Chapter 15. Buffering

Table of Contents

Stream buffering
Download buffering
Timeshift buffering
Live buffering
Buffering strategies
No-rebuffer strategy

The purpose of buffering is to accumulate enough data in a pipeline so that playback can occur smoothly and without interruptions. It is typically done when reading from a (slow) and non-live network source but can also be used for live sources.

GStreamer provides support for the following use cases:

GStreamer can provide the application with progress reports about the current buffering state as well as let the application decide on how to buffer and when the buffering stops.

In the most simple case, the application has to listen for BUFFERING messages on the bus. If the percent indicator inside the BUFFERING message is smaller than 100, the pipeline is buffering. When a message is received with 100 percent, buffering is complete. In the buffering state, the application should keep the pipeline in the PAUSED state. When buffering completes, it can put the pipeline (back) in the PLAYING state.

What follows is an example of how the message handler could deal with the BUFFERING messages. We will see more advanced methods in the section called “Buffering strategies ”.


  [...]

  switch (GST_MESSAGE_TYPE (message)) {
    case GST_MESSAGE_BUFFERING:{
      gint percent;

      /* no state management needed for live pipelines */
      if (is_live)
        break;

      gst_message_parse_buffering (message, &percent);

      if (percent == 100) {
        /* a 100% message means buffering is done */
        buffering = FALSE;
        /* if the desired state is playing, go back */
        if (target_state == GST_STATE_PLAYING) {
          gst_element_set_state (pipeline, GST_STATE_PLAYING);
        }
      } else {
        /* buffering busy */
        if (buffering == FALSE && target_state == GST_STATE_PLAYING) {
          /* we were not buffering but PLAYING, PAUSE  the pipeline. */
          gst_element_set_state (pipeline, GST_STATE_PAUSED);
        }
        buffering = TRUE;
      }
      break;
    case ...

  [...]

  

Stream buffering

      +---------+     +---------+     +-------+
      | httpsrc |     | buffer  |     | demux |
      |        src - sink      src - sink     ....
      +---------+     +---------+     +-------+
    

In this case we are reading from a slow network source into a buffer element (such as queue2).

The buffer element has a low and high watermark expressed in bytes. The buffer uses the watermarks as follows:

  • The buffer element will post BUFFERING messages until the high watermark is hit. This instructs the application to keep the pipeline PAUSED, which will eventually block the srcpad from pushing while data is prerolled in the sinks.

  • When the high watermark is hit, a BUFFERING message with 100% will be posted, which instructs the application to continue playback.

  • When during playback, the low watermark is hit, the queue will start posting BUFFERING messages again, making the application PAUSE the pipeline again until the high watermark is hit again. This is called the rebuffering stage.

  • During playback, the queue level will fluctuate between the high and the low watermark as a way to compensate for network irregularities.

This buffering method is usable when the demuxer operates in push mode. Seeking in the stream requires the seek to happen in the network source. It is mostly desirable when the total duration of the file is not known, such as in live streaming or when efficient seeking is not possible/required.

The problem is configuring a good low and high watermark. Here are some ideas:

  • It is possible to measure the network bandwidth and configure the low/high watermarks in such a way that buffering takes a fixed amount of time.

    The queue2 element in GStreamer core has the max-size-time property that, together with the use-rate-estimate property, does exactly that. Also the playbin buffer-duration property uses the rate estimate to scale the amount of data that is buffered.

  • Based on the codec bitrate, it is also possible to set the watermarks in such a way that a fixed amount of data is buffered before playback starts. Normally, the buffering element doesn't know about the bitrate of the stream but it can get this with a query.

  • Start with a fixed amount of bytes, measure the time between rebuffering and increase the queue size until the time between rebuffering is within the application's chosen limits.

The buffering element can be inserted anywhere in the pipeline. You could, for example, insert the buffering element before a decoder. This would make it possible to set the low/high watermarks based on time.

The buffering flag on playbin, performs buffering on the parsed data. Another advantage of doing the buffering at a later stage is that you can let the demuxer operate in pull mode. When reading data from a slow network drive (with filesrc) this can be an interesting way to buffer.