# Concurrency Control

> A mechanism that coordinates concurrent access to shared state, through locks, versions or compare-and-swap.

Record: `architecture:concurrency-control`
Kind: mechanism
Layer: [Atomic Boundary](https://banes-lab.com/records/layer/atomic-boundary.md)
Severity: mandatory
Scope: transaction, memory, distributed system
Canonical: https://banes-lab.com/ontology#architecture-concurrency-control

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Isolation](https://banes-lab.com/records/architecture/isolation.md) and before [Optimistic Locking](https://banes-lab.com/records/architecture/optimistic-locking.md).

## Repair

- Refactored by: Add Locking, Use Immutable State, Add CAS
- Detected by: race detectors, flaky concurrent tests
- Violated by: unsynchronized shared mutation
- Measured by: race count, contention
- Enforced by: thread-safety analysis, [tests](https://banes-lab.com/records/lexicon/tests.md)

## Requires

- [Shared State Identification](https://banes-lab.com/records/lexicon/shared-state-identification.md)

## Reinforces

- [Isolation](https://banes-lab.com/records/architecture/isolation.md)
- [Correctness](https://banes-lab.com/records/architecture/correctness.md)

## Enables

- [Safe Parallel Mutation](https://banes-lab.com/records/lexicon/safe-parallel-mutation.md)

## Conflicts with

- [Race Conditions](https://banes-lab.com/records/lexicon/race-conditions.md)
- [Lost Update](https://banes-lab.com/records/architecture/lost-update.md)

## In tension with

- [Performance](https://banes-lab.com/records/lexicon/performance.md)

## Tensions

- [Concurrency Control / Performance](https://banes-lab.com/records/tension/concurrency-control-performance.md)

## Severity

- [mandatory](https://banes-lab.com/records/vocabulary/severity-mandatory.md)

## Category

- [Transactions / State / Concurrency](https://banes-lab.com/records/architecture-category/transactions-state-concurrency.md)

## Required by

- [Concurrency](https://banes-lab.com/records/architecture/concurrency.md)

## Reinforced by

- [Optimistic Locking](https://banes-lab.com/records/architecture/optimistic-locking.md)

## Linked from

- [Coordination is software](https://banes-lab.com/disciplined-methodology/collaborate/coordination-is-software.md)
- [Anti-patterns](https://banes-lab.com/ontology/principles/architecture-category-anti-patterns.md)
- [Correctness / Determinism / Verification](https://banes-lab.com/ontology/principles/architecture-category-correctness-determinism-verification.md)
- [Scalability / Performance / Optimization](https://banes-lab.com/ontology/principles/architecture-category-scalability-performance-optimization.md)
- [Transactions / State / Concurrency](https://banes-lab.com/ontology/principles/architecture-category-transactions-state-concurrency.md)
- [Core Vocabulary](https://banes-lab.com/ontology/lexicon/lexicon-category-core-vocabulary.md)
- [Quality Attributes](https://banes-lab.com/ontology/lexicon/lexicon-category-quality-attributes.md)
- [Transactions / State / Concurrency](https://banes-lab.com/ontology/lexicon/lexicon-category-transactions-state-concurrency.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)