What is event driven io (context: Apache MINA, JBoss Netty)?

Question

I want to understand What is event driven io. I am hearing it is different than traditional blocking request/response model. Do we have any example to explain this? and how will it contribute to the increase in performance?

Examples will be highly appreciated.

Answer 1

This is a very old question but I assume this might help some body else to understand Event driven programming :

This following analogy might help you to understand event driven I/O programming by drawing a parallel to waiting line at Doctor's Reception desk.

Blocking I/O is like, if you are standing in the queue, receptionist asks a guy in front of you to fill in the form and she waits till he finishes. You have to wait for your turn till the guy finishes his form, this is blocking.

If single guy takes 3 mins to fill in, the 10th guy have to wait till 30 minutes. Now to reduce this 10th guys wait time, solution would be, increasing number of receptionist's, which is costly. This is what happens in traditional web servers. If you request for a user info, subsequent request by other users should wait till the current operation, fetching from Database, is completed. This increases the "time to response" of the 10th request and it increase exponentially for nth user. To avoid this traditional web servers creates thread (equivalent to increasing number of receptionists) for every single request, ie., basically it creates a copy of the server for each request which is costly interms of CPU consumption since every request will need a Operating systems thread. To scale up the app, you would have to throw lots of computation power at the app.

Event Driven: The other approach to scale up queue's "response time" is to go for event driven approach, where guy's in the queue will be handed over the form, asked to fill in and come back on completion. Hence receptionist can always take request. This is exactly what javascript has been doing since from it's inception. In browser, javascript would respond to user click event, scroll, swipe or database fetch and so on. This is possible in javascript inherently, because javascript treats functions as first class objects and they can be passed as a parameters to other functions (called callbacks), and can be called on completion of particular task. This is what exactly node.js does on the server. You can find more info about event driven programming and blocking i/o, in the context of node here

Answer 2

I'm guessing since it's been 4 months you've got your answers. Regardless here goes...

Netty http://www.jboss.org/netty

Mina http://mina.apache.org/

C10K http://www.kegel.com/c10k.html

To understand part of the problem that evented io is trying to solve take a look at the C10K link above. Scability is one of the main benefits of evented io.

A traditional web server will handle a request and then return a response (synchronous/blocking). Each request would typically require it's own thread.

An event driven web server will handle a request, then create an event (asynchronous/nonblocking io), and then return the response. Multiple requests are shared by a single thread/process.

Evented IO should be able to handle more requests per thread than a typical web server. You might not speed up your web application with evented IO, but it should handle large numbers of connections a lot easier than a traditional web server. This means requiring less machines for scaling.

Though I would argue that evented io architecture will force you to develop your web application to handle smaller chunks of data. Much like a google mail type application that uses a lot of ajax calls to poll for data on the server and then does small updates in the browser. This itself has many benefits that will help speed up AND improve scaling on your server.

Netty and Mina provide plenty of example code.