Question

I’m looking to write a NPM module that builds on top of node-http-proxy but adds caching support, like squid/varnish, but in node.js.

In my head, it will work like this:

• My NPM module won’t depend on http-proxy, with will just “wrap” any callback given to http.createServer. The idea is that you combine this with http-proxy, but it doesn’t assume that.
• It will look at the req and if it has a cached version, it will serve that.
• If it doesn’t, it will delegate to the original callback, inspecting the response to see if it can be cached.
• Caching takes place, and the actual response is returned.

The logic is pretty straightforward, but I’m not very familiar with node.js internals and concepts and I’m struggling to even figure out how to get the content of the response.

Given this example:

var http = require('http');

http.createServer(function(req, res) {
  res.writeHead(200, { 'Content-Type': 'text/plain; charset=utf-8' })
  res.write('Hello World!');
  res.end();
}).listen(80);

How would I get the data back from that response object? Ideally I’d just ‘observe’ the response and process data as it’s written to it, rather than retrospectively pull data back out of it.

I tried doing:

res.on('data', function(chunk) {
  console.log('Data written: ' + chunk);
});

But that never seems to get called.

I then tried:

res.connection.on('data', function(chunk) {
  console.log('Data written: ' + chunk);
});

But that just dumped the request data, not the response data. What’s the correct way to filter response data, assuming you know nothing about how the response was generated?

If I’m expressing my complete lack of understanding of node and should read some sort of “A newbie guide to node.js concepts” please direct me to this. I’m not lazy, I’m just lost.

EDIT | Looks like I can method-swizzle write() and writeHead() on the response to intercept the invocations to them, but this feels wrong.

Answer 1

Looks like redifinition write and writeHead methods of ServerResponse the only way to do that. Node API has no native events to handle outgoing data.