renenglish Wrote:
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> Sorry I can’t get it .
>
> If host A has added the counter and failed to response, the request
> would be failed over to host B with successful response, so the
> counter would be added twines. Wouldn’t it ?

Then a condition must occur where host A fails right after processing the request, they usually fail before accepting a request, it also depends on the timeout for a request. And it also depends what nginx considers a fail, nginx might not fail a host when it does not return from accepting a request.

You will have to simulate this to find out the conditions for a fail.