Creating Your First Mission Application

Mission applications are user-created programs which are used to control satellite behavior and execute mission logic.

These applications are registered with the applications service, which is responsible for tracking versioning and controlling application upgrades and rollbacks.

This tutorial guides the user through the process of creating a basic mission application using Python3.

At the end of the tutorial, the user will have a mission application which is capable of querying the monitor service for current system memory usage and then storing that data into the telemetry database.

You should be able to go through this tutorial entirely within your development environment. You do not need to have an OBC available.

Note

The iOBC does not support Python. If this is the board which you are using, please refer to the example Rust mission application for the specific application code. The rest of this document should still be useful for the high-level concepts which are involved when developing a mission application.

Setup

• Install the Kubos SDK or set up the dependencies required for a local dev environment

• If you have not done so already, create a clone of the KubOS source repo:

  $ git clone https://github.com/kubos/kubos
  

• Navigate to the kubos source directory and run the following commands to start the monitor service and telemetry database service in the background (the services may need to be built first, which will take several minutes to complete):

  $ cargo run --bin monitor-service -- -c tools/local_config.toml &
  $ cargo run --bin telemetry-service -- -c tools/local_config.toml &
  

• Navigate back out to the development directory of your choosing. This tutorial will use /home/user/my-app as the example development directory and will assume that the cloned kubos repo is in /home/user/kubos.

• Create a new file for this tutorial, my-mission-app.py. In order to allow the applications service to run our mission application, you’ll need to start by placing the following line at the top of the new file:

  #!/usr/bin/env python3
  

  This allows the file to be run like a normal executable, ./my-mission-app.py, rather than needing to explicitly call the Python interpreter with python my-mission-app.py.

• Since you’ll be calling the file as an executable, you’ll also need to update the file permissions:

  $ chmod +x my-mission-app.py
  

Kubos Services and GraphQL

A major component of most mission applications will be interacting with Kubos services.

These services provided interfaces to underlying hardware and other system resources.

All services work by consuming GraphQL requests over HTTP, running the requested operation, and then returning a JSON response.

GraphQL is a query language which allows users to create readable requests which will return only the data they specify.

GraphQL requests come in two varieties: queries and mutations.

Queries

GraphQL queries perform informational, read-only operations. For example, a query might request that an underlying piece of hardware be contacted for its current temperature or last data reading.

An example query for the telemetry database service might look like this:

{
    telemetry(subsystem: "EPS") {
        timestamp,
        parameter,
        value
    }
}

This translates to “please fetch all of the stored telemetry entries for the EPS subsystem and return only their timestamp, parameter, and value values.”

The response might look like this:

{
    "telemetry": [
        {
            "timestamp": 1100,
            "parameter": "voltage",
            "value": "4.4"
        },
        {
            "timestamp": 1100,
            "parameter": "current",
            "value": "0.25"
        },
        {
            "timestamp": 1002,
            "parameter": "voltage",
            "value": "4.5"
        },
        {
            "timestamp": 1002,
            "parameter": "current",
            "value": "0.20"
        }
    ]
}

Mutations

GraphQL mutations perform actions which can be invasive or destructive, for example, writing data to a file or rebooting a hardware device.

An example mutation for the telemetry database service might look like this:

mutation {
    insert(subsystem: "GPS", parameter: "lock_status", value: "good") {
        success,
        errors
    }
}

This translates to “please create a new telemetry database entry for the GPS subsystem’s lock status parameter with a value of ‘good’. Return the overall success of the operation and any errors.”

Worth noting, all mutation requests are prefixed with mutation to quickly indicate to the service what kind of action is being requested.

A successful response should look like this:

{
    "insert": {
        "success": true,
        "errors": ""
    }
}

If the request failed, the response might look like this:

{
    "insert": {
        "success": false,
        "errors": "Failed to connect to database"
    }
}

Schemas

Each service has a schema which defines all of its queries and mutations.

Users should refer to these to determine what actions are available for each service and how their requests should be structured.

Documentation for Kubos services can be found within the services section.

For example, links to the schemas for all of the pre-built hardware services can be found here.

Determining Service URLs

In order to communicate with a service, we need to know where to send our messages.

All services rely on a configuration file, config.toml, in order to determine which IP and port they should bind a listener thread to.

By default, this file is located in /etc/kubos-config.toml. Since we’re running these tutorials locally, that file location likely doesn’t exist, so instead we are using the tools/local_config.toml file in our cloned copy of the kubos repo.

We’ll need to pass our application this path when we go to run it locally.

Querying a Service

For this tutorial, we’ll be querying the monitor service to make sure it is successfully up and running.

The monitor service is a unique hardware service which communicates with the OBC itself in order to obtain information about current processes running and the amount of memory both available and generally present on the system. It is unique because it is not tied to a particular hardware device and can, instead, be run on any supported OBC (or in this instance, the local dev environment). Worth noting, the process of communicating with this service is the same as communicating with any other core or hardware service.

We intend for this to be an ad-hoc action, so we’ll be adding code to the on-command section of our program.

The all KubOS core services provide a ping query which can be used to verify that the service is currently running on the expected port. The request has the following format:

{
    ping
}

The response should return a single "pong" result:

{
  "data": {
    "ping": "pong"
  }
}

To make the communication process simpler, we’ll be using the Python app API to send our GraphQL requests.

For each request, it:

• Looks up the HTTP address of the service name which is given from the system’s config.toml file
• Wraps the given request into a proper HTTP packet and sends it to the target service
• Parses the response message and checks for errors
• Returns the message payload in the "data" field if the request was successful

To start, we’ll import the API:

import app_api

Then, we’ll add a new command line option -c to allow us to pass a non-default config file for testing purposes:

import argparse
parser = argparse.ArgumentParser()

parser.add_argument('--config', '-c')

args = parser.parse_args()

if args.config is not None:
    SERVICES = app_api.Services(args.config)
else:
    SERVICES = app_api.Services()

Then, we’ll create the query we want to send:

request = '{ ping }'

Next, we’ll send the request to the monitor service:

response = SERVICES.query(service="monitor-service", query=request)

And finally, we’ll parse the result to get our response string:

data = response["ping"]
if data == "pong":
    print("Successfully pinged monitor service")
else:
    print("Unexpected monitor service response: %s" % data)

After adding proper structure, our program should look like this:

#!/usr/bin/env python3

import argparse
import app_api

def main():

    parser = argparse.ArgumentParser()

    parser.add_argument('--config', '-c')

    args = parser.parse_args()

    if args.config is not None:
        SERVICES = app_api.Services(args.config)
    else:
        SERVICES = app_api.services()

    request = '{ ping }'

    response = SERVICES.query(service="monitor-service", query=request)

    data = response["ping"]

    if data == "pong":
        print("Successfully pinged monitor service")
    else:
        print("Unexpected monitor service response: %s" % data)

if __name__ == "__main__":
    main()

If we run our program, the output should look like this:

$ ./my-mission-app.py -c ../kubos/tools/local_config.toml
Successfully pinged monitor service

Writing Data to the Telemetry Database

Now that we have a data point, we need to save it somewhere useful. The telemetry database is the main storage location for all telemetry data. The telemetry database service is the preferred interface point for storing and retrieving that data.

We’ll be using the service’s insert mutation in order to add a new telemetry entry. This operation is a mutation rather than a query, because it will cause the system to perform a write, rather than simply reading data.

The mutation has the following schema:

mutation {
    insert(timestamp: Integer, subsystem: String!, parameter: String!, value: String!) {
        success: Boolean!,
        errors: String!
    }
}

This indicates that there are four possible input parameters, all of which are required except for timestamp, and two return fields which, when requested, will always return a value.

Our mutation will have the following parameters:

• subsystem: “OBC” - Indicating that our data point corresponds to the main OBC (other subsystem names might be things like “EPS” or “payload”)
• parameter: “available_mem” - Indicating that our data point represents the current amount of available memory
• value - The data value which was returned from our previous query

All together, our request should look like this:

request = '''
    mutation {
        insert(subsystem: "OBC", parameter: "status", value: "%s") {
            success,
            errors
        }
    }
    ''' % (status)

Like before, we’ll now use the app API to send our request, but this time we’ll be sending to the telemetry database service rather than the monitor service:

response = SERVICES.query(service="telemetry-service", query=request)

Finally, we’ll check the response to make sure the operation finished successfully:

data = response["insert"]
success = data["success"]
errors = data["errors"]

if success == False:
    print("Telemetry insert encountered errors: " + str(errors))
else:
    print("Telemetry insert completed successfully")

With some error handling, our final application looks like this:

#!/usr/bin/env python3

import argparse
import app_api
import sys

def main():

    parser = argparse.ArgumentParser()

    parser.add_argument('--config', '-c')

    args = parser.parse_args()

    if args.config is not None:
        SERVICES = app_api.Services(args.config)
    else:
        SERVICES = app_api.Services()

    request = '{ ping }'

    try:
        response = SERVICES.query(service="monitor-service", query=request)

        data = response["ping"]

        if data == "pong":
            print("Successfully pinged monitor service")
            status = "Okay"
        else:
            print("Unexpected monitor service response: %s" % data)
            status = "Unexpected"

    except Exception as e:
        print("Something went wrong: " + str(e))
        status = "Error"

    request = '''
        mutation {
            insert(subsystem: "OBC", parameter: "status", value: "%s") {
                success,
                errors
            }
        }
        ''' % (status)

    try:
        response = SERVICES.query(service="telemetry-service", query=request)
    except Exception as e:
        print("Something went wrong: " + str(e) )
        sys.exit(1)

    data = response["insert"]
    success = data["success"]
    errors = data["errors"]

    if success == False:
        print("Telemetry insert encountered errors: " + str(errors))
        sys.exit(1)
    else:
        print("Telemetry insert completed successfully")

if __name__ == "__main__":
    main()

If we run our program, the output should look like this:

$ ./my-mission-app.py -c ../kubos/tools/local_config.toml
Successfully pinged monitor service
Telemetry insert completed successfully

Creating the Manifest File

In order for the applications service to properly maintain versioning information, we’ll need to create a new file, manifest.toml, to accompany our mission app.

This file has the following key values:

• name - The name of the application
• executable - (Optional) The name of the file to be called to begin application execution
• version - The version number of the application
• author - The author of the application

Our file should look like this:

name = "my-mission-app"
executable = "my-mission-app.py"
version = "1.0"
author = "Me"

Next Steps

• Running an application on an OBC
• Registering a mission application with the applications service
• Fetching telemetry data from the database