Redis 是一个开源（BSD许可）的，内存中的数据结构存储系统，它可以用作数据库、缓存和消息中间件。那么redis的底层是如何来存储数据的呢？

一、redis如何在存储大量的key时候，查询速度还能接近O(1)呢？

查询速度接近O(1)的数据结构通常让我们想到的就是HashMap结构，那下面我从源码来追踪下redis到底是不是使用的HashMap结构呢？生成的全局hashTable的大小为4
redis的数据最外层的结构是redisDb（server.h文件） ，其定义如下：

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;                     /* Database ID */
    long long avg_ttl;          /* Average TTL, just for stats */
    unsigned long expires_cursor; /* Cursor of the active expire cycle. */
    list *defrag_later;         /* List of key names to attempt to defrag one by one, gradually. */
} redisDb;

从上面定义我们可以看出redisDb 的保存数据的结构是dict(dict.h)，那么我们从文件中获取

typedef struct dict {
    dictType *type;
    void *privdata;
    dictht ht[2];
    long rehashidx; /* rehashing not in progress if rehashidx == -1 */
    int16_t pauserehash; /* If >0 rehashing is paused (<0 indicates coding error) */
} dict;
/* This is our hash table structure. Every dictionary has two of this as we
 * implement incremental rehashing, for the old to the new table. */
typedef struct dictht {
    dictEntry **table;
    unsigned long size;
    unsigned long sizemask;
    unsigned long used;
} dictht;

dict 包含了两个hash表（dictht ht[2])，这里使用两个hash表就是为了后续给渐进式rehash来进行服务的.属性rehashidx == -1时候代表不是处于reshaing中。
dictht 就一个hashtable，其包含dictEntry 的数组。然后我们继续看下

   typedef struct dictEntry {
    void *key;
    union {
        void *val;
        uint64_t u64;
        int64_t s64;
        double d;
    } v;
    struct dictEntry *next;
} dictEntry;

dictEntry 的就是hash表中的一个键值对，那么根据上面的代码我们可以绘出redis中内存结构图。

redis的rehash过程怎么处理呢？

随着redis中key的数据量增多，随着key的增多，那么dictEntry 连越来越长，这个时候查询出来的性能将会越来越慢。这个时候就需要对hashTable进行扩容，在数据量大的时候如果等到所有的扩容完成，那么必然会导致redis长时间等待，那么这个时候我们就采用渐进式rehash方式来进行扩容。

什么是渐进式rehash呢？

Redis 默认使用了两个全局哈希表：dictht[0]和哈希表 dictht[1],一开始，当你刚插入数据时，默认使用dictht[0]，此时的dictht[1] 并没有被分配空间。随着数据逐步增多，Redis 开始执行 rehash，这个过程分为三步：

1、给dictht[1]分配更大的空间，一般是当前dictht[0]已使用大小的2倍，但是必须满足是2的幂次倍!
2、把哈希表0 中的数据重新映射并拷贝到哈希表1 中（在hash表1下进行重新计算hash值）；
3、释放哈希表 0 的空间
4、把dictht[0]指向刚刚创建好的dictht[1]

什么时候进行hash

• 1、在没有fork子进程进行RDS或者AOF数据备份的时候且ht[0] .used >= ht[0].size时
• 2、 在有fork子进程进行RDS或者AOF数据备份的时候且ht[0] .used > ht[0].size * 5时
  扩容，肯定是在添加数据的时候才会扩容，所以我们找一个添加数据的入口，我们从源码层面进行下验证：

int dictReplace(dict *d, void *key, void *val)
{
    dictEntry *entry, *existing, auxentry;

    /* Try to add the element. If the key
     * does not exists dictAdd will succeed. */
    entry = dictAddRaw(d,key,&existing);
    if (entry) {
        dictSetVal(d, entry, val);
        return 1;
    }

    /* Set the new value and free the old one. Note that it is important
     * to do that in this order, as the value may just be exactly the same
     * as the previous one. In this context, think to reference counting,
     * you want to increment (set), and then decrement (free), and not the
     * reverse. */
    auxentry = *existing;
    dictSetVal(d, existing, val);
    dictFreeVal(d, &auxentry);
    return 0;
}

然后继续查看dictAddRaw方法

dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing)
{
    long index;
    dictEntry *entry;
    dictht *ht;

    if (dictIsRehashing(d)) _dictRehashStep(d);

    /* Get the index of the new element, or -1 if
     * the element already exists. */
    if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1)
        return NULL;

    /* Allocate the memory and store the new entry.
     * Insert the element in top, with the assumption that in a database
     * system it is more likely that recently added entries are accessed
     * more frequently. */
    ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];
    entry = zmalloc(sizeof(*entry));
    entry->next = ht->table[index];
    ht->table[index] = entry;
    ht->used++;

    /* Set the hash entry fields. */
    dictSetKey(d, entry, key);
    return entry;
}

然后继续往下看_dictKeyIndex方法

static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing)
{
    unsigned long idx, table;
    dictEntry *he;
    if (existing) *existing = NULL;

    /* Expand the hash table if needed */
    if (_dictExpandIfNeeded(d) == DICT_ERR)
        return -1;
    for (table = 0; table <= 1; table++) {
        idx = hash & d->ht[table].sizemask;
        /* Search if this slot does not already contain the given key */
        he = d->ht[table].table[idx];
        while(he) {
            if (key==he->key || dictCompareKeys(d, key, he->key)) {
                if (existing) *existing = he;
                return -1;
            }
            he = he->next;
        }
        if (!dictIsRehashing(d)) break;
    }
    return idx;
}

从上面代码注释可以看出来，_dictExpandIfNeeded就是用来进行扩容的

   /* Expand the hash table if needed */
static int _dictExpandIfNeeded(dict *d)
{
    /* Incremental rehashing already in progress. Return. */
    if (dictIsRehashing(d)) return DICT_OK;

    /* If the hash table is empty expand it to the initial size. */
    if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);

    /* If we reached the 1:1 ratio, and we are allowed to resize the hash
     * table (global setting) or we should avoid it but the ratio between
     * elements/buckets is over the "safe" threshold, we resize doubling
     * the number of buckets. */
    if (!dictTypeExpandAllowed(d))
        return DICT_OK;
    if ((dict_can_resize == DICT_RESIZE_ENABLE &&
         d->ht[0].used >= d->ht[0].size) ||
        (dict_can_resize != DICT_RESIZE_FORBID &&
         d->ht[0].used / d->ht[0].size > dict_force_resize_ratio))
    {
        return dictExpand(d, d->ht[0].used + 1);
    }
    return DICT_OK;
}

• 1、在hashtable扩容的时候，如果正在扩容的时将不会出发扩容操作
• 2、DICT_HT_INITIAL_SIZE的大小为4，即默认创建的hashtable大小为4
• 3、dict_force_resize_ratio的值为5
  *这里需要关注dict_can_resize 这个字段什么时候被赋值了，

如何进行扩容？

对hashtable真正扩容的方法是dictExpand(d, d->ht[0].used + 1)

/* return DICT_ERR if expand was not performed */
int dictExpand(dict *d, unsigned long size) {
    return _dictExpand(d, size, NULL);
}
int _dictExpand(dict *d, unsigned long size, int* malloc_failed)
{
    if (malloc_failed) *malloc_failed = 0;

    /* the size is invalid if it is smaller than the number of
     * elements already inside the hash table */
    if (dictIsRehashing(d) || d->ht[0].used > size)
        return DICT_ERR;

    dictht n; /* the new hash table */
    unsigned long realsize = _dictNextPower(size);

    /* Detect overflows */
    if (realsize < size || realsize * sizeof(dictEntry*) < realsize)
        return DICT_ERR;

    /* Rehashing to the same table size is not useful. */
    if (realsize == d->ht[0].size) return DICT_ERR;

    /* Allocate the new hash table and initialize all pointers to NULL */
    n.size = realsize;
    n.sizemask = realsize-1;
    if (malloc_failed) {
        n.table = ztrycalloc(realsize*sizeof(dictEntry*));
        *malloc_failed = n.table == NULL;
        if (*malloc_failed)
            return DICT_ERR;
    } else
        n.table = zcalloc(realsize*sizeof(dictEntry*));

    n.used = 0;

    /* Is this the first initialization? If so it's not really a rehashing
     * we just set the first hash table so that it can accept keys. */
    if (d->ht[0].table == NULL) {
        d->ht[0] = n;
        return DICT_OK;
    }

    /* Prepare a second hash table for incremental rehashing */
    d->ht[1] = n;
    d->rehashidx = 0;
    return DICT_OK;
}

1、先定义一个新的全局表，大小2^2 到 2的n次幂跟size来进行比较，取第一次满足的时候的条件，_dictNextPower(size)的代码如下：

   while(1) {
        if (i >= size)
            return i;
        i *= 2;
    }

2、设置dictht 的size等于刚刚计算好的realSize,掩码等于realsize-1
3、给dictht 的table分配地址和做初始化操作
4、接下来就判断ht[0].table是否为null，如果为null说明是第一次进行初始存放数据，而不是真正的进行rehash。此时只需要将ht[0] = n，即把刚刚创建的全局hashtable赋值给ht[0]就可以了
5、如果不是那么把刚刚创建的全局hashtable赋值给ht[1],然后dict对应的rehashidx 值修改为0
至此我们完成了hash表的扩容

那redis的数据如何进行迁移的

答案就是我们刚刚说到的使用渐进式rehash方法，那具体是如何做的？
假如一次性把数据迁移会很耗时间，会让单条指令等待很久很久，会形成阻塞。所以，Redis采用的是渐进式Rehash,所谓渐进式，就是慢慢的，不会一次性把所有数据迁移。
那什么时候会进行渐进式数据迁移？
1.每次进行key的crud操作都会进行一个hash桶的数据迁移
2.定时任务，进行部分数据迁移

• 执行crud时候对数据的操作，进行rehash操作
  
  
  
  
• 定时任务执行
  源码来源于server.c

/* This function handles 'background' operations we are required to do
 * incrementally in Redis databases, such as active key expiring, resizing,
 * rehashing. */
void databasesCron(void) {
    /* Expire keys by random sampling. Not required for slaves
     * as master will synthesize DELs for us. */
    if (server.active_expire_enabled) {
        if (iAmMaster()) {
            activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);
        } else {
            expireSlaveKeys();
        }
    }

    /* Defrag keys gradually. */
    activeDefragCycle();

    /* Perform hash tables rehashing if needed, but only if there are no
     * other processes saving the DB on disk. Otherwise rehashing is bad
     * as will cause a lot of copy-on-write of memory pages. */
    if (!hasActiveChildProcess()) {
        /* We use global counters so if we stop the computation at a given
         * DB we'll be able to start from the successive in the next
         * cron loop iteration. */
        static unsigned int resize_db = 0;
        static unsigned int rehash_db = 0;
        int dbs_per_call = CRON_DBS_PER_CALL;
        int j;

        /* Don't test more DBs than we have. */
        if (dbs_per_call > server.dbnum) dbs_per_call = server.dbnum;

        /* Resize */
        for (j = 0; j < dbs_per_call; j++) {
            tryResizeHashTables(resize_db % server.dbnum);
            resize_db++;
        }

        /* Rehash */
        if (server.activerehashing) {
            for (j = 0; j < dbs_per_call; j++) {
                int work_done = incrementallyRehash(rehash_db);
                if (work_done) {
                    /* If the function did some work, stop here, we'll do
                     * more at the next cron loop. */
                    break;
                } else {
                    /* If this db didn't need rehash, we'll try the next one. */
                    rehash_db++;
                    rehash_db %= server.dbnum;
                }
            }
        }
    }
}

那接下来我们真正看下真正执行rehash操作的方法：

static void _dictRehashStep(dict *d) {
    if (d->pauserehash == 0) dictRehash(d,1);
}

/* Performs N steps of incremental rehashing. Returns 1 if there are still
 * keys to move from the old to the new hash table, otherwise 0 is returned.
 *
 * Note that a rehashing step consists in moving a bucket (that may have more
 * than one key as we use chaining) from the old to the new hash table, however
 * since part of the hash table may be composed of empty spaces, it is not
 * guaranteed that this function will rehash even a single bucket, since it
 * will visit at max N*10 empty buckets in total, otherwise the amount of
 * work it does would be unbound and the function may block for a long time. */
int dictRehash(dict *d, int n) {
    int empty_visits = n*10; /* Max number of empty buckets to visit. */
    if (dict_can_resize == DICT_RESIZE_FORBID || !dictIsRehashing(d)) return 0;
    if (dict_can_resize == DICT_RESIZE_AVOID && 
        (d->ht[1].size / d->ht[0].size < dict_force_resize_ratio))
    {
        return 0;
    }

    while(n-- && d->ht[0].used != 0) {
        dictEntry *de, *nextde;

        /* Note that rehashidx can't overflow as we are sure there are more
         * elements because ht[0].used != 0 */
        assert(d->ht[0].size > (unsigned long)d->rehashidx);
        while(d->ht[0].table[d->rehashidx] == NULL) {
            d->rehashidx++;
            if (--empty_visits == 0) return 1;
        }
        de = d->ht[0].table[d->rehashidx];
        /* Move all the keys in this bucket from the old to the new hash HT */
        while(de) {
            uint64_t h;

            nextde = de->next;
            /* Get the index in the new hash table */
            h = dictHashKey(d, de->key) & d->ht[1].sizemask;
            de->next = d->ht[1].table[h];
            d->ht[1].table[h] = de;
            d->ht[0].used--;
            d->ht[1].used++;
            de = nextde;
        }
        d->ht[0].table[d->rehashidx] = NULL;
        d->rehashidx++;
    }

    /* Check if we already rehashed the whole table... */
    if (d->ht[0].used == 0) {
        zfree(d->ht[0].table);
        d->ht[0] = d->ht[1];
        _dictReset(&d->ht[1]);
        d->rehashidx = -1;
        return 0;
    }

    /* More to rehash... */
    return 1;
}

• 1、基于rehashidx从0开始把数据从ht[0]转移到ht[1]中
• 2、当整个ht[0]中已使用的数量为0时，就会把原来ht[0]中所占用的内存进行释放，然后ht[0]指向ht[1],最后重置rehashIndex为-1
  根据上面的分析我们可以知道rehash过程中，在redis中设置值的操作大致如下：