Http status code 499 is a non-standard status code introduced by nginx for the case when a client closes the connection while nginx is processing the request.
I just encountered the 499 code in the nginx log a couple days ago. The reason of that is I wrote a Ajax to make a API call to the backend where nginx is the reverse proxy, but the page get refreshed before the backend sends back the response.
To fix it, I added ‘return false;‘ to stop the page to reload.
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