NodeJS middleware how to read from a writable stream (http.ServerResponse)

Question

I'm working on a app (using Connect not Express) composed of a set of middlewares plus node-http-proxy module, that is, I have a chain of middlewares like:

midA -> midB -> http-proxy -> midC

In this scenario, the response is wrote by the http-proxy that proxies the request to some target and returns the content.

I would like to create a middleware (say midB) to act as a cache. The idea is:

If url is cached the cache-middleware writes the response and avoids continuing the middleares chain.
If url is not cached the cache-middleware passes the request within the middlewares chain bit requires to read the final response content to be cached.

How can achieve this? Or there is another approach?

Cheers

So you don't want client-side caching, but save your db calls (or whatever)? I would say you can just save the response quite before sending it in some sort of hashmap and look it up in your "caching" middleware. — deyhle
– deyhle, Commented Aug 6, 2015 at 9:57
I just updated the question for a better explanation. Thanks — acanimal
– acanimal, Commented Aug 6, 2015 at 11:23
Maybe you can simply use github.com/jpodwys/superagent-cache to make the internal HTTP call – it got everything built-in. — deyhle
– deyhle, Commented Aug 6, 2015 at 11:28

acanimal · Accepted Answer · 2015-08-07 12:19:58Z

0

Answering myself. If you have a middleware like function(req, res, next){..} and need to read the content of the response object.

In this case the res is a http.ServerResponse object, a writable stream where every middleware in the chain is allowed to write content that will conform the response we want to return.

Do not confuse with the response you get when make a request with http.request(), that is a http.IncomingMessage which in fact is a readable stream.

The way I found to read the content all middlewares write to the response is redefining the write method:

var middleare = function(req, res, next) {

  var data = "";
  res._oldWrite = res.write;
  res.write = function(chunk, encoding, cb) {
    data += chunck;
    return res._oldWrite.call(res, chunck, encoding, cb);
  }

  ...
}

Any other solutions will be appreciated.

edited Aug 7, 2015 at 12:19

answered Aug 7, 2015 at 12:08

acanimal

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