什么是Blocking pop
在Redis中對(duì)于list中數(shù)據(jù)的pop有兩個(gè)命令
- rpop, 從List的右側(cè)彈出數(shù)據(jù)
- lpop, 從list的左側(cè)彈出數(shù)據(jù)
blocking pop的意思就是在pop的時(shí)候，如果list不存在，則客戶端阻塞等待，直到有其他客戶端進(jìn)行push的時(shí)候，這個(gè)阻塞的客戶端才會(huì)得到答復(fù), 當(dāng)然也可以設(shè)置阻塞的超時(shí)時(shí)間
- blpop list1[list2...] timeout
- rlpop list1[list2...] timeout
Redis2.6版本的變化
首先假設(shè)一個(gè)場(chǎng)景
客戶端1：blpop list1 0
客戶端2：lpush list1 v1 v2 v3
- Redis2.6版本之前, 客戶端1將會(huì)得到：v1, 并且v1的push和pop操作不會(huì)記錄aof
- Redis2.6版本及之后，客戶端1將會(huì)得到: v3，v1的push和婆婆操作會(huì)記錄到aof中
如何實(shí)現(xiàn)
在閱讀完代碼后發(fā)現(xiàn)，代碼實(shí)現(xiàn)中有以下幾種變化:
數(shù)據(jù)結(jié)構(gòu)的變化
可以看到2.6之后，redisDb結(jié)構(gòu)體中增加了ready_keys字典
/* 2.4 */
typedef struct redisDb {
dict *dict;
dict *expires;
dict *blocking_keys; /* Keys with clients waiting for data (BLPOP) */
dict *io_keys; /* Keys with clients waiting for VM I/O */
dict *watched_keys; /* WATCHED keys for MULTI/EXEC CAS */
int id;
} redisDb;
/* 2.6 */
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;
lpush/rpush邏輯的變化
Redis在2.6版本之前直接在push中處理blocking client, 所以blpop/brpop, 都是將push中的前幾個(gè)value返回給blocking client, 這幾個(gè)value沒有操作db，也不會(huì)進(jìn)入aof
而在2.6版本之后，則首先將數(shù)據(jù)插入到db的list中，然后在processCommand中再去處理，所以blpop/brpop和語義更相符
/* 2.6以前 */
/* 直接在push中處理block client */
void pushGenericCommand(redisClient *c, int where) {
int j, addlen = 0, pushed = 0;
robj *lobj = lookupKeyWrite(c->db,c->argv[1]);
int may_have_waiting_clients = (lobj == NULL);
if (lobj && lobj->type != REDIS_LIST) {
addReply(c,shared.wrongtypeerr);
return;
}
for (j = 2; j < c->argc; j++) {
c->argv[j] = tryObjectEncoding(c->argv[j]);
if (may_have_waiting_clients) {
if (handleClientsWaitingListPush(c,c->argv[1],c->argv[j])) {
addlen++;
continue;
} else {
may_have_waiting_clients = 0;
}
}
if (!lobj) {
lobj = createZiplistObject();
dbAdd(c->db,c->argv[1],lobj);
}
listTypePush(lobj,c->argv[j],where);
pushed++;
}
addReplyLongLong(c,addlen + (lobj ? listTypeLength(lobj) : 0));
if (pushed) signalModifiedKey(c->db,c->argv[1]);
server.dirty += pushed;
}
/* 2.6 */
/* 每個(gè)list在push的時(shí)候都會(huì)去檢查，是否在blocking keys字典中出現(xiàn)過,
* 如果出現(xiàn)過，則加入 ready_keys 字典中 */
void dbAdd(redisDb *db, robj *key, robj *val) {
sds copy = sdsdup(key->ptr);
int retval = dictAdd(db->dict, copy, val);
redisAssertWithInfo(NULL,key,retval == REDIS_OK);
if (val->type == REDIS_LIST) signalListAsReady(db, key);
}
/* 在processCommand中執(zhí)行如下操作 */
call(c,REDIS_CALL_FULL);
if (listLength(server.ready_keys))
handleClientsBlockedOnLists();
