缓存的Cache Aside模式

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

        <!-- https://mvnrepository.com/artifact/com.github.ben-manes.caffeine/caffeine -->
        <dependency>
            <groupId>com.github.ben-manes.caffeine</groupId>
            <artifactId>caffeine</artifactId>
            <version>2.5.5</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/com.google.guava/guava -->
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>22.0</version>
        </dependency>

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<Integer> db = new AtomicReference<>(1);

        LoadingCache<String, Integer> cache = CacheBuilder.newBuilder()
                .build(
                new CacheLoader<String, Integer>() {
                    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 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

使用caffeine

@Test
    public void testCacheDirty() throws InterruptedException, ExecutionException {
        AtomicReference<Integer> db = new AtomicReference<>(1);

        com.github.benmanes.caffeine.cache.LoadingCache<String, Integer> 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 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

这里可以看到invalidate的时候，loading又重新触发了一次，然后脏数据就清除了

doc

• 缓存更新的套路

• caffeine: Java 8高性能缓存库包