Consistency Models
Linearizable, sequential, causal, eventual, and the session guarantees between them.
5 to work through
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intermediate Multiple choice
After a user updates their profile, the page sometimes shows the old value. The team says this is normal eventual consistency. Is that acceptable?
2 min answer -
advanced
A crypto exchange must decide which operations require strong consistency and which can be eventually consistent. Where should the line be drawn, and what is the cost of drawing it wrong in each direction?
2 min answer -
advanced
A food-delivery platform must match orders to nearby couriers while supply and demand change every few seconds. Which information needs strong consistency, which can be stale, and how should dispatch handle a courier who accepted a job while their availability record was out of date?
2 min answer -
advanced Multiple choice
For a shopping cart at very large scale, would you choose a strongly consistent store or an always-writeable one with conflict resolution?
2 min answer -
advanced
Two people are editing the same page in a collaborative workspace. One loses connectivity for several minutes while continuing to type, then reconnects. What should happen, and what consistency guarantee is actually required?
2 min answer
7 terms in this topic
Amazon Dynamo: Always Writeable
Amazon chose an always-writeable shopping cart with application-level conflict resolution, accepting merge complexity to guarantee that "add to cart"…
conceptCausal Consistency
A model guaranteeing that operations which causally depend on one another are seen in the same order everywhere, while concurrent operations may be s…
conceptConsistent Prefix Read
A guarantee that if a sequence of writes happens in a given order, a reader sees a prefix of that sequence — never an out-of-order subset.
conceptInvariant-Aligned Partitioning
Choosing the partition key so that every strong-consistency invariant falls entirely inside one partition - which converts distributed coordination i…
conceptLinearizability
The strongest single-object guarantee — every operation appears to take effect instantaneously at some point between its call and its return.
patternOptimistic Claim
Ranking candidates from deliberately stale data and resolving the race atomically at the moment of commitment, rather than locking during selection.
conceptRead-Your-Writes Consistency
A guarantee that a client always sees its own prior writes, even in a system that is otherwise eventually consistent.
Neighbouring topics
Distributed Systems
General material on partial failure, coordination and distributed reasoning.
CAP & PACELC
What you must give up during a partition, and the latency choice the rest of the time.
Idempotency
Making an operation safe to repeat, because a client that times out cannot know.
Retries & Backoff
Exponential backoff, jitter, retry budgets, and how retries become the outage.
Timeouts & Deadlines
Per-hop timeouts that do not compose, and the deadline budget that replaces them.
Circuit Breakers
Failing fast on a broken dependency, and what you fail fast to.
Backpressure & Flow Control
Telling callers to slow down instead of buffering into congestion collapse.
Load Shedding
Rejecting some work deliberately so the rest can be served correctly.
Bulkheads & Isolation
Partitioning resources so one dependency cannot starve the others.
Leader Election
Agreeing who is in charge, and fencing the one who no longer is.
Consensus Protocols
Raft, Paxos and quorums — what they guarantee and what they cost.
Distributed Locking
Mutual exclusion across machines, and why it is harder than it looks.
Distributed Transactions
Two-phase commit, its blocking failure mode, and when it is still reasonable.
Sagas & Compensation
Replacing atomicity with semantic undo, and ordering the irreversible steps last.
Service Discovery
Finding a healthy address for something whose instances are ephemeral.
Messaging & Queues
Decoupling producer from consumer, and the semantics that come with it.
Event Streaming
Retained ordered logs, consumer offsets, partitions and replay.
Clocks & Ordering
Why wall clocks lie, and how logical clocks and versions restore order.
Failure Modes
Slow rather than down, partial, grey, and failing while reporting success.