fsfw-from-zero/start/01-tasks.md

Threads and Tasks

A satellite is a complex system which usually has a lot of tasks which need to be done simulatenously by a dedicated On-Board Computer (OBC). This can include for example:

• TMTC handling. This includes Telecommand (TC) reception and execution, and the (autonomous) generation of Telemetry (TM)
• Control Operations, for example execution of the Attitue Control System (ACS) loop
• Handling of connected physical devices like sensors or payloads

Usually, these systems oftentimes have soft and even hard real-time requirements where longer delays are not allowed and the system has an upper bound for response times.

This basically means that any software which does multiple non-trivial tasks needs a (real-time) operating system to perform multiple tasks consecutively, with deterministisc guarantees that these tasks are performed within a certain temporal bound.

Some common operating system in the Space domain able to do this:

• FreeRTOS for smaller MCUs (e.g. SOURCE CubeSat project)
• Embedded Linux (EIVE CubeSat project)
• RTEMS (FLP satellite project)

All these operating system use threads or tasks as the basic worker unit which is executing code. This chapter first introduces threads as they are exposed by the C++ standard library. After that, the code is transitioned to use the abstraction provided by the framework.

1. Scheduling a basic task using the C++ std::thread API

The goal of this task is to set up a basic thread which prints the following string every second: "Hello World".

• std::thread API
• Delaying a thread

2. Changing to the concept of executable objects

The goal of this task is to convert the code from task 1 so the [std::thread] API takes an executable object to move to a more object oriented task approach. The printout of the thread should remain the same. The executable objects should be named MyExecutableObject. It contains one function called periodicOperation which performs the printout, and a static function which takes the MyExecutableObject itself by reference and executes it in a permanent loop.

The executable object should be passed into the [std::thread] directly.

Hints

• std::reference_wrapper to pass references to the [std::thread] API
• std::chrono::milliseconds has a constructor where an uint32_t can be used to create the duration from a custon number.

Subtasks

1. Create a class called MyExecutableObject with a public block.
2. Add a static function called executeTask which expects itself (MyExecutableObject& self) as a parameter with an empty implementation
3. Add a regular method called performOperation which performs the printout
4. Implement executeTask. This function uses the passed object and performs the scheduling specific part by calling self.performOperation in a permanent loop with a delay between calls. You can hardcode the delay to 1000ms for the first implementation.
5. Add a constructor to MyExecutableObject which expects a millisecond delay as an uint32_t and cache it as a member variable. Then use this member variable in the executeTask implementation to make the task frequency configurable via the constructor (ctor) parameter.

With the conversion to executable object, we have reached a useful goal in object-oriented programming (OOP) in general: The application logic inside performOperation is now decoupled from the scheduling logic inside executeTask. This is also called seperation of concerns.

3. Making the executable objects generic

Our approach is useful buts lacks being generic as it relies on std library API. C++ as an OOP language provides abstraction in form of interfaces, which can be used to have different types of generic executable objects. Interfaces usually do not have a lot of source code on their own. They describe a design contract a class should have which implements the interface. In general, the FSFW relies heavily on subclassing and inheritance to provide adaptions point to users.

We are going to refactor our MyExecutableObject by introducing an interface for any executable object. We are then going to add a generic class which expects an object fulfilling this design contract and then executes that object.

Interfaces in C++ are implemented using abstract classes which only contains pure virtual functions.

Subtasks

1. Create an interface called MyExecutableObjectIF. You can create this like a regular class. As opposed to Java the differences between interfaces and classes are only by convention.

2. In general, it is recommended to add a virtual destructor to an interface. It looks like this:

   virtual ~<Class>() = default;
   

3. Add a abstract virtual function performOperation. Abstract virtual functions look like this in general

   virtual <functionName>(...) = 0;
   

4. Implement you custom interface for MyExecutableObject by re-using the exsiting performOperation function. In general, when implementing an interface or overriding a virtual function, it is recommended to add the override keyword to the function delaration. We do not have seperation between source and header files for our class yet, so you can add the override keyword after the function arguments and before the implementation block. The compiler will throw a compile error if a function is declared override but no base object function was actually overriden. This can prevent subtle bugs. Please note that MyExecutableObject is actually now forced to implement the performOperation function because that function is pure. The compiler makes sure we fulfill the design contract specified by the interface

5. Add a new class called MyPeriodicTask. Our executed object and the task abstraction are now explicitely decoupled by using composition. Composition means that we have a "has-a" relationship instead of a "is-a" relationship. In general, composition is preferable to inheritance for flexible software designs. The new MyPeriodicTask class should have a ctor which expects a MyExecutableObjectIF by reference. It caches that object and exposes a start method to start the task

3. Using the framework abstractions

Threads generally expect a function which is then directly executed. Sometimes, the execution of threads needs to be deferred. For example, this can be useful if the execution of tasks should only start after a certain condition.

Also, it might become useful to model any task in form of a class. An instantiation of that class would then be an executable object. This is precisely what the framework exposes in form of the ExecutableObjectIF.

It also offers a unform API to execute periodic tasks in form of the PeriodicTaskIF.

These tasks can then be created using the TaskFactory singleton.

An arbitrary number of executable objects can then be passed to a periodic task. These objects are then executed sequentially. This allows a granular design of executable tasks. For example, important tasks get an own dedicated thread while other low priority objects are scheduled consecutively in another thread.

The task abstractions have the following advantages:

• Task execution can be deferred until an explicit start method is called
• Same uniform API across multiple operating systems

The goal of this task is to implement the task specified in 1 using the abstractions provided in step 1.