Sync vs Async
Whether the caller's outcome depends on the callee's response.
6 to work through
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beginner
A team moves a slow 800 ms enrichment call out of the request path onto a queue. Request p99 drops immediately, total system throughput does not improve, and the time until the user sees the enriched result gets worse. Why does the queue not reduce the work?
3 min answer -
intermediate
A team debates whether an operation should be synchronous or asynchronous. What decides it, and what is the cost of getting it wrong in each direction?
2 min answer -
intermediate Multiple choice
Order service must notify inventory, billing and notifications when an order is placed. Synchronous calls or events? Justify.
2 min answer -
intermediate
Where should the boundary between synchronous and asynchronous processing sit in an API platform, and what determines it?
2 min answer -
advanced
A checkout flow makes 9 synchronous calls: inventory, pricing, tax, fraud, payment, loyalty, shipping, notification and analytics. What would you change?
2 min answer -
advanced
An order API calls six services synchronously and takes 3 seconds at p95, failing whenever any dependency is degraded. Redesign it.
2 min answer
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.
Strong vs Eventual Consistency
A per-operation decision, resolved by what a stale read would cost.
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.