Tasking Event-B Tutorial

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

Tasking Event-B Tutorial Overview

This code generation tutorial supplements 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 project which models the implementation. The Ada code is produced using a pretty printer tool from an intermediate model, the Common Language model (IL1), generated by a translation tool. An overview of Tasking Event-B can be found on the Tasking_Event-B_Overview page.

The Heating Controller development 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 that are to be implemented (and their seen Contexts) are selected and added to a Tasking Development; the Tasking Development files have the file extension .tasking. The machines in the Tasking Development are then extended with implementation details.

The example/tutorial projects are,

Heating_ControllerTutorial_Completed	An example project with a completed Tasking Development and IL1 model (post IL1 translation, but before Event-B translation).
Heating_ControllerTutorial_Completed_Gen	Same as the example project above, but with Event-B model translations. The difference being that this development includes a model of the implementation. These are refinements that include a program counter to describe flow of execution in each task.
Heating_ControllerTutorial_Step1	A bare project for step 1 of the tutorial.
Heating_ControllerTutorial_Step2	A partially completed tasking development for steps 2, 3 and 4 of the tutorial.

Preliminaries

Before further discussion of the modelling aspects, we take a look at the PrettyPrint viewers. The PrettyPrinters make the viewing of IL1 and tasking models easier; it also provides a route to generate source code. The source code can easily be pasted from the IL1 Pretty Printer window into an the Ada source file .

The PrettyPrint View of a Tasking Development

To open the Tasking PrettyPrint viewer,

• from the top-menu select Window/Show View/Other/Tasking Pretty Printer.

Note that the Tasking PrettyPrinter may have to be closed when editing the Tasking Development, since it can give rise to exceptions. The PrettyPrinter would need further work to make it robust, however it is intended only as a short-term solution.

• Open the Heating_ControllerTutorial_Completed Project and switch to the Resource Perspective.
• Open the .tasking model and inspect it. Clicking on the Main, Machine or Event nodes updates the pretty print window.

Viewing Source Code

aka. The PrettyPrint View of an IL1 Model.

To view Ada source code,

• from the top-menu select Window/Show View/Other/IL1 Pretty Printer.
• Open the Heating_ControllerTutorial_Completed Project and switch to the Resource Perspective.
• Open the .il1 model and inspect it. Clicking on the Protected, Main Entry, or Task nodes updates the pretty print window.

Cleaning the Tasking Development

If the .tasking file has errors, then it may need cleaning. To do this right-click on the Main node, select Epsilon Translation/CleanUp. If a model has errors it can still be viewed by clicking on the Selection tab at the bottom of the tasking editor window.

The Tutorial

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

• Step 1 - Create the tasking development.
• Step 2 - Add Tasking annotations.
• Step 3 - Add annotations for addressed variables (optional).
• Step 4 - Invoke translators.

Creating The Tasking Development

• Change to the Event-B Perspective.
• Open the Heating_ControllerTutorial_Step1 Project.
• Select the following Machines: Display_Update_Task1, Envir1, Heater_Monitor_Task1, Shared_Object1, Temp_Ctrl_Task1 and HC_CONTEXT.
• Right-click and select Make Tasking Development/Generate Tasking Development.

The new Tasking Development will not be visible in the Event-B perspective, change to the resource perspective, open and inspect the new .tasking file. The Tasking Development contains (the EMF representation of) the machines that we wish to provide implementations for. In order to introduce the new concepts we have prepared a partially complete development.

• Change the tasking development, if necessary, so that the machine that models the environment is at the top of the tree of machines.

In the simulation implementation, elements of the Environ machine must be declared first.

Change to the Project Heating_ControllerTutorial_Step2 to begin the next step.

Providing the Annotations for Implementations

• Close any Tasking Pretty Print Viewers that remain open. The incomplete model will give rise to exceptions.
• Go to the to the Resource Perspective.
• Open and inspect the .tasking machine.

The Temp_Ctrl_Task1Impl, Envir1 and Shared_Object1 machines are incomplete. We will take the necessary steps to provide implementation details.

The Temp_Ctrl_Task1Impl Machine

In the partially complete tutorial project we have already identified the Temp_Ctrl_Task1Impl as an Auto Task Tasking Machine, by adding the Auto Task extension. 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. We have added the Periodic Task extension to the Auto Task, and set a period of 250 milliseconds.

We have provided a PrettyPrint view of the Temp_Ctrl_Task1Impl.

We will now complete the sequence that has been partially defined in the task body.

• Add Sensing between TCSense_Temperatures and ENSense_Temperatures.
  • Expand the Temp_Ctrl_Task1Impl Auto Task Machine node.
  • Expand the Seq sub-tree.
  • Right-click on the Seq node and select New Child/Left Branch EventWrapper.
  • Provide the event label tc1 using the properties view.
  • Right-click on Event Wrapper and select New Child/ Synch Events.
  • Select Synch Events and go to the drop-down menu of the Local Event property.
  • At this point the drop-down box displays a number of event names, select the second TCSense_Temperatures event.
  • Go to the drop-down menu of the Remote Event property.
  • From the list of events select the first ENSense_Temperatures event.

By relating the sensing events in this way we describe a simulation of the interaction between the task and environment. The details of the interaction are embodied in the events themselves. This is implemented in the simulation code as a write to environment variables using a subroutine call.

Note that the Synch Events construct is used in several ways. We use it to implement Event Synchronisation; sensing and actuation; and as a simple event wrapper. An example of its use in a simple event wrapper follows. The simple event wrapper is used to update local state; there is no synchronisation, as such, but we re-use the constructs that already exist rather than create new ones. We now add a wrapped event to the sequence:

• Add the Wrapped Event TCCalculate_Average_Temperature.
  • Expand the sub-tree of the second Seq node.
  • Right-click on the Seq node and select New Child/Left Branch EventWrapper.
  • Provide the event label tc2 using the properties view.
  • Right-click on Event Wrapper and select New Child/ Synch Events.
  • Select Synch Events and go to the drop-down menu of the Local Event property.
  • From the list of events select the TCCalculate_Average_Temperature event.

The addition of the wrapped event, to the sequence, is simply specification of event ordering. It is implemented in code as a sequential subroutine call statement. We now specify event synchronisation between the task and shared object:

• Add Synchronisation between TCGet_Target_Temperature2 and SOGet_Target_Temperature2.
  • Further expand the Seq sub-tree until Eventwrapper tc3 appears.
  • Right-click on the sibling Seq node (lowest in the tree) and select New Child/Left Branch EventWrapper.
  • Provide the event label tc4 using the properties view.
  • Right-click on Event Wrapper and select New Child/ Synch Events.
  • Select Synch Events and go to the drop-down menu of the Local Event property.
  • At this point the drop-down box displays a number of event names, select the second TCGet_Target_Temperature2 event.
  • Go to the drop-down menu of the Remote Event property.
  • From the list of events select the second SOGet_Target_Temperature2 event.

We have now completed the task body, and next provide additional details for events. In the first instance we focus on the the TCGet_Target_Temperature2 event in Temp_Ctrl_Task1Impl which is to be synchronized with the SOGet_Target_Temperature2 event in Shared_ObjectImpl.

• Add The Event Synchronisation Extension.
  • Right-click on the TCGet_Target_Temperature2 Event node.
  • Select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to ProcedureSynch.

We have identified the event as one that partakes in a synchronisation.

• Identify a parameter direction.
  • Right-click on the tm node.
  • SelectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to actualIn.

We have now identified the parameter as an actualIn (this models a call's return value).

Next we look at the sensing event TCSense_Temperatures event in Temp_Ctrl_Task1Impl. Sensing (and actuating) can be viewed as a kind of synchronisation. The simulation code has a subroutine corresponding to the ENSense_Temperatures event in the Environ machine Envir1Impl:

• Add The Sensed Event Extension.
  • Right-click on the TCSense_Temperatures Event node.
  • Select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to Sensing.

We have identified the event as a sensing event. Now we add the parameter direction:

• Identify parameter directions.
  • Right-click on the t1 node.
  • SelectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to actualIn.
  • Right-click on the t2 node.
  • SelectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to actualIn.

We have now identified the parameters as an actualIn (modelling a simulation's subroutine call return value).

The Shared Machine

The next step is to identify the SharedObj machine as a Shared Machine. The SharedObj Machine will be extended using the Event-B EMF extension mechanism.

• Right-click on the SharedObj Machine node in the .tasking file.
• Select New Child/Extension.
• Right-click on the Extension node and select New Child/Shared Machine from the menu.

We now show how to extend the SWrite event of the Shared Machine with details about its implementation. The SWrite event in SharedObj is to be synchronized with the TWrite event in the WriterTsk.

• Identify SWrite as a Syncronisation.
  • Right-click on the SWrite Event node.
  • Select New Child/Extension.
  • Right-click on the Extension node and select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to ProcedureSynch.

• Identify incoming and outgoing parameters.
  • Right-click on the inFP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalIn.
  • Right-click on the outFP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalOut.

Optional Annotations for Addressed Variables

Link To Addressed Variables!!!!!

A Summary of Steps

If generating environment simulation code:

1. Ensure the Environ Machine is first machine in the development.

For a Tasking Machine definition:

1. Add the Tasking Machine type (Auto etc).
2. Add the task type (Periodic etc.).
3. Define the task priority.
4. Define the task body.
5. For each event, add the Event Type.
6. For each event parameter, add the Parameter Type.
7. Optionally define addressed variables.

For a Shared Machine definition:

1. Add the SharedMachine Machine type.
2. For each event, define the Event Type.
3. For each event parameter, define the Parameter Type.

Invoking the Translation

• To create the IL1 model,
  • Right-Click on the Main node, select Epsilon Translation/Translate Task Mch 2 IL1 EMF.
  • Open the Resource Perspective.
  • Right-click on the sharedbuffer20100819Tutorial2 project folder.
  • Select refresh, the .il1 file should appear in the project.
  • Open and inspect the file, and view the source code by opening the IL1 Pretty Print view if desired.

• To create the Event-B model of the implementation,
  • Return to the Rodin Modelling Perspective.
  • Right-Click on the Main node, select Epsilon Translation/Translate Task Mch 2 Event-B EMF.
  • The sharedbuffer20100819bTasking project is generated, it can be opened and inspected.

There are errors in the generated machines (not investigated the cause yet); these can be fixed in the following way.

• Open a Machine in the Event-B Machine Editor.
• Select the Edit tab.
• Open the REFINES section, the error lies here.
• The correct machine is refined, but choose a different machine to refine (any one, it doesn't matter).
• Select the original refined machine again.
• Save and clean the project, and the error should disappear.
• Repeat for the same errors in the other machines; save and clean again.
• The machines can viewed as normal using the Rodin editors.