Redis数据库定义：

dict

dict数组保存所有的数据库，Redis初始化的时候，默认会创建16个数据库

默认情况下，Redis客户端的目标数据库是0 号数据库，可以通过select命令切换。 注意，由于Redis缺少获取当前操作的数据库命令，使用select切换需要特别注意

读写数据库中的键值对的时候，Redis除了对键空间执行指定操作外，还有一些额外的操作：

• 读取键之后（读和写操作都会先读取），记录键空间命中或不命中次数
• 读取键之后，更新键的LRU
• 读取时发现已经过期，会先删除过期键
• 如果有客户端使用watch命令监视了key，会在修改后标记为dirty
• 修改之后，会对dirty键计数器加1，用于持久化和复制
• 如果开启了数据库通知，修改之后会发送相应通知

expires

通过exprire或者pexpire命令，可以设置键的TTL，如果键的TTL为0，会被自动删除。

expires字典保存了数据库中所有键的过期时间。

• 过期字典的键是指向某个数据中的键对象
• 过期字段的值是long long类型的整数，保存这个键的过期时间
  void expireCommand(redisClient *c) {
  expireGenericCommand(c,mstime(),UNIT_SECONDS);
  }
  void expireGenericCommand(redisClient *c, long long basetime, int unit) {
  robj *key = c->argv[1], *param = c->argv[2];
  long long when; /* unix time in milliseconds when the key will expire. */
  if (getLongLongFromObjectOrReply(c, param, &when, NULL) != REDIS_OK)
  return;
  if (unit == UNIT_SECONDS) when *= 1000;
  when += basetime;
  /* No key, return zero. */
  if (lookupKeyRead(c->db,key) == NULL) {
  addReply(c,shared.czero);
  return;
  }
  /* EXPIRE with negative TTL, or EXPIREAT with a timestamp into the past
  * should never be executed as a DEL when load the AOF or in the context
  * of a slave instance.
  *
  * Instead we take the other branch of the IF statement setting an expire
  * (possibly in the past) and wait for an explicit DEL from the master. */
  if (when <= mstime() && !server.loading && !server.masterhost) {
  robj *aux;
  redisAssertWithInfo(c,key,dbDelete(c->db,key));
  server.dirty++;
  /* Replicate/AOF this as an explicit DEL. */
  aux = createStringObject(“DEL“,3);
  rewriteClientCommandVector(c,2,aux,key);
  decrRefCount(aux);
  signalModifiedKey(c->db,key);
  notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,“del“,key,c->db->id);
  addReply(c, shared.cone);
  return;
  } else {
  //放到expires字典中
  setExpire(c->db,key,when);
  addReply(c,shared.cone);
  signalModifiedKey(c->db,key);
  notifyKeyspaceEvent(REDIS_NOTIFY_GENERIC,“expire“,key,c->db->id);
  server.dirty++;
  return;
  }
  }

过期键删除策略

• 惰性删除：每次执行命令前，都会调用expireIfNeeded函数检查是否过期，如果已经过期，改函数会删除过期键
• 定时删除：定时执行activeExpireCycleTryExpire函数
  

  expireIfNeeded

  int expireIfNeeded(redisDb *db, robj *key) {
  mstime_t when = getExpire(db,key);
  mstime_t now;
  if (when < 0) return 0; /* No expire for this key */
  /* Don’t expire anything while loading. It will be done later. */
  if (server.loading) return 0;
  /* If we are in the context of a Lua script, we claim that time is
  * blocked to when the Lua script started. This way a key can expire
  * only the first time it is accessed and not in the middle of the
  * script execution, making propagation to slaves / AOF consistent.
  * See issue #1525 on Github for more information. */
  now = server.lua_caller ? server.lua_time_start : mstime();
  /* If we are running in the context of a slave, return ASAP:
  * the slave key expiration is controlled by the master that will
  * send us synthesized DEL operations for expired keys.
  *
  * Still we try to return the right information to the caller,
  * that is, 0 if we think the key should be still valid, 1 if
  * we think the key is expired at this time. */
  if (server.masterhost != NULL) return now > when;
  /* Return when this key has not expired */
  if (now <= when) return 0;
  /* Delete the key */
  server.stat_expiredkeys++;
  propagateExpire(db,key);
  notifyKeyspaceEvent(REDIS_NOTIFY_EXPIRED,
  “expired“,key,db->id);
  return dbDelete(db,key);
  }

  activeExpireCycleTryExpire

  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) ttl = 0;
  ttl_sum += ttl;
  ttl_samples++;
  }

  AOF、RDB和复制功能对过期键的处理

• 生成RDB文件时，已过期的键不会被保存到新的RDB文件中
• 载入RDB文件：
  • 主服务器载入时，会忽略过期键
  • 从服务器载入时，都会被载入（但是很快会因为同步被覆盖）
• AOF写入，已过期未删除的键没有影响，被删除后，会追加一条del命令
• AOF重写，会对键进行检查，过期键不会保存到重写后的AOF文件
• 复制：
  • 主服务器删除一个过期键后，会显式向所有从服务器发送DEL命令
  • 从服务器执行读命令，及时过期也不会删除，只有接受到主服务器DEL命令才会删除