Tasking Event-B Overview

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_ControllerTutorial_Completed	An example project generating an environment simulation. Generates code, where environment variables are monitored and controlled using subroutine calls. Contains a completed Tasking Development with generated Event-B and Ada code.
Heating_ControllerTutorial_Step1	A bare project for step 1 of the tutorial.
Heating_ControllerTutorial_Step2	A partially completed tasking development for steps 2, and 4 of the tutorial (step 3 not required here).

Using the Tasking Extension

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 list of machines.

This is due to a temporary technical limitation in the translator.

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 completed Temp_Ctrl_Task1Impl here, it can be read in conjunction with the tutorial.

The next step is to construct the task body using control constructs such as sequence, branch, loop and output. These constructs are discussed in the overview of Tasking Event-B. We will now complete the sequence that has been partially defined in the task body.

• Synchronize 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 first 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 in the properties view.
  • 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 second 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; and this is implemented in the simulation code by reading the values of the environment variables.

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 this (same as above) 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 second 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_Object1Impl.

• Add The Event Synchronisation Extension.
  • Navigate to the list of events in the machine.
  • 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. The corresponding event in the Shared machine is dealt with here

• 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. Synchronisation between tasks and shared objects are represented as subroutine calls. The sensing/actuating synchronisations only occur between tasks and the environment.

• 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 received value from the environment).

The Shared Machine

The next step is to identify the Shared_Object1Impl machine as a Shared Machine. A PrettyPrint view of the Shared_Object1Impl shared machine can be read in conjunction with the text.

• Optionally collapse open branches of the EMF editor to remove clutter.
• Right-click on the Shared_Object Machine node in the .tasking file.
• Select New Child/Shared Machine from the menu.

We now show how to extend the SOGet_Target_Temperature2 event of the Shared Machine with details about its implementation. The SOGet_Target_Temperature2 event in Shared_Object1Impl synchronizes with the TCGet_Target_Temperature2 event in the Temp_Ctrl_Task1Impl.

• Identify SOGet_Target_Temperature2 as a Synchronized event.
  • Right-click on the SOGet_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.

• Identify the outgoing (return) parameter.
  • Right-click on the tm node.
  • Select New Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalOut.

The Environ Machine

In the prepared machine we have identified the Envir1Impl as an Environ Machine, by adding the Environ Machine extension. Envir1Impl 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; we provide details of non-simulated code using addressed variables later. As before, a PrettyPrint view is available here. In the prepared Environment Machine we have already added a Periodic Task extension, and set a period of 100 milliseconds.

We will now complete the sequence that has been partially defined in the task body. The following specification models simulation of a temperature change; the temperature value is represented by a monitored variable in the environment. The generated code simulates the temperature change in the environment by changing the monitored value.

• Model Temperature Change in the environment.
  • Optionally collapse open branches of the EMF editor to remove clutter.
  • Expand the Environ1Impl Environ Machine node.
  • Expand the Seq sub-tree fully.
  • Right-click on the last Seq node in the tree and and select New Child/Left Branch EventWrapper.
  • Provide the event label e4 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 ENAlter_Temperature_Sensor1 event.

Output to the screen during the simulation can be specified as follows:

• Text Output during Simulation..
  • Right-click on the last Seq node in the tree and and select New Child/Right Branch Output.
  • Select the Output node, and in the properties menu select the Element property drop down box.
  • Select the last variable ctd that appears in the list.
  • In the Text property field, add a textual description to accompany the text output.

The generated code will print the text, and the value of the variable, to the screen. The next step is to identify the ENAlter_Temperature_Sensor1 as a ProcedureDef. This event enables the environment to manipulate the monitored variable.

• Identify ENAlter_Temperature_Sensor1 as a ProcedureDef event.
  • Right-click on the ENAlter_Temperature_Sensor1 Event node.
  • Select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to ProcedureDef.

The final step is to complete the ENSense_Temperatures event. The event is a sensing event, sensing is a kind of synchronisation, it synchronises with the TCSense_Temperatures event in the Temp_Ctrl_Task1 tasking machine. We add formal parameters annotations corresponding to the actual parameters that we have already defined in the task.

• Add The Sensed Event Extension.
  • Right-click on the ENSense_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 formalOut.
  • Right-click on the t2 node.
  • SelectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalOut.

We have now identified the parameters as an formalOut (modelling a simulation's subroutine call return value). This completes the necessary annotations for the simulation, and we can proceed to the translation step. In the event that memory mapped IO is required (non-simulation) then addresses can be added to the model at this stage, before translation takes place. See section on Addressed Variables for details.

A Quick Check

It should now be possible to open the tasking PrettyPrinter view (Window menu) without errors. If the PrettyPrint fails, then this is a sign that the model has been incorrectly constructed. The point of failure (the extent of the printout before failure) may indicate the location of the error in the model.

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.

For an Environ Machine definition:

1. Make the type an Environ Machine type.
2. Make the task type Periodic; a shorter period than the shortest task period is best for simulation.
3. Define the task priority.
4. Define the task body, it will contain a simulation of changes in the environment.
5. For each event, add the Event Type.
6. For each event parameter, add the Parameter Type.
7. Optionally define addressed variables.

Invoking the Translators

• 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 Heating_ControllerTutorial_Step2 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.

In the event that the translator fails check that the correct events have been selected in the synchEvents construct. This can be done by looking at each task's taskBody construct, in the Tasking Pretty Printer view. In the PrettyPrinter view, each synchronization has a target and event seprated by dot-notation. The target is either the name of a shared machine or an environ machine. In this example check that the target refers to the correct machine, i.e. Envir1Impl rather than Envir1; Temp_Ctrl_Task1Impl rather than Temp_Ctrl_Task1; and SharedObject1Impl, rather than SharedObject1. After correcting any errors, invoke the translator again.

• 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 Heating_Controller5AGen project is generated, it can be opened and inspected.

There may be errors in the generated machines (the issue will disappear in a future release); 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.

Optional Annotations for Addressed Variables

To use memory mapped IO (Addressed Variables) in our generated code we can specify which addresses to use in our sensing and actuating events. The addresses are added to the event parameters of a Tasking Machine's sensing and actuating events. The addresses may also be added to the Environ machine's machine variables, for use in simulation. It should be noted that the use of addressed variables, in simulation, has to be done cautiously to prevent memory errors. In the current release the translator generates code for all of these situations, and the environment task should be discarded if simulation is not required.

We now add addressed variable to the TCSense_Temperatures event in Temp_Ctrl_Task1Impl, a PrettyPrint view is available here.

• Add Address Information to Event Parameters.
  • Right-click on the parameter node.
  • Select New Child/Addressed Variable.
  • Go to the Addressed Variable properties view and set the Address and Base properties to appropriate values.

Reads of the monitored variables of the sensing event can therefore be made directly from the address specified. Their is also a base property which can be set to indicate the base of the property value. The default value is 16. The environment simulation may also make use of addressed variables, but in this case the extension is made to the Environ Machine machine variables and used as shown here.

Invocation of the translators proceeds as detailed above.

Generated Code

The Ada Code generated by the translator is available at the following links:

for simulation of environment without addressed variables, Heating_ControllerTutorial_Completed

for simulation of environment with addressed variables, Heating_Controller5AddressedSim_Completed

Removal of the environment task from the Heating_Controller5AddressedSim_Completed should be deployable.