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Media Source Extensions

Media Source Extensions (MSE) is a JavaScript API that lets you build streams for playback from segments of audio or video. Although not covered in this article, understanding MSE is needed if you want to embed videos in your site that does such things as:

  • Adaptive streaming, which is another way of saying adapting to device capabilities and network conditions
  • Adaptive splicing, such as ad insertion
  • Time shifting
  • Control of performance and download size
Basic MSE data flow
Figure 1: Basic MSE data flow

You can almost think of MSE as a chain. As illustrated in the figure, between the downloaded file and the media elements are several layers.

  • An <audio> or <video> element to play the media.
  • A MediaSource instance with a SourceBuffer to feed the media element.
  • A fetch() or XHR call to retrieve media data in a Response object.
  • A call to Response.arrayBuffer() to feed MediaSource.SourceBuffer.

In practice, the chain looks like this:

var vidElement = document.querySelector('video');

if (window.MediaSource) {
  var mediaSource = new MediaSource();
  vidElement.src = URL.createObjectURL(mediaSource);
  mediaSource.addEventListener('sourceopen', sourceOpen);
} else {
  console.log("The Media Source Extensions API is not supported.")
}

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';
  var mediaSource = e.target;
  var sourceBuffer = mediaSource.addSourceBuffer(mime);
  var videoUrl = 'droid.webm';
  fetch(videoUrl)
    .then(function(response) {
      return response.arrayBuffer();
    })
    .then(function(arrayBuffer) {
      sourceBuffer.addEventListener('updateend', function(e) {
        if (!sourceBuffer.updating && mediaSource.readyState === 'open') {
          mediaSource.endOfStream();
        }
      });
      sourceBuffer.appendBuffer(arrayBuffer);
    });
}

If you can sort things out from the explanations so far, feel free to stop reading now. If you want a more detailed explanation, then please keep reading. I'm going to walk through this chain by building a basic MSE example. Each of the build steps will add code to the previous step.

A note about clarity

Will this article tell you everything you need to know about playing media on a web page? No, it's only intended to help you understand more complicated code you might find elsewhere. For the sake of clarity, this document simplifies and excludes many things. We think we can get away with this because we also recommend using a library such as Google's Shaka Player. I will note throughout where I'm deliberately simplifying.

A few things not covered

Here, in no particular order, are a few things I won't cover.

  • Playback controls. We get those for free by virtue of using the HTML5 <audio> and <video> elements.
  • Error handling.

For use in production environments

Here are some things I'd recommend in a production usage of MSE related APIs:

  • Before making calls on these APIs, handle any error events or API exceptions, and check HTMLMediaElement.readyState and MediaSource.readyState. These values can change before associated events are delivered.
  • Make sure previous appendBuffer() and remove() calls are not still in progress by checking the SourceBuffer.updating boolean value before updating the SourceBuffer's mode, timestampOffset, appendWindowStart, appendWindowEnd, or calling appendBuffer() or remove() on the SourceBuffer.
  • For all SourceBuffer instances added to your MediaSource, ensure none of their updating values are true before calling MediaSource.endOfStream() or updating the MediaSource.duration.
  • If MediaSource.readyState value is ended, calls like appendBuffer() and remove(), or setting SourceBuffer.mode or SourceBuffer.timestampOffset will cause this value to transition to open. This means you should be prepared to handle multiple sourceopen events.
  • When handling HTMLMediaElement error events, the contents of MediaError.message can be useful to determine the root cause of the failure, especially for errors that are hard to reproduce in test environments.

Attach a MediaSource instance to a media element

As with many things in web development these days, you start with feature detection. Next, get a media element, either an <audio> or <video> element. Finally create an instance of MediaSource. It gets turned into a URL and passed to the media element's source attribute.

var vidElement = document.querySelector('video');

if (window.MediaSource) {
  var mediaSource = new MediaSource();
  vidElement.src = URL.createObjectURL(mediaSource);
  // Is the MediaSource instance ready?
} else {
  console.log("The Media Source Extensions API is not supported.")
}
A source attribute as a blob
Figure 1: A source attribute as a blob

That a MediaSource object can be passed to a src attribute might seem a bit odd. They're usually strings, but they can also be blobs. If you inspect a page with embedded media and examine its media element, you'll see what I mean.

Is the MediaSource instance ready?

URL.createObjectURL() is itself synchronous; however, it processes the attachment asynchronously. This causes a slight delay before you can do anything with the MediaSource instance. Fortunately, there are ways to test for this. The simplest way is with a MediaSource property called readyState. The readyState property describes the relation between a MediaSource instance and a media element. It can have one of the following values:

  • closed - The MediaSource instance is not attached to a media element.
  • open - The MediaSource instance is attached to a media element and is ready to receive data or is receiving data.
  • ended - The MediaSource instance is attached to a media element and all of its data has been passed to that element.

Querying these options directly can negatively affect performance. Fortunately, MediaSource also fires events when readyState changes, specifically sourceopen, sourceclosed, sourceended. For the example I'm building, I'm going to use the sourceopen event to tell me when to fetch and buffer the video.

var vidElement = document.querySelector('video');

if (window.MediaSource) {
  var mediaSource = new MediaSource();
  vidElement.src = URL.createObjectURL(mediaSource);
  mediaSource.addEventListener('sourceopen', sourceOpen);
} else {
  console.log("The Media Source Extensions API is not supported.")
}

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  // Create a SourceBuffer and get the media file.
}

Notice that I've also called revokeObjectURL(). I know this seems premature, but I can do this any time after the media element's src attribute is connected to a MediaSource instance. Calling this method doesn't destroy any objects. It does allow the platform to handle garbage collection at an appropriate time, which is why I'm calling it immediately.

Create a SourceBuffer

Now it's time to create the SourceBuffer, which is the object that actually does the work of shuttling data between media sources and media elements. A SourceBuffer has to be specific to the type of media file you're loading.

In practice you can do this by calling addSourceBuffer() with the appropriate value. Notice that in the example below the mime type string contains a mime type and two codecs. This is a mime string for a video file, but it uses separate codecs for the video and audio portions of the file.

Version 1 of the MSE spec allows user agents to differ on whether to require both a mime type and a codec. Some user agents don't require, but do allow just the mime type. Some user agents, Chrome for example, require a codec for mime types that don't self-describe their codecs. Rather than trying to sort all this out, it's better to just include both.

var vidElement = document.querySelector('video');

if (window.MediaSource) {
  var mediaSource = new MediaSource();
  vidElement.src = URL.createObjectURL(mediaSource);
  mediaSource.addEventListener('sourceopen', sourceOpen);
} else {
  console.log("The Media Source Extensions API is not supported.")
}

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';
  // e.target refers to the mediaSource instance.
  // Store it in a variable so it can be used in a closure.
  var mediaSource = e.target;
  var sourceBuffer = mediaSource.addSourceBuffer(mime);
  // Fetch and process the video.
}

Get the media file

If you do an internet search for MSE examples, you'll find plenty that retrieve media files using XHR. To be more cutting edge, I'm going to use the Fetch API and the Promise it returns. If you're trying to do this in Safari, it won't work without a fetch() polyfill.

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';  
  var mediaSource = e.target;  
  var sourceBuffer = mediaSource.addSourceBuffer(mime);  
  var videoUrl = 'droid.webm'; 
  fetch(videoUrl)
    .then(function(response) {
      // Process the response object.
    });
}

A production quality player would have the same file in multiple versions to support different browsers. It could use separate files for audio and video to allow audio to be selected based on language settings.

Real world code would also have multiple copies of media files at different resolutions so that it could adapt to different device capabilities and network conditions. Such an application is able to load and play videos in chunks either using range requests or segments. This allows for adaption to network conditions while media are playing. You may have heard the terms DASH or HLS, which are two methods of accomplishing this. A full discussion of this topic is beyond the scope of this introduction.

Process the response object

The code looks almost done, but the media doesn't play. We need to get media data from the Response object to the SourceBuffer.

The typical way to pass data from the response object to the MediaSource instance is to get an ArrayBuffer from the response object and pass it to the SourceBuffer. Start by calling response.arrayBuffer(), which returns a promise to the buffer. In my code, I've passed this promise to a second then() clause where I append it to the SourceBuffer.

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';
  var mediaSource = e.target;
  var sourceBuffer = mediaSource.addSourceBuffer(mime);
  var videoUrl = 'droid.webm';
  fetch(videoUrl)
    .then(function(response) {
      return response.arrayBuffer();
    })
    .then(function(arrayBuffer) {
      sourceBuffer.appendBuffer(arrayBuffer);
    });
}

Call endOfStream()

After all ArrayBuffers are appended, and no further media data is expected, call MediaSource.endOfStream(). This will change MediaSource.readyState to ended and fire the sourceended event.

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';
  var mediaSource = e.target;
  var sourceBuffer = mediaSource.addSourceBuffer(mime);
  var videoUrl = 'droid.webm';
  fetch(videoUrl)
    .then(function(response) {
      return response.arrayBuffer();
    })
    .then(function(arrayBuffer) {
      sourceBuffer.addEventListener('updateend', function(e) {
        if (!sourceBuffer.updating && mediaSource.readyState === 'open') {
          mediaSource.endOfStream();
        }
      });
      sourceBuffer.appendBuffer(arrayBuffer);
    });
}

The final version

Here's the complete code example. I hope you have learned something about Media Source Extensions.

var vidElement = document.querySelector('video');

if (window.MediaSource) {
  var mediaSource = new MediaSource();
  vidElement.src = URL.createObjectURL(mediaSource);
  mediaSource.addEventListener('sourceopen', sourceOpen);
} else {
  console.log("The Media Source Extensions API is not supported.")
}

function sourceOpen(e) {
  URL.revokeObjectURL(vidElement.src);
  var mime = 'video/webm; codecs="opus, vp9"';
  var mediaSource = e.target;
  var sourceBuffer = mediaSource.addSourceBuffer(mime);
  var videoUrl = 'droid.webm';
  fetch(videoUrl)
    .then(function(response) {
      return response.arrayBuffer();
    })
    .then(function(arrayBuffer) {
      sourceBuffer.addEventListener('updateend', function(e) {
        if (!sourceBuffer.updating && mediaSource.readyState === 'open') {
          mediaSource.endOfStream();
        }
      });
      sourceBuffer.appendBuffer(arrayBuffer);
    });
}