gstreamer/markdown/design/overview.md

Overview

This part gives an overview of the design of GStreamer with references to the more detailed explanations of the different topics.

This document is intented for people that want to have a global overview of the inner workings of GStreamer.

Introduction

GStreamer is a set of libraries and plugins that can be used to implement various multimedia applications ranging from desktop players, audio/video recorders, multimedia servers, transcoders, etc.

Applications are built by constructing a pipeline composed of elements. An element is an object that performs some action on a multimedia stream such as:

• read a file
• decode or encode between formats
• capture from a hardware device
• render to a hardware device
• mix or multiplex multiple streams

Elements have input and output pads called sink and source pads in GStreamer. An application links elements together on pads to construct a pipeline. Below is an example of an ogg/vorbis playback pipeline.

+-----------------------------------------------------------+
|    ----------> downstream ------------------->            |
|                                                           |
| pipeline                                                  |
| +---------+   +----------+   +-----------+   +----------+ |
| | filesrc |   | oggdemux |   | vorbisdec |   | alsasink | |
| |        src-sink       src-sink        src-sink        | |
| +---------+   +----------+   +-----------+   +----------+ |
|                                                           |
|    <---------< upstream <-------------------<             |
+-----------------------------------------------------------+

The filesrc element reads data from a file on disk. The oggdemux element parses the data and sends the compressed audio data to the vorbisdec element. The vorbisdec element decodes the compressed data and sends it to the alsasink element. The alsasink element sends the samples to the audio card for playback.

Downstream and upstream are the terms used to describe the direction in the Pipeline. From source to sink is called "downstream" and "upstream" is from sink to source. Dataflow always happens downstream.

The task of the application is to construct a pipeline as above using existing elements. This is further explained in the pipeline building topic.

The application does not have to manage any of the complexities of the actual dataflow/decoding/conversions/synchronisation etc. but only calls high level functions on the pipeline object such as PLAY/PAUSE/STOP.

The application also receives messages and notifications from the pipeline such as metadata, warning, error and EOS messages.

If the application needs more control over the graph it is possible to directly access the elements and pads in the pipeline.

Design overview

GStreamer design goals include:

• Process large amounts of data quickly
• Allow fully multithreaded processing
• Ability to deal with multiple formats
• Synchronize different dataflows
• Ability to deal with multiple devices

The capabilities presented to the application depends on the number of elements installed on the system and their functionality.

The GStreamer core is designed to be media agnostic but provides many features to elements to describe media formats.

Elements

The smallest building blocks in a pipeline are elements. An element provides a number of pads which can be source or sinkpads. Sourcepads provide data and sinkpads consume data. Below is an example of an ogg demuxer element that has one pad that takes (sinks) data and two source pads that produce data.

 +-----------+
 | oggdemux  |
 |          src0
sink        src1
 +-----------+

An element can be in four different states: NULL, READY, PAUSED, PLAYING. In the NULL and READY state, the element is not processing any data. In the PLAYING state it is processing data. The intermediate PAUSED state is used to preroll data in the pipeline. A state change can be performed with gst_element_set_state().

An element always goes through all the intermediate state changes. This means that when en element is in the READY state and is put to PLAYING, it will first go through the intermediate PAUSED state.

An element state change to PAUSED will activate the pads of the element. First the source pads are activated, then the sinkpads. When the pads are activated, the pad activate function is called. Some pads will start a thread (GstTask) or some other mechanism to start producing or consuming data.

The PAUSED state is special as it is used to preroll data in the pipeline. The purpose is to fill all connected elements in the pipeline with data so that the subsequent PLAYING state change happens very quickly. Some elements will therefore not complete the state change to PAUSED before they have received enough data. Sink elements are required to only complete the state change to PAUSED after receiving the first data.

Normally the state changes of elements are coordinated by the pipeline as explained in states.

Different categories of elements exist:

• source elements: these are elements that do not consume data but only provide data for the pipeline.

• sink elements: these are elements that do not produce data but renders data to an output device.

• transform elements: these elements transform an input stream in a certain format into a stream of another format. Encoder/decoder/converters are examples.

• demuxer elements: these elements parse a stream and produce several output streams.

• mixer/muxer elements: combine several input streams into one output stream.

Other categories of elements can be constructed (see klass).

Bins

A bin is an element subclass and acts as a container for other elements so that multiple elements can be combined into one element.

A bin coordinates its children’s state changes as explained later. It also distributes events and various other functionality to elements.

A bin can have its own source and sinkpads by ghostpadding one or more of its children’s pads to itself.

Below is a picture of a bin with two elements. The sinkpad of one element is ghostpadded to the bin.

 +---------------------------+
 | bin                       |
 |    +--------+   +-------+ |
 |    |        |   |       | |
 |  /sink     src-sink     | |
sink  +--------+   +-------+ |
 +---------------------------+

Pipeline

A pipeline is a special bin subclass that provides the following features to its children:

• Select and manage a global clock for all its children.
• Manage running_time based on the selected clock. Running_time is the elapsed time the pipeline spent in the PLAYING state and is used for synchronisation.
• Manage latency in the pipeline.
• Provide means for elements to comunicate with the application by the GstBus.
• Manage the global state of the elements such as Errors and end-of-stream.

Normally the application creates one pipeline that will manage all the elements in the application.

Dataflow and buffers

GStreamer supports two possible types of dataflow, the push and pull model. In the push model, an upstream element sends data to a downstream element by calling a method on a sinkpad. In the pull model, a downstream element requests data from an upstream element by calling a method on a source pad.

The most common dataflow is the push model. The pull model can be used in specific circumstances by demuxer elements. The pull model can also be used by low latency audio applications.

The data passed between pads is encapsulated in Buffers. The buffer contains pointers to the actual memory and also metadata describing the memory. This metadata includes:

• timestamp of the data, this is the time instance at which the data was captured or the time at which the data should be played back.

• offset of the data: a media specific offset, this could be samples for audio or frames for video.

• the duration of the data in time.

• additional flags describing special properties of the data such as discontinuities or delta units.

• additional arbitrary metadata

When an element whishes to send a buffer to another element is does this using one of the pads that is linked to a pad of the other element. In the push model, a buffer is pushed to the peer pad with gst_pad_push(). In the pull model, a buffer is pulled from the peer with the gst_pad_pull_range() function.

Before an element pushes out a buffer, it should make sure that the peer element can understand the buffer contents. It does this by querying the peer element for the supported formats and by selecting a suitable common format. The selected format is then first sent to the peer element with a CAPS event before pushing the buffer (see negotiation).

When an element pad receives a CAPS event, it has to check if it understand the media type. The element must refuse following buffers if the media type preceding it was not accepted.

Both gst_pad_push() and gst_pad_pull_range() have a return value indicating whether the operation succeeded. An error code means that no more data should be sent to that pad. A source element that initiates the data flow in a thread typically pauses the producing thread when this happens.

A buffer can be created with gst_buffer_new() or by requesting a usable buffer from a buffer pool using gst_buffer_pool_acquire_buffer(). Using the second method, it is possible for the peer element to implement a custom buffer allocation algorithm.

The process of selecting a media type is called caps negotiation.

Caps

A media type (Caps) is described using a generic list of key/value pairs. The key is a string and the value can be a single/list/range of int/float/string.

Caps that have no ranges/list or other variable parts are said to be fixed and can be used to put on a buffer.

Caps with variables in them are used to describe possible media types that can be handled by a pad.

Dataflow and events

Parallel to the dataflow is a flow of events. Unlike the buffers, events can pass both upstream and downstream. Some events only travel upstream others only downstream.

The events are used to denote special conditions in the dataflow such as EOS or to inform plugins of special events such as flushing or seeking.

Some events must be serialized with the buffer flow, others don’t. Serialized events are inserted between the buffers. Non serialized events jump in front of any buffers current being processed.

An example of a serialized event is a TAG event that is inserted between buffers to mark metadata for those buffers.

An example of a non serialized event is the FLUSH event.

Pipeline construction

The application starts by creating a Pipeline element using gst_pipeline_new(). Elements are added to and removed from the pipeline with gst_bin_add() and gst_bin_remove().

After adding the elements, the pads of an element can be retrieved with gst_element_get_pad(). Pads can then be linked together with gst_pad_link().

Some elements create new pads when actual dataflow is happening in the pipeline. With g_signal_connect() one can receive a notification when an element has created a pad. These new pads can then be linked to other unlinked pads.

Some elements cannot be linked together because they operate on different incompatible data types. The possible datatypes a pad can provide or consume can be retrieved with gst_pad_get_caps().

Below is a simple mp3 playback pipeline that we constructed. We will use this pipeline in further examples.

+-------------------------------------------+
| pipeline                                  |
| +---------+   +----------+   +----------+ |
| | filesrc |   | mp3dec   |   | alsasink | |
| |        src-sink       src-sink        | |
| +---------+   +----------+   +----------+ |
+-------------------------------------------+

Pipeline clock

One of the important functions of the pipeline is to select a global clock for all the elements in the pipeline.

The purpose of the clock is to provide a stricly increasing value at the rate of one GST_SECOND per second. Clock values are expressed in nanoseconds. Elements use the clock time to synchronize the playback of data.

Before the pipeline is set to PLAYING, the pipeline asks each element if they can provide a clock. The clock is selected in the following order:

• If the application selected a clock, use that one.

• If a source element provides a clock, use that clock.

• Select a clock from any other element that provides a clock, start with the sinks.

• If no element provides a clock a default system clock is used for the pipeline.

In a typical playback pipeline this algorithm will select the clock provided by a sink element such as an audio sink.

In capture pipelines, this will typically select the clock of the data producer, which in most cases can not control the rate at which it produces data.

Pipeline states

When all the pads are linked and signals have been connected, the pipeline can be put in the PAUSED state to start dataflow.

When a bin (and hence a pipeline) performs a state change, it will change the state of all its children. The pipeline will change the state of its children from the sink elements to the source elements, this to make sure that no upstream element produces data to an element that is not yet ready to accept it.

In the mp3 playback pipeline, the state of the elements is changed in the order alsasink, mp3dec, filesrc.

All intermediate states are traversed for each element resulting in the following chain of state changes:

• alsasink to READY: the audio device is probed

• mp3dec to READY: nothing happens.

• filesrc to READY: the file is probed

• alsasink to PAUSED: the audio device is opened. alsasink is a sink and returns ASYNC because it did not receive data yet. mp3dec to PAUSED: the decoding library is initialized

• filesrc to PAUSED: the file is opened and a thread is started to push data to mp3dec

At this point data flows from filesrc to mp3dec and alsasink. Since mp3dec is PAUSED, it accepts the data from filesrc on the sinkpad and starts decoding the compressed data to raw audio samples.

The mp3 decoder figures out the samplerate, the number of channels and other audio properties of the raw audio samples and sends out a caps event with the media type.

Alsasink then receives the caps event, inspects the caps and reconfigures itself to process the media type.

mp3dec then puts the decoded samples into a Buffer and pushes this buffer to the next element.

Alsasink receives the buffer with samples. Since it received the first buffer of samples, it completes the state change to the PAUSED state. At this point the pipeline is prerolled and all elements have samples. Alsasink is now also capable of providing a clock to the pipeline.

Since alsasink is now in the PAUSED state it blocks while receiving the first buffer. This effectively blocks both mp3dec and filesrc in their gst_pad_push().

Since all elements now return SUCCESS from the gst_element_get_state() function, the pipeline can be put in the PLAYING state.

Before going to PLAYING, the pipeline select a clock and samples the current time of the clock. This is the base_time. It then distributes this time to all elements. Elements can then synchronize against the clock using the buffer running_time base_time (See also synchronisation).

The following chain of state changes then takes place:

• alsasink to PLAYING: the samples are played to the audio device

• mp3dec to PLAYING: nothing happens

• filesrc to PLAYING: nothing happens

Pipeline status

The pipeline informs the application of any special events that occur in the pipeline with the bus. The bus is an object that the pipeline provides and that can be retrieved with gst_pipeline_get_bus().

The bus can be polled or added to the glib mainloop.

The bus is distributed to all elements added to the pipeline. The elements use the bus to post messages on. Various message types exist such as ERRORS, WARNINGS, EOS, STATE_CHANGED, etc..

The pipeline handles EOS messages received from elements in a special way. It will only forward the message to the application when all sink elements have posted an EOS message.

Other methods for obtaining the pipeline status include the Query functionality that can be performed with gst_element_query() on the pipeline. This type of query is useful for obtaining information about the current position and total time of the pipeline. It can also be used to query for the supported seeking formats and ranges.

Pipeline EOS

When the source filter encounters the end of the stream, it sends an EOS event to the peer element. This event will then travel downstream to all of the connected elements to inform them of the EOS. The element is not supposed to accept any more data after receiving an EOS event on a sinkpad.

The element providing the streaming thread stops sending data after sending the EOS event.

The EOS event will eventually arrive in the sink element. The sink will then post an EOS message on the bus to inform the pipeline that a particular stream has finished. When all sinks have reported EOS, the pipeline forwards the EOS message to the application. The EOS message is only forwarded to the application in the PLAYING state.

When in EOS, the pipeline remains in the PLAYING state, it is the applications responsability to PAUSE or READY the pipeline. The application can also issue a seek, for example.

Pipeline READY

When a running pipeline is set from the PLAYING to READY state, the following actions occur in the pipeline:

• alsasink to PAUSED: alsasink blocks and completes the state change on the next sample. If the element was EOS, it does not wait for a sample to complete the state change.
• mp3dec to PAUSED: nothing
• filesrc to PAUSED: nothing

Going to the intermediate PAUSED state will block all elements in the _push() functions. This happens because the sink element blocks on the first buffer it receives.

Some elements might be performing blocking operations in the PLAYING state that must be unblocked when they go into the PAUSED state. This makes sure that the state change happens very fast.

In the next PAUSED to READY state change the pipeline has to shut down and all streaming threads must stop sending data. This happens in the following sequence:

• alsasink to READY: alsasink unblocks from the _chain() function and returns a FLUSHING return value to the peer element. The sinkpad is deactivated and becomes unusable for sending more data.
• mp3dec to READY: the pads are deactivated and the state change completes when mp3dec leaves its _chain() function.
• filesrc to READY: the pads are deactivated and the thread is paused.

The upstream elements finish their _chain() function because the downstream element returned an error code (FLUSHING) from the _push() functions. These error codes are eventually returned to the element that started the streaming thread (filesrc), which pauses the thread and completes the state change.

This sequence of events ensure that all elements are unblocked and all streaming threads stopped.

Pipeline seeking

Seeking in the pipeline requires a very specific order of operations to make sure that the elements remain synchronized and that the seek is performed with a minimal amount of latency.

An application issues a seek event on the pipeline using gst_element_send_event() on the pipeline element. The event can be a seek event in any of the formats supported by the elements.

The pipeline first pauses the pipeline to speed up the seek operations.

The pipeline then issues the seek event to all sink elements. The sink then forwards the seek event upstream until some element can perform the seek operation, which is typically the source or demuxer element. All intermediate elements can transform the requested seek offset to another format, this way a decoder element can transform a seek to a frame number to a timestamp, for example.

When the seek event reaches an element that will perform the seek operation, that element performs the following steps.

1. send a FLUSH_START event to all downstream and upstream peer elements.
2. make sure the streaming thread is not running. The streaming thread will always stop because of step 1).
3. perform the seek operation
4. send a FLUSH done event to all downstream and upstream peer elements.
5. send SEGMENT event to inform all elements of the new position and to complete the seek.

In step 1) all downstream elements have to return from any blocking operations and have to refuse any further buffers or events different from a FLUSH done.

The first step ensures that the streaming thread eventually unblocks and that step 2) can be performed. At this point, dataflow is completely stopped in the pipeline.

In step 3) the element performs the seek to the requested position.

In step 4) all peer elements are allowed to accept data again and streaming can continue from the new position. A FLUSH done event is sent to all the peer elements so that they accept new data again and restart their streaming threads.

Step 5) informs all elements of the new position in the stream. After that the event function returns back to the application. and the streaming threads start to produce new data.

Since the pipeline is still PAUSED, this will preroll the next media sample in the sinks. The application can wait for this preroll to complete by performing a _get_state() on the pipeline.

The last step in the seek operation is then to adjust the stream running_time of the pipeline to 0 and to set the pipeline back to PLAYING.

The sequence of events in our mp3 playback example.

                                   | a) seek on pipeline
                                   | b) PAUSE pipeline
+----------------------------------V--------+
| pipeline                         | c) seek on sink
| +---------+   +----------+   +---V------+ |
| | filesrc |   | mp3dec   |   | alsasink | |
| |        src-sink       src-sink        | |
| +---------+   +----------+   +----|-----+ |
+-----------------------------------|-------+
           <------------------------+
                 d) seek travels upstream

    --------------------------> 1) FLUSH event
    | 2) stop streaming
    | 3) perform seek
    --------------------------> 4) FLUSH done event
    --------------------------> 5) SEGMENT event

    | e) update running_time to 0
    | f) PLAY pipeline