1. ENMA 6010: State Transition Diagrams1 ENMA 6010 State Transition Diagrams Modeling Time-Dependent Behavior Reference: Wikipedia – State Transition DiagramsState Transition Diagrams © 2010 Mark Polczynski All rights reserved

2. ENMA 6010: State Transition Diagrams2 Characteristics of Good Models 1.Graphical, with support for detailed text descriptions: Picture is worth a thousand words  picture links to a thousand words. 2.Top-down partitionable: Globe  Continents  Countries  3.Minimally redundant: Make changes in just one place in the model. 4.Transparent: Requires no expertise in model building to understand the model.

3. ENMA 6010: State Transition Diagrams3 Goals of System Modeling: 1.To focus on important system features while downplaying less important features, 2.To discuss changes and corrections to the user’s requirements at low cost and with minimal risk, 3.To verify that we understand the user’s environment, 4.To verify that we have documented our understanding in a way that would allow others to construct/maintain the system.

4. ENMA 6010: State Transition Diagrams4 We will be studying four related modeling approaches: Data flow diagram – system function State transition diagrams – time dependent behavior Entity relationship diagram – stored data/material Process flow diagrams – actions and decisions How does this fit?

5. ENMA 6010: State Transition Diagrams5 State transition diagram Entity relationship diagram Process flow diagram Control flow Control function

6. ENMA 6010: State Transition Diagrams6 Control Function The control function coordinates the activities of the other DFD functions. Control function inputs and outputs are 1-bit on/off yes/no signals. No data/materials flowing on control lines! These signals “wake up” other DFD functions when the function needs to transform inputs to outputs. These could be expected or unexpected conditions. Typically there is only one control function for a particular data flow diagram.

7. ENMA 6010: State Transition Diagrams7 STD vs. DFD The state transition diagram goes inside the control function. The control function inputs and outputs connect to the state transition diagram. The STD defines how the DFD functions (bubbles) get turned on and off. Thus, the state transition diagram controls system sequencing and timing. Let’s see how this works…

8. ENMA 6010: State Transition Diagrams8 State Transition Diagrams State: Some behavior of a system that is observable and that lasts for some period of time. A state is when a system is: Doing something – e.g., heating oven, mixing ingredients, accelerating engine, Waiting for something to happen – Waiting for user to enter password, waiting for sensor reading. Transition: (Virtually) instantaneous change in state (behavior).

9. ENMA 6010: State Transition Diagrams9 Here’s a simple example STD for a washing machine. State Transition Condition Action

10. ENMA 6010: State Transition Diagrams10 A condition is typically some kind of event, e.g.: Signal Arrival of an object (data/material), Etc… Condition Action An action is the appropriate output or response to the event, e.g.: Signal or message Transfer of an object, Calculation, Etc…

11. ENMA 6010: State Transition Diagrams11 Here’s a simplified data flow diagram for our wash machine, which shows the three primary functions and associated material flows: How does this relate to our state transition diagram? State transition diagram

12. ENMA 6010: State Transition Diagrams12 Control lines in DFD are associated with conditions and actions in STD 1-bit on/off signals Timer

13. ENMA 6010: State Transition Diagrams13 Similarly… The DFD shows flow of entities for Make Lunch The STD shows timing for Make Lunch

14. ENMA 6010: State Transition Diagrams14 This is pretty boring, isn’t it? Couldn’t we just write this down as a sequence of text statements? Start Do until wash cycle complete Wash clothes Do until rinse cycle complete Rinse cloths Do until spin cycle complete Spin-dry clothes End

15. ENMA 6010: State Transition Diagrams15 Here’s a more interesting (simplified) diagram which shows exception handling. Text descriptions start getting messy when you have to account for every possible if-then-else exception to normal operation.

16. ENMA 6010: State Transition Diagrams16 Here’s an even more interesting example. It is now perfectly clear that you: Can go right to rinsing without washing, but.. Can never go to spin-drying without rinsing. So, what would I use such a modeling technique for?

17. ENMA 6010: State Transition Diagrams17 State Transition Diagram vs. DFD Context Diagram For DFDs, we used the context diagram to show how the system interacts with the external world (terminators). For STDs, a condition can be an event occurring in the environment external to the system being modeled, And actions can be responses sent back to the external environment. State Condition X = External event at terminator Action Y = Response to terminator

18. ENMA 6010: State Transition Diagrams18 State Transition Diagram Use State transition diagrams have bee used for a long time to model real-time systems such as: Process control, Telephone switching systems, Data acquisition systems, Military command and control systems. They are also quite useful in modeling “object oriented systems” such as those produced by Visual Basic. These are software program that basically sit there waiting until you initiate an event such as clicking an icon or hitting the “Enter” key.

19. ENMA 6010: State Transition Diagrams19 Condition: Select “File” Action: Open File dialog box PowerPoint waiting for user to select function PPT waiting for “File” function selection Several possible state transitions

20. ENMA 6010: State Transition Diagrams20 Building State Transition Models The literature and the references for this course provide details on rules and processes for building good state transition models. We will not cover these details here, but we will touch on some key points in the following slides…

21. ENMA 6010: State Transition Diagrams21 A transition arrow without an originating state is the system’s start state A system is not required to have a specified start state, but most do. For example, does your computer always start in a pre-defined state when power is applied? Or does it power up running PPT some days, and Outlook others?

22. ENMA 6010: State Transition Diagrams22 A state with no transition arrows out is the system’s end state A system can have multiple end states But…

23. ENMA 6010: State Transition Diagrams23 Systems typically do not have end states. The end state is essentially “dead”, since you can’t ever get back to any other state. Except if you are a cruise missile…

24. ENMA 6010: State Transition Diagrams24 Guards So far, we have seen the use of states, transitions, events, and actions. A guard is a True/False statement that enables a transition to occur. In this example of editing a PowerPoint presentation… no object in the ppt slide has been selected… so the Cut, Copy, and Clear functions are disabled. This is because a guard has detected that “object selected” is “false”, so transitions to these states are not allowed.

25. ENMA 6010: State Transition Diagrams25 Waiting for trigger pulled Trigger pulled & safety on Bullet fired disabled Trigger pulled & safety off Bullet fired Thug insults Dirty Harry “Make my day!” Guard Captain arrives Gun holstered Common example of the use of a state transition guard Common example of the use of a state transition guard

26. ENMA 6010: State Transition Diagrams26 Returning to our simplified washing machine… Guard Can’t start the spin cycle while the door is open. Timer

27. ENMA 6010: State Transition Diagrams27 Top-Down Partitionable Modeling Recall that STDs are inside of the control functions on DFDs. Now, DFDs can be decomposed into levels, and STDs follow the DFDs, so STDs are naturally decomposable. Furthermore! Sometimes it can help clarify things if you decompose an STD within a DFD…

28. ENMA 6010: State Transition Diagrams28 Top-Down Modeling: Just as for functions in data flow diagrams, states can be broken into sub-states.

29. ENMA 6010: State Transition Diagrams29 STD Logical Models vs. Physical Models For data flow diagrams, we distinguished between logical models and physical models (essential vs. environmental models). For DFDs, the major difference is usually the appearance of “implementation stores” which buffer objects (data/materials). We use these to store objects while we are “waiting for” something to happen. A similar situation holds for STDs. Waiting for lunch

30. ENMA 6010: State Transition Diagrams30 Logical Models vs. Physical Models In the logical/essential world, state transitions are virtually instantaneous. But in the real world, transitions may actually take some time, just like DFD functions take time. This may cause “Waiting for transition to complete” states to be added to the physical/environmental model used used for actual system implementation. This is just like adding implementation stores to DFDs.

31. ENMA 6010: State Transition Diagrams31 Here’s another difference between logical and physical models– Logical model condition: Select “File” Physical model condition: Left click on “File” Or keyboard Alt-F PowerPoint waiting for user to select function PPT waiting for “File” function selection Several possible state transitions Implementation independent Implementation dependent

32. ENMA 6010: State Transition Diagrams32 Building a State Transition Diagram Here is one approach: 1.Identify an initial state and walk through the sequence of states. 2.Look for all possible state transitions (branches) as each state is identified. For the second approach, states can be identified through a systematic process: 1.List all of the conditions that the system takes, 2.Identify all of the actions that result from each condition. 3.Identify the state that follows each condition/action pair: what is the system waiting for after the action takes place? 4.Look for additional condition/action pairs not initially identified.

33. ENMA 6010: State Transition Diagrams33 Building a State Transition Diagram For the second approach, you have a bunch of condition/action/state “genes”. Now, you need to link all of the genes together into a chromosome: For each state, what condition(s)/event(s) is the system waiting for? What chromosome(s) provide the event? State Y Condition Y Action Y State X Condition X Action X Add link when you find Action X which produced Condition Y. “Gene”

34. ENMA 6010: State Transition Diagrams34 Building a State Transition Diagram This process works pretty well for having at team jointly converge to a state transition diagram. Here, individuals write individual condition/action/state chromosomes on post- it notes. Then, the team compares individual chromosomes, modifying, combining and eliminating as needed. Chromosomes can then be assembled into a diagram. Do this by randomly selecting one chromosome, and then finding the chromosomes with the conditions that the chromosome state is waiting for. Repeat this for al the chromosomes you just added to the diagram, and iterate until all the chromosomes are used up. It can be tempting for individuals to assemble their chromosomes into diagrams before comparing with the team, but the team should reach consensus on chromosomes before assembling the diagram.

35. ENMA 6010: State Transition Diagrams35 Building a State Transition Diagram After you have constructed the diagram, check the following: 1.Do you have a start and one or more end states (if needed)? 2.Can you transition into all states except the start state? 3.Can you exit all states except end states? 4.Are there any other conditions not covered? 5.Are there any other actions not covered? If possible, check your diagram by doing a live process walk-through.

36. ENMA 6010: State Transition Diagrams36 List all events that occur in the system For each event, identify the corresponding action Put the event-action- state sets on self- stick notes For each event-action pair, identify a unique “following” state Link states with actions Compare all event- action-state sets Eliminate duplicate states Walk through system operation using the model Model accurate? Use the model Identify missing states EnterExit Build Diagram Identify States Review and Revise Test Model Yes No Process Flow Diagram for State Transition Diagramming

37. ENMA 6010: State Transition Diagrams37 State Transition Matrix As with DFDs and ERDs, things can get pretty messy. Plus, it’s nice to have a systematic way to check for consistency, And also to identify the myriad exception/unexpected situations. The state transition matrix can help.

38. ENMA 6010: State Transition Diagrams38 1. Primary State Transitions Do this first to define desired operation. Check blank cells – have we covered all possible desired transitions?

39. ENMA 6010: State Transition Diagrams39 2. Guards Do this next to incorporate guards.

40. ENMA 6010: State Transition Diagrams40 3. Exceptions Finally, do this to cover all the possible exceptions to the normal operating sequence.

41. ENMA 6010: State Transition Diagrams41 State Transition Diagram Example Here is part of a STD for a calculator, highlighting the function of the Clear and Off buttons, using UML (unified modeling language) notation (see: http://en.wikipedia.org/wiki/UML_state_machine). Start End

42. ENMA 6010: State Transition Diagrams42 State Transition Diagram Example Here is an interesting example of a state transition diagram: ““The Emotional State-Transition Diagram” http://syque.com/articles/state-transition/state-transition.htm The first diagram shows what happens when a customer calls in to a call center with a problem…

43. ENMA 6010: State Transition Diagrams43 Note: This is an “expected” sequence. The “unexpected” events and transitions have not been considered, e.g., customer dials wrong number. Call center system STD

44. ENMA 6010: State Transition Diagrams44 Customer STD This describes the states that the customer is in vs. the call center system states.

45. ENMA 6010: State Transition Diagrams45 Customer’s emotional state STD

46. ENMA 6010: State Transition Diagrams46 Finally: As mentioned, a state is when a system is: Doing something - Heating oven, mixing ingredients, accelerating engine, Waiting for something to happen – Waiting for user to enter password, waiting for sensor reading. Well, waiting for something to happen is just a waste of time, So state transition diagrams can be helpful in identifying and minimizing or eliminating time-wasting states. Just like data flow diagrams were useful in identifying and minimizing or eliminating inventory and work-in-process. You can do this by comparing logical/essential models with physical/environmental implementation models. Do you really need to wait for some thing to happen?

47. ENMA 6010: State Transition Diagrams47 Characteristics of Good Models 1.Graphical, with support for detailed text descriptions: Picture is worth a thousand words  picture links to a thousand words. 2.Top-down partitionable: Globe  Continents  Countries  3.Minimally redundant: Make changes in just one place in the model. 4.Transparent: Requires no expertise in model building to understand the model.

48. ENMA 6010: State Transition Diagrams48 Goals of System Modeling: 1.To focus on important system features while downplaying less important features, 2.To discuss changes and corrections to the user’s requirements at low cost and with minimal risk, 3.To verify that we understand the user’s environment, 4.To verify that we have documented our understanding in a way that would allow others to construct/maintain the system. What’s next?

49. ENMA 6010: State Transition Diagrams49 We will be studying five modeling approaches: Data flow diagram – system function State transition diagrams – time dependent behavior Entity relationship diagram – stored data/material Process flow diagrams – actions and decisions How does this fit?

50. ENMA 6010: State Transition Diagrams50 State transition diagram Entity relationship diagram Process flow diagram Control flow Control function