缓存的Cache Aside模式-白红宇

缓存的Cache Aside模式

阅读量：6835 次

发布时间：2019-06-26

本文共 5935 字，大约阅读时间需要 19 分钟。

序

本文主要讲述下缓存的Cache Aside模式。

Cache Aside

有两个要点：

应用程序先从cache取数据，没有得到，则从数据库中取数据，成功后，放到缓存中。

更新是先更新数据库，成功后，让缓存失效.为什么不是写完数据库后更新缓存？主要是怕两个并发的写操作导致脏数据。

public V read(K key) {  V result = cache.getIfPresent(key);  if (result == null) {    result = readFromDatabase(key);    cache.put(key, result);  }  return result;}public void write(K key, V value) {  writeToDatabase(key, value);  cache.invalidate(key);};

脏数据

一个是读操作，但是没有命中缓存，然后就到数据库中取数据，此时来了一个写操作，写完数据库后，让缓存失效，然后，之前的那个读操作再把老的数据放进去，所以，会造成脏数据。

这个case理论上会出现，不过，实际上出现的概率可能非常低，因为这个条件需要发生在读缓存时缓存失效，而且并发着有一个写操作。而实际上数据库的写操作会比读操作慢得多，而且还要锁表，而读操作必需在写操作前进入数据库操作，而又要晚于写操作更新缓存，所有的这些条件都具备的概率基本并不大。

maven


            
                
     
      com.github.ben-manes.caffeine
                 
     
      caffeine
                 
     
      2.5.5
             
            
            
                
     
      com.google.guava
                 
     
      guava
                 
     
      22.0

代码复现

这里使用代码复现一下这个脏数据场景。

读操作进来，发现没有cache，则触发loading，获取数据，尚未返回

写操作进来，更新数据源，invalidate缓存

loading获取的旧数据返回，cache里头存的是脏数据

@Test    public void testCacheDirty() throws InterruptedException, ExecutionException {        AtomicReference
    
      db = new AtomicReference<>(1);        LoadingCache
     
       cache = CacheBuilder.newBuilder()                .build(                new CacheLoader
      
       () {                    public Integer load(String key) throws InterruptedException {                        LOGGER.info("loading reading from db ...");                        Integer v = db.get();                        LOGGER.info("loading read from db get:{}",v);                        Thread.sleep(1000L); //这里1秒才返回,模拟引发脏缓存                        LOGGER.info("loading Read from db return : {}",v);                        return v;                    }                }        );        Thread t2 = new Thread(() -> {            try {                Thread.sleep(500L);            } catch (InterruptedException e) {                e.printStackTrace();            }            LOGGER.info("Writing to db ...");            db.set(2);            LOGGER.info("Wrote to db");            cache.invalidate("k");            LOGGER.info("Invalidated cached");        });        t2.start();        //这里在t2 invalidate 之前 先触发cache loading        //loading那里增加sleep,确保在invalidate之后,cache loading才返回        //此时返回的cache就是脏数据了        LOGGER.info("fire loading cache");        LOGGER.info("get from cache: {}",cache.get("k"));        t2.join();        for(int i=0;i<3;i++){            LOGGER.info("get from cache: {}",cache.get("k"));        }    }

输出

15:54:05.751 [main] INFO com.example.demo.CacheTest - fire loading cache15:54:05.772 [main] INFO com.example.demo.CacheTest - loading reading from db ...15:54:05.772 [main] INFO com.example.demo.CacheTest - loading read from db get:115:54:06.253 [Thread-1] INFO com.example.demo.CacheTest - Writing to db ...15:54:06.253 [Thread-1] INFO com.example.demo.CacheTest - Wrote to db15:54:06.253 [Thread-1] INFO com.example.demo.CacheTest - Invalidated cached15:54:06.778 [main] INFO com.example.demo.CacheTest - loading Read from db return : 115:54:06.782 [main] INFO com.example.demo.CacheTest - get from cache: 115:54:06.782 [main] INFO com.example.demo.CacheTest - get from cache: 115:54:06.782 [main] INFO com.example.demo.CacheTest - get from cache: 115:54:06.782 [main] INFO com.example.demo.CacheTest - get from cache: 1

使用caffeine

@Test    public void testCacheDirty() throws InterruptedException, ExecutionException {        AtomicReference
    
      db = new AtomicReference<>(1);        com.github.benmanes.caffeine.cache.LoadingCache
     
       cache = Caffeine.newBuilder()                .build(key -> {                    LOGGER.info("loading reading from db ...");                    Integer v = db.get();                    LOGGER.info("loading read from db get:{}",v);                    Thread.sleep(1000L); //这里1秒才返回,模拟引发脏缓存                    LOGGER.info("loading Read from db return : {}",v);                    return v;                });        Thread t2 = new Thread(() -> {            try {                Thread.sleep(500L);            } catch (InterruptedException e) {                e.printStackTrace();            }            LOGGER.info("Writing to db ...");            db.set(2);            LOGGER.info("Wrote to db");            cache.invalidate("k");            LOGGER.info("Invalidated cached");        });        t2.start();        //这里在t2 invalidate 之前 先触发cache loading        //loading那里增加sleep,确保在invalidate之后,cache loading才返回        //此时返回的cache就是脏数据了        LOGGER.info("fire loading cache");        LOGGER.info("get from cache: {}",cache.get("k"));        t2.join();        for(int i=0;i<3;i++){            LOGGER.info("get from cache: {}",cache.get("k"));        }    }

输出

16:05:10.141 [main] INFO com.example.demo.CacheTest - fire loading cache16:05:10.153 [main] INFO com.example.demo.CacheTest - loading reading from db ...16:05:10.153 [main] INFO com.example.demo.CacheTest - loading read from db get:116:05:10.634 [Thread-1] INFO com.example.demo.CacheTest - Writing to db ...16:05:10.635 [Thread-1] INFO com.example.demo.CacheTest - Wrote to db16:05:11.172 [main] INFO com.example.demo.CacheTest - loading Read from db return : 116:05:11.172 [main] INFO com.example.demo.CacheTest - get from cache: 116:05:11.172 [Thread-1] INFO com.example.demo.CacheTest - Invalidated cached16:05:11.172 [main] INFO com.example.demo.CacheTest - loading reading from db ...16:05:11.172 [main] INFO com.example.demo.CacheTest - loading read from db get:216:05:12.177 [main] INFO com.example.demo.CacheTest - loading Read from db return : 216:05:12.177 [main] INFO com.example.demo.CacheTest - get from cache: 216:05:12.177 [main] INFO com.example.demo.CacheTest - get from cache: 216:05:12.177 [main] INFO com.example.demo.CacheTest - get from cache: 2