Strong vs Eventual Consistency
A per-operation decision, resolved by what a stale read would cost.
5 to work through
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advanced
A booking platform must decide where strong consistency is required. How should the decision be made, and how is it communicated to application developers?
2 min answer -
advanced
A flash sale puts 500 units of one product in front of 200,000 concurrent shoppers. Design the inventory path.
2 min answer -
advanced
A grocery marketplace shows store availability from a read model fed by change-data-capture streams from each retailer's inventory system. Normal lag is 2 to 4 seconds. During the Saturday peak the stream backs up to 90 seconds and stays there for 20 minutes. Nothing errors and no alert fires. What happens, and which mechanism stops it?
3 min answer -
advanced Multiple choice
A lodging marketplace must show accurate availability across search, listing pages and booking. Which of these need strong consistency?
2 min answer -
advanced
Which operations in a ticketing system need strong consistency and which are correct with eventual consistency?
2 min answer
2 terms in this topic
Consistency Choices
Deciding, per operation, the weakest consistency guarantee that is still correct — rather than choosing one model for a system.
practiceConsistency Level Selection
Choosing the consistency guarantee per operation rather than per system, matching the cost of coordination to the business consequence of staleness.
Neighbouring topics
Architecture Decision-Making
General material on making and recording architectural decisions.
Architecture Decision Records
One decision, its context, alternatives and consequences, kept immutable.
Reversibility
One-way and two-way doors, and buying optionality deliberately.
Build vs Buy
Differentiation, five-year TCO, and the exit cost of each option.
Monolith vs Microservices
A team-topology decision far more often than a technology one.
SQL vs NoSQL
Decided by access patterns and query flexibility, not by data volume.
Sync vs Async
Whether the caller's outcome depends on the callee's response.
Managed vs Self-Managed
Trading control and unit cost against operational attention.
Serverless vs Containers
Spiky and event-driven versus sustained throughput.
Single vs Multi-Region
Driven by RTO, RPO and residency rather than by ambition.
Centralised vs Distributed
Shared platform leverage against team autonomy.
Performance vs Cost
Buying latency, and knowing what the last millisecond is worth.
Reliability vs Complexity
Mechanisms that add availability and add failure modes.
Security vs Usability
Varying control by the value of the action rather than uniformly.
Delivery vs Maintainability
Fast in the cheap places, careful in the expensive ones.
Deciding Under Uncertainty
Bounding the downside and buying information cheaply.
Trade-off Analysis Methods
ATAM, scenarios, and naming the points where qualities conflict.
Technology Selection
Evaluating options against drivers rather than against enthusiasm.
Decision Practice
Thresholds, review, supersession and keeping the log alive.