Handle Reactor errors by mapping to new return value without dropping any elements - spring-webflux

Question

In a scenario where an input value is mapped to a response object which contains either a success message or a failure message along with an error message, how can I handle errors correctly without dropping any elements from the publisher?
I have a domain object which follows the Builder pattern, and it performs validation on build, throwing an Exception which includes the object's ID.
To process this I've tried the following 2 attempts:
public Flux<GenericResponse> handle(Publisher<DomainDto> input) {
return Flux.from(input)
.map(c -> c.toDomain()) // this is what throws the exception
// some other processing here
.map(c -> GenericResponse.accepted(c.getId()))
.onErrorResume(e ->
Flux.just(GenericResponse.error(((BadRequestException)e).getId(), e.getMessage()))
);
}
public Flux<GenericResponse> handle(Publisher<DomainDto> input) {
return Flux.from(input)
.map(c -> c.toDomain()) // this is what throws the exception
// some other processing here
.concatMap(c ->
Flux.just(GenericResponse.accepted(c.getId()))
.onErrorResume(e ->
Flux.just(GenericResponse.error(((BadRequestException)e).getId(), e.getMessage()))
)
);
}
With the first, if I send through 5 inputs and the third is expected to throw an error, I get 2 success messages, and one failure message as expected:
[
{ "id": 1, "code": "ACCEPTED" },
{ "id": 2, "code": "ACCEPTED" },
{ "id": 3, "code": "ERROR", "description": "Some error message" }
]
However with the second attempt using the same inputs, I get an actual stack trace with no results:
{
"timestamp": 1627880204616,
"path": "/",
"status": 500,
"error": "Internal Server Error",
"message": "Some error message",
"requestId": "2917b3af-1",
"trace": "com.example.BadRequestException: Some error message..."
}
What can I do to get a response for all inputs as below:
[
{ "id": 1, "code": "ACCEPTED" },
{ "id": 2, "code": "ACCEPTED" },
{ "id": 3, "code": "ERROR", "description": "Some error message" },
{ "id": 4, "code": "ACCEPTED" },
{ "id": 5, "code": "ACCEPTED" }
]

Answer 1

If you want to manage error element per element, your best option is to use an intermediate Mono to manage errors like so :
public Flux<GenericResponse> handle(Publisher<DomainDto> input) {
return Flux.from(input)
.flatMap(c -> Mono.fromCallable(() -> c.toDomain())
.map(c -> GenericResponse.accepted(c.getId()))
.onErrorResume(BadRequestException.class, e
-> Mono.just(GenericResponse.error(e.getId(), e.getMessage())));
}
Another option is to use the onErrorContinue operator. It is very close to your original code. However, as stated by the documentation, it is not always safe to use, because in case an error has broken the upstream pipeline, it cannot properly "continue" the flow operations.
Example using onErrorContinue:
public Flux<GenericResponse> handle(Publisher<DomainDto> input) {
return Flux.from(input)
.map(c -> c.toDomain()) // this is what throws the exception
// some other processing here
.map(c -> GenericResponse.accepted(c.getId()))
.onErrorContinue(BadRequestException.class, e ->
Flux.just(GenericResponse.error(e.getId(), e.getMessage()))
);
}