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How do I pass an object to the next flow from a flow in spring integration?

Here I have 3 different flows and I'm using spring integration dsl. Let's assume we have prepared an object in flow 1 and I want to pass that object to other flows without disturbing the actual payload that's coming from the gateway. So I just want to add the object somehow in somewhere but not changing the actual payload so that I can use that object in subsequent flows. I can pass that in header but will that be correct to send a big object in header?
Here I'm using scatter gather pattern with three parallel flows.
#Bean
public IntegrationFlow flow() {
return flow ->
flow.handle(validatorService, "validateRequest")
.split()
.channel(c -> c.executor(Executors.newCachedThreadPool()))
.scatterGather(
scatterer ->
scatterer
.applySequence(true)
.recipientFlow(flow1())
.recipientFlow(flow2())
.recipientFlow(flow3()),
gatherer ->
gatherer
.releaseLockBeforeSend(true)
.releaseStrategy(group -> group.size() == 2))
.aggregate(lionService.someMethod())
// here I want to call other Integration flows
.gateway(someFlow())
.to(someFlow2());
}
//Here in this flow I'm calling prepareCDRequestFromLionRequest method in the handle(). This method returns an object1 which is one of the payload(among 3) that will be used after aggregation but I want to prepare another object2 in this method and somehow want to send that object2 to the someFlow() or someFlow2() but I want object1 as a payload.
#Bean
public IntegrationFlow flow1() {
return flow ->
flow.channel(c -> c.executor(Executors.newCachedThreadPool()))
.enrichHeaders(h -> h.errorChannel("flow1ErrorChannel", true))
.handle(cdRequestService, "prepareCDRequestFromLionRequest");
}
//same way I have flow2 and flow3
//validateRequest method
public Object1 validateRequest(LionRequest lionRequest) {
lionValidationHelper.validateRequestAttributes(lionRequest);
// validation code goes here after creating a new request object which will go to the parallel flows
Object1 obj1 = someLogicTocreateTheObject1
Object2 obj2 = someLogicTocreateTheObject2
return object1;
}
*UPDATE - Now as u see above I'm sending object1 as a payload but I need object2 to be sent from here somehow so that I can make use of this object2 in other subsequent flows. This Object2 is a POJO and having different fields so in other flows I'll be getting relevant information which I'll be setting to it's fields. And finally I'll be getting an object which then I'll be using in someFlow(). So I want object to be passed and enhancing that object in different flows that I have.
Or let's suppose after validateRequest I want to create an object and want to pass that to the parallel flows/someFlow somehow but I don't want to hamper the payload that will be coming to the flows. By using header it's achievable but is there a different way to achieve this?

We probably need to see your flows topology and more details about your solution, but there is indeed no problem to transfer big objects in the message headers.
You perhaps may also look into a publish-subscribe or recipient-list solution to send the same message to different sub-flows. This way you can do in those sub-flows whatever you need with request without looking into headers.
UPDATE
So, according your comment about scatter-gather, it sounds like you want to distribute a message to the recipients, gather their results and then continue together with an original, request message.
Since you say that you'd like do not send an extra header to those recipients, but still preserve a request message for post-gather processing, then you may look into a enrich() EI pattern, where your scatter-gather would be a request sub-flow. There you can decide to add a gather result into a header or property of the request payload (if that is possible).
See more info about Content Enricher in docs: https://docs.spring.io/spring-integration/docs/current/reference/html/message-transformation.html#payload-enricher

Spring Webflux Proper Way To Find and Save

I created the below method to find an Analysis object, update the results field on it and then lastly save the result in the database but not wait for a return.
public void updateAnalysisWithResults(String uuidString, String results) {
findByUUID(uuidString).subscribe(analysis -> {
analysis.setResults(results);
computeSCARepository.save(analysis).subscribe();
});
}
This feels poorly written to subscribe within a subscribe.
Is this a bad practice?
Is there a better way to write this?
UPDATE:
entry point
#PatchMapping("compute/{uuid}/results")
public Mono<Void> patchAnalysisWithResults(#PathVariable String uuid, #RequestBody String results) {
return computeSCAService.updateAnalysisWithResults(uuid,results);
}
public Mono<Void> updateAnalysisWithResults(String uuidString, String results) {
// findByUUID(uuidString).subscribe(analysis -> {
// analysis.setResults(results);
// computeSCARepository.save(analysis).subscribe();
// });
return findByUUID(uuidString)
.doOnNext(analysis -> analysis.setResults(results))
.doOnNext(computeSCARepository::save)
.then();
}

Why it is not working is because you have misunderstood what doOnNext does.
Lets start from the beginning.
A Flux or Mono are producers, they produce items. Your application produces things to the calling client, hence it should always return either a Mono or a Flux. If you don't want to return anything you should return a Mono<Void>.
When the client subscribes to your application what reactor will do is call all operators in the opposite direction until it finds a producer. This is what is called the assembly phase. If all your operators don't chain together you are what i call breaking the reactive chain.
When you break the chain, the things broken from the chain wont be executed.
If we look at your example but in a more exploded version:
#Test
void brokenChainTest() {
updateAnalysisWithResults("12345", "Foo").subscribe();
}
public Mono<Void> updateAnalysisWithResults(String uuidString, String results) {
return findByUUID(uuidString)
.doOnNext(analysis -> analysis.setValue(results))
.doOnNext(this::save)
.then();
}
private Mono<Data> save(Data data) {
return Mono.fromCallable(() -> {
System.out.println("Will not print");
return data;
});
}
private Mono<Data> findByUUID(String uuidString) {
return Mono.just(new Data());
}
private static class Data {
private String value;
public void setValue(String value) {
this.value = value;
}
}
in the above example save is a callable function that will return a producer. But if we run the above function you will notice that the print will never be executed.
This has to do with the usage of doOnNext. If we read the docs for it it says:
Add behavior triggered when the Mono emits a data successfully.
The Consumer is executed first, then the onNext signal is propagated downstream.
doOnNext takes a Consumer that returns void. And if we look at doOnNext we see that the function description looks as follows:
public final Mono<T> doOnNext(Consumer<? super T> onNext)`
THis means that it takes in a consumer that is a T or extends a T and it returns a Mono<T>. So to keep a long explanation short, you can see that it consumes something but also returns the same something.
What this means is that this usually used for what is called side effects basically for something that is done on the side that does not hinder the current flow. One of those things could for instance logging. Logging is one of those things that would consume for instance a string and log it, while we want to keep the string flowing down our program. Or maybe we we want to increment a number on the side. Or modify some state somewhere. You can read all about side effects here.
you can of think of it visually this way:
_____ side effect (for instance logging)
/
___/______ main reactive flow
That's why your first doOnNext setter works, because you are modifying a state on the side, you are setting the value on your class hence modifying the state of your class to have a value.
The second statement on the other hand, the save, does not get executed. You see that function is actually returning something we need to take care of.
This is what it looks like:
save
_____
/ \ < Broken return
___/ ____ no main reactive flow
all we have to do is actually change one single line:
// From
.doOnNext(this::save)
// To
.flatMap(this::save)
flatMap takes whatever is in the Mono, and then we can use that to execute something and then return a "new" something.
So our flow (with flatMap) now looks like this:
setValue() save()
______ _____
/ / \
__/____________/ \______ return to client
So with the use of flatMap we are now saving and returning whatever was returned from that function triggering the rest of the chain.
If you then choose to ignore whatever is returned from the flatMap its completely correct to do as you have done to call then which will
Return a Mono which only replays complete and error signals from this
The general rule is, in a fully reactive application, you should never block.
And you generally don't subscribe unless your application is the final consumer. Which means if your application started the request, then you are the consumerof something else so you subscribe. If a webpage starts off the request, then they are the final consumer and they are subscribing.
If you are subscribing in your application that is producing data its like you are running a bakery and eating your baked breads at the same time.
don't do that, its bad for business :D

Subscribe inside a subscribe is not a good practise. You can use flatMap operator to solve this problem.
public void updateAnalysisWithResults(String uuidString, String results) {
findByUUID(uuidString).flatMap(analysis -> {
analysis.setResults(results);
return computeSCARepository.save(analysis);
}).subscribe();
}

Spring WebFlux + ReactiveMongoDB, Can't save entity

I expect that after the execution of the program, Rubber will be saved in the mongo. The result is 200 O.K, but nothing was saved to the database, I suspect that the problem is in the doOnSuccess method. How to use it? Or what else could be the problem?
#PostMapping
public Mono<Rubber> create(#RequestBody Rubber rubber) {
return rubberService.create(rubber);
}
#Override
public Mono<Rubber> create(Rubber rubber) {
return Mono.just(rubber)
.map(rubberToRubberEntityConverter::convert)
.doOnSuccess(rubberRepository::save)
.doOnError((throwable) -> Mono.error(new ApplicationException("Can't create ruber :( ", throwable)))
.map(rubberEntityToRubberConverter::convert);
}
#Repository
public interface RubberRepository extends ReactiveMongoRepository<RubberEntity, String> {
}

Your reactive chain isn't set up correctly:
return Mono.just(rubber)
.map(rubberToRubberEntityConverter::convert)
.doOnSuccess(rubberRepository::save)
You're not actually doing anything reactive here - you're taking a value, wrapping it in a Mono, converting it (synchronously), then performing a side-effect (also synchronously.) In this case, your side-effect is simply setting up the reactive chain to save to the repository (which will return a Mono), but since that Mono is never subscribed to, the save never actually occurs.
Your doOnError() call has a similar issue - you're again returning a Mono rather than performing a side-effect. Instead, you almost certainly want to use onErrorMap() to convert between one error and another.
In short, any time you use doOnSuccess(), doOnError() etc. and use a method that returns a publisher of some description, it's almost always going to be the wrong thing to do. Using Mono.just() is also a hint that you're not starting with a reactive chain - not necessarily wrong in and of itself, but it can be a warning sign that you're not actually creating a "real" reactive chain.
Instead, you probably want something like:
return rubberRepository.save(rubberToRubberEntityConverter.convert(rubber))
.onErrorMap((throwable) -> new ApplicationException("Can't create rubber :( ", throwable))
.map(rubberEntityToRubberConverter::convert);

How are Mono<Void> and Mono.empty() different

As per my understanding, in Spring WebFlux reactor
Mono<Void> refers for a void Mono
Mono.empty() refers to void, as calling anything over this gives a null pointer.
How do these stand different in their usage ?

Mono<T> is a generic type - in your specific situation it represents Void type as Mono<Void>
Mono.empty() - return a Mono that completes without emitting any item.
Let's assume that you got a method:
private Mono<Void> doNothing() {
return Mono.empty();
}
Whe you want to chain anything after the method call it won't work with flatMap as it is a completed Mono.
In case you want continue another job after that method you can use operator then:
doNothing().then(doSomething())

Converting Mono to Pojo without block

Is there a way to convert Mono objects to java Pojo?
I have a web client connecting to 3rd party REST service and instead of returning Mono I have to extract that object and interrogate it.
All the examples I have found return Mono<Pojo> but I have to get the Pojo itself. Currently, I am doing it by calling block() on Pojo but is there a better way to avoid block?
The issue with the block is that after few runs it starts throwing some error like block Terminated with error.
public MyPojo getPojo(){
return myWebClient.get()
.uri(generateUrl())
.headers(createHttpHeaders(headersMap))
.exchange()
.flatMap(evaluateResponseStatus())
.block();
}
private Function<ClientResponse, Mono<? extends MyPojo>> evaluateResponseStatus() {
return response -> {
if (response.statusCode() == HttpStatus.OK) {
return response.bodyToMono(MyPojo.class);
}
if (webClientUtils.isError(response.statusCode())) {
throw myHttpException(response);
// This invokes my exceptionAdvice
// but after few runs its ignored and 500 error is returned.
}
return Mono.empty();
};
}

It's not a good idea to block to operate on value in a reactive stream. Project Reactor offers you a selection of operators for you to handle the objects within a stream.
In your case, you can write getPojo() method like:
public Mono<MyPojo> getPojo() {
return myWebClient.get()
.uri(generateUrl())
.headers(createHttpHeaders(headersMap))
.retrieve()
.onStatus(status -> webClientUtils.isError(status),
response -> Mono.error(myHttpException(response))
.bodyToMono(MyPojo.class)
}
Note that using onStatus method, we replaced the whole evaluateResponseStatus method in your example.
You would use this method like the following:
// some method
...
getPojo()
.map(pojo -> /* do something with the pojo instance */)
...
I strongly advise you to look into Transforming an existing sequence in Project Reactor docs.

Since Webclient.block() is not recommended, the other way to retrieve the values from incoming httpresponse is to create a POJO in the calling application having the required fields. Then once the Mono is received, use Mono.subscribe(), within subscribe add a lambda function, with input say x, to retrieve the individual fields using the x.getters(). The values could be printed on console or assigned to a local var for further processing. This helps in two ways:-
Avoid the dreaded .block()
Keep the call Asynchronous when pulling large volumes of data.
This is one of many other ways to achieve the desired outcome.