gstreamer/subprojects/gst-docs/markdown/additional/design/live-source.md

# Live sources

A live source is a source that cannot be arbitrarily `PAUSED` without
losing data.

A live source such as an element capturing audio or video, needs to be
handled in a special way. It does not make sense to start the dataflow
in the `PAUSED` state for those devices as the user might wait a long time
between going from `PAUSED` to `PLAYING`, making the previously captured
buffers irrelevant.

A live source therefore only produces buffers in the `PLAYING` state. This
has implications for sinks waiting for a buffer to complete the preroll
state since such a buffer might never arrive.

Live sources return `NO_PREROLL` when going to the `PAUSED` state to inform
the bin/pipeline that this element will not be able to produce data in
the `PAUSED` state. `NO_PREROLL` should be returned for both `READY→PAUSED`
and `PLAYING→PAUSED`.

When performing a `get_state()` on a bin with a non-zero timeout value,
the bin must be sure that there are no live sources in the pipeline
because otherwise, `get_state()` would block on the sinks.

A `GstBin` therefore always performs a zero-timeout `get_state()` on its
elements to discover the `NO_PREROLL` (and `ERROR`) elements before
performing a blocking wait.

## Scheduling

Live sources will not produce data in the `PAUSED` state. They block in
`get_range()` or in the loop function until they go to `PLAYING`.

## Latency

The live source timestamps its data with the time of the clock when
the data was captured. Normally, it will take some time to capture
the first sample of data and the last one. This means that when the
buffer arrives at the sink, it will already be late and will be dropped.

The latency is the time it takes to construct one buffer of data and it's
exposed with a `LATENCY` query.

See [latency](additional/design/latency.md)

## Timestamps

Live sources always timestamp their buffers with the `running_time` of
the pipeline. This is needed to be able to match the timestamps of
different live sources in order to synchronize them.

This is in contrast to non-live sources, which timestamp their buffers
starting from `running_time` 0.