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 */@Servicepublic 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();    }}