Possible reason for NGINX 499 error codes

HTTP 499 in Nginx means that the client closed the connection before the server answered the request. In my experience is usually caused by client side timeout. As I know it's an Nginx specific error code.

In my case, I was impatient and ended up misinterpreting the log.

In fact, the real problem was the communication between nginx and uwsgi, and not between the browser and nginx. If I had loaded the site in my browser and had waited long enough I would have gotten a "504 - Bad Gateway". But it took so long, that I kept trying stuff, and then refresh in the browser. So I never waited long enough to see the 504 error. When refreshing in the browser, that is when the previous request is closed, and Nginx writes that in the log as 499.

Elaboration

Here I will assume that the reader knows as little as I did when I started playing around.

My setup was a reverse proxy, the nginx server, and an application server, the uWSGI server behind it. All requests from the client would go to the nginx server, then forwarded to the uWSGI server, and then response was sent the same way back. I think this is how everyone uses nginx/uwsgi and are supposed to use it.

My nginx worked as it should, but something was wrong with the uwsgi server. There are two ways (maybe more) in which the uwsgi server can fail to respond to the nginx server.

1) uWSGI says, "I'm processing, just wait and you will soon get a response". nginx has a certain period of time, that it is willing to wait, fx 20 seconds. After that, it will respond to the client, with a 504 error.

2) uWSGI is dead, or uWSGi dies while nginx is waiting for it. nginx sees that right away and in that case, it returns a 499 error.

I was testing my setup by making requests in the client (browser). In the browser nothing happened, it just kept hanging. After maybe 10 seconds (less than the timeout) I concluded that something was not right (which was true), and closed the uWSGI server from the command line. Then I would go to the uWSGI settings, try something new, and then restart the uWSGI server. The moment I closed the uWSGI server, the nginx server would return a 499 error.

So I kept debugging with the 499 erroe, which means googling for the 499 error. But if I had waited long enough, I would have gotten the 504 error. If I had gotten the 504 error, I would have been able to understand the problem better, and then be able to debug.

So the conclusion is, that the problem was with uWGSI, which kept hanging ("Wait a little longer, just a little longer, then I will have an answer for you...").

How I fixed that problem, I don't remember. I guess it could be caused by a lot of things.

The "client" in "client closed the connection" isn't necessarily the Web browser!

You may find 499 errors in an Nginx log file if you have a load balancing service between your users and your Nginx -- using AWS or haproxy. In this configuration the load balancer service will act as a client to the Nginx server and as a server to the Web browser, proxying data back and forth.

For haproxy the default values for certain applicable timeouts are some 60 seconds for connecting to upstream and for reading from upstream (Nginx) or downstream (Web browser).

Meaning that if after some 60 seconds the proxy hasn't connected to the upstream for writing, or if it hasn't received any data from the downstream (Web browser) or upstream (Nginx) as part of a HTTP request or response, respectively, it will close the corresponding connection, which will be treated as an error by the Nginx, at least, if the latter has been processing the request at the time (taking too long).

Timeouts might happen for busy websites or scripts that need more time for execution. You may need to find a timeout value that will work for you. For example extending it to a larger number, like 180 seconds. That may fix it for you.

Depending on your setup you might see a 504 Gateway Timeout HTTP error in your browser which may indicate that something is wrong with php-fpm. That won't be the case, however, with 499 errors in your log files.