == NULL) {
        /* STORE option not specified, sent the sorting result to client */
        addReplyMultiBulkLen(c,outputlen);
        for (j = start; j <= end; j++) {
            listNode *ln;
            listIter li;

            if (!getop) addReplyBulk(c,vector[j].obj);
            listRewind(operations,&li);
            while((ln = listNext(&li))) {
                redisSortOperation *sop = ln->value;
                robj *val = lookupKeyByPattern(c->db,sop->pattern,
                    vector[j].obj);

                if (sop->type == REDIS_SORT_GET) {
                    if (!val) {
                        addReply(c,shared.nullbulk);
                    } else {
                        addReplyBulk(c,val);
                        decrRefCount(val);
                    }
                } else {
                    /* Always fails */
                    redisAssertWithInfo(c,sortval,sop->type == REDIS_SORT_GET);
                }
            }
        }
    } else {
        robj *sobj = createZiplistObject();

        /* STORE option specified, set the sorting result as a List object */
        for (j = start; j <= end; j++) {
            listNode *ln;
            listIter li;

            if (!getop) {
                listTypePush(sobj,vector[j].obj,REDIS_TAIL);
            } else {
                listRewind(operations,&li);
                while((ln = listNext(&li))) {
                    redisSortOperation *sop = ln->value;
                    robj *val = lookupKeyByPattern(c->db,sop->pattern,
                        vector[j].obj);

                    if (sop->type == REDIS_SORT_GET) {
                        if (!val) val = createStringObject("",0);

                        /* listTypePush does an incrRefCount, so we should take care
                         * care of the incremented refcount caused by either
                         * lookupKeyByPattern or createStringObject("",0) */
                        listTypePush(sobj,val,REDIS_TAIL);
                        decrRefCount(val);
                    } else {
                        /* Always fails */
                        redisAssertWithInfo(c,sortval,sop->type == REDIS_SORT_GET);
                    }
                }
            }
        }
        if (outputlen) {
            setKey(c->db,storekey,sobj);
            notifyKeyspaceEvent(REDIS_NOTIFY_LIST,"sortstore",storekey,
                                c->db->id);
            server.dirty += outputlen;
        } else if (dbDelete(c->db,storekey)) {
            signalModifiedKey(c->db,storekey);
            notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,"del",storekey,c->db->id);
            server.dirty++;
        }
        decrRefCount(sobj);
        addReplyLongLong(c,outputlen);
    }

    /* Cleanup */
    if (sortval->type == REDIS_LIST || sortval->type == REDIS_SET)
        for (j = 0; j < vectorlen; j++)
            decrRefCount(vector[j].obj);
    decrRefCount(sortval);
    listRelease(operations);
    for (j = 0; j < vectorlen; j++) {
        if (alpha && vector[j].u.cmpobj)
            decrRefCount(vector[j].u.cmpobj);
    }
    zfree(vector);
}

代码很长，我们细细来品味：

前面的注释作者也提到了，代码为了性能优化，牺牲了一点可读性。

1.先查找key对应的value.

2.假如value类型不是set/list/zset这种可排序的列表类型的，则返回客户端wrongtypeerr

3.开始解析key后面的参数列表，类似sql语句的asc/desc/limit/by这样的语法。

4.如果by语法后跟的参数不带'*',并且value类型为REDIS_SET，则根据字符串进行排序(ALPHA)。

5.获取需要排序对象集合的长度。

6.接下来的过程针对以下几种情况做了优化：

a)如果要排序的集合已经排序过了。

b)如果使用了by的集合dontsort为true

c)使用了LIMIT选项降低了需要排序的元素数量。

以上3种场景只会分配足够大小的内存，减少内存的浪费。

7.根据vectorlen初始化vector

8.把需要排序的数据转载到vector中

9.初始化vector[j].u.score，将by后面跟的key的值转换成浮点数。

10.调用pqsort或者qsort进行排序。(这里的pqsort使用了bsd的qsort实现，可以对指定的子集进行排序)

11.把要输出的内容写到buf中。

12.如果有STORE选项，把排序完的结果设置成list object.

13.清理vector.

格式为：

SCAN cursor [MATCH pattern] [COUNT count]

SCAN 命令及其相关的 SSCAN 命令、 HSCAN 命令和 ZSCAN 命令都用于增量地迭代（incrementally iterate）一集元素（a collection of elements）：

SCAN 命令用于迭代当前数据库中的数据库键。

SSCAN 命令用于迭代集合键中的元素。

HSCAN 命令用于迭代哈希键中的键值对。

ZSCAN 命令用于迭代有序集合中的元素（包括元素成员和元素分值）。

我们看最终的代码：

/* This command implements SCAN, HSCAN and SSCAN commands.
 * If object 'o' is passed, then it must be a Hash or Set object, otherwise
 * if 'o' is NULL the command will operate on the dictionary associated with
 * the current database.
 *
 * When 'o' is not NULL the function assumes that the first argument in
 * the client arguments vector is a key so it skips it before iterating
 * in order to parse options.
 *
 * In the case of a Hash object the function returns both the field and value
 * of every element on the Hash. */
void scanGenericCommand(redisClient *c, robj *o, unsigned long cursor) {
    int i, j;
    list *keys = listCreate();
    listNode *node, *nextnode;
    long count = 10;
    sds pat;
    int patlen, use_pattern = 0;
    dict *ht;

    /* Object must be NULL (to iterate keys names), or the type of the object
     * must be Set, Sorted Set, or Hash. */
    redisAssert(o == NULL || o->type == REDIS_SET || o->type == REDIS_HASH ||
                o->type == REDIS_ZSET);

    /* Set i to the first option argument. The previous one is the cursor. */
    i = (o == NULL) ? 2 : 3; /* Skip the key argument if needed. */

    /* Step 1: Parse options. */
    while (i < c->argc) {
        j = c->argc - i;
        if (!strcasecmp(c->argv[i]->ptr, "count") && j >= 2) {
            if (getLongFromObjectOrReply(c, c->argv[i+1], &count, NULL)
                != REDIS_OK)
            {
                goto cleanup;
            }

            if (count < 1) {
                addReply(c,shared.syntaxerr);
                goto cleanup;
            }

            i += 2;
        } else if (!strcasecmp(c->argv[i]->ptr, "match") && j >= 2) {
            pat = c->argv[i+1]->ptr;
            patlen = sdslen(pat);

            /* The pattern always matches if it is exactly "*", so it is
             * equivalent to disabling it. */
            use_pattern = !(pat[0] == '*' && patlen == 1);

            i += 2;
        } else {
            addReply(c,shared.syntaxerr);
            goto cleanup;
        }
    }

    /* Step 2: Iterate the collection.
     *
     * Note that if the object is encoded with a ziplist, intset, or any other
     * representation that is not a hash table, we are sure that it is also
     * composed of a small number of elements. So to avoid taking state we
     * just return everything inside the object in a single call, setting the
     * cursor to zero to signal the end of the iteration. */

    /* Handle the case of a hash table. */
    ht = NULL;
    if (o == NULL) {
        ht = c->db->dict;
    } else if (o->type == REDIS_SET && o->encoding == REDIS_ENCODING_HT) {
        ht = o->ptr;
    } else if (o->type == REDIS_HASH && o->encoding == REDIS_ENCODING_HT) {
        ht = o->ptr;
        count *= 2; /* We return key / value for this type. */
    } else if (o->type == REDIS_ZSET && o->encoding == REDIS_ENCODING_SKIPLIST) {
        zset *zs = o->ptr;
        ht = zs->dict;
        count *= 2; /* We return key / value for this type. */
    }

    if (ht) {
        void *privdata[2];
        /* We set the max number of iterations to ten times the specified
         * COUNT, so if the hash table is in a pathological state (very
         * sparsely populated) we avoid to block too much time at the cost
         * of returning no or very few elements. */
        long maxiterations = count*10;

        /* We pass two pointers to the callback: the list to which it will
         * add new elements, and the object containing the dictionary so that
         * it is possible to fetch more data in a type-dependent way. */
        privdata[0] = keys;
        privdata[1] = o;
        do {
            cursor = dictScan(ht, cursor, scanCallback, privdata);
        } while (cursor &&
              maxiterations-- &&
              listLength(keys) < (unsigned long)count);
    } else if (o->type == REDIS_SET) {
        int pos = 0;
        int64_t ll;

        while(intsetGet(o->ptr,pos++,&ll))
            listAddNodeTail(keys,createStringObjectFromLongLong(ll));
        cursor = 0;
    } else if (o->type == REDIS_HASH || o->type == REDIS_ZSET) {
        unsigned char *p = ziplistIndex(o->ptr,0);
        unsigned char *vstr;
        unsigned int vlen;
        long long vll;

        while(p) {
            ziplistGet(p,&vstr,&vlen,&vll);
            listAddNodeTail(keys,
                (vstr != NULL) ? createStringObject((char*)vstr,vlen) :
                                 createStringObjectFromLongLong(vll));
            p = ziplistNext(o->ptr,p);
        }
        cursor = 0;
    } else {
        redisPanic("Not handled encoding in SCAN.");
    }

    /* Step 3: Filter elements. */
    node = listFirst(keys);
    while (node) {
        robj *kobj = listNodeValue(node);
        nextnode = listNextNode(node);
        int filter = 0;

        /* Filter element if it does not match the pattern. */
        if (!filter && use_pattern) {
            if (kobj->encoding == REDIS_ENCODING_INT) {
                char buf[REDIS_LONGSTR_SIZE];
                int len;

                redisAssert(kobj->encoding == REDIS_ENCODING_INT);
                len = ll2string(buf,sizeof(buf),(long)kobj->ptr);
                if (!stringmatchlen(pat, patlen, buf, len, 0)) filter = 1;
            } else {
                if (!stringmatchlen(pat, patlen, kobj->ptr, sdslen(kobj->ptr), 0))
                    filter = 1;
            }
        }

        /* Filter element if it is an expired key. */
        if (!filter && o == NULL && expireIfNeeded(c->db, kobj)) filter = 1;

        /* Remove the element and its associted value if needed. */
        if (filter) {
            decrRefCount(kobj);
            listDelNode(keys, node);
        }

        /* If this is a hash or a sorted set, we have a flat list of
         * key-value elements, so if this element was filtered, remove the
         * value, or skip it if it was not filtered: we only match keys. */
        if (o && (o->type == REDIS_ZSET || o->type == REDIS_HASH)) {
            node = nextnode;
            nextnode = listNextNode(node);
            if (filter) {
                kobj = listNodeValue(node);
                decrRefCount(kobj);
                listDelNode(keys, node);
            }
        }
        node = nextnode;
    }

    /* Step 4: Reply to the client. */
    addReplyMultiBulkLen(c, 2);
    addReplyBulkLongLong(c,cursor);

    addReplyMultiBulkLen(c, listLength(keys));
    while ((node = listFirst(keys)) != NULL) {
        robj *kobj = listNodeValue(node);
        addReplyBulk(c, kobj);
        decrRefCount(kobj);
        listDelNode(keys, node);
    }

cleanup:
    listSetFreeMethod(keys,decrRefCountVoid);
    listRelease(keys);
}

1.解析参数，假如match后面跟的为'*'，那么use_pattern被禁用，相当于所有的元素都符合要求。

2.开始遍历元素

3.过滤元素

4.返回结果给客户端