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;

redis> set Ccww   5 2 0
ok
redis> expire Ccww 5
ok

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;
}

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);
}

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);
}