Performing actions optimistically

If one of your visitors on your website decided to leave a comment, they would click on a button that submits the form. When they do that, two actions take place. The form, using AJAX, sends a request to the server, and a loading graphic appears to notify the user that their submission is in the process. When the script hears back from the server that the task has been completed successfully, it updates the page alerting the visitor.

This is the way it's usually done, but maybe it's not the best way. This request, particularly on a high-latency network, can take several hundred milliseconds, which is a very noticeable delay for the person trying to submit their comment.

Instagram has taken a different approach to avoid that delay. As soon as the person submits the comment, it appears on the page. The request happens in the background. To the person submitting the comment, it looks like it happens instantaneously. In reality, it takes as long to process as...