The Node.js event loop - nginx/apache

Question

Both nginx and Node.js have event loops to handle requests. I put nginx in front of Node.js as has been recommended here

Using Node.js only vs. using Node.js with Apache/Nginx

with the setup shown here

Node.js + Nginx - What now?

1. How do the two event loops play together? Is there any risk of conflicts between the two? I wonder because Nginx may not be able to handle as many events per second as Node.js or vice versa. For example, if Nginx can handle 1000 events per second but node.js only 500, won't that cause issues? (I have no idea if 1000,500 are reasonable orders of magnitude, you could correct me on that.)

2. What about putting Apache in front of Node.js? Apache has no event loop. Just threads. So won't putting Apache in front of Node.js defeat the purpose?

3. In this 2010 talk, Node.js creator Ryan Dahl had vision to get rid of nginx/apache/whatever entirely and make node talk directly to the internet. When do you think this will be reality?

Answer 1

1. Both nginx and Node use an asynchronous and event-driven approach. The communication between them will go more or less like this:

   • nginx receives a request
   • nginx forwards the request to the Node process and immediately goes back to wait for more requests
   • Node receives the request from nginx
   • Node handles the request with minimal CPU usage, until at some point it needs to issue one or more I/O requests (read from a database, write the response, etc). At this point it launches all these I/O requests and goes back to wait for more requests.
   • The above can repeat lots of times. You could have hundreds of thousands of requests all in a non-blocking wait state where nginx is waiting for Node and Node is waiting for I/O. And while this happens both nginx and Node are ready to accept even more requests!
   • Eventually async I/O started by the Node process will complete and a callback function will get invoked.
   • If there are still I/O requests that haven't completed for this request, then Node goes back to its loop one more time. It can also happen that once an I/O operation completes this data is consumed by the Node callback and then new I/O needs to happen, so Node can start more async I/O requests before going back to the loop.
   • Eventually all I/O operations started by Node for a particular request will be complete, including those that write the response back to nginx. So Node ends this request, and then as always goes back to its loop.
   • nginx receives an event indicating that response data has arrived for a request, so it takes that data and writes it back to the client, once again in a non-blocking fashion. When the response has been written to the client and event will trigger and nginx will then end the request.

   You are asking about what would happen if nginx and Node can handle a different number of maximum connections. They really don't have a maximum, the maximum in general comes from operating system configuration, for example from the maximum number of open handles the system can have at a time or the CPU throughput. So your question does not really apply. If the system is configured correctly and all processes are I/O bound, neither nginx or Node will ever block.

2. Putting Apache in front of Node will only work well if you can guarantee that your Apache never blocks (i.e it never reaches its maximum connection limit). This is hard/impossible to achieve for large number of connections, because Apache uses an individual process or thread for each connection. nginx and Node scale really well, Apache does not.

3. Running Node without another server in front works fine and it should be okay for small/medium load sites. The reason putting a web server in front of it is preferred is that web servers like nginx come with features that Node does not have and you would need to implement yourself. Things like caching, load balancing, running multiple apps from the same server, etc.

Answer 2

I think your questions have been largely covered by some of the others answers, but there are a few pieces missing, and some that I disagree with, so here are mine:

1. The event loops are isolated from each other at the process level, but do interact. The issues you're most likely to encounter are around the configuration of nginx response buffers, chunked data, etc. but this is optimisation rather than error resolution.

2. As you point out, if you use Apache you're nullifying the benefit of using Node.js, i.e. massive concurrency and websockets. I wouldn't recommend doing that.

3. People are already using Node.js at the front of their stack. Searching for benchmarks returns some reasonable-looking results in Node's favour, so performance to my mind isn't an issue. However, there are still reasons to put Nginx in front of Node.

   1. Security - Node has been given increasing scrutiny, but it's still young. You may not have problems here, but caution is often your friend.

   2. Training - Ops staff that you hire will know how to manage Nginx, but the configuration and management of your custom Node app will only ever be understood by those people your developers successfully communicate it to. In some companies this is nobody.

   3. Operational Flexibility - If you reach scale you might want to split out the serving of static content, purely to reduce the load on your app servers. You might want to split content amongst different domains and have it managed separately, or have different SSL or proxying behaviour for different domains or URL patterns. These are the things that are easy for Ops guys to configure in Nginx, but you'd have to code manually in a Node app.

Answer 3

1. The event loops are independent. Event loops are implemented at the application level, so neither cares what sort of architecture the other uses.

2. NodeJS is good at many things, but there are some places where it still falters. Once example is serving static files. At the moment, nodejs performs fairly poorly in this test, so having a dedicated web server for your static files greatly improves response time. Also, nodejs is still in its infancy, and has not been "tested and hardened" in the matters of security like Apache on nginX.

3. It'll take a long time for people to consider fronting nodejs all by itself. The cluster module is a step in the right direction, but it'll take a long time even after it reaches v1 before it happens.

Answer 4

1. Both event loops are unrelated. They don't play together.
2. Yes, it is pretty useless. Apache is not a load balancer.
3. What Ryan Dahl said may be applicable already. The limit of concurrent users is definitely higher than that of Apache. Before node.js websites with fair amount of concurrent users had to use nginx to balance the load. For small to medium sized businesses it can be done with node.js alone. But ruling out nginx completely will take time. Let node.js be stable before it can follow this ambitious dream.