Advanced State Modeling

Transcription

1 Advanced State Modeling Or: Derek Zoolander Strikes Back Jonathan Sprinkle 1 University of Arizona Department of Electrical and Computer Engineering PO Box , Tucson, AZ 85721, USA Jonathan Sprinkle Advanced State Modeling 1

2 Abstract Abstract This lecture continues discussion of state modeling and state diagrams. Specific details to be discussed include concurrent models, and various instance models of states. Jonathan Sprinkle Advanced State Modeling 2

3 Previously, on ECE State models give the behavior of a particular class. States are an abstraction of the current value(s) of an object and its links Events are instantaneous (when they happen), and unique. Kinds of events can be classified Transitions can fire when an event occurs, and guard conditions are satisfied (recall event[guard]). Jonathan Sprinkle Advanced State Modeling 3

4 Outline 1 Reminders from State Modeling Definitions Sending Signals 2 Advanced State Modeling Concepts Nested State Diagrams Concurrency and States Concurrency within an Object Synchronization of Concurrent Activities 3 In-Class Exercises Jonathan Sprinkle Advanced State Modeling 4

5 Definitions Transitions happen. State1 entry/action do/activity exit/action event/action(arguments) event[guard] State2 entry/action do/activity exit/action Jonathan Sprinkle Advanced State Modeling 5

6 Definitions Activities happen. State1 entry/action do/activity exit/action event/action(arguments) event[guard]/action State2 entry/action do/activity exit/action Jonathan Sprinkle Advanced State Modeling 6

7 Definitions Completion transitions When an activity is complete, or no activity exists, an unlabeled transition called the completion transition can be taken when activity is done. This is in response to the completion event, which fires only once. State1 State2 entry/action do/activity exit/action However, it may be possible get a state stuck if guard conditions prevent taking the transition, and no more completion transitions fire. Completion transitions should be used with care or not at all!! You will be heavily penalized on exams and homework assignments for state models with ambiguous completion transitions. Jonathan Sprinkle Advanced State Modeling 7

8 Sending Signals Race conditions An object can send a signal to another object, or objects. Generally, if a signal is directed at a group of objects, then each object received a unique signal, and they are processed concurrently. If an object can receive signals from more than one object, then it may be possible for the order of receipt to affect the final state of the object. This effect is called a race condition. Definition A race condition is a design consequence, wherein a particular object s final state is nondeterministic when considering the order of receipt of messages from more than one source. It is interesting to note that race conditions are not necessarily design flaws, but that many real-time of concurrent systems prefer that race conditions do not exist. Jonathan Sprinkle Advanced State Modeling 8

9 Outline 1 Reminders from State Modeling Definitions Sending Signals 2 Advanced State Modeling Concepts Nested State Diagrams Concurrency and States Concurrency within an Object Synchronization of Concurrent Activities 3 In-Class Exercises Jonathan Sprinkle Advanced State Modeling 9

10 Advanced State Modeling Previously discussed concepts for state modeling are sufficient for toy examples, but systems which are deployed frequently require more advanced concepts. This information is from Chapter 6 of the text. Jonathan Sprinkle Advanced State Modeling 10

11 Nested State Diagrams Flat state diagrams can become unwieldy, and are often so large that they cannot be readily understood as a documentation tool. Furthermore, it may be the case that the complexity of a flattened state diagram may hide design flaws in the same way that the complexity of textual code, for a sufficiently large example. Enter, then, nested state diagrams, which allow subdiagrams to hide complexity, or refactor tasks. Consider a vending machine, with 10 different selections. The act of dispensing an item can be refactored as a single state, even though it may require several sub-states to complete its task. Jonathan Sprinkle Advanced State Modeling 11

12 Nested State Diagrams Example (From Figures 6.2, 6.3 in Chapter 6) Consider a vending machine, with more that one selectable item. Vending Machine start Idle cons in(amount) / set balance cancel / refund coins [item empty] Collecting money coins in(amount) / add to balance select(item) do / test item and compute change [change < 0] [change = 0] [change > 0] dispense : DispenseItem do / make change Figure: The Vending Machine state diagram Jonathan Sprinkle Advanced State Modeling 12

13 Nested State Diagrams A submachine: DispenseItem Example (The DispenseItem submachine) DispenseItem start do / move arm( item.getrow( ) ) arm ready do / move arm( item.getcol( ) ) end pushed do / push item off shelf arm ready Figure: The DispenseItem subdiagram Jonathan Sprinkle Advanced State Modeling 13

14 Concurrency and States Aggregation Concurrency State models by their appearance and definition support concurrency well; however, it is entirely possible that certain objects may require interdependence at certain points in their life. The aggregate state refers to the combined state of all state diagrams in a collection. Definition Aggregation Concurrency is an and relationship between concurrent states. Two state which employ aggregation concurrency are both active at the same time. Jonathan Sprinkle Advanced State Modeling 14

15 Concurrency and States When employing Aggregation Concurrency, it is most interesting for contained states to interact (e.g., transitions for one object may depend on the state of another object). Example (Car example.) Car Transmission Accelerator Brake Jonathan Sprinkle Advanced State Modeling 15

16 Concurrency and States Aggregation s Concurrent State Diagram Transmission start select(reverse) Park Reverse select(park)[v=0] select(park)[v=0] select(drive) Drive when( v > v_max ) First when( v < v_min ) Second Accelerator Idle do / decrease v push down Depressed do / increase v release Brake Idle Depressed push down do / decrease v release Jonathan Sprinkle Advanced State Modeling 16

17 Concurrency within an Object When modeling an object which may have a concurrent set of states, it is possible to partition the state with a dotted line, to show that the state of the object is a tuple of single states from each concurrent region. State State1 State2 StateA StateB The state can be State1 and StateB, for example, but not StateA and StateB since exactly one state from each partition must be active. Jonathan Sprinkle Advanced State Modeling 17

18 Concurrency within an Object Example (Tennis Game) Jonathan Sprinkle Advanced State Modeling 18

19 Synchronization of Concurrent Activities For atomic synchronization, it is possible to use splitting and merging. Definition Splitting control requires that all following activities occur concurrently. A splitting fork (heavy bar with one input and 2..* outputs) denotes splitting of control. Definition Merging control is the collection of completion of all incoming activities before continuing. A merging (or joining) fork (heavy bar with 2..* inputs and one output) denotes merging of control. Jonathan Sprinkle Advanced State Modeling 19

20 Synchronization of Concurrent Activities A few notes... Merge and split are themselves transitions, and fire based on events, guards, and can produce actions. It is not necessary for concurrent states (after splitting control) to occur simultaneously. Implicit merges and simple splitting can cause headaches if states (and substates) do not contain default states. You should specify a default state for all concurrent substates to avoid confusion. Jonathan Sprinkle Advanced State Modeling 20

21 Outline 1 Reminders from State Modeling Definitions Sending Signals 2 Advanced State Modeling Concepts Nested State Diagrams Concurrency and States Concurrency within an Object Synchronization of Concurrent Activities 3 In-Class Exercises Jonathan Sprinkle Advanced State Modeling 21

22 Example (Completion Confusion) Create 2-3 example state models where completion transitions are used to describe a specific design of your choosing. Have some be clear, and others be unclear. Create sequence diagrams for unclear state models. Example (Current Concurrent) Create a state model using concurrent substates, to describe how capacitors in an example circuit are charging. Make sure to show the example circuit, but no specific equations need be given. Example (Substate Mate) Create a state model with explicit substates, that describe a chess game, and various strategies that either player will use as parts of each substate. Be general in your algorithms, not specific. Jonathan Sprinkle Advanced State Modeling 22