This Is How Airbnb Adopted HTTP Streaming to Save 84 Million USD in Costs
#7: Read Now - Awesome Web Optimisation Technique (6 minutes)
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A 100 ms delay in website load time decreases sales by 1%. That’s an 84 million USD loss per year for Airbnb.1
Airbnb switched to HTTP streaming to improve its page load performance. It reduced the first contentful paint (FCP) metric by 100 milliseconds on every page.
The FCP measures the time from the start of the page load to the screen render.
This post outlines how Airbnb reduced latency by 100 ms with HTTP streaming. Consider sharing this post with somebody who wants to study modern web development.
What Is Critical Rendering Path?
I will give you an overview of the critical rendering path.
A browser performs a sequence of events before the screen render. This sequence of events is called the critical rendering path.
The browser receives HTML and CSS from the server. Then it translates HTML and CSS into DOM and CSSOM. This is called parsing.
Document Object Model (DOM) is the browser's internal representation of markup. CSS Object Model (CSSOM) is the browser's internal representation of styles. And they are independent tree data structures.
The browser parses HTML incrementally. So, DOM construction is incremental.
But the browser blocks the screen until the server sends CSS files. Also parsing CSS is not incremental.
The browser then combines the DOM and CSSOM to create the render tree.
The determination of the dimensions and node locations by browser is called layout.
The browser finally renders the individual nodes to the screen. This is called painting.
What Is HTTP Streaming?
I will teach you HTTP streaming with an analogy. Imagine there is a water tap and a tube. And you want to fill the tube with water. There are 2 options:
Buffering: Take a cup and fill it with water from the tap. And then pour it down the tube. This is called buffering. Everything happens in sequence. In computing, the server writes the entire response into a buffer. And then sends it to the browser.
Streaming: Connect the tap to the tube. And fill it with water. This is called streaming. Many steps happen in parallel. In computing, the server breaks the response into chunks. And sends it out as soon as they are ready. The browser handles the received chunks. This speeds up things. A popular method for HTTP streaming is Chunked transfer encoding.
How Airbnb Optimised Critical Rendering Path?
According to the performance golden rule, 90% load time of most websites is on the front end. Yet many websites today rely on buffering. Because they want to avoid extra engineering effort to enable streaming.
A switch to streaming might not be worth it if the page content depends on a slow backend query. Because nothing gets rendered until that query finishes. But Airbnb found a universal use case for HTTP streaming to improve performance.
And they reduced their network waterfall by switching to streaming. A waterfall happens when a network request triggers another request.
The browser parsed HTML incrementally. So, they used early flush to stream HTML to the browser.
Here's how they did it. Split the HTML document into 2 chunks and send them separately. They sent the HTML <header> in the first chunk. And it allowed the browser to download external files as soon as it parsed the first chunk.
In the meantime, the server generated the remaining HTML. And this reduced the network waterfall.
<link rel="stylesheet" href="your.css" />
<script src="script.js" defer></script>
But a problem remained - the user saw a blank page until the HTML <body> tag arrived. So, they rendered a loading state if there was no data. And the browser fetched data.
But this created a new problem - they sent an extra network request to fetch data. So, they streamed a third chunk of data instead. It goes only after all the visible content and doesn't block render. This eliminated an extra network request.
But they ran into a few problems with enabling HTTP streaming. And resolved the problems by:
Disabling response buffering in nginx
The takeaways from this case study are:
Stream HTML. It allows incremental construction of the page. And enables the early discovery of external files
Code split CSS files. And inline critical styles. This improves performance
Stream HTML. And request-response approach for the remaining content. This keeps it simple
Google Search uses HTTP streaming to load the headers even before the user types in a full query. This speeds up their render times.
Do you know any websites that use HTTP streaming? Leave a comment.
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Victor (2023). Improving Performance with HTTP Streaming. [online] The Airbnb Tech Blog. Available at: https://medium.com/airbnb-engineering/improving-performance-with-http-streaming-ba9e72c66408.
www.stevesouders.com. (n.d.). Flushing the Document Early | High-Performance Web Sites. [online] Available at: https://www.stevesouders.com/blog/2009/05/18/flushing-the-document-early/ [Accessed 9 Sep. 2023].
Optimization, W. (2013). Flush HTML Early and Often - flushing HTML to speed up start render times and page rendering. [online] WebSiteOptimization.com. Available at: https://www.websiteoptimization.com/speed/tweak/flush/ [Accessed 9 Sep. 2023].
developer.mozilla.org. (n.d.). Populating the page: how browsers work - Web Performance | MDN. [online] Available at: https://developer.mozilla.org/en-US/docs/Web/Performance/How_browsers_work.
www.youtube.com. (n.d.). Critical rendering path - Crash course on web performance (Fluent 2013). [online] Available at: youtube.com [Accessed 9 Sep. 2023].