Spring WebFlux with in memory cache

Question

I am using Spring web flux and guava cache. In some of the examples they are using CacheMono.lookup to retrieve the value from the cache. I tried on the same lines and have following code.

CacheMono.lookup(key -> Mono.justOrEmpty(guavaCache.get(id, PhoneNumber.class))
            .map(Signal::next), id)
            .onCacheMissResume(() -> {
              LOGGER.info("fetch from db");
              return phoneNumberRepository.findById(id);})
            .andWriteWith((key, signal) -> Mono.fromRunnable(() ->
                Optional.ofNullable(signal.get())
                    .ifPresent(value -> {
                      if(value == null){
                        LOGGER.info("value is null");
                      }
                      LOGGER.info("value is not null "+value);
                      guavaCache.put(key, value);}))))

For the initial flow when guava cache does not contain the value it is fetching from db and storing into the cache. But for the same key when I send the request again, the cache has the value for the key. But CacheMono.lookup is still executing the fetch from db ( I am seeing LOGGER.info("fetch from db");. But at the same time I am not seeing the logs LOGGER.info("value is not null "+value); I am confused with this behavior. Why onCacheMissResume is called 2nd time when the cache has the data already.

Answer 1

You see LOGGER.info("fetch from db") because it is called every time CacheMono creates Mono<PhoneNumber> to fetching from db. But this Mono<PhoneNumber> won't be subscribed if the value exists in your cache.

You could examine it with logging onSubscribe event:

.onCacheMissResume(() -> {
    LOGGER.info("fetch from db");
    return phoneNumberRepository.findById(id)
        .doOnSubscribe(ignored -> LOGGER.info("really fetching from db"));
})