Redis笔记之单机数据库

服务器的数据库

Redis服务器将所有数据库都保存在服务器状态redisServer的结构中：

// redis.h
struct redisServer {
    /* General */
    // ...
    redisDb *db;
    // ...
    int dbnum;                      /* Total number of configured DBs */
    // ...
};

db：指向redisDb数组的指针，也就是实际的数据库；

dbnum：数据库的个数，由数据库选项--dbnum决定，默认是16个；

单个数据库的定义

/* Redis database representation. There are multiple databases identified
 * by integers from 0 (the default database) up to the max configured
 * database. The database number is the 'id' field in the structure. */
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;

切换数据库

每个redis client同时只能访问一个数据库，默认选择的是0号数据库。

// networking.c
client *createClient(connection *conn) {
    client *c = zmalloc(sizeof(client));

    // ...

    selectDb(c,0);

    // ...
}

可以通过select命令选择目标数据库。

127.0.0.1:6379> set msg "hello world"
OK
127.0.0.1:6379> get msg
"hello world"
127.0.0.1:6379> select 1
OK
127.0.0.1:6379[1]> get msg
(nil)
127.0.0.1:6379[1]> set msg "db index 1"
OK
127.0.0.1:6379[1]> get msg
"db index 1"
127.0.0.1:6379[1]> select 0
OK
127.0.0.1:6379> get msg
"hello world"

关于切换数据细节见以下代码：

// db.c
int selectDb(client *c, int id) {
    if (id < 0 || id >= server.dbnum)
        return C_ERR;C
    c->db = &server.db[id];
    return C_OK;
}

从这里可以看出，所谓的切换数据库其实就是把对应数据库的指针赋值给redis client的指针。这个方式只能在单机的时候。