redis设计与实现
https://github.com/CyC2018/CS-Notes/blob/master/docs/notes/Redis.md
1. 数据结构
跳跃表
与红黑树等平衡树相比，跳跃表具有以下优点：
- 插入速度非常快速，因为不需要进行旋转等操作来维护平衡性；
- 更容易实现；
- 支持无锁操作。
哈希表
有两个hash表指针，每次添加的时候，如果正在rehash，则进行n步rehash操作 [https://github.com/linyiqun/Redis-Code/blob/master/struct/dict.c]
/* 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. */
/* hash重定位，主要从旧的表映射到新表中
* 如果返回1说明旧的表中还存在key迁移到新表中，0代表没有 */
int dictRehash(dict *d, int n) {
if (!dictIsRehashing(d)) return 0;
/* 根据参数分n步多次循环操作 */
while(n--) {
dictEntry *de, *nextde;
/* 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;
}
/* 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++;
de = d->ht[0].table[d->rehashidx];
/* Move all the keys in this bucket from the old to the new hash HT */
/* 移动的关键操作 */
while(de) {
unsigned int 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++;
}
return 1;
}
/* Low level add. This function adds the entry but instead of setting
* a value returns the dictEntry structure to the user, that will make
* sure to fill the value field as he wishes.
*
* This function is also directly exposed to user API to be called
* mainly in order to store non-pointers inside the hash value, example:
*
* entry = dictAddRaw(dict,mykey);
* if (entry != NULL) dictSetSignedIntegerVal(entry,1000);
*
* Return values:
*
* If key already exists NULL is returned.
* If key was added, the hash entry is returned to be manipulated by the caller.
*/
/* 添加一个指定key值的Entry */
dictEntry *dictAddRaw(dict *d, void *key)
{
int index;
dictEntry *entry;
dictht *ht;
if (dictIsRehashing(d)) _dictRehashStep(d);
/* Get the index of the new element, or -1 if
* the element already exists. */
/* 如果指定的key已经存在，则直接返回NULL说明添加失败 */
if ((index = _dictKeyIndex(d, key)) == -1)
return NULL;
/* Allocate the memory and store the new entry */
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;
}
2. 过期键删除策略
惰性删除 + 定期删除
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;
惰性删除只在碰到过期键时进行删除操作，定期删除每隔一段时间主动查找并删除过期键，redisDB结构内的expires字典保存了所有键的过期时间，称为过期字典
3. 持久化
RDB
将某个时间点的所有数据都存放到硬盘上。
可以将快照复制到其它服务器从而创建具有相同数据的服务器副本。
如果系统发生故障，将会丢失最后一次创建快照之后的数据。
如果数据量很大，保存快照的时间会很长。
AOF
将写命令添加到 AOF文件（Append Only File）末尾。
使用 AOF 持久化需要设置同步选项，从而确保写命令什么时候会同步到磁盘文件上。
Redis提供了 AOF 重写特性，能够减小 AOF 文件大小。
使用子进程而非子线程可以避免锁。
同时追加命令至AOF缓冲区，AOF重写缓冲区。