Manifest Parser Plugins

Overview

This tutorial shows how to make a basic manifest parser plugin. This allows an app to define a custom manifest format and still use Shaka Player to handle the streaming and track switching.

The function of a manifest parser is to take a URL that was passed to load() and give us back a manifest object. The parser should fetch the URL, parse the manifest, and convert it to our format.

function MyManifestParser() {
  this.curId_ = 0;
  this.config_ = null;
}

MyManifestParser.prototype.configure = function(config) {
  this.config_ = config;
};

MyManifestParser.prototype.start = async function(uri, playerInterface) {
  const type = shaka.net.NetworkingEngine.RequestType.MANIFEST;
  const request = {
    uris: [uri],
    method: 'GET',
    retryParameters: this.config_.retryParameters
  };
  const response =
      await playerInterface.networkingEngine.request(type, request).promise;
  return this.loadManifest_(response.data);
};

MyManifestParser.prototype.stop = function() {
  return Promise.resolve();
};


shaka.media.ManifestParser.registerParserByExtension('json', MyManifestParser);
shaka.media.ManifestParser.registerParserByMime(
    'application/json', MyManifestParser);

First, this defines a constructor called MyManifestParser. This is called by the Player to create new parser instances. A new instance is created for each load. This should setup any initial state that is needed.

configure

This method is called right after creating the object and when the configuration changes. This is passed a shaka.extern.ManifestConfiguration object from the Player.

start

This method is called to load the manifest. This is called with a string URI that is passed to load() and a shaka.extern.ManifestParser.PlayerInterface object. The interface object contains a number of fields that are used to interact with the Player. This includes the NetworkingEngine instance to make network requests. This also includes callback methods that allow the parser to raise Player events and filter the variants. This method should return a Promise that will resolve with the parsed manifest.

stop

This method is called as part of player.unload(). This method should stop any background timers and free any state. It is invalid to use the config object or anything from the Player interface given to start after this is called. We don't reuse parser instances, so we will not call start() again after this is called. This should return a Promise that resolves when this object is destroyed.

registration

At the end of the file, you should register the parser with the library. This will allow it to be used by the Player. There are two methods: registerParserByExtension and registerParserByMime. They both add parsers to a registry of manifest parsers. When the Player gets a URI, it will determine which parser to use. It will first try based on the file extension, then it will make a HEAD request to the URI to get back a MIME type.

If you wish to unregister a parser, you can do so with shaka.media.ManifestParser.unregisterParserByMime. Parsers registered by extension cannot be unregistered at this time.

Variants and Streams

The audio and video content of our Manifest structure is stored, on the highest level, in an array of Variants. A Variant represents an audio+video pair. The array holds all possible pairs the Player can choose from. While playing, we will give these to the app (through getVariantTracks) and will switch between them (if ABR is enabled).

A stream represents a collection of media data segments. The segments are all the same type (audio/video/text) and all from the same version of the media (e.g. English vs Spanish or 720p vs 1080p). A Stream object holds metadata that describes what the stream contains as well as how to get the segments. Only one stream of each type will be playing at once.

Multiple Variants can hold the same streams. For example, both the 720p and the 1080p variant can refer to the same audio stream. In this case, both Variant objects must refer to the same object. It is not enough to use the same stream ID; it must be the same object.

Periods

While some manifest formats, such as MPEG-DASH, have the concept of a period, our internal manifest structure (as of Shaka Player v3.0) does not organize streams based on periods. If you are writing a manifest parser for such a format, you will need to generate Variants yourself, combining together periods and their component streams as needed. The resulting shaka.extern.Stream objects should each span the entire presentation.

All media times in the manifest are relative to the presentation, not the Period. This means that you must account for period start times in your segment references. (This changed in Shaka Player v3.0.)

PresentationTimeline

shaka.media.PresentationTimeline

The segment index doesn't need to handle segment availability for live content. All the segment index needs to do is return the segment references. The presentationTimeline in the manifest will be used to handle availability. All times in the timeline are in seconds; 0 represents when the live stream started.

An availability window defines a moving time window in which a segment can be downloaded. This is defined by a segment availability duration that indicates the number of seconds that a segment will remain available. So if the availability duration is 60 seconds, then the last 60 seconds of content is available.

The same timeline class handles on-demand content, too. The availability window starts at 0 and ends at the duration of the media.

Media Segments

A Stream contains a number of segment references contained in a segment index. A segment reference contains important metadata about the segment: the start and end times, the URL, and optionally a byte range into that URL. A segment reference is created using the shaka.media.SegmentReference constructor.

Rather than storing the references in a raw array, the manifest parser stores them in a shaka.media.SegmentIndex object. This object is allowed to be null before createSegmentIndex() is called, and thus defer the translation of abstract segment descriptions (such as DASH's SegmentTemplate) into concrete ones until necessary.

Media segment times are all in terms of the presentation timeline. So if the content was originally multi-period, the period start time must be accounted for in the reference's timestampOffset field.

First we ask for the index that corresponds with a start time. Then on update, we increment the index and ask for segments in order. The value of the index doesn't matter, but indices must be sequential integers.

createSegmentIndex():Promise

This is called first before any other method. This allows an index to be fetched over the network, if needed. This method should return a Promise that will resolve when the segment index is ready. This is only ever called once.

segmentIndex

This is not a function, but a shaka.media.SegmentIndex tracking all available segments.

shaka.media.SegmentIndex

To help in handling segment references, there is a shaka.media.SegmentIndex type. This is given an array of references. It handles merging new segments, and expanding the list of segments for live streams.

const references = refs.map(function(r) {
  // Should return an array of possible URI choices; this is used for failover
  // in the event of network error.  This is a function to defer calculations.
  const getUris = function() { return [r.uri]; };

  return new shaka.media.SegmentReference(
      r.start, r.end, getUris,
      /* startByte */ 0,
      /* endByte */ null,
      initSegmentReference,
      /* timestampOffset */ 0,
      /* appendWindowStart */ 0,
      /* appendWindowEnd */ Infinity);
});

const index = new shaka.media.SegmentIndex(references);

To merge updates and remove old references to reduce the memory footprint, simply create a new array of segments and call mergeAndEvict. Any existing segments will be updated, new segments will be added, and old unavailable references will be removed.

To expand the list of references on a timer, as is done for DASH's SegmentTemplate, call index.updateEvery with a callback that evicts old references and returns an array of new references.

index.updateEvery(updateIntervalSeconds, () => {
  // Evict old references
  index.evict(windowStartTime);

  // Generate new references to append to the end of the index
  const references = [];
  // ...
  return references;
});

If the callback returns null, the update timer for this index will be stopped. (NOTE: This method was introduced in v3.0.0, but the interpretation of the callback's return changed in v3.0.8 to fix a bug in our DASH SegmentTemplate support. We apologize for any inconvenience.)

Manifest Updates

In order to support Live content, the manifest may need to be updated. In the start() method, the manifest parser should start its own timers (e.g. setInterval) to update the manifest. Then it should re-parse the manifest periodically. To add new segments to the streams, simply add them to the segment index. Because the original manifest object is modified in-place, adding them to the index will allow the Player to use them.

After adding new content, you should call the filter method on the player interface. This removes any streams that were added that are incompatible with the platform. Keep in mind that this is an asynchronous method, and thus you will need to use it in conjunction with .then() or await. You should also call the makeTextStreamsForClosedCaptions method on the player interface, which is required for embedded captions (for example, CEA 608) to work; it makes dummy text streams to represent these tracks.

Full Manifest Parser Example

MyManifestParser.prototype.loadManifest_ = function(data) {
  // |data| is the response data from load(); but in this example, we ignore it.

  // The arguments are only used for live.
  const timeline = new shaka.media.PresentationTimeline(null, 0);
  timeline.setDuration(3600);  // seconds

  return {
    presentationTimeline: timeline,
    minBufferTime: 5,  // seconds
    offlineSessionIds: [],
    variants: [
      this.loadVariant_(true, true),
      this.loadVariant_(true, false)
    ],
    textStreams: [
      this.loadStream_('text'),
      this.loadStream_('text')
    ]
  };
};

MyManifestParser.prototype.loadVariant_ = function(hasVideo, hasAudio) {
  console.assert(hasVideo || hasAudio);

  return {
    id:        this.curId_++,  // globally unique ID
    language:  'en',
    primary:   false,
    audio:     hasAudio ? this.loadStream_('audio') : null,
    video:     hasVideo ? this.loadStream_('video') : null,
    bandwidth: 8000,  // bits/sec, audio+video combined
    allowedByApplication: true,  // always initially true
    allowedByKeySystem:   true   // always initially true
  };
};

MyManifestParser.prototype.loadStream_ = function(type) {
  const getUris = function() { return ['https://example.com/init']; };
  const initSegmentReference = new shaka.media.InitSegmentReference(getUris,
      /* startByte= */ 0, /* endByte= */ null);

  const index = new shaka.media.SegmentIndex([
    // Times are in seconds, relative to the presentation
    this.loadReference_(0, 0, 10, initSegmentReference),
    this.loadReference_(1, 10, 20, initSegmentReference),
    this.loadReference_(2, 20, 30, initSegmentReference),
  ]);

  const id = this.curId_++;
  return {
    id: id,  // globally unique ID
    originalId: id, // original ID from manifest, if any
    createSegmentIndex:     function() { return Promise.resolve(); },
    segmentIndex:           index,
    mimeType: type == 'video' ?
        'video/webm' : (type == 'audio' ? 'audio/webm' : 'text/vtt'),
    codecs:    type == 'video' ? 'vp9' : (type == 'audio' ? 'vorbis' : ''),
    frameRate: type == 'video' ? 24 : undefined,
    pixelAspectRatio: type == 'video' ? 4 / 3 : undefined,
    bandwidth: 4000,  // bits/sec
    width:     type == 'video' ? 640 : undefined,
    height:    type == 'video' ? 480 : undefined,
    kind:      type == 'text' ? 'subtitles' : undefined,
    channelsCount: type == 'audio' ? 2 : undefined,
    encrypted: false,
    drmInfos:  [],
    keyIds:    new Set(),
    language:  'en',
    label:     'my_stream',
    type:      type,
    primary:   false,
    trickModeVideo: null,
    emsgSchemeIdUris: null,
    roles:     []
    channelsCount: type == 'audio' ? 6 : null,
    audioSamplingRate: type == 'audio' ? 44100 : null,
    closedCaptions: new Map(),
  };
};

MyManifestParser.prototype.loadReference_ =
    function(position, start, end, initSegmentReference) {
  const getUris = function() { return ['https://example.com/ref_' + position]; };
  return new shaka.media.SegmentReference(
      start, end, getUris,
      /* startByte */ 0,
      /* endByte */ null,
      initSegmentReference,
      /* timestampOffset */ 0,
      /* appendWindowStart */ 0,
      /* appendWindowEnd */ Infinity);
};

Encrypted Content

If your content is encrypted, there are a few changes to the manifest you need to do. First, for each Stream that contains encrypted content, you need to set stream.encrypted to true and put the key IDs that the stream is encrypted with in the stream.keyIds set. Filling out the keyIds is technically optional, but it allows the player to choose streams more intelligently based on which keys are available. If keyIds is not filled out, missing keys may cause playback to stall.

You must also set stream.drmInfos to an array of shaka.extern.DrmInfo objects. All the fields (except the key-system name) can be set to the default and will be replaced by settings from the Player configuration. If the drmInfos array is empty, the content is expected to be clear.

If you set drmInfo.initData to a non-empty array, we will use that to initialize EME. We will override any encryption info in the media (e.g. pssh boxes in MP4). If you don't set this field (and it isn't set in the app config), then we will initialize EME based on the encryption info in the media.