July 22, 2024
Enhancing Efficiency with HTTP Streaming
The Airbnb Tech Blog

How HTTP Streaming can enhance web page efficiency and the way Airbnb enabled it on an current codebase

By: Victor Lin

You could have heard a joke that the Internet is a series of tubes. On this weblog publish, we’re going to speak about how we get a cool, refreshing stream of Airbnb.com bytes into your browser as shortly as potential utilizing HTTP Streaming.

Let’s first perceive what streaming means. Think about we had a spigot and two choices:

  • Fill an enormous cup, after which pour all of it down the tube (the “buffered” technique)
  • Join the spigot on to the tube (the “streaming” technique)

Within the buffered technique, every part occurs sequentially — our servers first generate all the response right into a buffer (filling the cup), after which extra time is spent sending it over the community (pouring it down). The streaming technique occurs in parallel. We break the response into chunks, that are despatched as quickly as they’re prepared. The server can begin engaged on the following chunk whereas earlier chunks are nonetheless being despatched, and the consumer (e.g, a browser) can start dealing with the response earlier than it has been absolutely obtained.

Streaming has clear benefits, however most web sites at the moment nonetheless depend on a buffered strategy to generate responses. One purpose for that is the extra engineering effort required to interrupt the web page into unbiased chunks. This simply isn’t possible generally. For instance, if the entire content material on the web page depends on a sluggish backend question, then we gained’t have the ability to ship something till that question finishes.

Nonetheless, there’s one use case that’s universally relevant. We are able to use streaming to scale back community waterfalls. This time period refers to when one community request triggers one other, leading to a cascading collection of sequential requests. That is simply visualized in a device like Chrome’s Waterfall:

Chrome Community Waterfall illustrating a cascade of sequential requests

Most net pages depend on exterior JavaScript and CSS information linked throughout the HTML, leading to a community waterfall — downloading the HTML triggers JavaScript and CSS downloads. Consequently, it’s a greatest apply to position all CSS and JavaScript tags close to the start of the HTML within the <head> tag. This ensures that the browser sees them earlier. With streaming, we are able to cut back this delay additional, by sending that portion of the <head> tag first.

Essentially the most easy technique to ship an early <head> tag is by breaking a normal response into two components. This method known as Early Flush, as one half is shipped (“flushed”) earlier than the opposite.

The primary half incorporates issues which might be quick to compute and may be despatched shortly. At Airbnb, we embrace tags for fonts, CSS, and JavaScript, in order that we get the browser advantages talked about above. The second half incorporates the remainder of the web page, together with content material that depends on API or database queries to compute. The top outcome appears to be like like this:

Early chunk:

<script src=… defer />
<hyperlink rel=”stylesheet” href=… />
<!--lots of different <meta> and different tags… ->

Late chunk:

<!-- <head> tags that rely upon knowledge go right here ->
<! — Physique content material right here →

We needed to restructure our app to make this potential. For context, Airbnb makes use of an Specific-based NodeJS server to render net pages utilizing React. We beforehand had a single React part answerable for rendering the whole HTML doc. Nonetheless, this offered two issues:

  • Producing incremental chunks of content material means we have to work with partial/unclosed HTML tags. For instance, the examples you noticed above are invalid HTML. The <html> and <head> tags are opened within the Early chunk, however closed within the Late chunk. There’s no technique to generate this kind of output utilizing the usual React rendering capabilities.
  • We are able to’t render this part till we have now the entire knowledge for it.

We solved these issues by breaking our monolithic part into three:

  • an “Early <head>” part
  • a “Late <head>” part, for <head> tags that rely upon knowledge
  • a “<physique>” part

Every part renders the contents of the top or physique tag. Then we sew them collectively by writing open/shut tags on to the HTTP response stream. General, the method appears to be like like this:

  1. Write <html><head>
  2. Render and write the Early <head> to the response
  3. Anticipate knowledge
  4. Render and write the Late <head> to the response
  5. Write </head><physique>
  6. Render and write the <physique> to the response
  7. End up by writing </physique></html>

Early Flush optimizes CSS and JavaScript community waterfalls. Nonetheless, customers will nonetheless be observing a clean web page till the <physique> tag arrives. We’d like to enhance this by rendering a loading state when there’s no knowledge, which will get changed as soon as the info arrives. Conveniently, we have already got loading states on this state of affairs for consumer facet routing, so we might accomplish this by simply rendering the app with out ready for knowledge!

Sadly, this causes one other community waterfall. Browsers should obtain the SSR (Server-Facet Render), after which JavaScript triggers one other community request to fetch the precise knowledge:

Graph displaying a community waterfall the place SSR and client-side knowledge fetch occur sequentially

In our testing, this resulted in a slower complete loading time.

What if we might embrace this knowledge within the HTML? This may enable our server-side rendering and knowledge fetching to occur in parallel:

Graph displaying SSR and client-side knowledge fetch taking place in parallel

Provided that we had already damaged the web page into two chunks with Early Flush, it’s comparatively easy to introduce a 3rd chunk for what we name Deferred Knowledge. This chunk goes after the entire seen content material and doesn’t block rendering. We execute the community requests on the server and stream the responses into the Deferred Knowledge chunk. Ultimately, our three chunks appear to be this:

Early chunk

<hyperlink rel=”preload” as=”script” href=… />
<hyperlink rel=”stylesheet” href=… />
<! — numerous different <meta> and different tags… →

Physique chunk

    <! — <head> tags that rely upon knowledge go right here →
<! — Physique content material right here →
<script src=… />

Deferred Knowledge chunk

    <script sort=”utility/json” >
<!-- knowledge -->

With this carried out on the server, the one remaining job is to jot down some JavaScript to detect when our Deferred Knowledge chunk arrives. We did this with a MutationObserver, which is an environment friendly technique to observe DOM modifications. As soon as the Deferred Knowledge JSON factor is detected, we parse the outcome and inject it into our utility’s community knowledge retailer. From the applying’s perspective, it’s as if a standard community request has been accomplished.

Be careful for `defer`

You could discover that some tags are re-ordered from the Early Flush instance. The script tags moved from the Early chunk to the Physique chunk and not have the defer attribute. This attribute avoids render-blocking script execution by deferring scripts till after the HTML has been downloaded and parsed. That is suboptimal when utilizing Deferred Knowledge, as the entire seen content material has already been obtained by the top of the Physique chunk, and we not fear about render-blocking at that time. We are able to repair this by shifting the script tags to the top of the Physique chunk, and eradicating the defer attribute. Shifting the tags later within the doc does introduce a community waterfall, which we solved by including preload tags into the Early chunk.

Early Flush prevents subsequent modifications to the headers (e.g to redirect or change the standing code). Within the React + NodeJS world, it’s frequent to delegate redirects and error throwing to a React app rendered after the info has been fetched. This gained’t work when you’ve already despatched an early <head> tag and a 200 OK standing.

We solved this downside by shifting error and redirect logic out of our React app. That logic is now carried out in Express server middleware earlier than we try and Early Flush.

We discovered that nginx buffer responses by default. This has useful resource utilization advantages however is counterproductive when the purpose is sending incremental responses. We needed to configure these companies to disable buffering. We anticipated a possible enhance in useful resource utilization with this variation however discovered the impression to be negligible.

We observed that our Early Flush responses had an surprising delay of round 200ms, which disappeared after we disabled gzip compression. This turned out to be an interplay between Nagle’s algorithm and Delayed ACK. These optimizations try to maximise knowledge despatched per packet, introducing latency when sending small quantities of information. It’s particularly simple to run into this subject with jumbo frames, which will increase most packet sizes. It seems that gzip lowered the dimensions of our writes to the purpose the place they couldn’t fill a packet, and the answer was to disable Nagle’s algorithm in our haproxy load balancer.

HTTP Streaming has been a really profitable technique for bettering net efficiency at Airbnb. Our experiments confirmed that Early Flush produced a flat discount in First Contentful Paint (FCP) of round 100ms on each web page examined, together with the Airbnb homepage. Knowledge streaming additional eradicated the FCP prices of sluggish backend queries. Whereas there have been challenges alongside the way in which, we discovered that adapting our current React utility to help streaming was very possible and sturdy, regardless of not being designed for it initially. We’re additionally excited to see the broader frontend ecosystem development within the path of prioritizing streaming, from @defer and @stream in GraphQL to streaming SSR in Next.js. Whether or not you’re utilizing these new applied sciences, or extending an current codebase, we hope you’ll discover streaming to construct a sooner frontend for all!

If one of these work pursuits you, try a few of our associated positions here.

Elliott Sprehn, Aditya Punjani, Jason Jian, Changgeng Li, Siyuan Zhou, Bruce Paul, Max Sadrieh, and everybody else who helped design and implement streaming at Airbnb!

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