# Stream selection

This document describes the events and objects involved in stream selection in GStreamer pipelines, elements and applications

## History / Availability

Since 1.10:

- `GstStream` and `GstStreamCollection`
- `enum GstStreamType`
- `GST_EVENT_SELECT_STREAMS`, `GST_EVENT_STREAM_COLLECTION`, `GST_MESSAGE_STREAMS_SELECTED`, `GST_MESSAGE_STREAM_COLLECTION`
- `GstStream` present in `GST_EVENT_STREAM_START`
- `GST_BIN_FLAG_STREAMS_AWARE`

Since 1.22:

- `gst_query_new_selectable()`
- `gst_query_set_selectable()`
- `gst_query_parse_selectable()`

Initial version: June 2015 (Edward Hervey)

Last reviewed: March 2022 (Edward Hervey)

## Background

This new API is intended to address the use cases described in this section:

1. As a user/app I want an overview and control of the media streams that can be configured within a pipeline for processing, even when some streams are mutually exclusive or logical constructs only.

2. The user/app can disable entirely streams it's not interested in so they don't occupy memory or processing power - discarded as early as possible in the pipeline. The user/app can also (re-)enable them at a later time.

3. If the set of possible stream configurations is changing, the user/app should be aware of the pending change and be able to make configuration choices for the new set of streams, as well as possibly still reconfiguring the old set

4. Elements that have some other internal mechanism for triggering stream selections (DVD, or maybe some scripted playback playlist) should be able to trigger 'selection' of some particular stream.

5. Indicate known relationships between streams - for example that 2 separate video feeds represent the 2 views of a stereoscopic view, or that certain streams are mutually exclusive.

> Note: the streams that are "available" are not automatically the ones active, or present in the pipeline as pads. Think HLS/DASH alternate streams.

## Example use cases

1. Playing an MPEG-TS multi-program stream, we want to tell the app that there are multiple programs that could be extracted from the incoming feed.

   Furthermore, we want to provide a mechanism for the app to select which program(s) to decode, and once that is known to further tell the app which elementary streams are then available within those program(s) so the app/user can choose which audio track(s) to decode and/or use.

2. A new PMT arrives for an MPEG-TS stream, due to a codec or channel change. The pipeline will need to reconfigure to play the desired streams from new program. Equally, there may be multiple seconds of content buffered from the old program and it should still be possible to switch (for example) subtitle tracks responsively in the draining out data, as well as selecting which subs track to play from the new feed.

   This same scenario applies when doing gapless transition to a new source file/URL, except that likely the element providing the list of streams also changes as a new demuxer is installed.

3. When playing a multi-angle DVD, the DVD Virtual Machine needs to extract 1 angle from the data for presentation. It can publish the available angles as logical streams, even though only one stream can be chosen.

4. When playing a DVD, the user can make stream selections from the DVD menu to choose audio or sub-picture tracks, or the DVD VM can trigger automatic selections.

   In addition, the player UI should be able to show which audio/subtitle tracks are available and allow direct selection in a GUI the same as for normal files with subtitle tracks in them.

5. Playing a SCHC (3DTV) feed, where one view is MPEG-2 and the other is H.264 and they should be combined for 3D presentation, or not bother decoding 1 stream if displaying 2D. (bug https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/118)

6. Playing an adaptive stream (HLS or DASH), the user can know the various streams it can select, and only the required streams will be downloaded. This is independent of the bitrate selection mechanism.

## Design Overview

Stream selection in GStreamer is implemented in several parts:

1. Objects describing streams : `GstStream`
2. Objects describing a collection of streams : `GstStreamCollection`
3. Events from the app allowing selection and activation of some streams: `GST_EVENT_SELECT_STREAMS`
4. Messages informing the user/application about the available streams and current status: `GST_MESSAGE_STREAM_COLLECTION` and `GST_MESSAGE_STREAMS_SELECTED`
5. A field on `GST_EVENT_SELECT_STREAMS` to inform downstream elements that a certain element can handle the stream selection itself.

## GstStream objects

API:

    typedef enum {
      GST_STREAM_TYPE_UNKNOWN   = 1 << 0,
      GST_STREAM_TYPE_AUDIO     = 1 << 1,
      GST_STREAM_TYPE_VIDEO     = 1 << 2,
      GST_STREAM_TYPE_CONTAINER = 1 << 3,
      GST_STREAM_TYPE_TEXT      = 1 << 4
    } GstStreamType;

    struct {
    	const gchar *stream_id;
    } GstStream;

    GstStream *gst_stream_new(...);

    GstStream *gst_stream_get_*(...);

    GstStream *gst_stream_set_*(...);

    GstStream *gst_event_set_stream(GstEvent *stream_start, GstStream *stream)

    void gst_event_parse_stream (GstEvent * event, GstStream ** stream);

`GstStream` objects are a high-level convenience object containing information regarding a possible data stream that can be exposed by GStreamer elements.

They are mostly the aggregation of information present in other GStreamer components (`STREAM_START`, `CAPS`, `TAGS` events) but are not tied to the presence of a `GstPad`, and for some use-cases provide information that the existing components don't provide.

The various properties (available via setters/getters) of a `GstStream` object are:

- stream_id (from the `STREAM_START` event)
- flags (from the `STREAM_START` event)
- caps
- tags
- type (high-level type of stream: Audio, Video, Container,...)

`GstStream` objects can be subclassed so that they can be re-used by elements already using the notion of stream (which is common for example in demuxers).

Note: While the intent of the design was indeed to allow providers to subclass `GstStream`, it turns out it is better to not subclass it. Stream providers should instead have separate structures to track their internal properties of streams, and use a separate `GstStream` for storing information they want to share.

Elements that create `GstStream` should also set it on the `GST_EVENT_STREAM_START` event of the relevant pad. This helps downstream elements have all information in one location.

## Exposing collections of streams

API:

    struct GstStreamCollection;

    gst_stream_collection_new(...);
    gst_stream_collection_add_stream(...);
    gst_stream_collection_get_size(...);
    gst_stream_collection_get_stream(...);

    GST_MESSAGE_STREAM_COLLECTION
    gst_message_new_stream_collection(...)
    gst_message_parse_stream_collection(...)

    GST_EVENT_STREAM_COLLECTION
    gst_event_new_stream_collection(...)
    gst_event_parse_stream_collection(...)

Elements that create new streams (such as demuxers) or can create new streams (like the HLS/DASH alternative streams) can list the streams they can make available with the `GstStreamCollection` object.

Other elements that might generate `GstStreamCollections` are the DVD-VM, which handles internal switching of tracks, or `parsebin` and `decodebin3` when it aggregates and presents multiple internal stream sources as a single configurable collection.

The `GstStreamCollection` object is a flat listing of `GstStream` objects.

The various properties of a `GstStreamCollection` are:

- 'identifier'

  - the identifier of the collection (unique name)
  - Generated from the 'upstream stream id' (or stream ids, plural)

- the list of `GstStreams` in the collection.

- Not implemented : Flags

  For now, the only flag is `INFORMATIONAL` - used by container parsers to publish information about detected streams without allowing selection of the streams. Solving this might be possible through different means and needs more investigation.

- Not implemented yet : The relationship between the various streams

  This specifies which streams are exclusive (can not be selected at the same time), are related (such as `LINKED_VIEW` or `ENHANCEMENT`), or need to be selected together. This requires more investigation

An element will inform outside components about that collection of streams via:

- a `GST_MESSAGE_STREAM_COLLECTION` message on the bus.
- a `GST_EVENT_STREAM_COLLECTION` on each source pads.

Applications and container bin elements can listen and collect the various stream collections to know the full range of streams available within a bin/pipeline.

Once posted on the bus, a `GstStreamCollection` is immutable. It is updated by subsequent messages with a matching identifier.

If the element that provided the collection goes away, there is no way to know that the streams are no longer valid (without having the user/app track that element). The exception to that is if the bin containing that element (such as `parsebin` or `decodebin3`) informs that the next collection is a replacement of the former one.

The mutual exclusion and relationship lists use stream-ids rather than `GstStream` references in order to avoid circular referencing problems.

### Usage from elements

When a demuxer knows the list of streams it can expose, it creates a new `GstStream` for each stream it can provide with the appropriate information (stream id, flag, tags, caps, ...).

The demuxer then creates a `GstStreamCollection` object in which it will put the list of `GstStream` it can expose. That collection is then both posted on the bus (via a `GST_MESSAGE_COLLECTION`) and on each pad (via a `GST_EVENT_STREAM_COLLECTION`).

That new collection must be posted on the bus before the changes are made available. i.e. before pads corresponding to that selection are added/removed.

If an element can handle selecting a sub-set of those streams (for example a HLS/DASH demuxer which only download the required streams), it can inform downstream elements by answer the `GST_QUERY_SELECTABLE` with `TRUE`. It should therefore be able to handle `GST_EVENT_SELECT_STREAMS` at any point in time (including while posting the initial collection on the bus).

In order to be backwards-compatible and support elements that don't create streams/collection yet, the `parsebin` element (used by `decodebin3`) will automatically create the stream and collection if not provided by the elements within.

### Usage from application

Applications can know what streams are available by listening to the `GST_MESSAGE_STREAM_COLLECTION` messages posted on the bus.

The application can list the available streams per-type (such as all the audio streams, or all the video streams) by iterating the streams available in the collection by `GST_STREAM_TYPE`.

The application will also be able to use these stream information to decide which streams should be activated or not (see the stream selection event below).

### Backwards compatibility

Not all demuxers will create the various `GstStream` and `GstStreamCollection` objects. In order to remain backwards compatible, a parent bin (parsebin in decodebin3) will create the `GstStream` and `GstStreamCollection` based on the pads being added/removed from an element.

This allows providing stream listing/selection for any demuxer-like element even if it doesn't implement the `GstStreamCollection` usage.

## Adding and removing pads dynamically

API:

    GST_BIN_FLAG_STREAMS_AWARE

If an element is within a `GstBin` with the `GST_BIN_FLAG_STREAMS_AWARE` set, it can add and remove streams (and corresponding source `GstPad`) dynamically. It does not have to (re)add all pads of the new stream set, emit `no-more-pads` and then remove olds source pads.

This method is more efficient than the legacy behaviour, but is opt-in. Several elements (`playbin3`, `decodebin3`, `urisourcebin` and more) have this flag set by default. Application developers creating custom `GstBin` can set the flag to make compatible elements add/remove pads dynamically.

## Stream selection event

API:

    GST_EVENT_SELECT_STREAMS

    gst_event_new_select_streams(...)
    gst_event_parse_select_streams(...)

Stream selection events are generated by the application and sent into the pipeline to configure the streams.

The event carries:

- List of `GstStreams` to activate

  - a subset of the `GstStreamCollection`

- Not implemented List of `GstStreams` to be kept discarded

  - a subset of streams for which hot-swapping will not be desired, allowing elements (such as decodebin3, demuxers, ...) to not parse or buffer those streams at all. This will be solved in a different fashion.

### Usage from application

There are two use-cases where an application needs to specify in a generic fashion which streams it wants in output:

1. When there are several present streams of which it only wants a subset (such as one audio, one video and one subtitle stream). Those streams are demuxed and present in the pipeline.

2. When the stream the user wants require some element to undertake some action to expose that stream in the pipeline (such as DASH/HLS alternative streams).

From the point of view of the application, those two use-cases are treated identically. The streams are all available through the `GstStreamCollection` posted on the bus, and the applications selects a subset.

The application can select the streams it wants by creating a `GST_EVENT_SELECT_STREAMS` event with the list of stream-id of the streams it wants. That event is then sent on the pipeline, eventually traveling all the way upstream from each sink.

In some cases, selecting one stream may trigger the availability of other dependent streams, resulting in new `GstStreamCollection` messages. This can happen in the case where choosing a different DVB channel would create a new single-program collection.

### Usage in elements

Elements that receive the `GST_EVENT_SELECT_STREAMS` event and that can activate/deactivate streams need to look at the list of stream-id contained in the event and decide if they need to do some action.

In the standard demuxer case (demuxing and exposing all streams), there is nothing to do by default (see below).

In `decodebin3`, activating or deactivating streams is taken care of by linking only the streams present in the event to decoders and output ghostpad.

In the case of elements that can expose alternate streams that are not present in the pipeline as pads, they will take the appropriate action to add/remove those streams.

Containers that receive the event (via `GstElement::send_event()`) should pass it to any elements with no downstream peers, so that streams can be configured during pre-roll before a pipeline is completely linked down to sinks.

#### Which elements should handle stream-selection ?

By default demuxers must process and expose all streams contained within the multiplex. This is to allow downstream stream switching to happen as fast as possible.

The only exception to this is if extra processing (such as network download) can be avoided upstream as is the case with HLS/DASH sources. Those elements inform other downstream elements of this capability by replying to the `GST_QUERY_SELECTABLE`. They are also responsible for selecting a default selection of streams to output (one of each available `GstStreamType`) if no `GST_EVENT_SELECT_STREAMS` was received before.

Furthermore, for all elements, stream-selection can only happen if the `GstBin` they are in has the `GST_BIN_FLAG_STREAMS_AWARE` flag set.

Later design/API will be researched to allow use-cases where regular demuxers could completely disable outputting some of the contained streams (to allow more efficient I/O and memory usage).

## decodebin3 usage and example

This is an example of how decodebin3 works by using the above-mentioned objects/events/messages.

For clarity/completeness, we will consider a MPEG-TS stream that has multiple audio streams. Furthermore that stream might have changes at some point (switching video codec, or adding/removing audio streams).

### Initial differences with `decodebin2`

`decodebin3` is different, compared to `decodebin2`, in the sense that, by default:

- it will only expose as output ghost source pads one stream of each type (one audio, one video, ..).
- It will only decode the exposed streams

The `multiqueue` element is still used and takes in all elementary (non-decoded) streams. If parsers are needed/present they are placed before the multiqueue. This is needed in order for multiqueue to work only with packetized and properly timestamped streams.

Note that the whole typefinding of streams, and optional depayloading, demuxing and parsing are done in a new `parsebin` element.

Just like the current implementation, demuxers will expose all streams present within a program as source pads. They will connect to parsers and multiqueue.

Initial setup. 1 video stream, 2 audio streams.

      +---------------------+
      | parsebin            |
      | ---------           | +-------------+
      | | demux |--[parser]-+-| multiqueue  |--[videodec]---[
    ]-+-|       |--[parser]-+-|             |
      | |       |--[parser]-+-|             |--[audiodec]---[
      | ---------           | +-------------+
      +---------------------+

### Generating GstStreamCollection

When parsing the initial PAT/PMT, the demuxer will:

1. create the various `GstStream` objects for each stream.

2. create the `GstStreamCollection` for that initial PMT

3. post the `GST_MESSAGE_STREAM_COLLECTION`. Decodebin will intercept that message and know what the demuxer will be exposing.

4. The demuxer creates the various pads and sends the corresponding `STREAM_START` event (with the same stream-id as the corresponding `GstStream` objects), `CAPS` event, and `TAGS` event.

- `parsebin` will add all relevant parsers and expose those streams.

- `decodebin3` will be able to correlate, based on `STREAM_START` event stream-id, what pad corresponds to which stream. It links each stream from parsebin to multiqueue.

- `decodebin3` knows all the streams that will be available. Since by default it is configured to only expose a stream of each type, it will pick a stream of each for which it will complete the auto-plugging (finding a decoder and then exposing that stream as a source ghostpad.

> Note: If the demuxer doesn't create/post the `GstStreamCollection`, `parsebin` will create it on itself, as explained in the previous sections above.

### Changing the active selection from the application

The user wants to change the audio track. The application received the `GST_MESSAGE_STREAM_COLLECTION` containing the list of available streams. For clarity, we will assume those stream-ids are `"video-main"`, `"audio-english"` and `"audio-french"`.

The user prefers to use the french soundtrack (which it knows based on the language tag contained in the `GstStream` objects).

The application will create and send a `GST_EVENT_SELECT_STREAM` event containing the list of streams: `"video-main", "audio-french"`.

That event gets sent on the pipeline, the sinks send it upstream and eventually reach `decodebin3`.

`decodebin3` compares:

- The currently active selection (`"video-main", "audio-english"`)
- The available stream collection (`"video-main", "audio-english", "audio-french"`)
- The list of streams in the event (`"video-main", "audio-french"`)

`decodebin3` determines that no change is required for `"video-main"`, but sees that it needs to deactivate `"audio-english"` and activate `"audio-french"`.

It unlinks the `multiqueue` source pad connected to the current audio decoder. Then it queries that audio decoder, using the `GST_QUERY_ACCEPT_CAPS`, whether it can accept as-is the caps from the `"audio-french"` stream.

1. If it does, the multiqueue source pad corresponding to `"audio-french"` is linked to the decoder.

2. If it does not, the existing audio decoder is removed, a new decoder is selected (like during initial auto-plugging), and replaces the old audio decoder element.

The newly selected stream gets decoded and output through the same pad as the previous audio stream.

Note: The default behavior is to only expose one stream of each type. But nothing prevents `decodebin3` from outputting more/less of each type if the `GST_EVENT_SELECT_STREAM` event specifies that. This allows covering more use-case than the simple playback one. Such examples could be :

- Wanting just a video stream or just an audio stream
- Wanting all decoded streams
- Wanting all audio streams ...

### Changes of stream collection

At some point in time, a PMT change happens. Let's assume a change in video-codec and/or PID.

The demuxer creates a new `GstStream` for the changed/new stream, creates a new `GstStreamCollection` for the updated PMT and posts it.

`decodebin3` sees the new `GstStreamCollection` message.

The demuxer (and `parsebin`) then adds and removes pads:

1. `decodebin3` will match the new pads to `GstStream` in the new `GstStreamCollection` the same way it did for the initial pads previously.

2. `decodebin3` will see whether the new stream can re-use a `multiqueue` slot used by a stream of the same type no longer present (it compares the old collection to the new collection). In this case, `decodebin3` sees that the new video stream can re-use the same slot as the previous video stream.

3. If the new stream is going to be active by default (in this case it does because we are replacing the only video stream, which was active), it will check whether the caps are compatible with the existing video decoder (in the same way it was done for the audio decoder switch).

Finally, the stream that switched will be decoded and output through the same pad as the previous video stream in a gapless fashion.

### Further examples

##### DASH/HLS alternates (selection handled upstream of decodebin3)

There is a main (multi-bitrate or not) stream with audio and video interleaved in MPEG-TS. The manifest also indicates the presence of alternate language audio-only streams.

- (main) variant stream : `video`, `audio-eng`
- alternate rendition streams : `audio-kor`, `audio-fre`

A new element, `hlsdemux2`, used within `urisourcebin`, exposes one collection containing all individual elementary streams (regardless of bitrates). This is based on the contents of the manifest:

- `[ video , audio-eng, audio-kor, audio-fre ]`

Similar to other demuxers, it will post the collection on the bus, and will add/remove pads dynamically based on `GST_EVENT_SELECT_STREAMS` (or make a default choice if no event was received).

Since it can offer a selection of "elementary" streams, it will internally demux, parse and buffer the requested streams.

If ever it receives the `GST_QUERY_SELECTABLE`, it will set the `selectable` field of that query to TRUE. This informs `decodebin3` that it should handle all incoming streams and not do any selection (it is done upstream).

      +-----------+               + decodebin3 -----------------------------+
      | hlsdemux2 |               |                                         |
      |           |               |             +------------+              |
      |           +-   `video`   -+-[parsebin]--+ multiqueue +--[videodec]--[
      |           +- `audio-eng` -+-[parsebin]--+------------+--[audiodec]--[
      |           |               |                                         |
      +-----------+               +-----------------------------------------+

The user might want to use the korean audio track instead of the default english one.

      => SELECT_STREAMS ("video", "kor")

0. When `decodebin3` received `GST_EVENT_STREAM_START` on the initial incoming streams, it sent a `GST_QUERY_SELECTABLE` which `hlsdemux2` answered succesfully. `decodebin3` therefore knows that the upstream of that stream can handle stream-selection itself.

1. `decodebin3` receives the new `SELECT_STREAMS` event, and knows that upstream can handle stream selection and therefore forwards it upstream.

2. `hlsdemux2` receives the event, and prepares the `audio-kor` backing stream to be downloaded and returns. `decodebin3` also returns the event as is and doesn't do any selection itself.

3. Once `hlsdemux2` has downloaded enough from the target `audio-kor` stream, which it has demuxed, parsed and buffered internally, it will output it to the same output pad (replacing `audio-eng`):

- Send a new `GST_EVENT_STREAM_START` containing the information about `audio-kor`
- Send the various required events (caps, stream-collection,...)
- Send the buffers for the new stream

4. If the incoming stream isn't compatible with the existing `parsebin`, `decodebin3` will reset it so that it reconfigures itself.

      +-----------+               + decodebin3 -----------------------------+
      | hlsdemux2 |               |                                         |
      |           |               |             +------------+              |
      |           +-   `video`   -+-[parsebin]--+ multiqueue +--[videodec]--[
      |           +- `audio-kor` -+-[parsebin]--+------------+--[audiodec]--[
      |           |               |                                         |
      +-----------+               +-----------------------------------------+

#### Multi-program MPEG-TS

NOTE: Not properly handled yet.

Assuming the case of a MPEG-TS stream which contains multiple programs.

There would be three "levels" of collection:

1. The collection of programs presents in the stream
2. The collection of elementary streams presents in a stream
3. The collection of streams decodebin can expose

Initially `tsdemux` exposes the first program present (default)

        [ Collection 1 ]         [ Collection 2     ]        [ Collection 3    ]
        [  (tsdemux)   ]         [   (tsdemux)      ]        [ (decodebin)     ]
        [ id:Programs  ]<-\      [ id:BBC1          ]<-\     [ id:BBC1-decoded ]
        [ upstream:nil ]   \-----[ upstream:Programs]   \----[ upstream:BBC1   ]
        [              ]         [                  ]        [                 ]
        [ "BBC1" (C)   ]         [ id:"bbcvideo"(V) ]        [ id:"bbcvideo"(V)]
        [ "ITV"  (C)   ]         [ id:"bbcaudio"(A) ]        [ id:"bbcaudio"(A)]
        [ "NBC"  (C)   ]         [                  ]        [                 ]

At some point the user wants to switch to ITV (of which we do not know the topology at this point in time. A `SELECT_STREAMS` event is sent with "ITV" in it and the pointer to the Collection1.

1. The event travels up the pipeline until `tsdemux` receives it and begins the switch.

2. `tsdemux` publishes a new 'Collection 2a/ITV' and marks 'Collection 2/BBC' as replaced.

   1. App may send a `SELECT_STREAMS` event configuring which demuxer output streams should be selected (parsed)

3. `tsdemux` adds/removes pads as needed (flushing pads as it removes them?)

4. `decodebin3` feeds new pad streams through existing parsers/decoders as needed. As data from the new collection arrives out each decoder, `decodebin3` sends new `GstStreamCollection` messages to the app so it can know that the new streams are now switchable at that level.

   1. As new `GstStreamCollections` are published, the app may override the default decodebin stream selection to expose more/fewer streams. The default is to decode and output 1 stream of each type.

Final state:

        [ Collection 1 ]         [ Collection 4     ]        [ Collection 5    ]
        [  (tsdemux)   ]         [   (tsdemux)      ]        [ (decodebin)     ]
        [ id:Programs  ]<-\      [ id:ITV           ]<-\     [ id:ITV-decoded  ]
        [ upstream:nil ]   \-----[ upstream:Programs]   \----[ upstream:ITV    ]
        [              ]         [                  ]        [                 ]
        [ "BBC1" (C)   ]         [ id:"itvvideo"(V) ]        [ id:"itvvideo"(V)]
        [ "ITV"  (C)   ]         [ id:"itvaudio"(A) ]        [ id:"itvaudio"(A)]
        [ "NBC"  (C)   ]         [                  ]        [                 ]

### TODO

- Figure out how to handle mutual-exclusion and relationship API with collection of streams.

- ~~Add flags to `GstStreamCollection`~~

- Add helper API to figure out whether a collection is a replacement of another or a completely new one. This will require a more generic system to know whether a certain stream-id is a replacement of another or not.

### Open Questions

- ~~Is a `FLUSHING` flag for stream-selection required or not?~~

  This would make the handler of the `SELECT_STREAMS` event send `FLUSH START/STOP` before switching to the other streams. This is tricky when dealing where situations where we keep some streams and only switch some others. Do we flush all streams? Do we only flush the new streams, potentially resulting in delay to fully switch? Furthermore, due to efficient buffering in `decodebin3`, the switching time has been minimized extensively, to the point where flushing might not bring a noticeable improvement.

  Note: This flushing capability was not needed in the end. Leveraging minimal buffering in `decodebin3` and various other elements allows almost-instantaneous stream switching without this.

- Store the stream collection in bins/pipelines?

  A Bin/Pipeline could store all active collection internally, so that it could be queried later on. This could be useful to then get, on any pipeline, at any point in time, the full list of collections available without having to listen to all COLLECTION messages on the bus. This would require fixing the "is a collection a replacement or not" issue first.

- When switching to new collections, should `decodebin3` make any effort to map corresponding streams from the old to new PMT - that is, try and stick to the english language audio track, for example? Alternatively, rely on the app to do such smarts with stream-select messages?

  Note: If an application listens synchronously to the collection bus messages, it can indeed decide which streams it wants to select. And therefore stick to the (new) english audio track.

The results of the search are