Let me rephrase Hillel's intuition: suppose we have a nextstate relation
Next == A \/ B \/ ...
Given state s, TLC will process action A by computing the set
Post(s,A) == { t : (s,t) = A } [1]
Clearly, we have that
Post(s, A /\ B) \subseteq Post(s,A)
and therefore any state u such that (s,u) = A /\ B is a member of Post(s,A) and will be generated by TLC. Because it is also a member of Post(s,B), the state u will be regenerated when TLC processes action B.
Stephan [1] The restrictions that TLC imposes ensure that Post(s,A) is effectively computable and finite. In particular, it is well defined, which is all that matters here.
I'm having some trouble coming up with a valid spec where that
would matter. You're saying that OP1, OP2, and
OP1 /\ OP2 are all valid actions, but also that the
conjunction includes transitions that aren't included in either
individual action. As far as I know, the semantics of TLA+, in
particular that the nextstate relation must completely specify
all variables, make that impossible. But I'd be happy to be proven
wrong on this!
With that in mind, I suspect that TLC doesn't check those
transitions. You'd have to ask Markus to know for sure, though.
On 9/19/19 10:01 PM, Shiyao MA wrote:
Hi Hillel,
I was thinking about how the TLC checker searches for all
the possible states *regardless* what exactly an OP refers to.
So for example,
Given a spec:
Next == OP1 \/ OP2 \/ OP3
Spec == Init /\ [][Next]_vars
and consider the case where the checker is at state S. The
checker now proceeds to search all the *direct* child states
(in a BFS style).
My question and predication is, for the above Next
operation which contains a set of operations joined with *\/*,
a state transition will never be conducted by applying both
OP1 and OP2 (or any two of the three) on the current state S.
The checker enumerates the possible direct child states by
applying OP1, OP2, and OP3 separately.
Best,
On Friday, 20 September 2019 00:29:07 UTC+8, Hillel Wayne
wrote:
Hi Shiyao
related code:
VoteFor(a, b, v) ==
/\
maxBal[a] =< b
/\ \A vt \in votes[a] : vt[1] /= b
/\ \A c \in Acceptor \ {a} :
\A vt \in votes[c] : (vt[1] = b) => (vt[2] = v)
/\ \E Q \in Quorum : ShowsSafeAt(Q, b, v)
/\ votes' = [votes EXCEPT ![a] =
votes[a] \cup {<<b, v>>}]
/\ maxBal' = [maxBal EXCEPT ![a] = b]
If VoteFor specifies maxBal', and
IncreaseMaxBal specifies maxBal',
then they can only be true that the same time if there
is some value for maxBal' that
satisfies both actions. Given TLC isn't giving you an error, I'm guessing you
also have UNCHANGED votes in IncreaseMaxBal?
Then they definitely both can't be true at the same
time, as VoteFor would specify that votes
changes and IncreaseMaxBal does not.
H

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