java - 即使值存在，也无法单独从 Redis 加载值

Question

我正在使用Reactive Redis，我试图使用Redis作为数据库的缓存。我正在检查缓存中是否存在值？如果存在则返回它，否则如果结果返回则查询数据库；存储结果缓存并返回。

但是，即使 Redis 中存在值，它仍然会一直查询数据库。

public Mono<User> getUser(String email) {
    return reactiveRedisOperation.opsForValue().get("tango").switchIfEmpty(
        // Always getting into this block (for breakpoint) :(
        queryDatabase().flatMap(it -> {
            reactiveRedisOperation.opsForValue().set("tango", it, Duration.ofSeconds(3600)).then(Mono.just(it)); 
        })
    );
}

private Mono<User> queryDatabase() {
    return Mono.just(new User(2L,"test","test","test","test","test",true,"test","test","test"));
}

但是即使 Redis 中存在值，调用也始终会访问数据库。我在这里做错了什么？

Answer 1

基于this answer你可以尝试使用Mono.defer:

public Mono<User> getUser(String email) {
    return reactiveRedisOperation.opsForValue().get("tango").switchIfEmpty(Mono.defer(() -> {
        // Always getting into this block (for breakpoint) :(
        queryDatabase().flatMap(it -> {
            reactiveRedisOperation.opsForValue().set("tango", it, Duration.ofSeconds(3600)).then(Mono.just(it)); 
        })})
    );
}

更新:

我对 Mono 没有太多经验。我指出的答案解释了这一点:

... computation was already triggered at the point when we start composing our Mono types. To prevent unwanted computations we can wrap our future into a defered evaluation:

... is trapped in a lazy supplier and is scheduled for execution only when it will be requested.