/* 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;    /* 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;

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);
}

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;
}

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();

  /* 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);
}