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    <p>Cross-posted from NodeJS google groups:</p> <p>Okay i'll try my best to answer your question:</p> <p>Nginx is a web server that only proxies requests. Now if you take the example of Nginx+php+fpm or Nginx+wsgi+ruby you are having an asynchronous, evented web server sitting in front of webserver that is executing synchronously. So Nginx will accept() as many connections as possible and all of them would be queued. The requests from Nginx to your backend synchronous server would be asynchronous. But your backend synchronous server which also does accept() is not queuing any connections. It can serve only one request at a time (considering you are single threaded) and multiple requests at a time (prefork/fork(slow)/multithreaded -> has its own drawbacks like thread creation time(can be avoided with thread-pools but PITA to implement), context switches, thread deadlocks, number of connections accept()ed can never be greater than number of threads etc)</p> <p>Imagine you have 2 routes to your backend server that Nginx is hitting:</p> <p>/404, /login.</p> <p>If the /login route is doing a lot of I/O and if another request is made to /404, the rendering of the /404 page will depend on the completion of /login's request (because the process is blocked). So basically the response to any request will depend on the request that takes the longest time to do I/O. So even though Nginx is async and evented its response time for any request will depend entirely on that one request that takes the longest time to finish (culprit: the synchronous backend server).</p> <p>Now if you take the example of NodeJS, everything is asynchronous and evented. Be it File/Network I/O etc. So nothing blocks the process. So taking the previous example, even if /login route is doing a lot of I/O its all asynchronous and /404 page is rendered immediately.</p> <p>My explanation is quite rudimentary. But I think it should give you more clarity.</p>
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    1. POBeing both event-driven servers, why node.js needs async code where Nginx doesn't?
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    1. POBeing both event-driven servers, why node.js needs async code where Nginx doesn't?
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    1. COThank you! That explains right where the bottleneck is with php-fpm or others despite Nginx being event-driven.
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