gstreamer/docs/design/part-buffering.txt

Buffering
---------

This document outlines the buffering policy used in the GStreamer
core that can be used by plugins and applications.

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.

 Some use cases:

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

  In this case we are reading from a slow network source into a buffer element
  (such as queue). 
  
  The buffer element has a low and high watermark expressed in bytes. The
  buffer uses the watermarks as follows:
  
   - The buffer element will not produce data on the src pad until the high
     watermark is hit. While accumulating data in the buffer, progress is
     reported by posting BUFFERING messages.
   - The source will stop to produce data on the src pad when the low watermark
     is hit.


Messages
--------

A GST_MESSAGE_BUFFERING must be posted on the bus when playback temporarily
stops to buffer and when buffering finishes. When percentage field in the
BUFFERING message is 100, buffering is done. Values less than 100 mean that
buffering is in progress. 

The BUFFERING message should be intercepted and acted upon by the application.


Application
-----------

While data is buffered, the pipeline should remain in the PAUSED state. It is
also possible that more data should be buffered while the pipeline is PLAYING,
in which case the pipeline should be PAUSED until the buffering finished.

BUFFERING messages can be posted while the pipeline is prerolling. The
application should not set the pipeline to PLAYING before a BUFFERING message
with 100 percent value is received, which might only happen after the pipeline
prerolled.


Buffering Query
---------------

It is possible to query the amount of buffering performed in the pipeline, which
is defined as the amount of data made available at the source. This amount is
expressed in some GstFormat and is usually compared to the duration or position 
of the media stream in the same GstFormat.

The buffering query should return the following information:

 - format
 - position
 - duration

The format is of lesser importance, the ratio of position versus duration can be
used to calculate the percentage of available media. It should also be possible
for the application to calculate the expected time when the complete file will
be buffered. 



Incremental download
--------------------