This page is part of the documentation of auKsys/4

The computation server is part of the ros_pralin package, and it allows to run pralin composition as a service.

It is started with:

ros2 run ros_pralin computation_server

New computation can be started with the start_processing (ros_pralin_interfaces/srv/StartProcessing) service call. However, it can be difficult to pass computation as argument to ros2 service call, it is therefore recommended to use run_computation when starting computation from the command line, for instance, the following will run the computation defined in addition.yaml (you can use the computation from pralin/compose introduction as an example):

ros2 run ros_pralin run_computation --namespace addition addition.yaml

The --namespace is used to specify that the topic created for the computation server should be in the /addition namespace. To select the ROS namespace for the computation server, this is done with --ros-args -r __ns:=/server_namespace.

You can also run the following and it will read the computation from the terminal input:

ros2 run ros_pralin run_computation --stdin --namespace addition

When running the computation from the introduction tutorial, this will create three topics: /addition/a, /addition/b and /addition/c:

# In a terminal
ros2 topic echo /addition/output_c

# In an other terminal
ros2 topic pub --once /addition/input_b std_msgs/msg/Float64 "data: 2.0"
ros2 topic pub --once /addition/input_a std_msgs/msg/Float64 "data: 1.0"

The calculation will be completed once the computation server has receive one message on each topic. To keep the calculation running, we can pass the --repeat option:

ros2 run ros_pralin run_computation --repeat --namespace addition addition.yaml

Computation that are repeating needs to be stopped with a service call to stop_processing:

ros2 service call /stop_processing ros_pralin_interfaces/srv/ControlProcessing "uuid: 'COMPUTATION UUID'"

You need to replace 'COMPUTATION UUID' with the UUID show when executing run_computation.

Integration with kDB

Assuming kDB and the computation server are started in the same ROS namespace, it is possible to create a connection to a kDB server using the following pralin operation:

- ros_kdb/create_connection_handle:
    id: connection

For instance, the following composition can be used to extract all images from a dataset:

compose:
  variables:
    counter: &counter [0, 'add[0]']
  parameters:
    source_dataset_uri: !required null
  process:
    - ros_kdb/create_connection_handle:
        id: connection
    - kdb/sensing/query_image_dataset:
        id: qid
        inputs: ["connection[0]"]
        parameters:
          query: !param source_dataset_uri
    - for_each:
        id: iterate_image
        iterator: qid[0]
        process:
          - pralin/values/string/format:
              id: filename
              inputs: [ [*counter] ]
              parameters:
                format: "{}.jpg"
          - opencv/imgcodecs/imwrite:
              id: imwrite
              inputs: [ "filename[0]", "iterate_image[0]"]
          - pralin/arithmetic/addition:
              id: add
              inputs: [ *counter, 1]

Assuming a dataset with images was loaded in the database, through image recording or by importing, like it is done at the beginning of the dataset transfer tutorial. The following show how to pass the parameters as argument to run_computation:

ros2 run ros_pralin run_computation --parameters "{ source_dataset_uri: 'http://askco.re/examples#images_granso' }" --stdin

For more detail on how to process image datasets, check the process images datasets tutorial.

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