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.
Related
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.
We are learning in school that behavioral State diagram's transition has syntax:
list of events [guard condition] / list of actions
But I couldn't find any example on Internet where is used transition with multiple events. Is it really possible? If yes, how does it behave? Does it mean that transaction is realized when one of this events occurs (and of course condition is fulfilled)?
Yes, a transition can be triggered by one of many events in a list. You would use such a construct to avoid multiple lines between states, making a tidier diagram.
Here is what the 2.5 spec says:
14.2.3.9.2 Enabled Transitions
A Transition is enabled if and only if:
[ . . . ]
At least one of the triggers of the Transition has an Event that is matched by the Event type of the dispatched Event occurrence.
These logically OR'ed transitions are specified textually as a comma-separated list on the transition, as specified in §14.2.4.9:
[<trigger> [‘,’ <trigger>]* [‘[‘ <guard>’]’] [‘/’ <behavior-expression>]]
Unfortunately the UML spec is not specific in that respect (I thought, but Jim has the right answer). Anyway:
14.2.4.9 Transition
The default textual notation for a Transition is defined by the following BNF expression:
[<trigger> [‘,’ <trigger>]* [‘[‘ <guard>’]’] [‘/’ <behavior-expression>]]
Where is the standard notation for Triggers (see sub clause 13.3.4), is a Boolean expression for a guard, and the optional is an expression specifying the effect Behavior written in some vendor- specific or standard textual surface language (see sub clause 16.1). The trigger may be any of the standard trigger types. SignalEvent triggers and CallEvent triggers are not distinguishable by syntax and must be discriminated by their declaration elsewhere.
There are other places in the specs where this paragraph appears in similar way, but without explaining how multiple triggers will be treated. I assume that it's an OR-condition. But that's only an assumption. Alas, since you have not seen examples (me neither) it is probably an unknown fact. Just don't use it - that's indeed possible ;-) And if you happen to find an example, just ask the author what he meant. UML is about talking to each other.
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.).
Given this HSM example model - http://www.embedded.com/design/system-integration/4008251/8/A-crash-course-in-UML-state-machines-Part-2 - I'm wondering what should be the RIGHT moment of executing transition action. Let's say that this machine is in its initial state S211 and event G is dispatched (S21 -> S1) - let's assume that there's some action for this event. Should the sequence be like that:
exit to the least common ancestor (S) - transition action - entry to destination state (S1) - init actions of target...
or maybe like this:
exit to the source state of transition (S21) - transition action - exit to the least common ancestor (S) - entry to destination state (S1) - init action of target
?
In short - is the transition action associated with the "source of transition" or the "least common ancestor"?
The example with description at wikipedia is too trivial to give these details... The UML specs seem to suggest the LCA answer to be the right one, but I'm not sure.
Thx in advance (;
Automata theory suggests that the action does not occur in either the source or the target. But neither does automata theory generally consider hierarchical states. So yes, the LCA is as good a theoretical assumption as any for a transition action.
However, the state-machine code devised by Miro Samek (QHSM) actually performs the transition action within the source state. Please understand, this is not either a problem or a flaw.
As any particular implementation is concerned, all that matters is that the transition action occurs in a consistent way. That is the case with Miro's code.
If for whatever reason you really need the transition action to occur in the LCA or target state, you could use "extended state" to achieve the result. But in practice, it really isn't ever necessary.
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?