memcache过期时间的一点小小的分析
如果你清楚知道这一点，那就跳过吧！如果你不太清楚原因，那就往下看吧我们都在用MemCached，缓存有其过期时间，我们经常在配置中设置，如果有业务场景需要动态设置过期时间的时候，就可以通过接口直接设置过期时间
client.set(key, value, new Date(expireTime));
但这个时间如何设置，还是有一点区别，比如设置10分钟后过期，是应该设置date为System.currentTimeInMillis()+10*60*1000
还是10*60*1000
刚经过测试，两种竟然都是可以的，这是咋回事，那就看源码呗，网络上介绍的一些文档有时候需要鉴别，更可靠的还是自己下源码看，别被误导了.
/** Stores data on the server; the key, value, and an expiration time are specified.
* The server will automatically delete the value when the expiration time has been reached.
* If the value is not a String or numeric type, or if {@link #set_serial(boolean) set_serial(true)}
* has been called; the data will be Serialized prior to storage.
*
* If compression is enabled, and the data is longer than the compression threshold,
* and compresses by at least the compression savings, the data will be stored in
* compressed form.
*
* @param key key to store data under
* @param value value to store
* @param expiry when to expire the record
* @return true, if the data was successfully stored
*/
public boolean set(String key, Object value, Date expiry) {
return set("set",key,value,expiry,null);
}
这里过期时间只是简单的说when，没明确说明，在具体实现里面
try {
String cmd = cmdname + " " + key + " " + flags + " " +
expiry.getTime() / 1000 + " " + val.length + "\r\n";
sock.out().writeBytes(cmd);
sock.out().write(val);
sock.out().writeBytes("\r\n");
sock.out().flush();
String tmp = sock.in().readLine();
if (tmp.equals("STORED")) {
if (debug) {
System.out.println("MemCache: " + cmdname + " " + key + " = " + val);
}
return true;
} else {
System.out.println("MemCache:" + cmd + tmp);
}
实现中也是expiry.getTime() / 1000 传入到服务端
在客户端看不出到底怎么回事了，那就看服务端了，（网络上介绍的东西是copy来copy去的，有些都掉东西了，还是自己下代码来看）
#define REALTIME_MAXDELTA 60*60*24*30
/*
* given time value that's either unix time or delta from current unix time, return
* unix time. Use the fact that delta can't exceed one month (and real time value can't
* be that low).
*/
static rel_time_t realtime(const time_t exptime) {
/* no. of seconds in 30 days - largest possible delta exptime */
if (exptime == 0) return 0; /* 0 means never expire */
if (exptime > REALTIME_MAXDELTA) {
/* if item expiration is at/before the server started, give it an
expiration time of 1 second after the server started.
(because 0 means don't expire). without this, we'd
underflow and wrap around to some large value way in the
future, effectively making items expiring in the past
really expiring never */
if (exptime <= process_started)
return (rel_time_t)1;
return (rel_time_t)(exptime - process_started);
} else {
return (rel_time_t)(exptime + current_time);
}
}
看了这段c的代码，就明白服务端是两种方式都兼容的，一个是多少秒后过期，一个是什么时候过期，
但后者因为设置时间是在客户端，存储在服务端，假如两台服务器时间差别很大，就会导致数据的过期时间和我要求的时间点不符合。
个人觉得使用前者比较好！
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