Difference between pages "Tasking Event-B Overview" and "Tasking Event-B Tutorial"

From Event-B
(Difference between pages)
Jump to navigationJump to search
imported>Andy
 
imported>Andy
 
Line 1: Line 1:
−
=== Tasking Event-B ===
+
For more information contact Andy Edmunds - University of Southampton - mailto:ae2@ecs.soton.ac.uk
−
Tasking Event-B can be viewed as an extension of the existing Event-B language. We use the existing approaches of refinement and decomposition to structure a project that is suitable for a Tasking Development. During the modelling phase parameters are introduced to facilitate decomposition. As a result of the decomposition process, parameters become part of the interface that enables event synchronization. We make use of this interface and add information (see [[#Implementing Events]]) to facilitate code generation. The tasking extension consists of the constructs in the following table.
+
=== Tasking Event-B Tutorial Overview ===
 +
 
 +
<span style="color: RED">'''Caution''': This Page is under Construction - some parts are incomplete</span>
 +
 
 +
This tutorial follows on from the abstract development described [http://wiki.event-b.org/index.php/Development_of_a_Heating_Controller_System 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 [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview 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 [http://deploy-eprints.ecs.soton.ac.uk/304/ e-prints archive], or on [https://codegenerationd.svn.sourceforge.net/svnroot/codegenerationd/Examples/Heating_ControllerTutorial_v0.2.0/ SVN].
  
−
<center>
 
 
{| border="1"
 
{| border="1"
−
!Construct
+
|Heating_ControllerTutorial2_Completed
−
!Options
+
|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.
−
|-
 
−
|Machine Type
 
−
|DeclaredTask, [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Tasking_Machines AutoTask], [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Shared_Machines SharedMachine], [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#The_Environ_Machine Environ]
 
−
|-
 
−
|[http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Control_Constructs Control]
 
−
|Sequence, Loop, Branch, EventSynch, Output
 
 
|-
 
|-
−
|[http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Tasking_Machines Task Type]
+
|Heating_ControllerTutorial2_Partial1
−
|Periodic(n), Triggered, Repeating, OneShot
+
|A project with the final decomposition completed, ready to begin Tasking Event-B Development.
 
|-
 
|-
−
|Priority
+
|Heating_ControllerTutorial2_Partial2
−
| -
+
|A partially completed tasking specification for the continuation of the tutorial.
 
|-
 
|-
−
|[http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Implementing_Events Event Role]
+
|TheoriesForCG
−
|Branch, Loop, ProcedureDef, ProcedureSynch, Actuating, Sensing
+
|Contains the mathematical language translations; encoded as rules in a theory plug-in rule-base.
−
|-
 
−
|http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Implementing_Events Parameter Type]
 
−
|ActualIn, ActualOut, FormalIn, FormalOut
 
−
|-
 
−
|Addressed Variable
 
−
|Address, Base
 
 
|}
 
|}
−
</center>
 
  
−
=== Tasking Developments ===
+
== Using the Tasking Extension ==
−
A Tasking Development is modelling component that is generated programmatically, at the direction of the user. The Tasking Development consists of a number of machines (and perhaps associated contexts). We make use of the Event-B EMF extension mechanism <ref name = "EBEMF">[[EMF framework for Event-B]]</ref>, which allows addition of new constructs to a model. The machines in the Tasking Development are extended with the constructs shown in the table, and may be viewed as keywords in a textual representation of the language. With extensions added, a Tasking Development can be translated to a common language model for mapping to implementation source code. There is also a translator that constructs new machines/contexts modelling the implementation, and these should refine/extend the existing elements of the Event-B project.
+
The steps needed to generate code from an Event-B model, in this tutorial, are as follows,
 +
* Step 1 - [[Tasking Event-B_Tutorial#Adding the Implementation Level Refinement|Adding the Implementation Level Refinement]]
 +
* Step 2 - [[Tasking Event-B_Tutorial#Pre-processing|Pre-processing]]
 +
* Step 3 - [[Tasking Event-B_Tutorial#Providing the Annotations for Implementations|Add Tasking annotations]].
 +
* Step 4 - [[Tasking Event-B_Tutorial#Invoking the Translation|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. Two rule files are included for the example, and are available in the [https://codegenerationd.svn.sourceforge.net/svnroot/codegenerationd/Examples/Heating_ControllerTutorial_v0.2.0/ 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.
  
−
==== Tasking Machines ====
+
Alter_Temperature_Sensor1 in Envir1Impl: action becomes ts1 := ts1 + 1
−
The following constructs relate only to Tasking Machines, and provide implementation details. Timing of periodic tasks is not modelled formally. Tasking Machines are related to the concept of an Ada task. These can be implemented in Ada using tasks, in C using the pthread library C, or in Java using threads.
+
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
  
−
* Tasking Machines may be characterised by the following types:
+
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.
−
** AutoTasks - Singleton Tasks.
 
−
** Declared tasks - (Not currently used) A task template relating to an Ada ''tasktype'' declaration.  
 
−
** TaskType - Defines the scheduling, cycle and lifetime of a task. i.e. one-shot periodic or triggered. The period of a task is specified in milliseconds.
 
−
** Priority - An integer value is supplied, the task with the highest value priority takes precedence when being scheduled. For the demonstrator tool the default priority is 5.
 
  
−
''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.
+
* 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.
  
−
==== Shared Machines ====
+
=== Pre-processing ===
−
A Shared Machine corresponds to the concept of a protected resource, such as a monitor. They may be implemented in Ada as a Protected Object, in C using mutex locking, or in Java as a monitor.
 
  
−
* Applied to the Shared Machine we have:
+
The pre-processing step should be a temporary, the solutions can be incorporated into the tool to automatically perform the changes that are required.  
−
** A SharedMachine ''keyword'' that identifies a machine as a Shared Machine.
 
  
−
==== The Environ Machine ====
+
* The Code Generator requires a flattened version of each machine; all of the Event-B elements should be available in the implementation level machine.
−
An Environ machine is a model of the environment. It can be used to generate code for use in a simulation, or discarded (from the IL1 model) in the case that a simulated environment is not required.
+
* 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.
  
−
* Applied to the Environ Machine we have:
+
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)
−
** An Environ Machine ''keyword'' that identifies a machine as an Environ Machine.
 
  
−
=== Implementation Specifics ===
+
===== Providing the correct Composed Machine =====
−
At the stage where we are considering how to implement the Event-B development we need to think about controlling the flow of execution, and how events should be implemented. The following section describes the constructs that we have introduced to facilitate this.
 
−
==== Control Constructs ====
 
−
Each Tasking Machine has a ''task body'' which contains the flow control (algorithmic constructs).
 
  
−
* We have the following constructs available in the Tasking Machine body:
+
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.
−
** Sequence - for imposing an order on events.
 
−
** Branch - choice between a number of mutually exclusive events.
 
−
** Loop - event repetition while it's guard remains true.
 
−
** EventWrapper - a wrapper for the Event-B element (soon to be redundant).
 
−
** Synch Events - synchronizes two events.
 
−
** Text Output - writes textual output to the screen.
 
  
−
Synchronization corresponds to:
+
We must manually add the information to the composed machines to address these two problems.
−
* a subroutine call from task to shared machine, or,
 
−
* sensing or actuating of environment variables.
 
  
−
In the case of a subroutine call the subroutine is an atomic (with respect to an external viewer) update to state. The updates in the protected resource are implemented by a procedure call to a protected object, and tasks do no share state.  The synchronization construct also provides the means to specify parameter passing, both in and out of the task.
+
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.
  
−
In the case of a sensing or actuating event, the updates of the action correspond to reads of monitored variables, and writes to controlled variables of the environment.
+
=== Adding Tasking Event-B ===
 +
Each Machine should be completed as follows.
 +
==== The Temp_Ctrl_Task1Impl 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 the ''completed'' model when specifying the task bodies to save typing.
  
−
Event wrappers:
 
−
* The event synchronization construct is contained in an event wrapper. The wrapper may also contain a single event (we re-use the synchronization construct, but do not use it for synchronizing). The event may belong to the Tasking Machine, a Shared Machine that is visible to the task, or the Environ machine. Single events in a wrapper correspond to a subroutine call in an implementation.
 
  
−
When Editing the EMF model the constructs have the following names:
+
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 set the task type to ''Periodic'', and set a period of 250 milliseconds. We have provided a screenshot of the completed ''Temp_Ctrl_Task1Impl'' [http://wiki.event-b.org/images/Temp_Ctrl_Task1Impl.pdf here], it can be read in conjunction with the tutorial.
  
−
<center>
+
The next step is to construct the task body using the TaskBody Editor, with control constructs such as sequence, branch, loop and output. These constructs are discussed in the [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview overview] of Tasking Event-B. We will now complete the sequence that has been partially defined in the task body.
−
{| border="1"
 
−
!Construct
 
−
!EMF name
 
−
|-
 
−
|Sequence
 
−
|Seq
 
−
|-
 
−
|Branch
 
−
|Branch
 
−
|-
 
−
|Loop
 
−
|Do
 
−
|-
 
−
|Text Output
 
−
|Output
 
−
|-
 
−
|EventWrapper
 
−
|EventWrapper
 
−
|-
 
−
|SynchEvents
 
−
|SynchEvents
 
−
|-
 
−
|}
 
−
</center>
 
  
−
== The Abstract Syntax ==
+
?????????????????????????????
  
−
The following BNF syntax relates to the EMF model that developers will use in the tutorial. The task body is used to specify the ordering of events. The use of the EMF editor should be seen as a short term solution, in the longer term a suitable form-based or text-based editor is envisaged. Text, Variable and Event nodes are terminals in the syntax, they relate to Event-B elements.
+
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.  
  
−
<pre>
+
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.
−
    TaskBody ::= Seq | Branch | Do | EventWrapper | Output
 
  
−
    Seq  ::=  TaskBody TaskBody
+
*'''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''.
  
−
    Branch ::= Body [SubBranch] Else
+
==== The Shared Machine ====
  
−
    SubBranch ::= Body [SubBranch]
+
The next step is to identify the ''Shared_Object1Impl'' machine as a ''Shared Machine''. A screenshot of the [http://wiki.event-b.org/images/Shared_Object1Impl.pdf 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 Rose Editor.
 +
* Select ''New Child/Shared Machine'' from the menu.
  
−
    Else ::= EventWrapper
+
==== 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 screenshot is available [http://wiki.event-b.org/images/Envir1Impl_2.pdf here]. In the prepared Environment Machine we have already set task type to ''Periodic'' 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.  
  
−
    Body ::=  EventWrapper
+
*'''Model Temperature Change in the environment'''.
 +
??????
  
−
    Do ::=  Body [Finally]
+
* Output to the screen during the simulation can be specified as follows:
 +
??????
  
−
    Finally ::=  EventWrapper
+
The generated code will print the text, and the value of the variable, to the screen.
  
−
    Output ::=  Text Variable
+
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.
  
−
    EventWrapper ::=  SynchEvents
+
*'''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''.
  
−
    SynchEvents ::=  Local Remote
+
We have identified the event as a sensing event. Now we add the parameter direction:
  
−
    Local ::=  Event
+
=== A Summary of Steps ===
 +
For a Tasking Machine definition:
 +
# Add the Tasking Machine type (Auto etc).
 +
# Set the task type (Periodic etc.).
 +
# Set the task priority.
 +
# Specify the task body.
 +
# For sensing/actuating events, add the Event Type.
  
−
    Remote ::=  Event
+
For a Shared Machine definition:
−
</pre>
+
# Add the ''SharedMachine'' Machine type.
  
−
==== Implementing Events ====
+
For an Environ Machine definition:
−
An event's role in the implementation is identified using the following extensions which are added to the event. Events used in task bodies are 'references' that make use of existing event definitions from the abstract development. The events are extended. to assist with translation, with a keyword indicating their role in the implementation.
+
# Make the type an Environ Machine type.
 +
# Set the task type Periodic; a shorter period than the shortest task period is best for simulation.
 +
# Set the task priority.
 +
# Specify the task body, it will contain a simulation of changes in the environment.
 +
# For each sensing/actuating event, add the Event Type.
  
−
* Event implementation role.
+
== Invoking the Translators ==
−
** Branch - In essence a task's event is split in the implementation; guards are mapped to branch conditions and actions are mapped to the branch body. If the branch refers to a Shared Machine event (procedureDef) then this is mapped to a simple procedure call.
 
−
** Loop - The task's event guard maps to the loop condition and actions to to loop body. If the loop refers to a Shared Machine event then it is mapped to a simple procedure call.
 
−
** ProcedureSynch - This usually indicates to the translator that the event maps to a subroutine, but an event in a task may not require a subroutine implementation if its role is simply to provide parameters for a procedure call.
 
−
** ProcedureDef - Identifies an event that maps to a (potentially blocking) subroutine definition. Event guards are implemented as a conditional wait; in Ada this is an entry barrier, and in C may use a pthread condition variable .
 
−
** Sensing - Identifies an event that maps to a read from the environment. If the environment is simulated without address variables then the sensing event is similar to a ProcedureSynch event, in that it has an update action that models assignment of a return value from a subroutine call. The event parameters act like the ''actualIn'' parameters of a ProcedureSynch event. On the other hand, if the event has addressed variables associated with its event parameters, then they map to direct reads from memory mapped variables in the generated code.
 
−
** Actuating - Identifies an event that maps to a write to the environment. If the environment is simulated without address variables then the actuating event has no update action, the parameters act like ''actualOut'' parameters of a ProcedureSynch event. If a sensing event has addressed variables associated with its parameters then they map to direct writes to memory mapped variables in the generated code.
 
  
−
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. In implementable code, when an subroutine is defined, its formal parameters are replaced by actual parameter values at run-time. To assist the code generator we extend the Event-B parameters. We identify formal and actual parameters in the implementation, and add the following keywords to the event parameters, as follows:
+
* 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.
  
−
* Event parameter types - Note: formal parameters are place-holders in a subroutine; they are replaced by the actual parameters at call time.
+
* To create the Event-B model of the implementation,
−
** FormalIn or FormalOut - event parameters are extended with the ParameterType construct. Extension with formal parameters indicates a mapping to formal parameters in 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''.
−
** ActualIn or ActualOut - Extension with an actual parameter indicates a mapping to an actual parameter in the implementation.
+
** 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''
  
−
===== Addressed Variables =====
+
== Generated Code ==  
−
When sensing monitored variables, or actuating controlled variables in the environment, we may wish to use explicit memory addresses for use in the final implementation, or perhaps in the environment simulation too. We can link a task's event parameters, and an Environ machines variables, with specific addresses and use these in the generated code.
+
The Ada Code generated by the translator is available at the following links:
  
−
== References ==
+
for simulation of environment without addressed variables, [http://wiki.event-b.org/images/Code_Heating_ControllerTutorial_Completed.pdf Heating_ControllerTutorial_Completed]
  
−
<references/>
+
for simulation of environment with addressed variables, [http://wiki.event-b.org/images/Code_Heating_Controller5AddressedSim_Completed.pdf Heating_Controller5AddressedSim_Completed]
  
 +
Removal of the environment task from the ''Heating_Controller5AddressedSim_Completed'' should be deployable.
  
 
[[Category:User documentation]]
 
[[Category:User documentation]]

Revision as of 10:45, 1 December 2011

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

Tasking Event-B Tutorial Overview

Caution: This Page is under Construction - some parts are incomplete

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,

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. Two 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

Each Machine should be completed as follows.

The Temp_Ctrl_Task1Impl 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 the completed model when specifying the task bodies to save typing.


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 set the task type to Periodic, and set a period of 250 milliseconds. We have provided a screenshot 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 the TaskBody Editor, with 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.

?????????????????????????????

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.

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.

The Shared Machine

The next step is to identify the Shared_Object1Impl machine as a Shared Machine. A screenshot 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 Rose Editor.
  • Select New Child/Shared Machine from the menu.

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 screenshot is available here. In the prepared Environment Machine we have already set task type to Periodic 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.

??????

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

??????

The generated code will print the text, and the value of the variable, to the screen.

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:

A Summary of Steps

For a Tasking Machine definition:

  1. Add the Tasking Machine type (Auto etc).
  2. Set the task type (Periodic etc.).
  3. Set the task priority.
  4. Specify the task body.
  5. For sensing/actuating events, add the Event Type.

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

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.

Retrieved from ‘https://wiki.event-b.org/index.php?title=Tasking_Event-B_Overview&oldid=10018’