How to deal with nested levels of Unis

[Moicen]

I need to implement a @ServerRequestFilter, and it calls several levels of async calls, which generates nested Unis, and here is the screenshot of the code:

However this makes a problem that makes the return type has nested level of Unis :

And this breaks the convention of @ServerRequestFilter:

Is there any solution to my problem?

[Moicen]

If I add an await() to reduce one level of Uni for nested Uni<Uni<...>>:

Then it generates following problem:

So it’s not a possible solution.

[Moicen]

Here are the dependencies I’m using:

<dependency>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-rest-client-reactive</artifactId>
</dependency>
<dependency>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-resteasy-reactive</artifactId>
</dependency>

In addition I use ReactiveRedisClient:

[benkard]

You can always do uni.flatMap(Function.identity()) to go from Uni<Uni<T>> uni to Uni<T>.

In your particular case, it may be sufficient to replace one of the .transform calls with .transformToUni.

[Moicen]

I replaced transform() with transformToUni() and it works:

Thanks for help!

[Moicen]

@benkard Now I have further problem:

From above screenshot, you can see that by using transformToUni(), the above filter logic can work. But I have to comment out the tokenCacheeService.setRequestSudoerToken(token) method in the nested uc.verifyJwtToken() block.

If I enable it by using transform() method, and do a return null inside the block(here subscribe().with() together with a sleep() is not working because the subscribe() method doesn’t executed at all even after sleep() a while):

then it will produce the following error:

This is because now there is one extra level of nested onItem().invoke() which makes the return type changed. So I use transformToUni() in this level but it generates following error:

I have no idea how to further fixing it.

[Moicen]

I have separated out the root cause:

From above screenshot you can see I have aligned two nested levels of Uni<Uni<...>>, and now the error becomes:

So the transformToUni() seems not working for this two nested levels of Unis, is there any solution to this?

[benkard]

I don’t know what all the types are here, but I suspect that inner .transform again needs to be .transformToUni.

Aside: Personally, I tend to split any such Uni pipeline into smaller methods and compose those with the .transform etc. combinators. That way it is much easier to locate problems like this. Method references are super useful here – my own code tends to look more like return doThis.flatMap(this::doThat).flatMap(this::doAnotherThing); and so on.

[Moicen]

@benkard I simplified the code did some further tests, and here’s the result:

So the root cause seems because of the onFailure().recoverWithItem() method provided:

Do you have any idea on this?

[benkard]

null tends to throw off type inference. Try casting it to the desired type.

[Moicen]

@benkard By casting the null in onItem() and using transformToUni(), seems it works:

I guess this problem is solved, if I have any more problem I’ll start a new thread. Thanks for help!

In addition here’s all the working combinations if anyone else need it:

// All the working methods

@ServerRequestFilter
public Uni<RestResponse<Void>> check(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem();
}

@ServerRequestFilter
public Uni<RestResponse<Void>> check2(ContainerRequestContext ctx) throws Exception {
    return null;
}

@ServerRequestFilter
public Uni<RestResponse<Void>> check3(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem().onItem().transform(
            _ok -> null
    );
}

@ServerRequestFilter
public Uni<RestResponse<Void>> check4(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem().onItem().transformToUni(
            _ok -> (Uni<RestResponse<Void>>) null
    ).onFailure().recoverWithItem(
            _bad -> null
    );
}

@ServerRequestFilter
public Uni<RestResponse<Void>> check4b(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem().onItem().transformToUni(
            _ok -> (Uni<RestResponse<Void>>) null
    ).onFailure().recoverWithItem(
            _bad -> RestResponse.status(Response.Status.FORBIDDEN)
    );
}

@ServerRequestFilter
public Uni<RestResponse<Void>> check5(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem().onItem().transformToUni(
            _ok -> Uni.createFrom().nullItem().onItem().transform(_ok2 -> null)
    );
}

[Moicen]

Additional example:

@ServerRequestFilter
public Uni<RestResponse<Void>> check6(ContainerRequestContext ctx) throws Exception {
    return Uni.createFrom().nullItem().onItem().transformToUni(
            _ok -> Uni.createFrom()
                    .nullItem()
                    .onItem()
                    .transformToUni(_ok2 -> (Uni<RestResponse<Void>>) null)
                    .onFailure().recoverWithItem(
                            err -> null
                    )
    ).onFailure().recoverWithItem(
            err -> null
    );
}

[Moicen]

After applying what I have learned from this thread, I finally make my filter work.

But here is one more note to take:

As the screenshot shown above, I can’t directly use null in onItem().transformToUni() method, as it will produce mapper error on null value.

In addition, I can’t use Uni.createFrom().nullitem() too, because it’s implementation is:

And it doesn’t conform to the return type, so I have to cast the type by myself:

return Uni.createFrom().item((RestResponse<Void>) null);

At last, in onFailure().recoverWithItem() block, I can’t use Uni.createFrom() same with onItem() method, which is confusing to me, and I need to dig more into detail implementation of Mutiny.

Anyway I have learned a lot and it’s fun!

[benkard]

Uni.createFrom().<RestResponse<Void>>nullItem();

[PrakharSrivastav]

This is really great. Well documented and I learned so much in this single thread than what I did in part week.