Shared-State Concurrency Control Design

Wrote docs/architecture/tasks/banking-concurrency-001/concurrency-design/concurrency-contract-spec.md. Complete shared-state concurrency control specification establishing adaptive pessimistic/optimistic locking, lexicographical multi-resource ordering, 250ms lock timeouts, and deadlock elimination.

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skill: concurrency-design

Shared-State Concurrency Control Spec: Digital Wallet Deduction [CONCUR-BANK-001]

Summary

This specification establishes the shared-state concurrency control contract, locking mechanisms, transaction isolation levels, and deadlock prevention framework for wallet-balance-deduction-service v3.0 under run ID banking-concurrency-001. It governs balance debit and transfer operations across 15 million digital wallet accounts sustaining 14,000 peak mutations/second. It decisively eliminates the lost updates and infinite retry storms demonstrated in incident INC-4899 (where 2,500 concurrent buyers accessing a single merchant promotional wallet triggered an optimistic locking storm that pinned database CPU at 100%, causing $840,000 in failed transactions). The contract enforces an

adaptive concurrency model (applying row-level pessimistic locking with SELECT ... FOR UPDATE NOWAIT for high-contention merchant accounts, and versioned

Optimistic Concurrency Control (OCC) for low-contention retail wallets), mandates strict

lexicographical lock acquisition ordering, enforces a

250-millisecond lock timeout, and sets the transaction isolation floor to

REPEATABLE READ.

Detailed Description

In high-concurrency financial ledgers, naive concurrency control leads directly to data corruption or service collapse. Unprotected read-modify-write sequences allow race conditions where two simultaneous transactions read balance $100, calculate $50 deduction, and write $50 sequentially, losing $50 of ledger value. Conversely, naive optimistic locking with blind retries collapses into thrashing under hot-spot contention. A formal concurrency contract categorizes contention zones, binds lock timeouts, and eliminates cyclic deadlocks through strict resource ordering.

Incoming Concurrent Debits on Account `acc_merch_8812` (2,500 TPS)
                               │
                               ▼
[ Concurrency Router: Contention Classifier ]
  ├── Hot-Spot Detected (Contention > 25 TPS / Account)
  └── Routes to: Pessimistic Row Lock with Bounded Wait
                               │
                               ▼
[ PostgreSQL Transaction Boundary (`REPEATABLE READ`) ]
  ├── Step 1: Acquires Ordered Locks: `ORDER BY account_id ASC`
  │      (Acquires `acc_buyer_104` THEN `acc_merch_8812` -> Zero Cyclic Deadlocks)
  ├── Step 2: Executes: `SELECT balance FROM accounts WHERE id = :id FOR UPDATE NOWAIT`
  └── Step 3: Enforces Lock Timeout: `SET LOCAL lock_timeout = '250ms'`
                               │
       ┌───────────────────────┴───────────────────────┐
       ▼ (Lock Acquired in < 2ms)                      ▼ (Lock Contended > 250ms)
[ Mutates Balance & Commits ]                   [ Graceful Lock Contention Reject ]
  ├── Asserts: Balance >= DebitAmount             ├── Emits HTTP 409 Conflict / 429
  └── Emits HTTP 200 OK (Latency <= 8 ms)         └── Client Backs Off Exponentially

Criteria and weights

Comparison

Result

Option C is selected. Standard retail wallets utilize lightweight optimistic versioning; hot merchant wallets switch to database row locks with NOWAIT and strict lexicographical ordering.

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Required Mechanisms

1. Adaptive Concurrency Selection Contract [MC-AC-01]

Path 1: Standard Retail Wallets (Low Contention, < 10 tx/sec)

• Uses Optimistic Concurrency Control (OCC):

  UPDATE accounts
  SET balance = balance - :amount, version = version + 1
  WHERE account_id = :id AND version = :expected_version AND balance >= :amount;
  

• If rows affected == 0, throws OptimisticLockException. Retries maximum

2 times with exponential jitter (10ms–40ms).

Path 2: Merchant & Promotional Wallets (High Contention, >= 10 tx/sec)

• Automatically routes to Pessimistic Row Locking:

  SELECT balance, version FROM accounts
  WHERE account_id = :id
  FOR UPDATE NOWAIT;
  

• If lock is held, throws immediate LockNotAvailableException in $< 1$ ms instead of queuing behind threads.

2. Lexicographical Multi-Resource Lock Ordering [MC-LO-01]

The Deadlock Invariant: When a transaction mutates two or more accounts (e.g. peer-to-peer transfer from Account A to Account B):
LockOrder = sort_ascending(account_id_A, account_id_B)

• Execution:
  • The transaction always acquires the lock on the smaller account UUID first, followed by the larger account UUID.
  • Mathematically eliminates the circular wait condition ($T1: A \to B$ vs $T2: B \to A$), guaranteeing

zero database deadlocks.

3. Bounded Lock Timeouts & Fail-Fast Bounds [MC-LT-01]

• Every database transaction configures a strict local timeout ceiling:

  SET LOCAL statement_timeout = '1500ms';
  SET LOCAL lock_timeout = '250ms';
  

• If a lock cannot be acquired within

250 milliseconds, PostgreSQL aborts the query immediately, preventing connection pool saturation.

4. Transaction Isolation Level [MC-IL-01]

• All financial deduction transactions execute at REPEATABLE READ isolation level:
  • Prevents non-repeatable reads and phantom reads.
  • Guarantees that balance calculations are derived from a single consistent snapshot.

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Invariants and Contracts

Acyclic Lexicographical Lock Ordering [INV-CONCUR-01]
  Multi-resource locking operations must acquire locks in strict ascending lexicographical key order.
  Acquiring resource locks in arbitrary or caller-determined order is strictly prohibited.

Two-Hundred-Fifty Millisecond Lock Timeout [INV-CONCUR-02]
  Database lock acquisition wait time must never exceed 250 milliseconds.
  Executing blocking `SELECT FOR UPDATE` queries without explicit bounded timeouts is barred.

Zero Blind Retry Storm Invariant [INV-CONCUR-03]
  Optimistic concurrency retries must be bounded to at most 2 attempts with exponential random jitter.
  Unbounded retry loops that amplify database CPU load under contention are strictly prohibited.

Explicit Unknowns

• PostgreSQL page-level lock escalation behavior when 50,000 distinct accounts reside on the same physical table heap page (G-1).
• Redis Redlock clock drift risks across AWS availability zones during NTP leap-second synchronization events (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against concurrency design standards:

• Deadlock Immunity: PASS. Strict ascending UUID lock ordering mathematically eliminates deadlocks.
• Race Protection: PASS. Eliminates lost updates via REPEATABLE READ and row locks.
• Contention Safety: PASS. 250ms lock timeouts and NOWAIT prevent CPU lockup cascades.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-CONCUR-01: David O'Reilly to determine whether Redis advisory distributed locks should be deployed ahead of database queries for ultra-hot merchant wallets exceeding 500 TPS (Owner: David O'Reilly).

Next steps

1. Core Engineering implements the lexicographical lock sorting utility in the transaction interceptor.
2. Database team configures lock_timeout = '250ms' in production Aurora PostgreSQL parameter groups.
3. Conduct staging concurrency stress test firing 2,500 simultaneous transfers at a single account to verify zero deadlocks and sub-12ms p99 execution.