Adding a simple services implementation

[haixuanTao]

Is your feature request related to a problem? Please describe.
Implementing a service features could simplify implementing certain dataflows, including supporting ROS 2 legacy applications.

API details

I think that we can actually build around our current pub-sub pattern to support request-reply.

What we could do is to make a new method called send_request or request that will send an output and receive an input in one blocking function call. The function will return the reply from the request.

To do that, we could add the following dataflow pattern:

  - id: webcam
    custom:
      source: ./webcam.py
      inputs:
        tick:
          source: dora/timer/millis/50
          queue_size: 1000
      outputs:
        - image
  - id: object_detection
    operator:
      python: ../operators/webcam/object_detection.py
      services:
        get_image:
           reply: webcam/image
           request: webcam/tick

With the following python operator request code:

image = send_service("get_image", tick)


## alternatively
image = send("service", "get_image", tick)
image = send("request", "get_image", tick)
image = send_request("get_image", tick)
image = request("get_image", tick)

With the following python custom node request code:

image = node.send_service("get_image", tick)


## alternatively
image = node.send("service", "get_image", tick)
image = node.send("request", "get_image", tick)
image = node.send_request("get_image", tick)
image = node.request("get_image", tick)```

The webcam code does need to be changed. And the underlying communication design does not need to change as well so we can still make it flexible.

Edge cases details

• We should try to keep message that has been received in between the request-reply, so that to not miss out on important messages. And so some work has to be done within the message queue.

Open for discussions