A module with functionalities that allow to represent a topology of a pipeline/bin.

The children specification (commonly referred to as a "children_spec") is represented by the following type: t/0. It consists of two parts - a children's specification and the children's specification options.

The children's specification describes the desired topology and can be incorporated into a pipeline or a bin by returning Membrane.Pipeline.Action.spec/0 or Membrane.Bin.Action.spec/0 action, respectively. This commonly happens within Membrane.Pipeline.handle_init/2 and Membrane.Bin.handle_init/2, but can be done in any other callback also.

children-s-specification

Children's specification

The children's specification allows specifying the children that need to be spawned in the action, as well as links between the children (both the children spawned in that action, and already existing children).

The children's processes are spawned with the use of child/1, child/2, child/3 and child/4 functions. These functions can be used for spawning nodes of a link in an inline manner:

  spec = [child(:source, Source) |> child(:filter, %Filter{option: 1}) |> child(:sink, Sink)]

or just to spawn children processes, without linking the newly created children:

  spec = [child(:source, Source),
    child(:filter, Filter),
    child(:sink, Sink)
  ]

Providing a child name is not necessary - you can spawn an anonymous child if you do not need to refer to that child later on:

  spec = child(Source) |> child(Filter)

Children created that way will have their automatically generated identifier consisting of a module name and a random unique reference number, so that you will be able to distinguish between anonymous children i.e. in log prints.

In case you need to refer to an already existing child (which could be spawned, i.e. in the previous spec action), use get_child/1 and get_child/2 functions, as in the example below:

  spec = [get_child(:already_existing_source) |> child(:this_filter_will_be_spawned, Filter) |> get_child(:already_existing_sink)]

The child functions allow specifying :get_if_exists option. It might be helpful when you are not certain if the child with the given name exists, and, therefore, you are unable to choose between get_child and child functions. After setting the get_if_exists: true option in child/3 and child/4 functions you can be sure that in case a child with a given name already exists, you will simply refer to that child instead of respawning it.

  spec = [child(:sink, Sink),
    child(:sink, Sink, get_if_exists: true) |> child(:source, Source)]

In the example above you can see, that the :sink child is created in the first element of the spec list. In the second element of that list, the get_if_exists: true option is used within child/3, which will have the same effect as if get_child(:sink) was used. At the same time, if the :sink child wasn't already spawned, it would be created in that link definition. Please note that it makes sense to use :get_if_exists option only with named children.

links-between-pads

Links between pads

via_in/2 and via_out/2 functions allow specifying pads' names and parameters. If pads are not specified, name :input is assumed for inputs and :output for outputs.

Sample definition:

[
  get_child(:source_a)
  |> get_child(:converter)
  |> via_in(:input_a, target_queue_size: 20)
  |> get_child(:mixer),
  get_child(:source_b)
  |> via_out(:custom_output)
  |> via_in(:input_b, options: [mute: true])
  |> get_child(:mixer)
  |> via_in(:input, toilet_capacity: 500)
  |> get_child(:sink)
]

See the docs for via_in/3 and via_out/3 for details on pad properties that can be set. Links can also contain children's definitions, for example:

[
  child(:first_element, %Element.With.Options.Struct{option_a: 42})
  |> child(:some_element, Element.Without.Options)
  |> get_child(:element_specified_before)
]

bins

Bins

For bin boundaries, there are special links allowed. The user should define links between the bin's input and the first child's input (input-input type) and the last child's output and bin output (output-output type). In this case, bin_input/1 and bin_output/2 should be used.

Sample definition:

[
  bin_input() |> get_child(:filter1) |> get_child(:filter2) |> bin_output(:custom_output)
]

dynamic-pads

Dynamic pads

In most cases, dynamic pads can be linked the same way as static ones, although in the following situations, an exact pad reference must be passed instead of a name:

• When that reference is needed later, for example, to handle a notification related to that particular pad instance

pad = Pad.ref(:output, make_ref())
[
  get_child(:tee) |> via_out(pad) |> get_child(:sink)
]

• When linking dynamic pads of a bin with its children, for example in Membrane.Bin.handle_pad_added/3

@impl true
def handle_pad_added(Pad.ref(:input, _) = pad, _ctx, state) do
  spec = [bin_input(pad) |> get_child(:mixer)]
  {{:ok, spec: spec}, state}
end

children-s-specification-options

Children's specification options

stream-sync

Stream sync

:stream_sync field can be used for specifying elements that should start playing at the same moment. An example can be audio and video player sinks. This option accepts either :sinks atom or a list of groups (lists) of elements. Passing :sinks results in synchronizing all sinks in the pipeline, while passing a list of groups of elements synchronizes all elements in each group. It is worth mentioning that to keep the stream synchronized all involved elements need to rely on the same clock.

By default, no elements are synchronized.

Sample definitions:

children = ...
  {children, stream_sync: [[:element1, :element2], [:element3, :element4]]}
  {children, stream_sync: :sinks}

clock-provider

Clock provider

A clock provider is an element that exports a clock that should be used as the pipeline clock. The pipeline clock is the default clock used by elements' timers. For more information see Membrane.Element.Base.def_clock/1.

children-groups

Children groups

Children groups allow aggregating the spawned children into easily identifiable groups. With the use of them, it is possible to refer to all the children of the group with a single identifier. Example:

  spec1 = {links1, group: :first_children_group}
  spec2 = {links2, group: :second_children_group}

The children spawned within links1 specification will be put inside :first_children_group, whereas the children spawned within links2 specification will be put inside second_children_group.

Later on, the children from a given group can be referred with their group, as in the example below:

  actions = [remove_children: :first_children_group]

With the action defined above, all the children from the :first_children_group can be removed at once.

crash-groups

Crash groups

A crash group is a logical entity that prevents the whole pipeline from crashing when one of components crashes. A crash group is defined with the use of two children specification options:

• group - which acts as a crash group identifier
• crash_group_mode - its value specifies the behavior of children in the crash group. Currently, we support only :temporary mode which means that Membrane will not make any attempts to restart crashed child.

Adding children to a crash group

spec = [
  child(:some_element_1, %SomeElement{
    # ...
  },
  child(:some_element_2, %SomeElement{
    # ...
  }
]

spec = {spec, group: group_id, crash_group_mode: :temporary}

In the above snippet, we create new children - :some_element_1 and :some_element_2, we add them to the crash group with id group_id. Crash of :some_element_1 or :some_element_2 propagates only to the rest of the members of the crash group and the pipeline stays alive.

Handling crash of a crash group

When any of the members of the crash group goes down, the callback: handle_crash_group_down/3 is called.

@impl true
def handle_crash_group_down(crash_group_id, ctx, state) do
  # do some stuff in reaction to the crash of the group with id crash_group_id
end

Limitations

At this moment crash groups are only useful for elements with dynamic pads. Crash groups work in pipelines and bins as well.

log-metadata

Log metadata

:log_metadata field can be used to set the Membrane.Logger metadata for all children in the given children specification.

nesting-children-s-specifications

Nesting children's specifications

The children's specifications can be nested within themselves.

Consider the following children's specifications:

{[
  child(:a, A) |> child(:b, B),
  {child(:c, C), group: :second, crash_group_mode: :temporary}
], group: :first, crash_group_mode: :temporary, node: some_node}

Child :c will be spawned in the :second crash group, while children :a and :b will be spawned in the :first crash group. Furthermore, since the inner children specification does not define the :node option, it will be inherited from the outer children specification. That means that child :c will be spawned on the some_node node, along with children :a and :b.