#Har chrome webtools how to#

#Har chrome webtools how to#

So for example a wide horizontal gap in the network activity, with many bursts of pink, indicates a CPU bottleneck: low-end mobile devices might have a hard time processing this particular request.ДЇor a thorough analysis of visual patterns that can be found in a waterfall chart, I recommend the talk How to read a WebPageTest waterfall chart by Matt Hobbs or the associated blog post. The waterfall generated by WebPageTest is particularly useful here, because it shows the JavaScript execution in pink. When looking at the overall picture you can take note of how many parallel requests there are, how many TCP connections, how many HTTP redirects, and whether there are wide horizontal gaps in the chart. When looking at a single row, you may want to focus on the response size (body and headers), Cache-Control directives, cookies, and the TTFB. The waterfall chart contains a lot of valuable information, and itвЂ™s important to have a look both at individual rows and at the overall picture. But the waterfall chart generated by WebPageTest is by far the one with the most details. I like Compare because it lets me load either one or two HAR files, and it lets meвЂ¦ wellвЂ¦ compare them.

a UI component like Network-Viewer for React.

an online tool like WebPageTest, Compare, HAR Analyzer.

You have several options to view a HAR file: That's another reason why I prefer a tool like WebPageTest for this task. I find this quite annoying and easy to forget. вљ пёЏ - You should always create a HAR file in an incognito tab, to avoid having requests made by Chrome extensions show up in your waterfall. Replicating the same options in Chrome would mean having to implement custom profiles for CPU throttling, network throttling and location. I like to use WebPageTest to generate a HAR file because of its many test locations and the many options for device emulation.

a library that extracts the HTTP transactions from the Chrome DevTools Protocol, like chrome-har-capturer or chrome-har.

a web performance tool like WebPageTest or sitespeed.io.

a browser automation tool like Puppeteer, Cypress or Browsertime.

Also, HAR files generated by browsers are not perfect This is convenient, but donвЂ™t forget to warm up the DNS cache by making multiple runs. You can use several tools to create a HAR file: So letвЂ™s see which tools we can use for that. Chrome DevTools uses the term Content Download.Д«y looking at these waterfalls we can understand a lot about the performance of a web page, but first we need to generate a HAR file.

Receive: the browser is receiving the response.

Note that other definitions of TTFB include the DNS lookup time. Chrome DevTools calls this time TTFB (Time To First Byte).

Wait: the server received the request and it is generating a response, while the browser is waiting for the first byte of a response.

If itвЂ™s a PUT or POST request, then this will also include the time spent uploading any data with that request.

Send: the browser is sending the request to the server.

SSL (TLS): browser and server are negotiating an SSL certificate with the SSL/TLS handshake.

Connect: the browser is establishing a TCP connection, including TCP handshakes/retries.

ThatвЂ™s why most tools perform at least three runs before generating a waterfall. DNS requests are cached, so DNS lookup times may differ in subsequent tests.

DNS: the browser is performing a DNS lookup, i.e.

Chrome DevTools refers to this case with the terms Queueing and Stalled.

Blocked: the request is blocked for some reason: maybe the browser is fetching higher priority requests, there are no TCP connections available, there are too many TCP connections already open (applies only to HTTP/1.0 and HTTP/1.1).

The colors of the horizontal bars represent: This is usually the case with HAR file viewers, even if there are some tools that use a different color scheme. Note how Chrome DevTools and Compare share the same color scheme for the horizontal bars. The blue vertical line represents the DOMContentLoaded event, while the red one the Load event. Thanks to this relationship the browser-or any tool that can process and show HAR files-can reconstruct a waterfall of HTTP requests.Дown below you can see how Chrome DevTools displays the waterfall chart of the Google homepage.

Inside a HAR file you can find both coarse data regarding every single page visited-like timings for the DOMContentLoaded and load events-and granular data describing every single HTTP request made by the browser.Д®very entry in a HAR file is tied to a page via a pageref field.

The HAR file format is based on JSON, and the current version of the specification (1.2) can be found here. Generating and analyzing a HAR file is important when troubleshooting performance issues. A HAR (Http ARchive) file contains data related to the HTTP transactions that occur between a web browser and a website or web app.