External image cacheing

.js

var Promise = require("bluebird");
var bhttp = require("bhttp");

var currentImage;

function fetchNewImage(){
	var fetchPromise = Promise.try(function(){
		/* You can replace this with anything that returns a promise, really... */
		return bhttp.get("http://remote-site.com/image.jpg");
	}).then(function(response){
		/* We just want the body, which is the image data as a Buffer... */
		return response.body;
	});
	
	if (currentImage == null) {
		/* First time we're fetching the image, set the promise immediately. */
		currentImage = fetchPromise;
	} else {
		/* Not the first time; we only update the currentImage value once we're done fetching it. */
		fetchPromise.then(function(image){
			currentImage = fetchPromise;
		})
	}
	
	setTimeout(fetchNewImage, 10 * 60 * 1000); // every 10 minutes
}

fetchNewImage(); // fetch it the first time

/* Actually using the image: */
app.get("/image", function(req, res){
	currentImage.then(function(image){ // this will only run once we *have* an image
		res.set("Content-Type", "image/png"); // or whatever content-type is appropriate for the image
		res.send(image);
	})
})

/* Possible scenarios here:
 * 1) Image fetched for the first time, not done yet; the /image route will wait for completion.
 * 2) Image fetched for the first time, done; the .then in the /image route triggers immediately.
 * 3) Image fetched for the Nth time, not done yet; currentImage still has the old image, and triggers immediately.
 * 4) Image fetched for the Nth time, done; currentImage is replaced with the new image we completed fetching just now.
 */