mediapipe/docs/framework_concepts/graphs.md

---
layout: default
title: Graphs
parent: Framework Concepts
nav_order: 2
---

# Graphs
{: .no_toc }

1. TOC
{:toc}
---

## GraphConfig

A `GraphConfig` is a specification that describes the topology and functionality
of a MediaPipe graph. In the specification, a node in the graph represents an
instance of a particular calculator. All the necessary configurations of the
node, such its type, inputs and outputs must be described in the specification.
Description of the node can also include several optional fields, such as
node-specific options, input policy and executor, discussed in
[Synchronization](synchronization.md).

`GraphConfig` has several other fields to configure the global graph-level
settings, eg, graph executor configs, number of threads, and maximum queue size
of input streams. Several graph-level settings are useful for tuning the
performance of the graph on different platforms (eg, desktop v.s. mobile). For
instance, on mobile, attaching a heavy model-inference calculator to a separate
executor can improve the performance of a real-time application since this
enables thread locality.

Below is a trivial `GraphConfig` example where we have series of passthrough
calculators :

```proto
# This graph named main_pass_throughcals_nosubgraph.pbtxt contains 4
# passthrough calculators.
input_stream: "in"
node {
    calculator: "PassThroughCalculator"
    input_stream: "in"
    output_stream: "out1"
}
node {
    calculator: "PassThroughCalculator"
    input_stream: "out1"
    output_stream: "out2"
}
node {
    calculator: "PassThroughCalculator"
    input_stream: "out2"
    output_stream: "out3"
}
node {
    calculator: "PassThroughCalculator"
    input_stream: "out3"
    output_stream: "out4"
}
```

## Subgraph

To modularize a `CalculatorGraphConfig` into sub-modules and assist with re-use
of perception solutions, a MediaPipe graph can be defined as a `Subgraph`. The
public interface of a subgraph consists of a set of input and output streams
similar to a calculator's public interface. The subgraph can then be included in
an `CalculatorGraphConfig` as if it were a calculator. When a MediaPipe graph is
loaded from a `CalculatorGraphConfig`, each subgraph node is replaced by the
corresponding graph of calculators. As a result, the semantics and performance
of the subgraph is identical to the corresponding graph of calculators.

Below is an example of how to create a subgraph named `TwoPassThroughSubgraph`.

1.  Defining the subgraph.

    ```proto
    # This subgraph is defined in two_pass_through_subgraph.pbtxt
    # and is registered as "TwoPassThroughSubgraph"

    type: "TwoPassThroughSubgraph"
    input_stream: "out1"
    output_stream: "out3"

    node {
        calculator: "PassThroughCalculator"
        input_stream: "out1"
        output_stream: "out2"
    }
    node {
        calculator: "PassThroughCalculator"
        input_stream: "out2"
        output_stream: "out3"
    }
    ```

    The public interface to the subgraph consists of:

    *   Graph input streams
    *   Graph output streams
    *   Graph input side packets
    *   Graph output side packets

2.  Register the subgraph using BUILD rule `mediapipe_simple_subgraph`. The
    parameter `register_as` defines the component name for the new subgraph.

    ```proto
    # Small section of BUILD file for registering the "TwoPassThroughSubgraph"
    # subgraph for use by main graph main_pass_throughcals.pbtxt

    mediapipe_simple_subgraph(
        name = "twopassthrough_subgraph",
        graph = "twopassthrough_subgraph.pbtxt",
        register_as = "TwoPassThroughSubgraph",
        deps = [
                "//mediapipe/calculators/core:pass_through_calculator",
                "//mediapipe/framework:calculator_graph",
        ],
    )
    ```

3.  Use the subgraph in the main graph.

    ```proto
    # This main graph is defined in main_pass_throughcals.pbtxt
    # using subgraph called "TwoPassThroughSubgraph"

    input_stream: "in"
    node {
        calculator: "PassThroughCalculator"
        input_stream: "in"
        output_stream: "out1"
    }
    node {
        calculator: "TwoPassThroughSubgraph"
        input_stream: "out1"
        output_stream: "out3"
    }
    node {
        calculator: "PassThroughCalculator"
        input_stream: "out3"
        output_stream: "out4"
    }
    ```

## Cycles

<!-- TODO: add discussion of PreviousLoopbackCalculator -->

By default, MediaPipe requires calculator graphs to be acyclic and treats cycles
in a graph as errors. If a graph is intended to have cycles, the cycles need to
be annotated in the graph config. This page describes how to do that.

NOTE: The current approach is experimental and subject to change. We welcome
your feedback.

Please use the `CalculatorGraphTest.Cycle` unit test in
`mediapipe/framework/calculator_graph_test.cc` as sample code. Shown
below is the cyclic graph in the test. The `sum` output of the adder is the sum
of the integers generated by the integer source calculator.

![a cyclic graph that adds a stream of integers](../images/cyclic_integer_sum_graph.svg "A cyclic graph")

This simple graph illustrates all the issues in supporting cyclic graphs.

### Back Edge Annotation

We require that an edge in each cycle be annotated as a back edge. This allows
MediaPipe’s topological sort to work, after removing all the back edges.

There are usually multiple ways to select the back edges. Which edges are marked
as back edges affects which nodes are considered as upstream and which nodes are
considered as downstream, which in turn affects the priorities MediaPipe assigns
to the nodes.

For example, the `CalculatorGraphTest.Cycle` test marks the `old_sum` edge as a
back edge, so the Delay node is considered as a downstream node of the adder
node and is given a higher priority. Alternatively, we could mark the `sum`
input to the delay node as the back edge, in which case the delay node would be
considered as an upstream node of the adder node and is given a lower priority.

### Initial Packet

For the adder calculator to be runnable when the first integer from the integer
source arrives, we need an initial packet, with value 0 and with the same
timestamp, on the `old_sum` input stream to the adder. This initial packet
should be output by the delay calculator in the `Open()` method.

### Delay in a Loop

Each loop should incur a delay to align the previous `sum` output with the next
integer input. This is also done by the delay node. So the delay node needs to
know the following about the timestamps of the integer source calculator:

*   The timestamp of the first output.

*   The timestamp delta between successive outputs.

We plan to add an alternative scheduling policy that only cares about packet
ordering and ignores packet timestamps, which will eliminate this inconvenience.

### Early Termination of a Calculator When One Input Stream is Done

By default, MediaPipe calls the `Close()` method of a non-source calculator when
all of its input streams are done. In the example graph, we want to stop the
adder node as soon as the integer source is done. This is accomplished by
configuring the adder node with an alternative input stream handler,
`EarlyCloseInputStreamHandler`.

### Relevant Source Code

#### Delay Calculator

Note the code in `Open()` that outputs the initial packet and the code in
`Process()` that adds a (unit) delay to input packets. As noted above, this
delay node assumes that its output stream is used alongside an input stream with
packet timestamps 0, 1, 2, 3, ...

```c++
class UnitDelayCalculator : public Calculator {
 public:
  static absl::Status FillExpectations(
      const CalculatorOptions& extendable_options, PacketTypeSet* inputs,
      PacketTypeSet* outputs, PacketTypeSet* input_side_packets) {
    inputs->Index(0)->Set<int>("An integer.");
    outputs->Index(0)->Set<int>("The input delayed by one time unit.");
    return absl::OkStatus();
  }

  absl::Status Open() final {
    Output()->Add(new int(0), Timestamp(0));
    return absl::OkStatus();
  }

  absl::Status Process() final {
    const Packet& packet = Input()->Value();
    Output()->AddPacket(packet.At(packet.Timestamp().NextAllowedInStream()));
    return absl::OkStatus();
  }
};
```

#### Graph Config

Note the `back_edge` annotation and the alternative `input_stream_handler`.

```proto
node {
  calculator: 'GlobalCountSourceCalculator'
  input_side_packet: 'global_counter'
  output_stream: 'integers'
}
node {
  calculator: 'IntAdderCalculator'
  input_stream: 'integers'
  input_stream: 'old_sum'
  input_stream_info: {
    tag_index: ':1'  # 'old_sum'
    back_edge: true
  }
  output_stream: 'sum'
  input_stream_handler {
    input_stream_handler: 'EarlyCloseInputStreamHandler'
  }
}
node {
  calculator: 'UnitDelayCalculator'
  input_stream: 'sum'
  output_stream: 'old_sum'
}
```