Statecharts are everywhere! #6 - Verification example
As we mentioned in a former post, Bence, who is our colleague, has developed a model transformation plugin for YAKINDU as well. The plugin transforms a chosen SCT model into timed automata, and automatically generates CTL expressions for formal verification.
Letâs have a look at its theoretical background and a practical example.
Theoretical approach
The workflow of the transformation is depicted above. First, the YAKINDU Statecharts to UPPAAL Transformation Engine parses the Statechart design model (the SCT file), and looks for different mathces of structural graph patterns in it.
The patterns were composed in the Query Language of EMF-IncQuery. For composing these patterns the metamodel of the YAKINDU Statecharts were used, because the statcharts in the SCT model are the instances of that metamodel.
After that, the Transformation Engine transforms the given statecharts to timed automata acceptable by UPPAAL. In order to achieve it, we used the metamodel of UPPAAL. The metamodel is publicly available and was designed at the University of Padernborn. The transformation code was written in Xtend and it extensively uses the EMF Reflective API.
Finally, the Transformation Engine creates UPPAAL automata that adhere to the statecharts, and generates CTL expressions to make model checking automatic. You can find details on model checking in a former blogpost.
Practical approach
Now let us show you in an example what our YAKINDU Statecharts to UPPAAL Transformation Engine is capable of. First of all, we need a Yakindu model that we want to transform. We have made a simple timer statemachine just for this demonstration.
As you can see, the model consists of two states. In Idle state the system does not do anything. In Running state the system measures time. Also, there are two events that can be used for controlling the system. Finally, there is an integer variable that symbolizes elapsed time.
Let us transform this statemachine to UPPAAL. This is what we get:
Control template:
As you can see, the transformer creates two templates. The first one is the UPPAAL equivalent of the YAKINDU statechart. The second one is a control template that is used for firing events.
Let us see what transformation rules the transformer uses:
Events are transformed to synchnronization channels
Integer variables are transformed to int variables
Vertices are transformed to locations
Transitions are transformed to edges
Updates of transitions are transformed to updates on edges
Time events (after .. s) are transformed to clock variables, location invariants and guards on outgoing edges
So what can we use this UPPAAL model for? First, you can run simulations on it. With the control template you can fire events, so transitions and actions can take place in the primary process. You can see the actual active locations of templates, you can follow changes in values of variables etc. Here is a picture of a simulation on the timer model:
Second, queries can be created for checking certain properties of our models. Let us say you want to know whether your statemachine is able to reach a certain state, or a value of a variable can be negative. You just have to create a query that describes the condition and the Verifier will tell you whether it is possible or not in your model. If it is possible, even a simulation trace is created that shows you the transitions taking your model into the certain state. Let us show you some queries:
The first query checks whether there can be a deadlock in the system. Fortunately, the model has been designed well, so deadlocks cannot take place.
The second and third queries check whether Running or Idle states can be reached. We create states because we want to reach them. If a state is unreachable, then something is wrong with the model.
The fourth query checks whether time can be measured for longer than 9 seconds. We created this model to measure time, and according to the Verifier we can do so for at least 10 seconds.
The fifth query checks whether the elapsed time can be a negative value. If it was possible, then something would be very wrong with the model. Fortunately, this is not the situation now.
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As we mentioned earlier, we used YAKINDU Statechart Tools for the safety logic, that controls the model railways. In this post we are going to show, how we use MQTT in YAKINDU Statechart Tools, how we combine model based tools with IoT technologies.
We implemented data classes that are converted into the JSON format automatically by Gson. There is a general Command class which identifies what type of JSON message we are sending, and it determines the Payload of the message transfers.
E.g. if we transfer a direction status of a turnout, then the corresponding JSON looks like: {âcommandâ:âSEND_TURNOUT_STATUSâ, âcontentâ:â{âidâ:12, âstatusâ: âSTRAIGHTâ}â} . It means that the turnout that has the ID 12 is set into the STRAIGHT direction.
Besides, we have a general PayloadHelper class whose sendCommandWithContent method can create a correct JSON message from a command and a jsonConvertible object. Note that jsonConvertible objects are those that are depicted on the figure above. First, it creates the corresponding JSON message from the command and the jsonConvertible, and finally it publishes the message to the broker through an MQTT Publisher object.
There is another method, called getPayloadFromMessage that extracts the payload from an MQTT Message and converts it to a Payload object whose contentâs type always depends on the corresponding commandâs value. In this way we implemented a general JSON to Java object conversion solution with the help of Gson.
Finally, letâs see how the Yakindu model uses these functions. On the following figure a safety-logic specific command, called PASSAGE_REQUEST is sent to the corresponding direction: PASSAGE_REQUEST_TOP, PASSAGE_REQUEST_STRAIGHT, PASSAGE_REQUEST_DIVERGENT. It means the turnout asks the next turnout whether the train can enter its section. The content of the JSON message is generated and sent at the end of the method.
This is where statechart modelling in Yakindu meets IoT!
You can find all the sources codes, along with other modules (MQTT clients, YAKINDU Statechart Tools models, OpenCV codes, Complex Event Processing codes and models, etc) in our GitHub repository at https://github.com/FTSRG/BME-MODES3.
Integrating model railways with IBM Bluemix and Node-RED
Cyber-Physical Systems (CPS) are on one hand close to embedded systems as they are also built from sensors, controllers and actuators, where the sensors gather heterogeneous information from the environment, the controllers observe the gathered information and order the actuator to modify the environment according to the observed information. On the other hand, CPS systems are aiming to harvest the benefits of elastic cloud based resources to provide more sophisticated automation services.
As part of the MoDeS3 project we successfully integrated the safety logic that controls our model railways, with IBM Bluemix and Node-RED.
IBM Bluemix
Bluemix is an open standards, cloud platform for building, running, and managing apps and services. Bluemix is designed to make developersâ lives easier. Thatâs why it provides developer teams of all sizes with the flexibility to scale compute power at a very granular level, seamlessly collaborate on source code and shared APIs, and manage appsâ performance, logs and costs from a single dashboard.
Bluemix has three open source compute options to power your applications:
Cloud Foundry: Cloud Foundry is an open source PaaS that offers devs the ability to quickly compose their apps without worrying about the underlying infrastructure. Bluemix extends Cloud Foundry with a number of managed runtimes and services, enterprise-grade DevOps tooling, and a seamless overall developer experience.
IBM Containers: IBM Containers allow portability and consistency regardless of where they are runâbe it on bare metal servers in Bluemix, your companyâs data center, or on your laptop. Easily spin up images from our public hub or your own private registry.
VMs: Virtual machines offer the most control over your apps and middleware. Bluemix uses industry-leading OpenStack software to run and manage VMs in a public cloud, a dedicated cloud, or your own on-premises cloud. Key OpenStack services such as Auto Scaling, Load Balancing, and Object Storage can be used in conjunction with Bluemix services to build and run hybrid apps.
Source of information: IBM Bluemix homepage
 Node-RED
Node-RED is a tool for wiring together hardware devices, APIs and online services in new and interesting ways. It provides a browser-based flow editor that makes it easy to wire together flows using the wide range nodes in the palette. Flows can be then deployed to the runtime in a single-click.With over 225,000 modules in Nodeâs package repository, it is easy to extend the range of palette nodes to add new capabilities.
The light-weight runtime is built on Node.js, taking full advantage of its event-driven, non-blocking model. This makes it ideal to run at the edge of the network on low-cost hardware such as the Raspberry Pi as well as in the cloud.
Source of information: Node-RED homepage
 How were model railways integrated with the cloud?
As we describer earlier in a blogpost we designed the model railway control logic, also known as safety logic, with YAKINDU Statechart Tools. YAKINDU Statecharts enabled automatic code generation from the designed statecharts. This way we could directly create the implementation of the safety logic, based on statechart semantics.
However, generating codes automatically were not enough. We had to integrate custom Java codes to the generated codes through an interface. In this way we could connect the statecharts with the physical model railway track, including the BeagleBone Black based embedded controllers.
After that as an experiment we designed the communication module, originally implemented in Java and that is neccessary for communication between the statecharts, in Node-RED. The high-level signals used in the communication have been constructed as flows in Node-RED, depicted on the following figure.
As you can see, although different requests, originating from the left-hand side, has different flows containing some functions, they all end in the same response node. It was in order to make the design easier and less redundant, excluding the status and error logging nodes used for debug purposes.
So, as you may have already guessed the Node-RED flows and the generated statechart codes have been deployed into IBM Bluemix. Each turnout has its own statechart and they run separately, connected through the Node-RED flow, to make the communication easier and use cutting-edge Internet of Things technolgy!
We deployed each component (statecharts that control the turnouts and their nearby sections, and the communication module designed in Node-RED) into the IBM Bluemix as a container. Six containers were running the generated Java codes, each in its own, and the seventh container was deploying the Node-RED flow. They were put in the same subnetwork, so all the statecharts could communicate with the Node-RED flow as described above.
On a local machine working at the Fault Tolerant Systems Research Group at BME, only a proxy module was running that received signals from the YAKINDU statecharts to stop the trains if neccessary. This module periodically transmitted status about the track to the cloud, so the statecharts could make decisions based on the real-world sensor information.
 Conclusions
Although deploying a safety-critical system into the cloud is strongly opposed, due to network latencies, nondeterministic instabilities of the cloud, noisy neighbours in the virtual machines running in the cloud, etc, it was an interesting experimentation. It was fascinating to see that the section in danger, where the trains could collide, was disabled from the cloud. Despite we did not know where the containers were running exactly in the cloud (e.g. in the EU/USA/Asia), the network latency was low enough not to have serious implications in our case.
We were more than satisfied with the avilability and the stability of IBM Bluemix, so we strongly recommend to give it a try. It has a strong community which is eager to help you if you have any difficulties with the cloud services offer by IBM.
Last but not least, we would like to recommend Node-RED as well. If you are either a Java Script developer, or you would like to connect your embedded systems together, you may find it really useful.
Statecharts are everywhere! #5 - Validation example
As we mentioned in a former post, Bence, who is our colleague, has extended the validation rules embedded into YAKINDU Statechart Tools with new ones. The high-level purposes of the validation rules are to reduce the ambiguity and nondeterminism of the design models and to avoid bad design practises and structures.
Letâs see how it works in practise.
Letâs image we designed a statechart, that contains a composite state with two parallel regions as depicted on the next figure. Each region has two states, which are connected through transitions. Although most triggers are different in the two regions, there is a trigger, called Protocol.response, that is the same for both regions. Although these two transitions have the same trigger (Protocol.response) the respective actions are different: in the first first region Protocol.actionA, in the other region Protocol.actionB is the respective action. Since the execution order of transitions with the same trigger in parallel regions is undefined, it is a bad design pattern (so called antipattern).
Bence designed an EMF-IncQuery validation pattern to automatically recognize these kinds of design patterns. More on EMF-IncQuery validation patterns can be read in the respective blogpost here.
As the validation rules are evaluated incrementally by the EMF-IncQuery Validation Framework, warnings are showed in the Problems view of Eclipse. If we double click on a warning message, it automatically jumps to the respective model element, to help correct the error easily.
Statecharts are everywhere! #4 - Validation & Verification
In the previous parts of Statecharts are everywhere series we discussed a lot of things. First, we talked about what xtUML BridgePoint Editor is, and how we designed the âsafety logicâ with it. Second, we talked about the reasons why we switched from BridgePoint to YAKINDU Statechart Tools (SCT). Third, we introduced YAKINDU Statechart Tools.
Now, we are going to talk about the validation features delivered with SCT, and our extensions: validation based on incremental graph matching, and verification based on timed automaton formalism.
YAKINDU Statechart Tools (SCT) comes with a Validation framework that ensures the detection of unreachable states, dead ends and references to unknown events. These validation constraints are checked live during editing [1].
Xtext
This framework is based on the Xtextâs Validation features that provide both syntactical and semantical validation. The syntactical correctness of any textual input is validated automatically by the parser and any broken cross-references can be checked generically [2].Â
Semantical validation means the developer of the grammar (that can be either textual, or graphical) can write validation rules in Java language for model elements. These rules are applied on the model and if they are not justified, then an error message is shown in the editor. More on validation can be read in the documentation of Xtext.
This way it can be ensured, that the model is valid, because it meets the well-formedness constraints it was set against.
Built-in validation rules
After the introduction, let us see, what validation rules are built in SCT [3]. They mostly inherit from the semantics of (UML)Â statecharts, that is a well researched area.
A state must have a name.
Node is not reachable.
A final state should have no outgoing transition.
A state should have at least one outgoing transition.
Initial entry should have no incoming transition.
Initial entry should have a single outgoing transition
Entries must not have more than one outgoing transition
Outgoing transitions from entries can not have a trigger or guard.
Outgoing transitions from entries can only target to sibling or inner states.
Exit node should have no outgoing transition.
Exit node should have at least one incoming transition
Exit node in top level region not supported - use final states instead.
A choice must have at least one outgoing transition.
The region canât be entered using the shallow history. Add a default entry node.
The region canât be entered using the shallow history. Add a transition from default entry to a state.
The source / target states of a synchronization must be orthogonal.
The source / target states of a synchronization have to be contained in the same parent state within different regions.
Some other rules that are applied on the SCT model as well [4], but they do not necessarily come from the statechart semantics.
A choice should have at least one outgoing default transition.
In/Out declarations are not allowed in internal scope.
Local declarations are not allowed in interface scope.
The evaluation result of a time expression must be of type integer.
The evaluation result of a guard expression must be of type boolean.
Target state has regions without âdefaultâ entries.
Region must have a âdefaultâ entry.
The named entry is not used by incoming transitions.
The named exit is not used by outgoing transitions.
The parent composite state has no âdefaultâ exit transition.
The left-hand side of an assignment must be a variable.
Missing trigger. Transisition is never taken. Use âoncycleâ or âalwaysâ instead.
EMF IncQuery
Bence, who is a friend of ours has extended the validation rules mentioned before with new ones. But these rules were composed using EMF IncQuery and are applied by its Validation Framework, rather than the Xtext Validation Framework.
EMF-IncQuery is a framework for defining declarative graph queries over EMF models, and executing them efficiently without manual coding in an imperative programming language such as Java.
With EMF-IncQuery, you can:
Define model queries using a high level yet powerful query language (supported by state-of-the-art Xtext-based development tools)
Execute the queries efficiently and incrementally, with proven scalability for complex queries over large instance models
Integrate queries into your applications using essential feature APIs including IncQuery Viewers, Databinding, Validation and Query-based derived features with notifications.
More on the EMF IncQuery Validation Framework can be read in its documentation.
EMF-IncQuery Validation Framework
Bence composed new validation rules in the IncQueryâs declarative query specification laguage. These rules are applied on the statechart model. If a rule is violated by the referred model then an error message is displayed next to it.
So how is IncQuery supposed to be used for validation? We are going to show you in an example.
The first pattern is responsible for returning declared events that are not used in the state machine. The validation message is defined by the Constraint annotation above the first pattern. The second pattern is an auxiliary pattern.
First of all, let us talk about how to define IncQuery patterns. Let us take a look at the first pattern. Note, that the pattern name must be preceded by the âpatternâ keyword. Between parantheses we must declare what nodes of the instance model we want access to. Naturally, the given name must appear in the pattern body and refer to a node type.
âEvent : EventDefinitionâ says that we will want access to the nodes that we refer to as âeventâ in the pattern body and they are instances of âEventDefinitionâ. Then, the pattern body must be constructed. Each line states something about the result set. âEventDefinition(event);â states, that through âeventâ we only want to refer to nodes that are instances of âEventDefinitionâ. Patterns can be reused with the âfindâ keyword. This can be imagined as a function call: all of the statements of the called pattern will be applied in the pattern we call it from. To invert the meaning of the find keyword it must be preceded by the ânegâ modifier. It forbids the match of the called pattern with the given parameterization.
In the second pattern we want to return nodes, instances ofâEventDefinitionâ, that are not referred by any instance of âFeatureCallâ or âElementReferenceExpressionâ. FeatureCall.feature(_featureCall, event);â states that there must be a âFeatureCallâ instance that has a âfeatureâ reference to âeventâ (which is as stated before is an instance of EventDefinition). The statements are in a logical AND relation. As you can see, OR relation can be declared as well using the âorâ keyword.
To mark the unused events for the Yakindu user a Constraint annotation must be defined. In the annotation the target editor, the message and the severity must be given apart from the elements that we want to mark. The elements are returned by the pattern the annotation is bound to.
New validation rules
The high-level purposes of the validation rules are to reduce the ambiguity and nondeterminism of the design models and to avoid bad design practises and structures.
The transition has the same trigger as another transition in the parallel region, but different action.
This transition is in a circle of always triggered transition causing a livelock.
Ambiguity. This transition is not the only default transition of the exit event.
The transition has the same trigger as another transition, which is on a higher level in the state hierarchy.
Same trigger used on outgoing transitions.
The transition is covered by an always triggered transition.
This choice should have at least two outgoing transitions.
This final state should have no outgoing transition.
This exit should have no outgoing transition.
This entry should have no incoming transition.
This entry has more than one outgoing transition.
Missing trigger. Transition is never taken. Use âalwaysâ instead.
This region has no entry node.
This region has more than one entry node.
Unreachable vertex.
Unused variable.
Unused event.
Verification
Verification is used for making sure the designed system meets its requirements, and work as we imagined. Although there are a lot of verification techniques, we use timed automaton formalism and computational tree logic.
UPPAAL
We chose timed automaton formalism because it is similar to the formal operation of YAKINDU statecharts (in theory), and we already have practise with UPPAAL. UPPAAL is an industry-wide known tool for formal verification of real-timed systems by timed automata.
UPPAAL uses a subset of Computational Tree Logic (CTL for short). The UPPAAL query language consists of a path formula quantified over none, one, more or all paths of the model. Specifications used in UPPAAL can be classified into reachability, safety and liveness properties. Next figure illustrates the formulae supported by UPPAAL. Filled circles represent those states for which a given state formulae hold.
Model transformation
Besides the validation rules, Bence developed a model transformation plugin for Yakindu as well. The plugin transforms a chosen SCT model into a timed automaton, and automatically generates CTL expressions for formal verification.
The model transformation plugin is continuously evolving to support all features (e.g. choice, entry, exit nodes, parallel regions, etc) of statecharts.
As an example the design and formal models of main region of Section statechart is depicted in the next figures.
Design model:
Formal model:
Finally, the formal verification result of the timed automata transformed from the Section statechart is depicted in the next figure.
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Statecharts are everywhere! #3 - Getting started with YAKINDU Statechart Tools
After leaving BridgePoint, it was time to get started with YAKINDU Statechart Tools. Letâs start with an official introduction of YAKINDU Statechart Tools from its website:
The free to use, open source toolkit YAKINDU Statechart Tools (SCT) provides an integrated modeling environment for the specification and development of reactive, event-driven systems based on the concept of statecharts.
Following figures are from the website of YAKINDU Statechart Tool.
Editing
SCT features an intuitive combination of graphical and textual notation. While states, transitions and state hierarchies are graphical elements, all declarations and actions are specified using a textual notation. The usability of the statechart editor is simply fascinating.
Validation
The validation of state machines includes syntax and semantic checks of the full model. Examples of built in validation rules are the detection of unreachable states, dead ends and references to unknown events. These validation constraints are checked live during editing.
Simulation
The simulation of state machine models allows the dynamic semantics to be checked. Active states are directly highlighted in the statechart editor and a dedicated simulation perspective features access to execution controls, inspection and setting variables, as well as raising events.
Code generation
SCT includes code generators for Java, C and C++ out of the box. The code generators follow a code-only approach and do not rely on any additional runtime library. The generated code provides a well-defined interface and can be integrated easily with any client code.
After the marketing part, letâs get some hands-on experience with Yakindu. It has a really intuitive and exhaustive documentation with lots of tutorials and video instructions. One can get familiar with Yakindu in an hour or so.
Transforming BridgePoint models to Yakindu statecharts
 As I mentioned in the previous parts of this post series (#1, #2), I designed a lot of state machines in xtUML BridgePoint Editor. However, we had some serious issues with BridgePoint, so we switched for Yakindu Statechart Tool. We hoped it would be a statechart designer suite that enables simulation and code generation too.
Since BridgePoint follows the xtUML methodology is based on Shlaer-Mellor Method of Model-Driven Architecture that is an object-oriented software development methodology (more on Wikipedia). This means we can design the structure of the state machine hierarchy in a class diagram manner depicted in the next figure.
As you can see classes can inherit from each other, however multiple inheritance is not allowed. This inheritance is similar to the virtual function concept of C++, and the abstract method concept of Java and C#. This means if the superclass does not handle an event, then its subclass must handle it. However it is not possible to redefine an event in subclass, that has already been handled by the superclass.
In Yakindu Statecharts this object-oriented concept is not followed, because it would be quite weird. Instead, we had to translate this in some respect hierarchical structure to a more convenient one.
Thatâs why we created a separate SCT model for the Section and the Turnout. Within each model we designed a hierachical statechart that consists of parallel regions, letâs see.
Section
We introduced a general statechart for the Sections. It means in the figure above you can see that the Section was connected to the Turnout through three associations, which describes from which direction the Section connects to the Turnout (of course only one association was valid for each Section instance).
Now, this information is stored in a variable, called direction, which may have three values: STRAIGHT, DIVERGENT, TOP. In the statechart all the messages are compared to this direction value, and in this way the concept of direction has been preserved for the âsafety logicâ protocol.
The statechart of the OccupiedSection, depicted below, can be compared to the former version here that was designed in BridgePoint.
Turnout
The highest-level statechart of the Turnout is responsible for differentiating the recent direction of the Turnout. It means, whether the Turnout connects the straight - top or the divergent - top sections. In this way the former large columns of inheritance hierarhcy (depicted above) have been replaced with two composite states which contain parallel regions.
As an example the StraightTurnout state now handles the possible events in five parallel regions. This way multiple events can be handled from the safety logic protocolâs perspective, at one time. In the former version, if a lock request has been received by the Turnout, then it rejects and further requests. In the recent version it now handles the parallel requests independently. The statechart is too complicated to be depicted in this post, but you can see it on an external link here.
Validation
Yakindu Statechart Tool has a built-in validation framework for syntax and semantic checks of the created models.
It includes the detection of unreachable states, dead ends and references to unknown events. These validation constraints are checked live during editing.
E.g. for the Turnout model it found that several choice nodes, do not have a default outgoing transition. So it can be a problem, if at runtime there is no valid outgoing transition from a choice node.
Simulation
We have not tried the Simulator for our models, because we were so eager to see the generated code working with the model railway track, that we skipped this stage.
Code generation
We used the built-in Java code generator to generate code from the statecharts. We connected the Section and Turnout models through the interfaces of the generated codes.
If they raise an event, that should be dispatched to each other, then it goes through the interfaces of the separate statecharts. In this way all statecharts are separated, can be configured and run individually.
Besides, we integrated our codebase, that is necessary to communicate with the model railway trackâs embedded controllers, with the statecharts, so we managed to get the statecharts control and stop the trains if any dangerous situation emerges.
This way the âsafety logicâ controls the model railway track finally.
