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<a href="/专栏/Redis 核心原理与实战/16 Redis 事务深入解析.md.html">16 Redis 事务深入解析.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/17 Redis 键值过期操作.md.html">17 Redis 键值过期操作.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/21 游标迭代器(过滤器)——Scan.md.html">21 游标迭代器(过滤器)——Scan.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/22 优秀的基数统计算法——HyperLogLog.md.html">22 优秀的基数统计算法——HyperLogLog.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/23 内存淘汰机制与算法.md.html">23 内存淘汰机制与算法.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/24 消息队列——发布订阅模式.md.html">24 消息队列——发布订阅模式.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/25 消息队列的其他实现方式.md.html">25 消息队列的其他实现方式.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/26 消息队列终极解决方案——Stream(上).md.html">26 消息队列终极解决方案——Stream(上).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/27 消息队列终极解决方案——Stream(下).md.html">27 消息队列终极解决方案——Stream(下).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/28 实战:分布式锁详解与代码.md.html">28 实战:分布式锁详解与代码.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/29 实战:布隆过滤器安装与使用及原理分析.md.html">29 实战:布隆过滤器安装与使用及原理分析.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/30 完整案例:实现延迟队列的两种方法.md.html">30 完整案例:实现延迟队列的两种方法.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/31 实战:定时任务案例.md.html">31 实战:定时任务案例.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/32 实战:RediSearch 高性能的全文搜索引擎.md.html">32 实战:RediSearch 高性能的全文搜索引擎.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/33 实战:Redis 性能测试.md.html">33 实战:Redis 性能测试.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/34 实战:Redis 慢查询.md.html">34 实战:Redis 慢查询.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/35 实战:Redis 性能优化方案.md.html">35 实战:Redis 性能优化方案.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/36 实战:Redis 主从同步.md.html">36 实战:Redis 主从同步.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/37 实战:Redis哨兵模式(上).md.html">37 实战:Redis哨兵模式(上).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/38 实战:Redis 哨兵模式(下).md.html">38 实战:Redis 哨兵模式(下).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/39 实战:Redis 集群模式(上).md.html">39 实战:Redis 集群模式(上).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/40 实战:Redis 集群模式(下).md.html">40 实战:Redis 集群模式(下).md.html</a>
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<a href="/专栏/Redis 核心原理与实战/41 案例:Redis 问题汇总和相关解决方案.md.html">41 案例:Redis 问题汇总和相关解决方案.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/42 技能学习指南.md.html">42 技能学习指南.md.html</a>
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<a href="/专栏/Redis 核心原理与实战/43 加餐:Redis 的可视化管理工具.md.html">43 加餐:Redis 的可视化管理工具.md.html</a>
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let overlay = document.querySelector('.off-canvas-overlay')
|
||
|
||
sidebar.classList.add('show')
|
||
|
||
overlay.classList.add('show')
|
||
|
||
}
|
||
|
||
function hide_canvas() {
|
||
|
||
let sidebar = document.querySelector('.book-sidebar')
|
||
|
||
let overlay = document.querySelector('.off-canvas-overlay')
|
||
|
||
sidebar.classList.remove('show')
|
||
|
||
overlay.classList.remove('show')
|
||
|
||
}
|
||
|
||
|
||
|
||
</script>
|
||
|
||
|
||
|
||
<div class="off-canvas-content">
|
||
|
||
<div class="columns">
|
||
|
||
<div class="column col-12 col-lg-12">
|
||
|
||
<div class="book-navbar">
|
||
|
||
<!-- For Responsive Layout -->
|
||
|
||
<header class="navbar">
|
||
|
||
<section class="navbar-section">
|
||
|
||
<a onclick="open_sidebar()">
|
||
|
||
<i class="icon icon-menu"></i>
|
||
|
||
</a>
|
||
|
||
</section>
|
||
|
||
</header>
|
||
|
||
</div>
|
||
|
||
<div class="book-content" style="max-width: 960px; margin: 0 auto;
|
||
|
||
overflow-x: auto;
|
||
|
||
overflow-y: hidden;">
|
||
|
||
<div class="book-post">
|
||
|
||
<p id="tip" align="center"></p>
|
||
|
||
<div><h1>18 Redis 过期策略与源码分析</h1>
|
||
|
||
<p>在 Redis 中我们可以给一些元素设置过期时间,那当它过期之后 Redis 是如何处理这些过期键呢?</p>
|
||
|
||
<h3>过期键执行流程</h3>
|
||
|
||
<p>Redis 之所以能知道那些键值过期,是因为在 Redis 中维护了一个字典,存储了所有设置了过期时间的键值,我们称之为过期字典。</p>
|
||
|
||
<p>过期键判断流程如下图所示:</p>
|
||
|
||
<p><img src="assets/3bf71ae0-5de7-11ea-9e57-957b6467a3fc" alt="内存过期策略-过期键判断流程.png" /></p>
|
||
|
||
<h3>过期键源码分析</h3>
|
||
|
||
<p>过期键存储在 redisDb 结构中,源代码在 src/server.h 文件中:</p>
|
||
|
||
<pre><code class="language-c">/* 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; /* 数据库键空间,存放着所有的键值对 */
|
||
|
||
dict *expires; /* 键的过期时间 */
|
||
|
||
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 */
|
||
|
||
int id; /* Database ID */
|
||
|
||
long long avg_ttl; /* Average TTL, just for stats */
|
||
|
||
list *defrag_later; /* List of key names to attempt to defrag one by one, gradually. */
|
||
|
||
} redisDb;
|
||
|
||
|
||
|
||
</code></pre>
|
||
|
||
<blockquote>
|
||
|
||
<p>小贴士:本文的所有源码都是基于 Redis 5。</p>
|
||
|
||
</blockquote>
|
||
|
||
<p>过期键数据结构如下图所示:</p>
|
||
|
||
<p><img src="assets/4f62e050-5de7-11ea-97d6-db1217470ff3" alt="微信截图_20191116185218.png" /></p>
|
||
|
||
<h3>过期策略</h3>
|
||
|
||
<p>Redis 会删除已过期的键值,以此来减少 Redis 的空间占用,但因为 Redis 本身是单线的,如果因为删除操作而影响主业务的执行就得不偿失了,为此 Redis 需要制定多个(过期)删除策略来保证糟糕的事情不会发生。</p>
|
||
|
||
<p>常见的过期策略有以下三种:</p>
|
||
|
||
<ul>
|
||
|
||
<li>定时删除</li>
|
||
|
||
<li>惰性删除</li>
|
||
|
||
<li>定期删除</li>
|
||
|
||
</ul>
|
||
|
||
<p>下面分别来看每种策略有何不同。</p>
|
||
|
||
<h4><strong>定时删除</strong></h4>
|
||
|
||
<p>在设置键值过期时间时,创建一个定时事件,当过期时间到达时,由事件处理器自动执行键的删除操作。</p>
|
||
|
||
<ul>
|
||
|
||
<li>**优点:**保证内存可以被尽快地释放。</li>
|
||
|
||
<li>**缺点:**在 Redis 高负载的情况下或有大量过期键需要同时处理时,会造成 Redis 服务器卡顿,影响主业务执行。</li>
|
||
|
||
</ul>
|
||
|
||
<h4><strong>惰性删除</strong></h4>
|
||
|
||
<p>不主动删除过期键,每次从数据库获取键值时判断是否过期,如果过期则删除键值,并返回 null。</p>
|
||
|
||
<ul>
|
||
|
||
<li>**优点:**因为每次访问时,才会判断过期键,所以此策略只会使用很少的系统资源。</li>
|
||
|
||
<li>**缺点:**系统占用空间删除不及时,导致空间利用率降低,造成了一定的空间浪费。</li>
|
||
|
||
</ul>
|
||
|
||
<p><strong>源码解析</strong></p>
|
||
|
||
<p>惰性删除的源码位于 src/db.c 文件的 expireIfNeeded 方法中,源码如下:</p>
|
||
|
||
<pre><code class="language-c">int expireIfNeeded(redisDb *db, robj *key) {
|
||
|
||
// 判断键是否过期
|
||
|
||
if (!keyIsExpired(db,key)) return 0;
|
||
|
||
if (server.masterhost != NULL) return 1;
|
||
|
||
/* 删除过期键 */
|
||
|
||
// 增加过期键个数
|
||
|
||
server.stat_expiredkeys++;
|
||
|
||
// 传播键过期的消息
|
||
|
||
propagateExpire(db,key,server.lazyfree_lazy_expire);
|
||
|
||
notifyKeyspaceEvent(NOTIFY_EXPIRED,
|
||
|
||
"expired",key,db->id);
|
||
|
||
// server.lazyfree_lazy_expire 为 1 表示异步删除(懒空间释放),反之同步删除
|
||
|
||
return server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :
|
||
|
||
dbSyncDelete(db,key);
|
||
|
||
}
|
||
|
||
// 判断键是否过期
|
||
|
||
int keyIsExpired(redisDb *db, robj *key) {
|
||
|
||
mstime_t when = getExpire(db,key);
|
||
|
||
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;
|
||
|
||
mstime_t now = server.lua_caller ? server.lua_time_start : mstime();
|
||
|
||
return now > when;
|
||
|
||
}
|
||
|
||
// 获取键的过期时间
|
||
|
||
long long getExpire(redisDb *db, robj *key) {
|
||
|
||
dictEntry *de;
|
||
|
||
/* No expire? return ASAP */
|
||
|
||
if (dictSize(db->expires) == 0 ||
|
||
|
||
(de = dictFind(db->expires,key->ptr)) == NULL) return -1;
|
||
|
||
/* The entry was found in the expire dict, this means it should also
|
||
|
||
* be present in the main dict (safety check). */
|
||
|
||
serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
|
||
|
||
return dictGetSignedIntegerVal(de);
|
||
|
||
}
|
||
|
||
|
||
|
||
</code></pre>
|
||
|
||
<p>所有对数据库的读写命令在执行之前,都会调用 expireIfNeeded 方法判断键值是否过期,过期则会从数据库中删除,反之则不做任何处理。</p>
|
||
|
||
<p>惰性删除执行流程,如下图所示:</p>
|
||
|
||
<p><img src="assets/a5e135d0-5de7-11ea-8972-8fbbcd885f6f" alt="内存过期策略-惰性删除执行流程.png" /></p>
|
||
|
||
<h4><strong>定期删除</strong></h4>
|
||
|
||
<p>每隔一段时间检查一次数据库,随机删除一些过期键。</p>
|
||
|
||
<p>Redis 默认每秒进行 10 次过期扫描,此配置可通过 Redis 的配置文件 redis.conf 进行配置,配置键为 hz 它的默认值是 <code>hz 10</code>。</p>
|
||
|
||
<p>需要注意的是:Redis 每次扫描并不是遍历过期字典中的所有键,而是采用随机抽取判断并删除过期键的形式执行的。</p>
|
||
|
||
<p><strong>定期删除流程</strong></p>
|
||
|
||
<ol>
|
||
|
||
<li>从过期字典中随机取出 20 个键;</li>
|
||
|
||
<li>删除这 20 个键中过期的键;</li>
|
||
|
||
<li>如果过期 key 的比例超过 25%,重复步骤 1。</li>
|
||
|
||
</ol>
|
||
|
||
<p>同时为了保证过期扫描不会出现循环过度,导致线程卡死现象,算法还增加了扫描时间的上限,默认不会超过 25ms。</p>
|
||
|
||
<p>定期删除执行流程,如下图所示:</p>
|
||
|
||
<p><img src="assets/c511d450-5de7-11ea-a9fa-c1c510366b3a" alt="内存过期策略-执行流程2.png" /></p>
|
||
|
||
<ul>
|
||
|
||
<li>**优点:**通过限制删除操作的时长和频率,来减少删除操作对 Redis 主业务的影响,同时也能删除一部分过期的数据减少了过期键对空间的无效占用。</li>
|
||
|
||
<li>**缺点:**内存清理方面没有定时删除效果好,同时没有惰性删除使用的系统资源少。</li>
|
||
|
||
</ul>
|
||
|
||
<p><strong>源码解析</strong></p>
|
||
|
||
<p>定期删除的核心源码在 src/expire.c 文件下的 activeExpireCycle 方法中,源码如下:</p>
|
||
|
||
<pre><code class="language-c">void activeExpireCycle(int type) {
|
||
|
||
static unsigned int current_db = 0; /* 上次定期删除遍历到的数据库ID */
|
||
|
||
static int timelimit_exit = 0; /* Time limit hit in previous call? */
|
||
|
||
static long long last_fast_cycle = 0; /* 上一次执行快速定期删除的时间点 */
|
||
|
||
int j, iteration = 0;
|
||
|
||
int dbs_per_call = CRON_DBS_PER_CALL; // 每次定期删除,遍历的数据库的数量
|
||
|
||
long long start = ustime(), timelimit, elapsed;
|
||
|
||
if (clientsArePaused()) return;
|
||
|
||
if (type == ACTIVE_EXPIRE_CYCLE_FAST) {
|
||
|
||
if (!timelimit_exit) return;
|
||
|
||
// ACTIVE_EXPIRE_CYCLE_FAST_DURATION 是快速定期删除的执行时长
|
||
|
||
if (start < last_fast_cycle + ACTIVE_EXPIRE_CYCLE_FAST_DURATION*2) return;
|
||
|
||
last_fast_cycle = start;
|
||
|
||
}
|
||
|
||
if (dbs_per_call > server.dbnum || timelimit_exit)
|
||
|
||
dbs_per_call = server.dbnum;
|
||
|
||
// 慢速定期删除的执行时长
|
||
|
||
timelimit = 1000000*ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC/server.hz/100;
|
||
|
||
timelimit_exit = 0;
|
||
|
||
if (timelimit <= 0) timelimit = 1;
|
||
|
||
if (type == ACTIVE_EXPIRE_CYCLE_FAST)
|
||
|
||
timelimit = ACTIVE_EXPIRE_CYCLE_FAST_DURATION; /* 删除操作的执行时长 */
|
||
|
||
long total_sampled = 0;
|
||
|
||
long total_expired = 0;
|
||
|
||
for (j = 0; j < dbs_per_call && timelimit_exit == 0; j++) {
|
||
|
||
int expired;
|
||
|
||
redisDb *db = server.db+(current_db % server.dbnum);
|
||
|
||
current_db++;
|
||
|
||
do {
|
||
|
||
// .......
|
||
|
||
expired = 0;
|
||
|
||
ttl_sum = 0;
|
||
|
||
ttl_samples = 0;
|
||
|
||
// 每个数据库中检查的键的数量
|
||
|
||
if (num > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP)
|
||
|
||
num = ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP;
|
||
|
||
// 从数据库中随机选取 num 个键进行检查
|
||
|
||
while (num--) {
|
||
|
||
dictEntry *de;
|
||
|
||
long long ttl;
|
||
|
||
if ((de = dictGetRandomKey(db->expires)) == NULL) break;
|
||
|
||
ttl = dictGetSignedInteger
|
||
|
||
// 过期检查,并对过期键进行删除
|
||
|
||
if (activeExpireCycleTryExpire(db,de,now)) expired++;
|
||
|
||
if (ttl > 0) {
|
||
|
||
/* We want the average TTL of keys yet not expired. */
|
||
|
||
ttl_sum += ttl;
|
||
|
||
ttl_samples++;
|
||
|
||
}
|
||
|
||
total_sampled++;
|
||
|
||
}
|
||
|
||
total_expired += expired;
|
||
|
||
if (ttl_samples) {
|
||
|
||
long long avg_ttl = ttl_sum/ttl_samples;
|
||
|
||
if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
|
||
|
||
db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);
|
||
|
||
}
|
||
|
||
if ((iteration & 0xf) == 0) { /* check once every 16 iterations. */
|
||
|
||
elapsed = ustime()-start;
|
||
|
||
if (elapsed > timelimit) {
|
||
|
||
timelimit_exit = 1;
|
||
|
||
server.stat_expired_time_cap_reached_count++;
|
||
|
||
break;
|
||
|
||
}
|
||
|
||
}
|
||
|
||
/* 每次检查只删除 ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP/4 个过期键 */
|
||
|
||
} while (expired > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP/4);
|
||
|
||
}
|
||
|
||
// .......
|
||
|
||
}
|
||
|
||
|
||
|
||
</code></pre>
|
||
|
||
<p>activeExpireCycle 方法在规定的时间,分多次遍历各个数据库,从过期字典中随机检查一部分过期键的过期时间,删除其中的过期键。</p>
|
||
|
||
<p>这个函数有两种执行模式,一个是快速模式一个是慢速模式,体现是代码中的 timelimit 变量,这个变量是用来约束此函数的运行时间的。快速模式下 timelimit 的值是固定的,等于预定义常量 ACTIVE_EXPIRE_CYCLE_FAST_DURATION,慢速模式下,这个变量的值是通过 1000000*ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC/server.hz/100 计算的。</p>
|
||
|
||
<h4><strong>Redis 使用的过期策略</strong></h4>
|
||
|
||
<p>Redis 使用的是惰性删除加定期删除的过期策略。</p>
|
||
|
||
<h3>小结</h3>
|
||
|
||
<p>通过本文可知 Redis 是通过设置过期字典的形式来判断过期键的,Redis 采用的是惰性删除和定期删除的形式删除过期键的,Redis 的定期删除策略并不会遍历删除每个过期键,而是采用随机抽取的方式删除过期键,同时为了保证过期扫描不影响 Redis 主业务,Redis 的定期删除策略中还提供了最大执行时间,以保证 Redis 正常并高效地运行。</p>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
<div>
|
||
|
||
<div style="float: left">
|
||
|
||
<a href="/专栏/Redis 核心原理与实战/17 Redis 键值过期操作.md.html">上一页</a>
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|
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</div>
|
||
|
||
<div style="float: right">
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||
|
||
<a href="/专栏/Redis 核心原理与实战/19 Redis 管道技术——Pipeline.md.html">下一页</a>
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|
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</div>
|
||
|
||
</div>
|
||
|
||
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
</div>
|
||
|
||
|
||
|
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<a class="off-canvas-overlay" onclick="hide_canvas()"></a>
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||
|
||
gtag('config', 'G-NPSEEVD756');
|
||
|
||
var path = window.location.pathname
|
||
|
||
var cookie = getCookie("lastPath");
|
||
|
||
console.log(path)
|
||
|
||
if (path.replace("/", "") === "") {
|
||
|
||
if (cookie.replace("/", "") !== "") {
|
||
|
||
console.log(cookie)
|
||
|
||
document.getElementById("tip").innerHTML = "<a href='" + cookie + "'>跳转到上次进度</a>"
|
||
|
||
}
|
||
|
||
} else {
|
||
|
||
setCookie("lastPath", path)
|
||
|
||
}
|
||
|
||
|
||
|
||
function setCookie(cname, cvalue) {
|
||
|
||
var d = new Date();
|
||
|
||
d.setTime(d.getTime() + (180 * 24 * 60 * 60 * 1000));
|
||
|
||
var expires = "expires=" + d.toGMTString();
|
||
|
||
document.cookie = cname + "=" + cvalue + "; " + expires + ";path = /";
|
||
|
||
}
|
||
|
||
|
||
|
||
function getCookie(cname) {
|
||
|
||
var name = cname + "=";
|
||
|
||
var ca = document.cookie.split(';');
|
||
|
||
for (var i = 0; i < ca.length; i++) {
|
||
|
||
var c = ca[i].trim();
|
||
|
||
if (c.indexOf(name) === 0) return c.substring(name.length, c.length);
|
||
|
||
}
|
||
|
||
return "";
|
||
|
||
}
|
||
|
||
|
||
|
||
</script>
|
||
|
||
|
||
|
||
</html>
|
||
|