dimgigov
39e07b542d
- twopc.tla: Add coordinatorLog (persistent WAL for decisions) - twopc.tla: Add CrashCoordinator action - twopc.tla: Add RecoverCoordinator action (reads from coordinatorLog) - twopc.tla: Add ParticipantTimeout (only if coordinatorLog = Nil) - twopc.tla: Add RecoveryConsistency invariant - models/twopc.cfg: Add RecoveryConsistency to checked invariants - Update PLAN.md, ANALYSIS.md, README.md with new metrics
227 lines
10 KiB
Plaintext
227 lines
10 KiB
Plaintext
-------------------------------- MODULE twopc --------------------------------
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(*
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TLA+ specification of the Two-Phase Commit (2PC) distributed transaction
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protocol as implemented in BaraDB (core/disttxn.nim).
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Key properties verified:
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- Atomicity : all participants commit, or all abort.
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- NoOrphanBlocks : a committed txn implies all participants committed.
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- CoordinatorConsistency : once decided, the coordinator never changes decision.
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- NoDecideWithoutConsensus: coordinator only decides when all votes are collected.
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- ParticipantStateValid : participant state transitions are valid.
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- RecoveryConsistency : after coordinator crash+recovery, decision is unchanged.
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*)
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EXTENDS Integers, Sequences, FiniteSets, TLC
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CONSTANTS Participants, \* set of participant node IDs
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Nil, \* distinguished nil value (model value)
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MaxTxnId \* bound transaction IDs for model checking
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ASSUME IsFiniteSet(Participants)
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VARIABLES
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txnState, \* txnState[t] ∈ {"Active","Preparing","Prepared","Committing",
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\* "Committed","Aborting","Aborted"}
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participantState, \* participantState[t][p] ∈ {"Active","Prepared","Committed","Aborted"}
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coordinatorDecided, \* coordinatorDecided[t] ∈ {TRUE, FALSE}
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decidedAction, \* decidedAction[t] ∈ {"Commit","Abort", Nil}
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coordinatorLog, \* coordinatorLog[t] ∈ {"Commit","Abort", Nil} — persistent WAL
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coordinatorAlive \* coordinatorAlive[t] ∈ BOOLEAN
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vars == <<txnState, participantState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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-----------------------------------------------------------------------------
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\* Helper operators
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AllPrepared(t) ==
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\A p \in Participants : participantState[t][p] \in {"Prepared", "Committed", "Aborted"}
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AnyPrepareFailed(t) ==
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\E p \in Participants : participantState[t][p] = "Aborted"
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AllResponded(t) ==
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\A p \in Participants : participantState[t][p] /= "Active"
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-----------------------------------------------------------------------------
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\* Initial state
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Init ==
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/\ txnState = [t \in 1..MaxTxnId |-> "Active"]
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/\ participantState = [t \in 1..MaxTxnId |-> [p \in Participants |-> "Active"]]
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/\ coordinatorDecided = [t \in 1..MaxTxnId |-> FALSE]
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/\ decidedAction = [t \in 1..MaxTxnId |-> Nil]
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/\ coordinatorLog = [t \in 1..MaxTxnId |-> Nil]
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/\ coordinatorAlive = [t \in 1..MaxTxnId |-> TRUE]
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-----------------------------------------------------------------------------
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\* State transitions
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\* Phase 1a: Coordinator sends PREPARE to all participants.
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SendPrepare(t) ==
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/\ coordinatorAlive[t]
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/\ txnState[t] = "Active"
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/\ txnState' = [txnState EXCEPT ![t] = "Preparing"]
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/\ UNCHANGED <<participantState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Phase 1b: Participant p receives PREPARE and votes Yes.
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ParticipantPrepare(t, p) ==
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/\ txnState[t] = "Preparing"
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/\ participantState[t][p] = "Active"
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/\ participantState' = [participantState EXCEPT ![t][p] = "Prepared"]
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/\ UNCHANGED <<txnState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Phase 1c: Participant p votes No (aborts locally).
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ParticipantAbort(t, p) ==
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/\ txnState[t] \in {"Preparing", "Active"}
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/\ participantState[t][p] = "Active"
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/\ participantState' = [participantState EXCEPT ![t][p] = "Aborted"]
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/\ UNCHANGED <<txnState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Phase 2a: Coordinator decides COMMIT (all voted Yes).
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\* The decision is first persisted to coordinatorLog before being sent.
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DecideCommit(t) ==
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/\ coordinatorAlive[t]
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/\ txnState[t] = "Preparing"
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/\ AllPrepared(t)
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/\ ~AnyPrepareFailed(t)
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/\ txnState' = [txnState EXCEPT ![t] = "Committing"]
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/\ coordinatorDecided' = [coordinatorDecided EXCEPT ![t] = TRUE]
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/\ decidedAction' = [decidedAction EXCEPT ![t] = "Commit"]
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/\ coordinatorLog' = [coordinatorLog EXCEPT ![t] = "Commit"]
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/\ UNCHANGED <<participantState, coordinatorAlive>>
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\* Phase 2a-alt: Coordinator decides ABORT (at least one No or timeout).
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DecideAbort(t) ==
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/\ coordinatorAlive[t]
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/\ txnState[t] = "Preparing"
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/\ AnyPrepareFailed(t)
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/\ txnState' = [txnState EXCEPT ![t] = "Aborting"]
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/\ coordinatorDecided' = [coordinatorDecided EXCEPT ![t] = TRUE]
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/\ decidedAction' = [decidedAction EXCEPT ![t] = "Abort"]
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/\ coordinatorLog' = [coordinatorLog EXCEPT ![t] = "Abort"]
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/\ UNCHANGED <<participantState, coordinatorAlive>>
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\* Phase 2b: Participant receives COMMIT decision.
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ReceiveCommit(t, p) ==
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/\ txnState[t] = "Committing"
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/\ decidedAction[t] = "Commit"
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/\ participantState[t][p] \in {"Prepared", "Active"}
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/\ participantState' = [participantState EXCEPT ![t][p] = "Committed"]
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/\ UNCHANGED <<txnState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Phase 2b-alt: Participant receives ABORT decision.
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ReceiveAbort(t, p) ==
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/\ txnState[t] = "Aborting"
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/\ decidedAction[t] = "Abort"
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/\ participantState[t][p] \in {"Prepared", "Active", "Aborted"}
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/\ participantState' = [participantState EXCEPT ![t][p] = "Aborted"]
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/\ UNCHANGED <<txnState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Finalize: transaction moves to terminal state when all participants are done.
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FinalizeCommit(t) ==
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/\ txnState[t] = "Committing"
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/\ \A p \in Participants : participantState[t][p] = "Committed"
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/\ txnState' = [txnState EXCEPT ![t] = "Committed"]
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/\ UNCHANGED <<participantState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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FinalizeAbort(t) ==
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/\ txnState[t] = "Aborting"
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/\ \A p \in Participants : participantState[t][p] = "Aborted"
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/\ txnState' = [txnState EXCEPT ![t] = "Aborted"]
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/\ UNCHANGED <<participantState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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\* Coordinator crashes after deciding but possibly before all participants are informed.
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CrashCoordinator(t) ==
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/\ coordinatorAlive[t]
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/\ coordinatorAlive' = [coordinatorAlive EXCEPT ![t] = FALSE]
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/\ UNCHANGED <<txnState, participantState, coordinatorDecided, decidedAction, coordinatorLog>>
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\* Coordinator recovers from crash and reads its decision from the persistent log.
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RecoverCoordinator(t) ==
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/\ ~coordinatorAlive[t]
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/\ coordinatorAlive' = [coordinatorAlive EXCEPT ![t] = TRUE]
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/\ coordinatorDecided' = [coordinatorDecided EXCEPT ![t] = coordinatorLog[t] /= Nil]
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/\ decidedAction' = [decidedAction EXCEPT ![t] = coordinatorLog[t]]
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/\ UNCHANGED <<txnState, participantState, coordinatorLog>>
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\* Participant p times out waiting for the coordinator decision and aborts.
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\* This can only happen if the coordinator crashed BEFORE deciding (coordinatorLog = Nil).
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\* If the coordinator already decided, the participant must wait for recovery.
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ParticipantTimeout(t, p) ==
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/\ participantState[t][p] = "Prepared"
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/\ ~coordinatorAlive[t]
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/\ coordinatorLog[t] = Nil
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/\ participantState' = [participantState EXCEPT ![t][p] = "Aborted"]
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/\ UNCHANGED <<txnState, coordinatorDecided, decidedAction, coordinatorLog, coordinatorAlive>>
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-----------------------------------------------------------------------------
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\* Next-state relation
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Next ==
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\/ \E t \in 1..MaxTxnId : SendPrepare(t)
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\/ \E t \in 1..MaxTxnId : \E p \in Participants : ParticipantPrepare(t, p)
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\/ \E t \in 1..MaxTxnId : \E p \in Participants : ParticipantAbort(t, p)
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\/ \E t \in 1..MaxTxnId : DecideCommit(t)
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\/ \E t \in 1..MaxTxnId : DecideAbort(t)
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\/ \E t \in 1..MaxTxnId : \E p \in Participants : ReceiveCommit(t, p)
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\/ \E t \in 1..MaxTxnId : \E p \in Participants : ReceiveAbort(t, p)
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\/ \E t \in 1..MaxTxnId : FinalizeCommit(t)
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\/ \E t \in 1..MaxTxnId : FinalizeAbort(t)
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\/ \E t \in 1..MaxTxnId : CrashCoordinator(t)
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\/ \E t \in 1..MaxTxnId : RecoverCoordinator(t)
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\/ \E t \in 1..MaxTxnId : \E p \in Participants : ParticipantTimeout(t, p)
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-----------------------------------------------------------------------------
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\* Safety properties
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\* Atomicity: it is never the case that one participant committed while another aborted.
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Atomicity ==
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\A t \in 1..MaxTxnId :
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~(\E p1, p2 \in Participants :
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participantState[t][p1] = "Committed" /\ participantState[t][p2] = "Aborted")
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\* No orphan blocks: once a transaction is committed, every participant is committed.
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NoOrphanBlocks ==
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\A t \in 1..MaxTxnId :
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txnState[t] = "Committed" =>
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\A p \in Participants : participantState[t][p] = "Committed"
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\* Once coordinator decides, the decision is immutable.
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CoordinatorConsistency ==
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\A t \in 1..MaxTxnId :
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coordinatorDecided[t] = TRUE =>
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(decidedAction[t] = "Commit" => txnState[t] \in {"Committing", "Committed"})
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/\ (decidedAction[t] = "Abort" => txnState[t] \in {"Aborting", "Aborted"})
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\* Coordinator only decides when all participants have responded.
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NoDecideWithoutConsensus ==
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\A t \in 1..MaxTxnId :
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coordinatorDecided[t] = TRUE => AllPrepared(t) \/ AnyPrepareFailed(t)
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\* Participant state transitions are consistent with coordinator.
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ParticipantStateValid ==
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\A t \in 1..MaxTxnId :
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\A p \in Participants :
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(participantState[t][p] = "Committed" => decidedAction[t] = "Commit")
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/\ (participantState[t][p] = "Aborted" /\ decidedAction[t] /= Nil => decidedAction[t] = "Abort")
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\* RecoveryConsistency: if the coordinator has a logged decision, recovery restores it exactly.
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RecoveryConsistency ==
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\A t \in 1..MaxTxnId :
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coordinatorLog[t] /= Nil => decidedAction[t] = coordinatorLog[t]
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\* Type invariant
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TypeOk ==
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/\ txnState \in [1..MaxTxnId -> {"Active","Preparing","Prepared","Committing",
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"Committed","Aborting","Aborted"}]
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/\ participantState \in [1..MaxTxnId -> [Participants -> {"Active","Prepared",
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"Committed","Aborted"}]]
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/\ coordinatorDecided \in [1..MaxTxnId -> BOOLEAN]
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/\ decidedAction \in [1..MaxTxnId -> {"Commit","Abort", Nil}]
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/\ coordinatorLog \in [1..MaxTxnId -> {"Commit","Abort", Nil}]
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/\ coordinatorAlive \in [1..MaxTxnId -> BOOLEAN]
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=============================================================================
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