Last Seen

Apr 18, 2021 java guava caches removal-listener

Use Case

Suppose your manager tells you

“Virg, do you remember where we store MaxMind data?

Can you figure out when we saw an IP the last time around and send it on a stream?

Gini will write a consumer to look at some patterns, also… don’t bombard him”.

Thinking Process

Wait, what is MaxMind? MaxMind is a company that provides location data given an IP. Something like 205.101.100.247 -> Istanbul.

Ok, need a code refresher now.

LoadingCache<String, String> cache =
    CacheBuilder.newBuilder()
                .expireAfterWrite(5, TimeUnit.MINUTES)
                .build(new CacheLoader<String, String>() {
                    @Override
                    public String load(String key) {
                        // Query Table and then MaxMind
                        return UUID.randomUUID() + "_place";
                    }
                });

We have a cache (essentially a key-value map) that stores IP and its location.

After 5 minutes, the cache forgets the mapping.

Coming back to “when did I see it last”? The obvious answer is whenever the cache forgets about it, right?

What can I do about it?

Wonder if this cache has a hook that is invoked when it forgets something?

It has built-in support for it - “removal listeners”.

Code

LoadingCache<String, String> cache =
    CacheBuilder.newBuilder()
                .expireAfterWrite(5, TimeUnit.SECONDS) // changed to SECONDS for local testing
                .removalListener((RemovalListener<String, String>) notification -> {
                    System.out.printf("Removal listener called since %s with %s %s \n", 
                                      notification.getKey(), 
                                      notification.getValue(), 
                                      notification.getCause());
                    // Wrap key, value and time in an object and pass it along the stream for Gini.
                })
                .build(new CacheLoader<String, String>() {
                    @Override
                    public String load(String key) {
                        // Query Table and then MaxMind
                        return UUID.randomUUID().toString().split("-")[0] + "_place";
                    }
                });

Testing

System.out.println(cache.get("205.101.100.247")); // -> prints 26be9f3a_place (totally random place)
System.out.println(cache.get("205.101.100.247")); // -> prints 26be9f3a_place (same random place)

Thread.sleep(10_000);                             // sleep for 10 seconds

System.out.println(cache.get("205.101.100.247")); // -> Removal listener called since 205.101.100.247 with 26be9f3a_place EXPIRED
                                                  // -> and prints 14e3c237_place (totally random place again)

The inline testing comments above should make the behavior of the removal listener hook clear.

Let me know if it doesn't. I'm @anirudhonezero.

Thanks for reading along.

Gotchas

1. The above hook does not get called always (implementation detail) because guava doesn't have its own clean-up threads. It relies on get/write to happen on the same segment of the expired key.
2. The last get call invokes the segment read and the hook. Ref