gstreamer/docs/design/part-push-pull.txt

push-pull
---------

Normally a source element will push data to the downstream element using
the gst_pad_push() method. The downstream peer pad will receive the
buffer in the Chain function. In the push mode, the source element is the
driving force in the pipeline as it initiates data transport.

It is also possible for an element to pull data from an upstream element.
The downstream element does this by calling gst_pad_pull_range() on one
of its sinkpads. In this mode, the upstream element is the driving force 
in the pipeline as it initiates data transfer.

It is important that the elements are in the correct state to handle a
push() or a pull_range() from the peer element. For push() based elements
this means that all downstream elements should be in the correct state and
for pull_range() based elements this means the upstream elements should
be in the correct state.

Most sinkpads implement a chain function. This is the most common case.
sinkpads implementing a loop function will be the exception. Likewise
srcpads implementing a getrange function will be the exception.


state changes
~~~~~~~~~~~~~

The GstBin sets the state of all the sink elements. These are the elements
without source pads.

Setting the state on an element will first activate all the srcpads and then
the sinkpads. For each of the sinkpads, gst_pad_check_pull_range() is
performed. If the sinkpad supports a loopfunction and the peer pad returns TRUE
from the GstPadCheckPullRange function, then the peer pad is activated first as
it must be in the right state to handle a _pull_range(). Note that the
state change of the element is not yet performed, just the activate function
is called on the source pad. This means that elements that implement a
getrange function must be prepared to get their activate function called
before their state change function.

Elements that have multiple sinkpads that require all of them to operate
in the same mode (push/pull) can use the _check_pull_range() on all
their pads and can then remove the loop functions if one of the pads does
not support pull based mode.