今天在给同事做CodeReview时,发现了一个问题。代码大致如下:
public boolean delete(String key){
boolean isCacheDeleted = memcachedClient.delete(key);
if(isCacheDeleted){
//delete stuffs from DB
}
//other stuffs
}
1.Memcache客户端采用的gwhalin / Memcached-Java-Client v2.6.3:https://github.com/gwhalin/Memcached-Java-Client
2.问题分析:
若对应的key在缓存服务器中已经不存在,或者memCachedClient.get(key)返回值为null,那么delete()方法的返回值为false。试想,如果缓存过期、或被算法逐出、或因故障缓存数据丢失等等情况,这些情况发生时memCachedClient.get(key)都会返回null,memCachedClient.delete(key)都会返回false,所以以上代码块中的delete stuffs from DB分支都无法执行。
所以根据delete的返回值来决定是否进行DB中数据的删除,那么可能会导致DB中的数据在一段时间内甚至永远得到删除,影响数据一致性。
3.改进:
delete之前先get一下,若为null,则认为删除成功。
public boolean delete(String key){
boolean isCacheDeleted = false;
Object obj = memcachedClient.delete(key);
if(obj == null){
isCacheDeleted = true;
}else{
isCacheDeleted = memcachedClient.delete(key);
}
if(isCacheDeleted){
//delete stuffs from DB
}
//other stuffs....
}
4.其他
还未来得及看gwhalin / Memcached-Java-Client v2.6.3的源码,如果get为null,delete返回值到底如何处理的,后续再更新
to be continued....
>>>>>>>>>>更新<<<<<<<<<<<
查看MemcachedClient源码public boolean delete( String key, Integer hashCode, Date expiry )部分,当server端返回“NOT_FOUND”时,未做任何处理,最后返回false。
else if ( NOTFOUND.equals( line ) ) {
if ( log.isInfoEnabled() )
log.info( "++++ deletion of key: " + key + " from cache failed as the key was not found" );
}