当遇到美女面试官之如何理解Redis的Expire Key(过期键)

typedef struct redisDb {
     dict *dict;     /* 键空间 key space */
     dict *expires;    /* 过期字典 */
     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;
复制代码

redis> set Ccww   5 2 0  
ok  
redis> expire Ccww 5  
ok  
复制代码

robj *lookupKeyRead(redisDb *db, robj *key) {
          robj *val;
   expireIfNeeded(db,key); // 切入点
   val = lookupKey(db,key);
   if (val == NULL)
    server.stat_keyspace_misses++;
   else
    server.stat_keyspace_hits++;
   return val;
}
复制代码

int expireIfNeeded(redisDb *db, robj *key) {
     /* 取出键的过期时间 */
    mstime_t when = getExpire(db,key);
    mstime_t now;
     /* 没有过期时间返回0*/
    if (when < 0) return 0; /* No expire for this key */
    /* 服务器loading时*/
    if (server.loading) return 0;
    /* 根据一定规则获取当前时间*/
    now = server.lua_caller ? server.lua_time_start : mstime();
    /* 如果当前的是从(Slave)服务器
     * 0 认为key为无效
     * 1 if we think the key is expired at this time. 
     * */
    if (server.masterhost != NULL) return now > when;
     /* key未过期，返回 0 */
    if (now <= when) return 0;
     /* 删除键 */
    server.stat_expiredkeys++;
    propagateExpire(db,key,server.lazyfree_lazy_expire);
    notifyKeyspaceEvent(NOTIFY_EXPIRED,
        "expired",key,db->id);
    return server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :
                                         dbSyncDelete(db,key);
}
复制代码

for (j = 0; j < dbs_per_call; j++) {
 int expired;
 redisDb *db = server.db+(current_db % server.dbnum);
 current_db++;
 /* 超过25％的key已过期，则继续. */
 do {
  unsigned long num, slots;
  long long now, ttl_sum;
  int ttl_samples;
  /* 如果该db没有设置过期key，则继续看下个db*/
  if ((num = dictSize(db->expires)) == 0) {
   db->avg_ttl = 0;
   break;
  }
  slots = dictSlots(db->expires);
  now = mstime();
  /*但少于1%时，需要调整字典大小*/
  if (num && slots > DICT_HT_INITIAL_SIZE &&
   (num*100/slots < 1)) break;
  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;
   /样本获取移动平均值 */
   if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
   db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);
  }
  iteration++;
  if ((iteration & 0xf) == 0) { /* 每迭代16次检查一次 */
   long long elapsed = ustime()-start;
   latencyAddSampleIfNeeded("expire-cycle",elapsed/1000);
   if (elapsed > timelimit) timelimit_exit = 1;
  }
 /* 超过时间限制则退出*/
  if (timelimit_exit) return;
  /* 在当前db中，如果少于25%的key过期，则停止继续删除过期key */
 } while (expired > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP/4);
}
复制代码