Spring Webflux + Sleuth Zipkin: Proof of duplicate requests?

Small question regarding duplicates requests with Spring Webflux + Sleuth Zipkin server please.

I have a server, which code is super simple:

    @PostMapping("/question")
    public Mono<String> question() {
        LOGGER.info("This has been called!");
        return someService.getResponse();
    }

Every hour, I expect only one client that I know to call this endpoint only once.

Therefore, every hour, I do see this in my log:

INFO [myservice,c3a25fb0fb7426b7,c3a25fb0fb7426b7] 10 --- [or-http-epoll-3] c.my.Controller  : This has been called!

So far so good.

The issue is that several times, I did see in my logs:

INFO [myservice,5278cfd673fddc60,1582c3da8d01adaa] 10 --- [or-http-epoll-2] c.my.Controller  : This has been called!
INFO [myservice,5278cfd673fddc60,c8a85b0275b6bfdd] 10 --- [or-http-epoll-3] c.my.Controller  : This has been called!

Very naturally, I assume the only client I know, instead of calling me once as expected, called me twice.

However, the logs on client side shows only one http outbound request has been made.

May I ask, seeing same trace ID, but different Span ID is enough to prove, be hard evidence there is at least two requests sent?

Can the [or-http-epoll-2] and [or-http-epoll-3] help proving as well?

With the only information written here, is it possible to prove anything regarding the duplicates please?

Thank you

Solution

You can prove this by turning on access logs. Having the same traceID for two different log events does not prove anything, it can happen that:

The client called you twice
The client called you once but you created another span
The client called you and another client which also called you

You can enable access logs which can prove this or you can use a rq/rs log library (like logbook) that does this for you. I recommend simply enabling the access logs.