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Im using Express. i can't figure out how to send an image file to client in a way that it will be displayed to HTML tag <img src='/preview/?doc=xxxxxx&image=img1.jpg'>. I'm using Cradle getAttachment function to communicate with Couchdb https://github.com/flatiron/cradle 

db.getAttachment(id, filename, function (err, reply) {
    set('Content-Type', 'image/png');
    res.end(reply);
});

i don't know what reply is exactly and how to transfer that image to client without buffer

DennyHiu
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  • Actually, `reply` is equal to the contents of the file that got requested via `db.getAttachment`. As binary files happen to be quite large, I guess this variant is supposed to be used for text files mostly... – skiqh Jul 26 '14 at 13:23

1 Answers1

6

To transfer an attachment from cradle to a client without buffering, you can pipe its readableStream to the reponse's writableStream.

The long version

A variant of cradle's db.getAttachment returns a readableStream (see streaming from cradle's docs). express' res object on the other hand serves as a writableStream. This means you should be able* to pipe an attachment to it like this:

// respond to a request like '/preview/?doc=xxxxxx&image=img1.jpg'
app.get('/preview/', function(req, res){

  // fetch query parameters from url (?doc=...&image=...)
  var id = req.query.doc
  var filename = req.query.image

  // create a readableStream from the doc's attachment
  var readStream = db.getAttachment(id, filename, function (err) { 
    // note: no second argument
    // this inner function will be executed 
    // once the stream is finished
    // or has failed
    if (err)
      return console.dir(err)
    else
      console.dir('the stream has been successfully piped')
  })
  // set the appropriate headers here
  res.setHeader("Content-Type", "image/jpeg")

  // pipe the attachment to the client's response
  readStream.pipe(res)
})

Or, slightly shorter:

app.get('/preview/', function(req, res){
  res.setHeader("Content-Type", "image/png")
  db.getAttachment(req.query.doc, req.query.image, someErrorHandlerFunction).pipe(res)
})

*I'm not at work, so sadly I cannot verify this code will run. Drop me a line if you have problems.

skiqh
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