View Source Track Lifecycle

There are two types of tracks - audio and video.

Video tracks might be in multiple qualities/resolutions. We call them variants and indentify them using atoms - :high, :medium, :low.

In general:

  • :high - main track resolution
  • :medium - main track resolution scaled down by 2
  • :low - main track resolution scaled down by 4

Audio tracks can have only one variant - :high.

At first, each track is created with It is important that the engine supports only RTP tracks so each endpoint that publishes some track has to publish it in the form of RTP packets.

Unpacking RTP stream

If you are writing an endpoint that consumes data from the engine you can get media out of RTP packets with the support of Membrane.RTC.Engine.Track.get_depayloader/1.

After creating a track it is published using Membrane.RTC.Engine.publish_action_t/0. This causes the engine to notify other endpoints about new tracks and allows other endpoints to subscribe to them.

Next, each track variant is marked as ready with Membrane.RTC.Engine.track_ready_action_t/0. The action informs the engine that it can link bin's output pad to the engine itself.

At this point, the track is published and marked as ready. Next section outlines how resuming, pausing and requesting track variants looks like.

Managing track variants

Resuming and pausing track variant

Endpoint ---- TrackVariantResumed (:medium) ---> Engine
Endpoint ----        media (:medium)        ---> Engine
Endpoint ---- TrackVariantResumed (:low)    ---> Engine
Endpoint ----        media (:low)           ---> Engine
Endpoint ---- TrackVariantPaused (:medium)  ---> Engine

General rules:

  • media can flow only when a track variant is in the resumed state
  • when a track variant becomes inactive for some reason (e.g. browser stops sending it to the server because of bandwidth limitation), such variant is marked as paused with Membrane.RTC.Engine.Event.TrackVariantPaused.

Requesting track variant

Engine ---- TrackVariantResumed (:medium) ---> Endpoint
Engine <--- RequestTrackVariant (:medium) ---- Endpoint
Engine ---- TrackVariantResumed (:low)    ---> Endpoint
Engine ---- TrackVariantSwitched          ---> Endpoint
Engine ----        media                  ---> Endpoint
Engine ---- TrackVariantPaused  (:medium) ---> Endpoint
Engine <--- RequestTrackVariant (:low)    ---- Endpoint

General rules: