gstreamer/markdown/additional/design/draft-push-pull.md

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# DRAFT push-pull scheduling
Status
DRAFT. DEPRECATED by better current implementation.
Observations:
- The main scheduling mode is chain based scheduling where the source
element pushes buffers through the pipeline to the sinks. this is
called the push model
- In the pull model, some plugin pulls buffers from an upstream peer
element before consuming and/or pushing them further downstream.
Usages of pull based scheduling:
- sinks that pull in data, possibly at fixed intervals driven by some
hardware device (audiocard, videodevice, …).
- Efficient random access to resources. Especially useful for certain
types of demuxers.
API for pull-based scheduling:
- an element that wants to pull data from a peer element needs to call
the `pull_range()` method. This method requires an offset and a
size. It is possible to leave the offset and size at -1, indicating
that any offset or size is acceptable, this of course removes the
advantages of getrange based scheduling.
Types of pull based scheduling:
- some sources can do random access (file source, …)
- some sources can read a random number of bytes but not at a random
offset. (audio cards, …) Audio cards using a ringbuffer can however
do random access in the ringbuffer.
- some sources can do random access in a range of bytes but not in
another range. (a caching network source).
- some sources can do a fixed size data and without an offset. (video
sources, …)
Current scheduling decision:
- core selects scheduling type starting on sinks by looking at
existence of loop function on sinkpad and calling
`_check_pull_range()` on the source pad to activate the pads in
push/pull mode.
- element proxies pull mode pad activation to peer pad.
Problems:
- core makes a tough desicion without knowing anything about the
element. Some elements are able to deal with a `pull_range()` without
offset while others need full random access.
Requirements:
- element should be able to select scheduling method itself based on
how it can use the peer element `pull_range()`. This includes if the
peer can operate with or without offset/size. This also means that
the core does not need to select the scheduling method anymore and
allows for more efficient scheduling methods adjusted for the
particular element.
Proposition:
- pads are activated without the core selecting a method.
- pads queries scheduling mode of peer pad. This query is rather
finegrained and allows the element to know if the peer supports
offsets and sizes in the `get_range()` function. A proposition for the
query is outlined in draft-query.txt.
- pad selects scheduling mode and informs the peer pad of this
decision.
Things to query:
- pad can do real random access (downstream peer can ask for offset `!= -1`)
- min offset
- suggest sequential access
- max offset
- align: all offsets should be aligned with this value.
- pad can give ranges from A to B length (peer can ask for `A ⇐ length ⇐ B`)
- min length
- suggested length
- max length
Use cases:
- An audio source can provide random access to the samples queued in
its DMA buffer, it however suggests sequential access method. An
audio source can provide a random number of samples but prefers
reading from the hardware using a fixed segment size.
- A caching network source would suggest sequential access but is
seekable in the cached region. Applications can query for the
already downloaded portion and update the GUI, a seek can be done in
that area.
- a live video source can only provide buffers sequentialy. It exposes
offsets as -1. lengths are also -1.