Why does Google Analytics use __utm.gif?

Question

Just trying to understand why they didn't use a REST API.

Answer 1

In REST, clients initiate requests to servers for resources; servers process those requests and return appropriate responses.

The utm.gif is not involved in server-to-client data transfer, but instead it's involved in moving data in the other direction.

Of course REST has HTTP methods for the client to communicate with servers (GET and POST) and indeed, Google Analytics directs the client's browser to send all analytics data to the GA servers via a GET Request. More precisely, a GET Request is comprised of a Request URL and Request Headers (e.g., Referer and User-Agent Headers).

All GA data--every single item--is assembled and packed into the Request URL's query string (everything after the '?'). But in order for that data to go from the client (where it is created) to the GA server (where it is logged and aggregated) there must be an HTTP Request, so the ga.js (google analytics script that's downloaded, unless it's cached, by the client, as a result of a function called when the page loads) directs the client to assemble all of the analytics data--e.g., cookies, location bar, request headers, etc.--concatenate it into a single string and append it as a query string to a URL (http://www.google-analytics.com/__utm.gif?) and that becomes the Request URL.

Of course there can't be an HTTP Request without a resource; so resource is the client requesting from the server? It doesn't need anything from the server, instead it wants to send information to the server. So the actual server resource requested by the client is purely pretextual--the resource isn't even needed by the client, it's solely requested to comply with the transmission protocol operator. Therefore, it makes sense to make that resource as small and as unobtrusive as possible, which is why it's a 1 x 1 transparent pixel in gif format. It is the smallest possible size and the least dense image format (bytes/pixel); I think it's a little over 30 bytes. A 1 x 1 image in the other common formats (e.g., jpeg, png, tiff) are larger.

This general scheme for transferring data between a client and a server has been around forever; there could very well be a better way of doing this, but it's the only way I know of (that satisfies the constraints imposed by a hosted analytics service).

(Google Analytics does indeed have two APIs--"Data Export" and "Management"--which are both RESTful Web Services.)