void saveCommand(redisClient *c) {

    // BGSAVE 已经在执行中，不能再执行 SAVE
    // 否则将产生竞争条件
    if (server.rdb_child_pid != -1) {
        addReplyError(c,"Background save already in progress");
        return;
    }

    // 执行 
    if (rdbSave(server.rdb_filename) == REDIS_OK) {
        addReply(c,shared.ok);
    } else {
        addReply(c,shared.err);
    }
}

void bgsaveCommand(redisClient *c) {

    // 不能重复执行 BGSAVE
    if (server.rdb_child_pid != -1) {
        addReplyError(c,"Background save already in progress");

    // 不能在 BGREWRITEAOF 正在运行时执行
    } else if (server.aof_child_pid != -1) {
        addReplyError(c,"Can't BGSAVE while AOF log rewriting is in progress");

    // 执行 BGSAVE
    } else if (rdbSaveBackground(server.rdb_filename) == REDIS_OK) {
        addReplyStatus(c,"Background saving started");

    } else {
        addReply(c,shared.err);
    }
}

/* Dup object types to RDB object types. Only reason is readability (are we
 * dealing with RDB types or with in-memory object types?).
 *
 * 对象类型在 RDB 文件中的类型
 */
#define REDIS_RDB_TYPE_STRING 0
#define REDIS_RDB_TYPE_LIST   1
#define REDIS_RDB_TYPE_SET    2
#define REDIS_RDB_TYPE_ZSET   3
#define REDIS_RDB_TYPE_HASH   4

/* Object types for encoded objects.
 *
 * 对象的编码方式
 */
#define REDIS_RDB_TYPE_HASH_ZIPMAP    9
#define REDIS_RDB_TYPE_LIST_ZIPLIST  10
#define REDIS_RDB_TYPE_SET_INTSET    11
#define REDIS_RDB_TYPE_ZSET_ZIPLIST  12
#define REDIS_RDB_TYPE_HASH_ZIPLIST  13

/* Load a Redis object of the specified type from the specified file.
 *
 * 从 rdb 文件中载入指定类型的对象。
 *
 * On success a newly allocated object is returned, otherwise NULL. 
 *
 * 读入成功返回一个新对象，否则返回 NULL 。
 */
robj *rdbLoadObject(int rdbtype, rio *rdb) {
    robj *o, *ele, *dec;
    size_t len;
    unsigned int i;

    // 载入字符串对象
    if (rdbtype == REDIS_RDB_TYPE_STRING) {
        /* Read string value */
        if ((o = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;
        o = tryObjectEncoding(o);

    // 载入列表对象
    } else if (rdbtype == REDIS_RDB_TYPE_LIST) {

        /* Read list value 
         *
         * 读入列表的节点数
         */
        if ((len = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;

        /* Use a real list when there are too many entries 
         *
         * 根据节点数，创建对象的编码
         */
        if (len > server.list_max_ziplist_entries) {
            o = createListObject();
        } else {
            o = createZiplistObject();
        }

        /* Load every single element of the list 
         *
         * 载入所有列表项
         */
        while(len--) {

            // 载入字符串对象
            if ((ele = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;

            /* If we are using a ziplist and the value is too big, convert
             * the object to a real list. 
             *
             * 根据字符串对象，
             * 检查是否需要将列表从 ZIPLIST 编码转换为 LINKEDLIST 编码
             */
            if (o->encoding == REDIS_ENCODING_ZIPLIST &&
                sdsEncodedObject(ele) &&
                sdslen(ele->ptr) > server.list_max_ziplist_value)
                    listTypeConvert(o,REDIS_ENCODING_LINKEDLIST);

            // ZIPLIST
            if (o->encoding == REDIS_ENCODING_ZIPLIST) {
                dec = getDecodedObject(ele);

                // 将字符串值推入 ZIPLIST 末尾来重建列表
                o->ptr = ziplistPush(o->ptr,dec->ptr,sdslen(dec->ptr),REDIS_TAIL);

                decrRefCount(dec);
                decrRefCount(ele);
            } else {
                // 将新列表项推入到链表的末尾
                ele = tryObjectEncoding(ele);
                listAddNodeTail(o->ptr,ele);
            }
        }

    // 载入集合对象
    //还有很多不同类型的处理.....
    
    }
    return o;
}