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如果一個(gè)鍵過(guò)期了,那么它什么時(shí)候會(huì)被刪除呢?
這個(gè)問(wèn)題有三種可能的答案,它們分別代表了三種不同的刪除策略:
在這三種策略中,第一種和第三種為主動(dòng)刪除策略, 而第二種則為被動(dòng)刪除策略。
前言
使用Redis時(shí)我們可以使用EXPIRE或EXPIREAT命令給key設(shè)置過(guò)期刪除時(shí)間,結(jié)構(gòu)體redisDb中的expires字典保存了所有key的過(guò)期時(shí)間,這個(gè)字典(dict)的key是一個(gè)指針,指向redis中的某個(gè)key對(duì)象,過(guò)期字典的value是一個(gè)保存過(guò)期時(shí)間的整數(shù)。
/* 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; /* 過(guò)期字典*/ 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 */ struct evictionPoolEntry *eviction_pool; /* Eviction pool of keys */ int id; /* Database ID */ long long avg_ttl; /* Average TTL, just for stats */ } redisDb;
設(shè)置過(guò)期時(shí)間
不論是EXPIRE,EXPIREAT,還是PEXPIRE,PEXPIREAT,底層的具體實(shí)現(xiàn)是一樣的。在Redis的key空間中找到要設(shè)置過(guò)期時(shí)間的這個(gè)key,然后將這個(gè)entry(key的指針,過(guò)期時(shí)間)加入到過(guò)期字典中。
void setExpire(redisDb *db, robj *key, long long when) { dictEntry *kde, *de; /* Reuse the sds from the main dict in the expire dict */ kde = dictFind(db->dict,key->ptr); redisAssertWithInfo(NULL,key,kde != NULL); de = dictReplaceRaw(db->expires,dictGetKey(kde)); dictSetSignedIntegerVal(de,when); }
過(guò)期刪除策略
如果一個(gè)key過(guò)期了,何時(shí)會(huì)被刪除呢?在Redis中有兩種過(guò)期刪除策略:(1)惰性過(guò)期刪除;(2)定期刪除。接下來(lái)具體看看。
惰性過(guò)期刪除
Redis在執(zhí)行任何讀寫(xiě)命令時(shí)都會(huì)先找到這個(gè)key,惰性刪除就作為一個(gè)切入點(diǎn)放在查找key之前,如果key過(guò)期了就刪除這個(gè)key。
robj *lookupKeyRead(redisDb *db, robj *key) { robj *val; expireIfNeeded(db,key); // 切入點(diǎn) val = lookupKey(db,key); if (val == NULL) server.stat_keyspace_misses++; else server.stat_keyspace_hits++; return val; }
定期刪除
key的定期刪除會(huì)在Redis的周期性執(zhí)行任務(wù)(serverCron,默認(rèn)每100ms執(zhí)行一次)中進(jìn)行,而且是發(fā)生Redis的master節(jié)點(diǎn),因?yàn)閟lave節(jié)點(diǎn)會(huì)通過(guò)主節(jié)點(diǎn)的DEL命令同步過(guò)來(lái)達(dá)到刪除key的目的。
依次遍歷每個(gè)db(默認(rèn)配置數(shù)是16),針對(duì)每個(gè)db,每次循環(huán)隨機(jī)選擇20個(gè)(ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP)key判斷是否過(guò)期,如果一輪所選的key少于25%過(guò)期,則終止迭次,此外在迭代過(guò)程中如果超過(guò)了一定的時(shí)間限制則終止過(guò)期刪除這一過(guò)程。
for (j = 0; j < dbs_per_call; j++) { int expired; redisDb *db = server.db+(current_db % server.dbnum); /* Increment the DB now so we are sure if we run out of time * in the current DB we'll restart from the next. This allows to * distribute the time evenly across DBs. */ current_db++; /* Continue to expire if at the end of the cycle more than 25% * of the keys were expired. */ do { unsigned long num, slots; long long now, ttl_sum; int ttl_samples; /* 如果該db沒(méi)有設(shè)置過(guò)期key,則繼續(xù)看下個(gè)db*/ if ((num = dictSize(db->expires)) == 0) { db->avg_ttl = 0; break; } slots = dictSlots(db->expires); now = mstime(); /* When there are less than 1% filled slots getting random * keys is expensive, so stop here waiting for better times... * The dictionary will be resized asap. */ if (num && slots > DICT_HT_INITIAL_SIZE && (num*100/slots < 1)) break; /* The main collection cycle. Sample random keys among keys * with an expire set, checking for expired ones. */ expired = 0; ttl_sum = 0; ttl_samples = 0; if (num > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP) num = ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP;// 20 while (num--) { dictEntry *de; long long ttl; if ((de = dictGetRandomKey(db->expires)) == NULL) break; ttl = dictGetSignedIntegerVal(de)-now; if (activeExpireCycleTryExpire(db,de,now)) expired++; if (ttl > 0) { /* We want the average TTL of keys yet not expired. */ ttl_sum += ttl; ttl_samples++; } } /* Update the average TTL stats for this database. */ if (ttl_samples) { long long avg_ttl = ttl_sum/ttl_samples; /* Do a simple running average with a few samples. * We just use the current estimate with a weight of 2% * and the previous estimate with a weight of 98%. */ if (db->avg_ttl == 0) db->avg_ttl = avg_ttl; db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50); } /* We can't block forever here even if there are many keys to * expire. So after a given amount of milliseconds return to the * caller waiting for the other active expire cycle. */ iteration++; if ((iteration & 0xf) == 0) { /* 每迭代16次檢查一次 */ long long elapsed = ustime()-start; latencyAddSampleIfNeeded("expire-cycle",elapsed/1000); if (elapsed > timelimit) timelimit_exit = 1; } // 超過(guò)時(shí)間限制則退出 if (timelimit_exit) return; /* 在當(dāng)前db中,如果少于25%的key過(guò)期,則停止繼續(xù)刪除過(guò)期key */ } while (expired > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP/4); }
總結(jié)
惰性刪除:讀寫(xiě)之前判斷key是否過(guò)期
定期刪除:定期抽樣key,判斷是否過(guò)期
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