I have a system with 3 states. I wanted to handle failures. That is, when the system reboots, it doesn't know the state it's in. Is the following state diagram correct?
This not a valid UML State Machine Diagram for several reasons:
The start node is the wrong symbol. It should be a bullet.
The arrows fork. Each arrow (transition) should begin and end on a node.
The Y and N don't have square brackets.
Regarding the semantics:
The decisions don't have meaningful text (should refer to previously stored state). They may be combined to one decision "storedState = " which has four outgoing transitions guarded as [S1], [S2], [S3] and [empty].
The actions to store the state in persistent storage, in order to be restored in case of crash, are not present.
In case all decisions yield N, the object is destroyed immediately, instead of ending in some default state.
I don't understand the intention of A1, A2 and A3.
Perhaps it would be good to first show the diagram without reboot logic and then tell us what you try to add to that diagram to handle the failures.
Related
I'm trying to model a state machine which reuses a state in order to reduce complexity.
I've got three states: State A, B and X.
My state X can either be entered via a transaction from state A or B.
State X includes multiple substates with lots of complexity and I don't wont to implement it twice.
After the process in state X is completed I need to transition back to back to state A or B based on which one was the previous state.
Is there a elegant way to solve this?
State X includes multiple substates with lots of complexity and I don't wont to implement it twice
Define a submachine corresponding to your state X and in your current machine use submachine state to instantiate it where you need
See §14.2.3.4.7 Submachine States and submachines page 311 in formal-17-12-05 :
Submachines are a means by which a single StateMachine specification can be reused multiple times. They are similar to encapsulated composite States in that they need to bind incoming and outgoing Transitions to their internal Vertices.
...
NOTE. Each submachine State represents a distinct instantiation of a submachine, even when two or more submachine States reference the same submachine.
A SubMachine will help you to reuse several time part of your state modelling.
But if you want to be able to enter into your state X from A or B and then retun to the previous state, ShallowHistory Would be a good idea.
In the following state machine, I modeled a SubMachine X referenced by both states X1 and X2. I also wanted to model the fact that state X2 in processed after A or B and then next state if the previous one.
Another solution consists in playing with transition guards or events/triggers. You must keep in mind that transitions are triggered when specific events occurs or when its guard is true cf. following screenshot.
In UML 2.5.1, the initial pseudostate of a state machine is defined as follows:
An initial Pseudostate represents a starting point for a Region; that
is, it is the point from which execution of its contained behavior
commences when the Region is entered via default activation. It is the
source for at most one Transition, which may have an associated effect
Behavior, but not an associated trigger or guard. There can be at
most one initial Vertex in a Region.
In other words, a UML state machine should almost always contain exactly one initial pseudostate, which should have exactly one outgoing transition.
However, can an initial pseudostate have incoming transitions as well? For example:
I cannot find anything forbidding it in the UML specification, yet I cannot find any example online where this case happen, therefore I was wondering whether or not I overlooked anything.
EDIT: To go into more detail, if we look into the OCL constraints stated in the specification, we can only find the following one that affects outgoing transitions (section 14.5.6.7):
inv: (kind = PseudostateKind::initial) implies (outgoing->size() <= 1)
but I cannot find any constraint regarding incoming transitions
EDIT2: I have just realized that my model is wrong! Considering this sentence of the specification (cited above): "It is the source for at most one Transition, which may have an associated effect Behavior, but not an associated trigger or guard."
Therefore the transition between init and s1 should actually have zero triggers, instead of having e1 as a trigger.
Note that while this does not invalidate the initial question.
I see nothing in the UML 2.5.1 Specification that prohibits a transition whose target is the initial pseudostate.
Such a transition would be meaningless at best and confusing at worst, which is likely why no examples are found.
Edit: see the comments!
On p. 423 UML 2.5:
15.7.18 InitialNode [Class]
15.7.18.4 Constraints
• no_incoming_edges
An InitialNode has no incoming ActivityEdges.
inv: incoming->isEmpty()
N.B. If you intend to have a self-transition for e1 then why not just using that? The Initial can anyway have only on singular outgoing edge, namely to the first state (here s1).
No this is not allowed. And why would one Do that? As you already stated in the cited text,it can only have one outgoing edge without any guard. So what is the added value, as you cannot reuse anything.
I think the text is pretty clear as-is: "[An initial Pseudostate] is the point from which execution of its contained behavior commences when the Region is entered via default activation." If you connect a transition back around to the initial psuedostate, the initial psuedostate is no longer "the point from which execution of its contained behavior commences," it is something else, and is therefore undefined.
I am working my way through an MIT OCW course, Introduction to Electrical Engineering and Computer Science I, in which state machines are employed. I have noticed that the course instructors do not draw state transition diagrams for most of the state machines they discuss.
One problem is to design & Python code a state machine whose state is the input from two time intervals in the past. I think that this is an infinite state machine for which a state transition diagram might be useful for getting the general idea while showing only a few of the states.
I am wondering if a state transition diagram can be drawn for such double delay machine. All the examples, so far, have a transition line emerging from a state bubble marked with an input and the resulting output and then pointing at the next state. For a double delay machine the input of consequence is entered two time periods previous. The problem instructions state that all state memory for the machine be in one argument. No mention is made of input memory, which I would think necessary.
My questions:
Can a state transition diagram be drawn for this state machine?
Is it necessarily the case that input memory be a part of this design?
It is impossible to draw a diagram since the set of all possible states includes any value of any data type, given in the example for the (single) delay state machine in the readings. So the number of possible states can't be defined. See Chapter 4: State Machines.
In the problem description it states that:
It is essential that the init and getNextValues methods in any state machine not set or read any instance variables except self.startState (not even self.state). All memory (state) must be in the state argument to getNextValues. Look at the examples in the course notes, section 4.1.
So the state is all the memory you need. There is no reason not to use an array as state to keep the last two inputs.
First we save both values in memory (state)
class Delay2Machine(StateMachine):
def __init__(self, val0, val1):
self.startState = (val0, val1)
Following the super class SM step function implementation also given in the readings:
def step(self, inp):
(s, o) = self.getNextValues(self.state, inp)
self.state = s
return o
The output will be the first of the values saved in memory, and the state will be updated to include the new input
def getNextValues(self, state, inp):
return ((state[1], inp), state[0])
I'm writing a simple finite state machine and realized that there are situations where an event can take a state to more than one possible results. Basically, from state A, if Event E happens, the state could be either C or D.
I'm currently using the Javascript Finite State Machine code written here: https://github.com/jakesgordon/javascript-state-machine
From the documentation I don't see an obvious way that makes this possible. More so, I feel like maybe this is actually a flow in my original design.
Essentially, in a Finite State Machine, should there be a situation where a transition happens, and based on some logic result in one of multiple states (1 to many), or should it be that we check the logic to see which transition needs to takes place (1 to 1)?
Congratulations, you've just discovered non-deterministic finite state machines! The ideas are similar to that of a deterministic state machine, except that there may be multiple ways to transition from a state given the same input symbol. How this is actually done is unspecified (randomness, user input, branch out and run them all at once, etc.).
I'd like to know whether I have correctly understood the ShallowHistory syntax or not.
Is this the right way to use it?
In the UML spec. it is said it can be used instead of the initial psuedo-state. I guess that in that case there would be no way to reset the State1's memory while in this case the transition from State0 always starts from State1.1. Am I right?
Your interpretation seems correct. From the Superstructure:
Upon entering a composite state, the following cases are differentiated:
• Default entry: Graphically, this is indicated by an incoming transition that terminates on the outside edge of the composite state. In this case, the default entry rule is applied (see Semantic variation point (default entry rule)).
And
Semantic variation point (default entry rule)
If a transition terminates on an enclosing state and the enclosed regions do not have an initial pseudostate, the interpretation of this situation is a semantic variation point.
In some interpretations, this is considered an ill-formed model. That is, in those cases the initial pseudostate is mandatory. An alternative interpretation allows this situation and it means that, when such a transition is taken, the state machine stays in the composite state, without entering any of the regions or their substates.
And finally:
Shallow history entry: If the transition terminates on a shallow history pseudostate, the active substate becomes the most recently active substate prior to this entry, unless the most recently active substate is the final state or if this is the first entry into this state. In the latter two cases, the default history state is entered. This is the substate that is target of the transition originating from the history pseudostate. (If no such transition is specified, the situation is ill-defined and its handling is not defined.) If the active substate determined by history is a composite state, then it proceeds with its
default entry.
Note that from the last paragraph, it seems you should ALWAYS have a transition from the H pseudostate, at least to the same state pointed by the initial pseudostate, otherwise you may have an ill-defined machine.
I didn't find where it says that you can use H* instead of the initial pseudo-state. Where did you see this?