The WHATWG Blog

Progressing Streams

Back in 2014 we announced the Streams Standard. It's about time for an update on where we are and what's coming up.

Streaming the response to fetch() via the response.body attribute was standardized last year and is now implemented in several major browsers. Recently streaming uploads have been added to the Fetch Standard. fetch(url, {method: 'POST', body: readable}) will start an upload. The expected properties of a streaming function apply:

• Bytes will be written as they become available.
• Chunks do not need to be kept in memory after they have been uploaded.
• Backpressure is applied: the source can stop generating new data when the network or server is slow to accept it.

Service Workers are another area where Streams are indispensable. They are used internally to allow the page to start processing bytes as soon as any are available, and are being used by developers as a powerful tool to synthesize responses. A Response object can be constructed with a ReadableStream body and then passed to fetchEvent.respondWith() like any other response.

The real power of Streams is unlocked when sources, sinks and transforms from disparate authors are combined in novel ways. The most exciting action is not in platform built-ins but in streams created in the wider developer ecosystem. With this in mind, every aspect of the standard has been fine-tuned for productivity. Take the following example:

let appendChildWritableStream = new WritableStream({
  write(domNode) {
   parentNode.appendChild(domNode);
  }
});

Notice what isn't there:

• No setup code is needed. Boilerplate is kept to an absolute minimum.
• No type conversions. Streams handle whatever types you throw at them.
• We aren't interested in backpressure here, so nothing needs to be done about it.
• Our data sink is synchronous, so no async code needs to be written.

With a recent browser you can see this in action in a live demo (video). At time of writing, this demo and the others in this post work in Chrome stable version 59 and Safari stable version 10.1.

The WritableStream API is now stable. We've added a getWriter() method which is the analogue of ReadableStream's getReader(). It adds locking semantics so that multiple writers cannot interfere with each other. Recent work has focused on predictability, for example by preventing underlying sink methods from running concurrently, and robustness, like dealing with badly-behaving strategy size functions that call into other methods reentrantly.

The strength of the algorithmic style of specification is that even unintended behavior will be the same between implementations. On the other hand, when specifying the pipeTo() method of ReadableStream, providing latitude for browsers to optimize was a high priority. As well as bypassing JavaScript when copying data between built-in streams, user agents may need to change the timing or ordering of calls to underlying methods to get the best performance for their architecture. For this reason, we specified pipeTo() in a requirements style. This presents its own challenges, for example how to specify the "least work" that an implementation can do and still be compliant.

Streams also challenge our fundamental assumptions about how the web platform works. You may not want to have to modify the DOM directly if you already have a template engine producing HTML. Shouldn't you be able to pipe a stream of HTML to an element?

We don't yet know how this capability would fit in the web platform, but Jake Archibald has created a custom element providing a compelling vision of what we could do with it. The demo demonstrates inserting a stream of HTML directly from the server.

Depending on your environment, you may have seen some significant jank in that demo. The problem is that the server supplies data faster than the browser can layout and render it. This is where backpressure comes in. Any data sink can apply backpressure just by returning a promise from its write() method. In many cases this happens as a natural consequence of the implementation. In this case, we want to delay until the browser has had a chance to render the HTML. A slight modification to the custom element and the page becomes much smoother: demo (side-by-side video).

It's clear that we should prioritize interactivity when adding content to an existing page. Maybe browsers need a special low-jank path for streaming HTML. But what about initial page load? You've probably seen pages that didn't respond to input because they were still performing some expensive layout below the fold. Should we prioritize interactivity there, too? We're still working through all the implications.

Ensuring low friction for all participants has really helped drive the progress of the standard. From bug fixes to the BYOB readable byte stream design from implementers, to large scale contributions from external contributors, the benefits of the community process are clear.

Transform streams are the final key piece needed to make the stream ecosystem complete. We have a working, tested reference implementation that we are using as the basis for active design discussions. Full standardization and implementer adoption is expected to follow in the next few months.

In past two years streams have gone from being a promising idea to having multiple independent implementations and wide adoption. Implementation work is accelerating, and there is already a critical mass of shipping functionality.

We're looking to widen developer involvement with Streams. Check out the examples, contribute some web platform tests, or help improve the documentation.

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