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Compose Compute, Detect Termination (EWD998), and Shutdown.
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‎specifications/composition/Composition.tla

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(**************************************************************************)
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(* Background reading: Conjoining Specifications by Abadi and Lamport *)
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(* https://lamport.azurewebsites.net/pubs/abadi-conjoining.pdf *)
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(* *)
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(* Spec A and B are two components that together are composed into a *)
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(* larger system. It's toy example in the sense that minimize *)
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(* interaction of A and B to behaviors where first A takes its steps *)
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(* followed by B. Variable y in A and B represents the control wire *)
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(* that A and B use to synchronize. *)
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(* Variable z has been added to make the example more interesting--it *)
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(* only appears in spec A but *not* B. *)
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(* *)
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(**************************************************************************)
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EXTENDS Naturals, Sequences
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(* COMPONENT Spec A *)
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------------ MODULE A ------------
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VARIABLES x, \* Abstract/very high-level representation of the overall computation.
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\* Think of it as some computation going on. At a certain state
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\* of the computation, the composed system transitions into the
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\* next phase.
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z \* z is the variable that is only going to be present in spec A.
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varsA == <<x, z>>
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InitA ==
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EXTENDS Integers, Sequences
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Nodes == 1..3 \* Fixed to 1..3 here but should be a constant.
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(* COMPONENT Spec A (Computation) *)
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------ MODULE A --------
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VARIABLES x, \* Private to this component.
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active \* Shared with component spec B.
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vars == <<x,active>>
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Type ==
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/\ x \in 0..10
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/\ active \in [ Nodes -> BOOLEAN ]
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Init ==
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/\ x = 0
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/\ z = TRUE
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NextA ==
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/\ x < 6
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/\ x' = x + 1
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/\ z' = ~ z
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/\ active \in [ Nodes -> BOOLEAN ]
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Next ==
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\* Not all nodes are inactive.
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/\ \E i \in DOMAIN active : active[i] = TRUE
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\* Mimic some computation.
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/\ x' \in 1..10
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\* Computation changed the activation of 0 to N nodes.
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/\ active' \in [ Nodes -> BOOLEAN ]
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==================================
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(* COMPONENT Spec B *)
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VARIABLES x
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varsB == <<x>>
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(* COMPONENT Spec B (Safra's termination detection) *)
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------------ MODULE B ------------
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VARIABLES active, \* Shared with component A.
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inactive \* Shared with component C.
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vars == <<inactive, active>>
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\* ++Observation: This is most likely not the original Init predicate of a
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\* standalone B component, unless perhaps we consider spec A
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\* the environment of spec B. In this particular example,
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\* InitB is superfluouos anyway. However, if one would want
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\* to prove something about spec B, we probably need an init
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\* predicate (=> Conjoining Specifications).
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InitB ==
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/\ x \in Nat \* Spec B may starts with x being any natural number,
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\* which is where A left off.
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Type ==
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/\ inactive \subseteq Nodes
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/\ active \in [ Nodes -> BOOLEAN ]
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NextB ==
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/\ x < 10
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/\ x' = x + 1
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Init ==
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/\ inactive = {}
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/\ active \in [ Nodes -> BOOLEAN ]
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Next ==
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\* Since this high-level composition abstracts away communication,
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\* we could abstract termination detection into a single step:
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\* /\ \A i \in DOMAIN active : active[i] = FALSE
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\* /\ inactive' = Nodes
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\* However, we want to model the possible interleavings of the
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\* A and B spec.
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/\ \E i \in DOMAIN active:
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active[i] = FALSE /\ i \notin inactive
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/\ inactive' = inactive \cup {
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CHOOSE i \in DOMAIN active:
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active[i] = FALSE /\ i \notin inactive
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}
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/\ UNCHANGED active
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==================================
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-----------------------------------------------------------------------------
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(* COMPONENT Spec C (orderly/graceful shutdown) *)
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------------ MODULE C ------------
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VARIABLES inactive \* Shared with component spec B.
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vars == <<inactive>>
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(* This *extends* component spec B to defines what happens to the variables
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in spec A, which are not defined in B, when B takes a step. *)
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VARIABLES restOfTheUniverse
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Type ==
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/\ inactive \subseteq Nodes
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\* Note that TLC doesn't complain about restOfTheUniverse
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\* not appearing in InitB.
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OpenNextB ==
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/\ NextB
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/\ UNCHANGED <<restOfTheUniverse>>
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Init ==
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/\ inactive \subseteq Nodes
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IOExit == TRUE \* Shutdown via sys.exit
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Next ==
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/\ inactive = Nodes
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/\ UNCHANGED inactive
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/\ IOExit
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==================================
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-----------------------------------------------------------------------------
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(* Component specification. *)
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VARIABLES x, active, inactive
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vars ==
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<<x,restOfTheUniverse>>
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<<inactive, x, active>>
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A == INSTANCE A
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B == INSTANCE B
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C == INSTANCE C
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TypeOK ==
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/\ A!Type
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/\ B!Type
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/\ C!Type
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Init ==
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/\ A!Init
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/\ B!Init
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/\ C!Init
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\* PlusPy will most certainly not be able to evaluate this
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\* Next formula. Might have to introduce an auxiliary variable
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\* that represents the phase/stage of composed system.
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Next ==
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\/ /\ ENABLED A!Next
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/\ [A!Next]_vars
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/\ UNCHANGED <<inactive>>
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\/ /\ ENABLED B!Next
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/\ [B!Next]_vars
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/\ UNCHANGED <<x>>
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\/ /\ [C!Next]_vars
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/\ UNCHANGED <<x, active>>
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Spec == Init /\ [][Next]_vars /\ WF_vars(Next)
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(* Composition of A and B (A /\ B) *)
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(* Down here we know about the internals *)
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(* of spec A and B (whitebox component). *)
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THEOREM Spec => []TypeOK
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INSTANCE A WITH z <- restOfTheUniverse
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(* The system eventually converges to shutdown. This, however, does
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*not* hold because component spec A could keep computing forever. *)
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AlwaysShutdown ==
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<>[][C!Next]_vars
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Spec == InitA /\ InitB /\ [][ IF ENABLED NextA THEN [NextA]_vars
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ELSE [OpenNextB]_vars
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]_vars
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(* Unless all nodes are inactive at startup, some computation will occur. *)
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ComputationIfSomeActive ==
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[n \in Nodes |-> FALSE] # active => <>[](x \in 1..10)
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THEOREM Spec => ComputationIfSomeActive
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Inv == x \in 0..10
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THEOREM Spec => Inv
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(* Iff - from some point onward - all nodes will become
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inactive, the composed system will eventually shutdown. *)
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ShutdownIfNoComputation ==
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<>[][~ A!Next]_vars => <>[][C!Next]_vars
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THEOREM Spec => ShutdownIfNoComputation
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\* Not a theorem due to no fairness constraint.
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Live ==
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<>[](x = 10)
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=============================================================================
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\* Modification History
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\* Last modified Fri Jun 12 17:34:09 PDT 2020 by markus
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\* Last modified Mon Jun 15 17:37:21 PDT 2020 by markus
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\* Last modified Fri Jun 12 16:30:19 PDT 2020 by Markus Kuppe
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\* Created Fri Jun 12 10:30:09 PDT 2020 by Leslie Lamport

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