Consistency model
A consistency model is a formal specification of how operations on state may be ordered and what values those operations can observe.
Modern C
A C memory model is the language-level model that specifies how objects occupy storage and how their locations can be accessed during execution.
- How it is used
- Provides rules for reasoning about visibility and timing when threads access shared state.
- How deep it goes
- Compares sequential and acquire-release consistency and their costs, while setting aside consume consistency and its dependency ordering.
- Examples
- Uses a two-thread diagram showing modifications to a shared object and information about each thread's state.
- What you can do after
- Evaluate/understand cross-thread timing and why explicit acquire-release operations need careful design.
Modern C, Third Edition
The C memory model is the language abstraction that defines how objects, storage, pointers, and program state relate without requiring a particular physical-memory arrangement.
- How it is used
- Models C program state and pointer-based access without committing programs to physical memory locations.
- How deep it goes
- Descends from physical and virtual memory through storage, objects, representations, alignment, and aliasing, while deferring detailed lifetime rules.
- Examples
- Uses pointer-based functions, aliasing calls, unions, byte arrays, addresses, and an int32_t representation diagram.
- What you can do after
- Evaluate pointer accesses and object representations while recognizing portability, aliasing, type, alignment, and storage constraints.
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Microsoft Azure in Action
A consistency level is a Cosmos DB setting that defines the guarantees governing how current and ordered reads are relative to writes.
- How it is used
- Guides Cosmos DB choices when applications must balance data accuracy, performance, availability, and user expectations.
- How deep it goes
- Compares five levels through definitions, read behavior, trade-offs, and practical uses.
- Examples
- Uses inventory updates, order confirmations, user sessions, ordered reads, noncritical applications, and Azure CLI configuration.
- What you can do after
- Operate/configure Cosmos DB consistency with Azure CLI and select a level for application freshness and performance needs.
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Latency
A consistency model is a set of guarantees defining the ordering and visibility of operations and data updates in a distributed system.
- How it is used
- Guides replication designs by defining update visibility and operation ordering across copies.
- How deep it goes
- Defines distributed models, trade-offs, examples, and implementation mechanisms, then connects them to hardware memory ordering.
- Examples
- Uses collaborative document editing, shopping carts, vector clocks, and data-plus-flag publication ordering.
- What you can do after
- Evaluate consistency choices against application needs for latency, stale reads, ordering, and availability.
Related concepts
- eventual consistencyincludes · Eventual consistency is a distributed-data consistency model that allows replicas to differ temporarily but guarantees they converge to a consistent state after updates propagate, typically when no further updates occur.
- strong consistencyincludes · Strong consistency is a data consistency guarantee under which operations observe completed writes according to a coordinated, authoritative order, so reads reflect current rather than stale state.
- meancontrasts with · A mean is a measure of central tendency, calculated as the arithmetic average of values or, for a random variable, as the probability-weighted average of its possible values.
- linearizabilityincludes · Linearizability is a consistency property in which each operation appears to take effect atomically at a point between invocation and completion while preserving real-time order.
- sequential consistencyincludes · Sequential consistency is a memory model in which participating operations appear in a single total order that preserves each thread’s program order.
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