View Source Membrane.ChildrenSpec (Membrane Core v1.1.0)

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

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.

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

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

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)
]
@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

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

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 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

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

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.

Summary

Functions

Begins a link with a bin's pad.

Ends a link with a bin's output.

Used to spawn an anonymous child at the beggining of the link specification.

Used to spawn a named child at the beggining of the link specification or to spawn an anynomous child.

Used to spawn a named child or an anonymous child in the middle of the link specification.

Used to spawn a named child in the middle of a link specification.

Used to refer to an existing child at a beginning of a link specification.

Used to refer to an existing child in a middle of a link specification.

Specifies input pad name and properties of the subsequent child.

Specifies output pad name and properties of the preceding child.

Types

@opaque builder()
@type child_definition() :: struct() | module()
@type child_options() :: [{:get_if_exists, boolean()}]
Link to this type

children_spec_options()

View Source
@type children_spec_options() :: [
  group: Membrane.Child.group(),
  crash_group_mode: crash_group_mode(),
  stream_sync: :sinks | [[Membrane.Child.name()]],
  clock_provider: Membrane.Child.name() | nil,
  node: node() | nil,
  log_metadata: Keyword.t()
]
@type crash_group_mode() :: :temporary | nil
@type pad_options() :: Keyword.t()
@type t() :: builder() | [t()] | {t(), children_spec_options()}

Used to describe the inner topology of the pipeline/bin.

Functions

Link to this function

bin_input(pad \\ :input)

View Source
@spec bin_input(Membrane.Pad.name() | Membrane.Pad.ref()) :: builder() | no_return()

Begins a link with a bin's pad.

See the Children's specification section of the moduledoc for more information.

Link to this function

bin_output(builder, pad \\ :output)

View Source
@spec bin_output(builder(), Membrane.Pad.name() | Membrane.Pad.ref()) ::
  builder() | no_return()

Ends a link with a bin's output.

See the Children's specification section of the moduledoc for more information.

@spec child(child_definition()) :: builder()

Used to spawn an anonymous child at the beggining of the link specification.

See the Children's specification section of the moduledoc for more information.

Link to this function

child(child_name, child_definition)

View Source
@spec child(Membrane.Child.name(), child_definition()) :: builder()
@spec child(builder(), child_definition()) :: builder()
@spec child(child_definition(), child_options()) :: builder()

Used to spawn a named child at the beggining of the link specification or to spawn an anynomous child.

See the Children's specification section of the moduledoc for more information.

Link to this function

child(first_arg, second_arg, third_arg)

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Used to spawn a named child or an anonymous child in the middle of the link specification.

See the Children's specification section of the moduledoc for more information.

Link to this function

child(builder, child_name, child_definition, opts)

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Used to spawn a named child in the middle of a link specification.

See the Children's specification section of the moduledoc for more information.

@spec get_child(Membrane.Child.name()) :: builder()

Used to refer to an existing child at a beginning of a link specification.

See the Children's specification section of the moduledoc for more information.

Link to this function

get_child(builder, child_name)

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@spec get_child(builder(), Membrane.Child.name()) :: builder()

Used to refer to an existing child in a middle of a link specification.

See the Children's specification section of the moduledoc for more information.

Link to this function

via_in(builder, pad, props \\ [])

View Source
@spec via_in(builder(), Membrane.Pad.name() | Membrane.Pad.ref(),
  options: pad_options(),
  toilet_capacity: non_neg_integer() | nil,
  target_queue_size: non_neg_integer() | nil,
  min_demand_factor: non_neg_integer() | nil,
  auto_demand_size: non_neg_integer() | nil,
  throttling_factor: non_neg_integer() | nil
) :: builder() | no_return()

Specifies input pad name and properties of the subsequent child.

The possible properties are:

Additionally, the following properties can be used to adjust the flow control parameters. If set within a bin on an input that connects to the bin input, they will be overridden if set when linking to the bin in its parent.

  • toilet_capacity - Used when a toilet is created, that is for pull input pads (with :auto or :manual flow control), that have push output pads (with :push flow control) linked to them. When a push output produces more buffers than the pull input can consume, the buffers are accumulated in a queue called a toilet. If the toilet size grows above its capacity, it overflows by raising an error.
  • target_queue_size - The size of the queue of the input pad that Membrane will try to maintain. That allows for fulfilling the demands of the element by taking data from the queue while the actual sending of demands is done asynchronously, smoothing the processing. Used only for pads working in :manual flow control mode. See Membrane.Pad.flow_control/0 for more information.
  • min_demand_factor - A factor used to calculate minimal demand (minimal_demand = target_queue_size * min_demand_factor). Membrane won't send smaller demand than minimal demand, to reduce demands' overhead. However, the user will always receive as many buffers, as demanded, all excess buffers will be queued internally. Used only for pads working in :manual flow control mode. See Membrane.Pad.flow_control/0 for more info. Defaults to 0.25 (the default may change in the future).
  • auto_demand_size - Size of automatically generated demands. Used only for pads working in :auto flow control mode. See Membrane.Pad.flow_control/0 for more info.
  • throttling_factor - an integer specifying how frequently should a sender update the number of buffers in the Toilet. Defaults to 1, meaning, that the sender will update the toilet with each buffer being sent. Setting that factor for elements, which are running on the same node, does not have an impact of performance. However, once the sending element and the receiving element are put on different nodes, the sender updates the toilet with interprocess messages and setting a bigger throttling_factor can reduce the number of messages in the system. At the same time, setting a greater throttling_factor can result in a toilet overflow being detected later.

See the Children's specification section of the moduledoc for more information.

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via_out(builder, pad, props \\ [])

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@spec via_out(builder(), Membrane.Pad.name() | Membrane.Pad.ref(), [
  {:options, pad_options()}
]) ::
  builder() | no_return()

Specifies output pad name and properties of the preceding child.

The possible properties are:

See the Children's specification section of the moduledoc for more information.