Why does Google Analytics use __utm.gif?

Question

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

Answer 1

You can use __utm.gif in browsers that don't support javascript using the <noscript> tag (with some work on the server), as well as in email messages (with some work before sending the email).

How are you gonna make a REST request in an email message?

Answer 2

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.)

Answer 3

Because it's an image you can stick it anywhere you can use and image tag even if you can't execute JS. Many years back this Google pushed this for tracking of email campaigns. You could stick this formatted string in an html email message and then any client that displays the message will send that request to the GA servers and you will get at a minimum IP info (which get's you geo location also) depending on client you may also get OS, language and all the other browser settings. You don't get all the fancy analytics you get from the modern JS tracking scripts but if still has it's uses.

Here is a site that will help you format the request string and also has some more details. Google pixel generator