Memcached 使用记录（以及解决数据不同步的问题的方案）

因为使用了memcached， 所以第一次查找的时候把内容缓存到了memcached里面去了，这样第二次查看的时候就是从memcached里面去取，但是有个坏处是可能第一次查找完，立即内容有更新了，所以导致取到的数据不是最新的。

解决数据不同步的问题

常规做法有两种：
1、数据有改变则在添加、更新、删除时数据的同时更新memcache

2、数据有改变则数据库有一个字段来标识更新时间，例如update_time。当有查询时候，把memcache里的update_time与数据库的update_time比较。如果发生变化则更新memcache

超期时间设置，应根据数据访问的频繁度和内存大小来权衡一下。

注意：memcache内存申请后，映像中不会被释放。它是通过 如果有一部分数据超时，则新写入的数据覆盖超时数据来完成重复使用的。那么理论上来讲，内存使用情况是数据被memcache占用最多的那一刻！

据说不是分布式的话就没必要弄什么memcached.还不如优化数据库.数据结构 来得实在点.

eg:数据有改变则在添加、更新、删除时数据的同时更新memcache的应用场景：

package com.school.business.impl;

import java.util.List;

import net.spy.memcached.MemcachedClient;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.school.business.NavigationManager;
import com.school.dao.NavigationMapper;
import com.school.domain.Navigation;
import com.school.domain.NavigationExample;
import com.school.stereotype.NavigationType;
import com.school.stereotype.YesNoStatus;

/**
* {@link NavigationManager}接口的实现。
* @author
*/
@Service
public class NavigationManagerImpl implements NavigationManager {

private static final Logger LOG = LoggerFactory.getLogger(NavigationManagerImpl.class);

@Autowired
private NavigationMapper navigationMapper;

@Autowired
private MemcachedClient memcachedClient;

@Override
@SuppressWarnings("unchecked")
public List<Navigation> getNavigations(String schoolId, NavigationType navigationType) {
String key = this.getNavigationsMemcachedKey(schoolId, navigationType);
Object obj = null;
try {
obj = memcachedClient.get(key);
} catch (Exception e) {
LOG.warn("getNavigations from Memcached error. Cause: ", e);
}

if (null != obj) {
return (List<Navigation>) obj;
} else {
NavigationExample example = new NavigationExample();
example.createCriteria().andSchoolIdEqualTo(schoolId).andTypeEqualTo(navigationType.getValue())
.andStatusEqualTo(YesNoStatus.YES.getValue());
example.setOrderByClause(" ordered ASC ");

List<Navigation> navigations = navigationMapper.selectByExample(example);

int exp = 60 * 60 * 10; // 10小时后过期//超期时间设置，应根据数据访问的频繁度和内存大小来权衡一下
try {
memcachedClient.set(key, exp, navigations);
} catch (Exception e) {
LOG.warn("set getNavigations to Memcached error. Cause: ", e);
}

return navigations;
}
}

@Override
public List<Navigation> getNavigations(NavigationExample example) {
example.setOrderByClause(" ordered ASC ");
return navigationMapper.selectByExample(example);
}

@Override
public Navigation getNavigation(String navigationId) {
return navigationMapper.selectByPrimaryKey(navigationId);
}
//添加数据
@Override
public boolean addNavigation(String schoolId, Navigation navigation) {
int rows = navigationMapper.insertSelective(navigation);

try {
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.HEADER));
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.FOOTER));
} catch (Exception e) {
LOG.warn("addNavigation from Memcached error. Cause: ", e);
}

return (rows > 0);
}
<span style="white-space:pre">	</span>//更新数据时，删掉memcached缓存
@Override
public boolean updateNavigation(String schoolId, Navigation navigation) {
int rows = navigationMapper.updateByPrimaryKeySelective(navigation);

try {
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.HEADER));
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.FOOTER));
} catch (Exception e) {
LOG.warn("updateNavigation from Memcached error. Cause: ", e);
}

return (rows > 0);
}
// 删除数据
@Override
public boolean deleteNavigation(String schoolId, String navigationId) {
int rows = navigationMapper.deleteByPrimaryKey(navigationId);

try {
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.HEADER));
memcachedClient.delete(this.getNavigationsMemcachedKey(schoolId, NavigationType.FOOTER));
} catch (Exception e) {
LOG.warn("deleteNavigation from Memcached error. Cause: ", e);
}

return (rows > 0);
}

/**
* 构建缓存在Memcached中的Key。
*/
private String getNavigationsMemcachedKey(String schoolId, NavigationType navigationType) {
return schoolId + navigationType.getValue();
}
}