What is the easiest way to make an informative progress bar for long ajax requests?

Question

progress bar

The management set me the task: to make a parser-processor of switch logs with the output of results via Ajax.

Parsing is performed by the user upon request, through an html form, where the necessary parameters are set. There are tens and hundreds of thousands of lines in the logs, this business will not be quick. Previously, I've only dealt with short, asynchronous requests that have an unambiguous response. Now I need to perform some operations on the logs one by one during the request and report their result on the page in the browser. Those. essentially make a progress bar with intermediate messages and a percentage.

How it is better to make, where to dig?