redis文档翻译_key设置过期时间

Available since 1.0.0.    使用開始版本号1.01

Time complexity: O(1)  时间复杂度O（1）

出处：http://blog.csdn.net/column/details/redisbanli.html

Set a timeout on key.
After the timeout has expired, the key will automatically be deleted. A key with an associated timeout is often said to be volatile in
Redis terminology.

在key上设置一个超时时间。

这个时间期满后，key会自己主动被删除。key关联一个超时时间，在Redis术语中叫volatile （易挥发的）

The timeout is cleared only when the key is removed using the DEL command
or overwritten using the SET or GETSET commands.
This means that all the operations that conceptually alter the value stored at the key without replacing it with a new one will leave the timeout untouched.
For instance, incrementing the value of a key with INCR, pushing a new value into a list with LPUSH,
or altering the field value of a hash with HSET are all operations that will
leave the timeout untouched.

这个超时时间只在使用 DEL命令或者使用SET或者GETSET命名重写时被清除。全部操作，从概念上在key没有替换，改变存储的值将与一个新的超时时间关联。比如，使用INCR增量操作一个值，使用LPUSH为list添加新值。或者使用HEST设置hash字段的新值，全部的操作又一次关联一个超时时间。

The timeout can also be cleared, turning the key back into a persistent key, using thePERSIST command.

也能够使用PERSIST命令将有过期时间的key转换成永久的key。

If a key is renamed with RENAME,
the associated time to live is transferred to the new key name.

假设key被RENAME重命令，那么关联的时间将被转移到这个新的key名字上。

If a key is overwritten by RENAME,
like in the case of an existing key Key_A that is overwritten by a call like RENAME
Key_B Key_A, it does not matter if the original Key_Ahad a timeout associated or not, the new key Key_A will
inherit all the characteristics of Key_B.

假设一个key被RENAME命令覆盖，比如已经存在了一个Key_A，使用命令RENAME Key_B Key_A(将key_B重名为KEY_A),无论原来的Key_A有没有关联超时时间，新的Key_A将继承Key_B的全部特性。

Refreshing expires  刷新到期

It is possible to call EXPIRE using
as argument a key that already has an existing expire set. In this case the time to live of a key is updated to the new value. There are many useful applications
for this, an example is documented in the Navigation session pattern section below.

能够使用命令EXPIRE又一次为已经存在的超时时间的key设置新的超时时间。

假如这样做了。关联这个key的超时时间将被更新成新设置的值。

在很多应用很实用。比例如以下一节的样例。

Differences in Redis prior 2.1.3在Redis的2.1.3之前的差别

In Redis versions prior 2.1.3 altering a key with an expire set using a command altering its value had the effect of removing the key entirely. This
semantics was needed because of limitations in the replication layer that are now fixed.

在Redis2.1.3版本号之前，使用命令改变key超时时间设置。这个值将明白影响key的删除。

由于在那时固定复制层的限制，这样的语义是必要的。

Return value  返回值

Integer reply, specifically:特别的Iteger答复

• 1 if the timeout was set.    1代表设置了超时限制
• 0 if key does not exist or
  the timeout could not be set. 0表示key不存在或者key没有设置超时时间。

Examples

redis> SET mykey "Hello"

OK

redis> EXPIRE mykey 10

(integer) 1

redis> TTL mykey

(integer) 10

redis> SET mykey "Hello World"

OK

redis> TTL mykey

(integer) -1

redis> 

Pattern: Navigation session 图像session导航

Imagine you have a web service and you are interested in the latest N pages recently visited by your users, such that each adjacent page view was not performed more than 60 seconds after the previous. Conceptually
you may think at this set of page views as a Navigation session if your user, that may contain interesting information about what kind of products he or she
is looking for currently, so that you can recommend related products.

想象一下。你有一个web服务，而且你对你的用户最后訪问的N个网页非常感兴趣，使得通过前面的每一个相邻页面訪问的运行没有超过60秒。从概念上讲你可能觉得。页面浏览量大的就是你的用户设置页面的导航会话，那么可能包括他或她眼下正在寻找关于产品种类的有趣信息，因此你就能够推荐相关的产品。

You can easily model this pattern in Redis using the following strategy: every time the user does a page view you call the following commands:

在Redis中你能够使用一个简单的模式实现：每次用户訪问网页就运行以下的命令：

MULTI
RPUSH pagewviews.user:<userid> http://.....
EXPIRE pagewviews.user:<userid> 60
EXEC

If the user will be idle more than 60 seconds, the key will be deleted and only subsequent page views that have less than 60 seconds of difference will be recorded.

假设用户闲置网页超过60秒，key将要被删除而且仅仅有兴许訪问不同的网页少于60秒才会被又一次记录。

This pattern is easily modified to use counters using INCR instead
of lists usingRPUSH.

这个模式非常easy地使用了list的RPUSH代码自曾INCR。

Appendix: Redis expires  附件：Redis到期

Keys with an expire  key的到期

Normally Redis keys are created without an associated time to live. The key will simply live forever, unless it is removed by the user in an explicit way, for instance using the DEL command.

一般key创建没关联过期时间。这样的key将永远存在，除非被用户明白地删除，比如用户使用DEL命令。

The EXPIRE family
of commands is able to associate an expire to a given key, at the cost of some additional memory used by the key. When a key has an expire set, Redis will make sure to remove the key when the specified amount of time elapsed.

EXPIRE命令族能去关联给出给出key，有额外的内存开销。

当一个key有设置过期时间，Redis确保当过期时间过去时删除这个key。

The key time to live can be updated or entirely removed using the EXPIRE andPERSIST command
(or other strictly related commands).

key的过期时间被更新或者删除。使用命令EXPIRE 和PERSIST命令实现。

Expire accuracy  到期时间精确性

In Redis 2.4 the expire might not be pin-point accurate, and it could be between zero to one seconds out.

在2.4版本号之前，到期时间可能不太精确，可能会有0到1秒的误差。

Since Redis 2.6 the expire error is from 0 to 1 milliseconds.

从2.6版本号之后误差在0到1毫米之间。

Expires and persistence 到期和持久

Keys expiring information is stored as absolute Unix timestamps (in milliseconds in case of Redis version 2.6 or greater). This means that the time is flowing even when the Redis instance is not active.

key的过期时间信息是使用Unix的时间戳保存的（从2.6開始使用毫米级别）。

意思是即使Redis实例没有执行时间也是在流失的。

For expires to work well, the computer time must be taken stable. If you move an RDB file from two
computers with a big desync in their clocks, funny things may happen (like all the keys loaded to be expired at loading time).

为了使到期时间工作稳定，必须保证计算机时间稳定。

假设你在两个计算机之间移动RDB文件的时间延迟非常大。那么非常多有趣是的事情可能就会发生（比方全部设置到期时间的key刚被载入就过期了）。

Even running instances will always check the computer clock, so for instance if you set a key with
a time to live of 1000 seconds, and then set your computer time 2000 seconds in the future, the key will be expired immediately, instead of lasting for 1000 seconds.

甚至Redis总是在检測计算机时间的，以至于假设你设置一个key的存活为1000秒，然后你设置你的计算机时间为2000秒之后的时间，那么这个key就会立马过期。并且已经过期1000秒了。

How Redis expires keys Redis的key是怎样过期的

Redis keys are expired in two ways: a passive way, and an active way.

Redis key的过期方式有两种：被动过期和主动过期

A key is actively expired simply when some client tries to access it, and the key is found to be timed out.

主动方式过期比較简单。当有client去訪问这个key时，找到这个key而且使之超时。

Of course this is not enough as there are expired keys that will never be accessed again. These keys should be expired anyway, so periodically Redis tests a few keys at random among keys with an expire set. All the keys
that are already expired are deleted from the keyspace.

当然这还不够，由于有些key可能在设置过期时间之后，就一直都没有被訪问。

这些key不管怎样都应该被过期的，因此Redis会定期随机抽取设置有过期时间的key进行检查。过期的key过期将被从 key空间删除掉。

Specifically this is what Redis does 10 times per second:

Redis每秒进行10次检測：

   1：Test 20 random keys from the set of keys with an associated expire.

        随机抽取设置有过期时间中的20个key。

   2：Delete all the keys found expired.

    删除已经过期的key。

  3：If more than 25% of keys were expired, start again from step 1.

 假设有超过25%的key被超时。那么開始步骤1。

This is a trivial probabilistic algorithm, basically the assumption is that our sample is representative
of the whole key space, and we continue to expire until the percentage of keys that are likely to be expired is under 25%

这个概率算法是不重要的。主体上就是如果我们的样品代表了整个key空间。而且继续使key过期。直到有25%以上的key过期，然后開始第一步。

This means that at any given moment the maximum amount of keys already expired that are using memory
is at max equal to max amount of write operations per second divided by 4.

意思是不论什么时刻，给出已经过期的最大数量key，这些过期key使用的最大内存等于每秒最大数量写操作除以4倍。

How expires are handled in the replication link and AOF file

在AOF文件和复制连接中过期是怎样处理的

In order to obtain a correct behavior without sacrificing consistency, when a key expires, a DEL operation
is synthesized in both the AOF file and gains all the attached slaves. This way the expiration process is centralized in the master instance, and there is no chance of consistency errors.

为了获得不牺牲一致性的正确做法，当一个key过期时，DEL操作将在AOF文件和全部的 slaves中同步。这样的方式中，过期在master实例集中处理，而且避免产生改变一致性错误。

However while the slaves connected to a master will not expire keys independently (but will wait for the DEL coming
from the master), they'll still take the full state of the expires existing in the dataset, so when a slave is elected to a master it will be able to expire the keys independently, fully acting as a master.

然而，当slaves连接到master上时。就不会独立地使key过期（可是会等待master DEL的到来），他们将使用全部在数据设置中有过期状态的key，因此当一个slave被推选成为master时，它将開始独立地去过期key，充当maste的角色。

出处：http://blog.csdn.net/column/details/redisbanli.html