Tasking Event-B Tutorial

For more information contact Andy Edmunds - University of Southampton - mailto:ae2@ecs.soton.ac.uk

Tasking Event-B Tutorial Overview

This tutorial follows on from the abstract development described here.

This code generation tutorial extends the Heating Controller tutorial example, and makes use of example projects from the download site. The code generation stage produces implementable Ada code, and also an Event-B model. It is a model of the implementation, and contains flow control variables that model the flow of execution through the task body. The Ada code is produced from an intermediate model that is not visible to the user. The Common Language model (CLM), is generated from the Tasking Event-B by a translation tool. Ada (and other implementations) may be generated from the CLM. An overview of Tasking Event-B can be found here.

In the example so far, the Heating Controller has been refined to the point where we wish to add implementation constructs. The Event-B language is not expressive enough to fully describe the implementation. Tasking Event-B facilitates this final step to implementation, by extending Event-B with the necessary constructs. Event-B machines modelling tasks, shared objects and the environment are identified, and extended with the appropriate implementation details.

The example/tutorial projects are are available in the e-prints archive, or on SVN.

Heating_ControllerTutorial2_Completed	An example project with an environment simulation. The environment variables are monitored and controlled using subroutine calls. The project contains a complete Tasking Development with generated Event-B and Ada code.
Heating_ControllerTutorial2_Partial1	A project with the final decomposition completed, ready to begin Tasking Event-B Development.
Heating_ControllerTutorial2_Partial2	A partially completed tasking specification for the continuation of the tutorial.
TheoriesForCG	Contains the mathematical language translations; encoded as rules in a theory plug-in rule-base.

Using the Tasking Extension

The steps needed to generate code from an Event-B model, in this tutorial, are as follows,

• Step 1 - Adding the Implementation Level Refinement
• Step 2 - Pre-processing
• Step 3 - Add Tasking annotations.
• Step 4 - Invoke translators.

Download and Copy the Theories

The translations of the Event-B mathematical language to the target language constructs are specified as rules in the theory plug-in. Three rule files are included for the example, and are available in the SVN. The files can be downloaded and copied into an Event-B project called MathExtensions. The theory must then be deployed. Right-Click on the theory file and select deploy to do this. The non-Event-B project, the original download may now be deleted.

Adding the Implementation Level Refinement

The final decomposition generates the machines that are required for code generation. However, it is not possible to edit the machines since they are machine generated, and therefore this is prohibited. In order to be able to modify the models we will refine the generated machines. This is where we begin with the Heating_ControllerTutorial2_Partial1 project. To refine the machines we can use the automatic refinement feature, but this presents us with two problems that are dealt with in the pre-processing step. It is also at this stage that any remaining non-deterministic constructs should be removed by replacing them with deterministic constructs.

TIP: Non-deterministic constructs cause strange characters to appear in the source code. If you see strange characters in the generated code, check for non-deterministic constructs in the implementation level machines.

Alter_Temperature_Sensor1 in Envir1Impl: action becomes ts1 := ts1 + 1
Alter_Temperature_Sensor2 in Envir1Impl: action becomes ts1 := ts1 + 1
Alter_Heater_Status in Envir1Impl: action becomes hss := FALSE
INITIALISATION in Heater_Monitor_TaskImpl: becomes shs := FALSE

We also need to add a typing flag to an invariant. We need to add it in only one place, and this is where an invariant is used type a variable, in the Heating Controller machine. The flag is used to guide the translator to the typing invariant. This is because there is more than one invariant involving that particular variable. They may also be added to guards where parameters are typed in guards, and the parameters are referred to in more than one guard.

• Go to the Heater_Monitor_TaskImpl typing_shs invariant.
• Add the typing flag, by right-clicking on the invariant and selecting typing from the menu.

Pre-processing

The pre-processing step should be a temporary, the solutions can be incorporated into the tool to automatically perform the changes that are required.

• The Code Generator requires a flattened version of each machine; all of the Event-B elements should be available in the implementation level machine.
• Composed machines are not currently able to be refined, so anything that requires synchronization of events requires some manual updates.

'Flattening' the Implementation Machines

The temporary solution for flattening:

• Make events not extended.
• Copy missing invariants.

I found the Event-B Machine Editor's synthesis view useful for this. Invariants can be copy-pasted into the implementation machine from the abstraction. (A dummy invariant can be added and selected for pasting)

Providing the correct Composed Machine

The composed machine problem is sub-divided into two sub-problems. Firstly composed machines cannot be refined, and secondly when a machine is further decomposed there is no link between the first composed machine and the newly generated composed machine. So one or both of these problems may occur, depending on the number of decompositions.

We must manually add the information to the composed machines to address these two problems.

The temporary solution for composed machines:

• Modify the lowest level decomposed machine, HCtrl_M1_cmp, to include the implementation level machines (task names ending in *Impl). To do this,
• open the composed machine editor. Open the INCLUDES edit feature.
• Select the second drop-down box and find the *Impl version of each machine.
• Save the composed machine.
• Now add missing synchronizations to the composed machine. Add the Envir1Impl to the includes of HCtrl_M1_cmp.
• Each composed event in the task, that synchronizes with the Environ machine, must have the remote event synchronization added manually. This can only be done by inspection of each composed event. We need to update Sense_Temperatures, Display_Current_Temperature, Actuate_OverHeat_Alram, Actuate_Heat_Source, Sense_Heater_Status, Actuate_NoHeat_Alarm, Sense_PressIncrease_Target_Temperature, Sense_PressDecrease_Target_Temperature, Display_Target_Temperature. One by one, expand the events in the composed events section of the composed machine editor; add a new event in the combines events section, select Envir1Impl and add the synchronizing event from the list-box to the right.

Adding Tasking Event-B

Open the context and ensure Axioms axm3 and axm4, for typing Min and Max are Typing axioms, as indicated by the drop down selection.

Each Machine should be completed as follows.

The Temp_Ctrl_TaskImpl Machine

Continuing with the tutorial project Heating_ControllerTutorial2_Partial2, we need to make changes to the following machines. During the tutorial we will cut and paste from Heating_ControllerTutorial2_Completed model when, specifying the task bodies, to save typing.

• Set the Task Type to Auto Task
  • Open The Tasking section of the Event-B editor.
  • Click on + to add a new Machine Type.
  • Ensure Auto Task is selected in the Drop-down box.

Auto Tasks are tasks that will be declared and defined in the Main procedure of the implementation. The effect of this is that the Auto Tasks are created when the program first loads, and then activated (made ready to run) before the Main procedure body runs.

• Add a new TaskBody.
  • Open Task Type in the Machine Type element
  • Set the task type to Periodic,
  • Set a period of 250 milliseconds.
  • Add a new Task Body section.
  • Copy and paste the task body from Heating_ControllerTutorial2_Completed/Temp_Ctrl_TaskImpl
  • Save the model and rectify any problems highlighted.

The Shared Machine

The next step is to identify the Shared_ObjectImpl machine as a Shared Machine.

• Open the Tasking Section.
• Add a Machine Type by clicking +.
• Select Shared in the Machine Type drop Down box.

The Environ Machine

In the prepared machine we identify the Envir1Impl as an Environ Machine,

• Open the Tasking Section in the Machine Editor
• Click on + to add a machine type.
• Select Environ Machine in the drop down box.

In the implementation, Environ Tasks are declared and defined in the Main procedure . The Envir1Impl machine models a task that simulates the environment, and can be used to generate simulation code. For deployment in a non-simulated environment the environ machine's generated code can be ignored. To specify the Environment task's behaviour we add edit the task body.

• Add the Task Details.
  • Open the Task Type section.
  • Set the task type to Periodic,
  • Set a period of 100 milliseconds.
  • Add a new Task Body by clicking + in the Task Body section
  • Copy and paste the task body from Heating_ControllerTutorial2_Completed/Envir1Impl
  • Save the model.
  • Resolve any problems that are highlighted.

When saving, the task body text is sent to the parser. If parsing is successful then a builder adds the structure to the underlying EMF tree. If parsing fails then an error panel displays the source of the error.

A Summary of Steps

For a Tasking Machine definition:

1. Select the Tasking Machine type (Auto etc).
2. Set the task type (Periodic etc.).
3. Set the task priority.
4. Specify the task body.

For a Shared Machine definition:

1. Add the SharedMachine Machine type.

For an Environ Machine definition:

1. Make the type an Environ Machine type.
2. Set the task type Periodic; a shorter period than the shortest task period is best for simulation.
3. Set the task priority.
4. Specify the task body, it will contain a simulation of changes in the environment.
5. For each sensing/actuating event, add the Event Type.

Invoking the Translators

• To generate Ada code,
  • Right-Click on the composed machine, or any tasking machine in the development, select Code Generation/Translate Event-B to Ada.
  • Open the generated code directory in the project to view the source files. A refresh will be necessary to make the code visible. The .gpr file has been provided for AdaCore GPS users.

• To create the Event-B model of the implementation,
  • Right-Click on the composed machine, or any tasking machine in the development, select Code Generation/Translate Tasking Event-B to Event-B.
  • The Event-B model should be updated with the flow control variables. Users are not able to manually edit the generated elements. The additions can be removed using the menu option Code Generation/Remove Generated Event-B

Generated Code

Generated code will be visible in the code directory, in the Event-B project. However a refresh of the workspace is required. The directory is visible in the resource view; or alternatively, click on the view menu in the Event-B perspective, select customize view, and uncheck the all file and folders filter.