摘要：序本文主要講述下緩存的模式。更新是先更新數(shù)據(jù)庫，成功后，讓緩存失效為什么不是寫完數(shù)據(jù)庫后更新緩存主要是怕兩個并發(fā)的寫操作導(dǎo)致臟數(shù)據(jù)。

本文主要講述下緩存的Cache Aside模式。

有兩個要點：

應(yīng)用程序先從cache取數(shù)據(jù)，沒有得到，則從數(shù)據(jù)庫中取數(shù)據(jù)，成功后，放到緩存中。

更新是先更新數(shù)據(jù)庫，成功后，讓緩存失效.為什么不是寫完數(shù)據(jù)庫后更新緩存？主要是怕兩個并發(fā)的寫操作導(dǎo)致臟數(shù)據(jù)。

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);
};

一個是讀操作，但是沒有命中緩存，然后就到數(shù)據(jù)庫中取數(shù)據(jù)，此時來了一個寫操作，寫完數(shù)據(jù)庫后，讓緩存失效，然后，之前的那個讀操作再把老的數(shù)據(jù)放進(jìn)去，所以，會造成臟數(shù)據(jù)。

這個case理論上會出現(xiàn)，不過，實際上出現(xiàn)的概率可能非常低，因為這個條件需要發(fā)生在讀緩存時緩存失效，而且并發(fā)著有一個寫操作。而實際上數(shù)據(jù)庫的寫操作會比讀操作慢得多，而且還要鎖表，而讀操作必需在寫操作前進(jìn)入數(shù)據(jù)庫操作，而又要晚于寫操作更新緩存，所有的這些條件都具備的概率基本并不大。

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

這里使用代碼復(fù)現(xiàn)一下這個臟數(shù)據(jù)場景。

讀操作進(jìn)來，發(fā)現(xiàn)沒有cache，則觸發(fā)loading，獲取數(shù)據(jù)，尚未返回

寫操作進(jìn)來，更新數(shù)據(jù)源，invalidate緩存

loading獲取的舊數(shù)據(jù)返回，cache里頭存的是臟數(shù)據(jù)

@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); //這里1秒才返回,模擬引發(fā)臟緩存
                        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();

        //這里在t2 invalidate 之前 先觸發(fā)cache loading
        //loading那里增加sleep,確保在invalidate之后,cache loading才返回
        //此時返回的cache就是臟數(shù)據(jù)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"));
        }
    }

輸出

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

@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); //這里1秒才返回,模擬引發(fā)臟緩存
                    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();

        //這里在t2 invalidate 之前 先觸發(fā)cache loading
        //loading那里增加sleep,確保在invalidate之后,cache loading才返回
        //此時返回的cache就是臟數(shù)據(jù)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"));
        }
    }

輸出

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

這里可以看到invalidate的時候，loading又重新觸發(fā)了一次，然后臟數(shù)據(jù)就清除了

緩存更新的套路

caffeine: Java 8高性能緩存庫包