How to log error to DB and resume execution in project reactor?

Question

I want to log an error to a mongo db collection and continue with the error propagation in the pipeline. How can I do that in project reactor?

Current code makes use of an extractor(Mono.block()), which to my knowledge, is not advised to use. This is what current code looks like:

        testMono.doOnError(throwable -> {
            var errorObject = ErrorObject.builder()
                    .message(throwable.getLocalizedMessage())
                    .className(throwable.getClass().getSimpleName())
                    .build();
            errorMessageRepository.save(errorObject).block();
        })

Is this correct way to do it or should I use a Mono.subscribe() here?

Answer 1

You should avoid using block().

If you need a fire-and-forget call you could use subscribe():

 testMono.doOnError(throwable -> {
            var errorObject = ErrorObject.builder()
                    .message(throwable.getLocalizedMessage())
                    .className(throwable.getClass().getSimpleName())
                    .build();
            errorMessageRepository.save(errorObject).subscribe();
        })

Note that in that case, you lose backpressure support.

Alternatively, you could use onErrorResume operator like this:

testMono.onErrorResume(throwable -> {
    var errorObject = ErrorObject.builder()
            .message(throwable.getLocalizedMessage())
            .className(throwable.getClass().getSimpleName())
            .build();
    return errorMessageRepository.save(errorObject).then(Mono.error(throwable)));
});

That way, then operator replays the error as is.