Wednesday, June 12, 2013

How bigpipe works

Bigpipe is a concept invented by facebook to help speed up page load times. It paralellizes browser rendering and server processing to achieve maximum efficiency. To understand bigpipe lets see how the a user request-response cycle is executed in the current scenario

  • Browser sends an HTTP request to web server.
  • Web server parses the request, pulls data from storage tier then formulates an HTML document and sends it to the client in an HTTP response.
  • HTTP response is transferred over the Internet to browser.
  • Browser parses the response from web server, constructs a DOM tree representation of the HTML document, and downloads CSS and JavaScript resources referenced by the document.
  • After downloading CSS resources, browser parses them and applies them to the DOM tree.
  • After downloading JavaScript resources, browser parses and executes them.

In this scenario, while the web server is processing and creating the HTML document, the browser is idle and when the browser is rendering the html page, the web server remains idle.

Bigpipe concept breaks the page into smaller chunks known as pagelets. And makes page rendering on browser and processing on server side as parallel processes speeding up the page load time.

The request response cycle in the bigpipe scenario is as follows.

  • The browser sends an HTTP request to web server.
  • Server quickly renders a page skeleton containing the tags and a body with empty div elements which act as containers to the pagelets. The HTTP connection to the browser stays open as the page is not yet finished.
  • Browser will start downloading the bigpipe javascript library and after that it'll start rendering the page
  • The PHP server process is still executing and its building the pagelets. Once a pagelet has been completed it's results are sent to the browser inside a BigPipe.onArrive(...) javascript tag.
  • Browser injects the html code for the pagelet received into the correct place. If the pagelet needs any CSS resources those are also downloaded.
  • After all pagelets have been received the browser starts to load all external javascript files needed by those pagelets asynchronously.
  • After javascripts are downloaded browser executes all inline javascripts.

This results in a parallel system where as the pagelets are being generated the browser is rendering the pagelets. From the user's perspective the page is rendered progressively. The initial page content becomes visible much earlier, which dramatically improves user perceived latency of the page.

Source :
open bigpipe implementation :

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