02Key conceptsEntities and APIs
02 · Core method
Entities and APIs
Derive durable nouns and high-value operations directly from the agreed functional requirements.Lesson spine
What you need to understand.
Entities and APIs translate product language into durable contracts before storage and services distract the discussion.
Find the nouns
Start from requirements and identify objects with identity, lifecycle, ownership, or relationships. Avoid premature tables.
Name the state owner
For every entity, say which boundary can create or advance it. Derived stores may copy state but do not become truth by accident.
Design task-shaped APIs
Expose the user operation, not internal service topology. Include identifiers, pagination, filters, version checks, and idempotency where relevant.
Choose an API style deliberately
REST fits resource operations, RPC fits internal commands, GraphQL fits varied client reads, and streaming protocols fit long-lived updates.
Protect evolution
Use opaque public IDs, additive schemas, stable error semantics, and explicit bulk contracts.
Before the boxes
Frame the decision.
What must work
Derive durable nouns and high-value operations directly from the agreed functional requirements.
What changes the design
Keep the model to 3–6 entities and 4–8 high-value interfaces.
What owns the truth
Identify the component that commits authoritative state, then separate synchronous confirmation from derived work.
What stays simple
Do not add global coordination, multi-region writes, or a specialized store until a requirement earns the complexity.
Architecture map
Trace ownership, not just traffic.
Follow the decision from left to right. Every arrow should have a reason.
Underline durable nouns
Assign identity and lifecycle
Mark ownership
Define state changes
Add pagination and freshness
Name asynchronous facts
Walk one representative request across every arrow. Say whether the handoff is synchronous or asynchronous, what identity makes a retry safe, and which step changes authoritative state.
- 01
Requirements — Underline durable nouns Define the output contract before moving to the next owner.
- 02
Entities — Assign identity and lifecycle Define the output contract before moving to the next owner.
- 03
Relationships — Mark ownership Define the output contract before moving to the next owner.
- 04
Commands — Define state changes Define the output contract before moving to the next owner.
- 05
Queries — Add pagination and freshness Define the output contract before moving to the next owner.
- 06
Events — Name asynchronous facts Confirm the result and emit the evidence needed to reconcile it.
Decision table
Make the trade-offs explicit.
| Decision | Defensible position | Cost to acknowledge |
|---|---|---|
| Primary mechanism | Use domain operations when they protect an invariant; do not force every workflow into raw CRUD. | The stronger guarantee usually adds coordination, latency, state, or operational work. |
| Sync vs. async | Keep only correctness-critical confirmation synchronous. Move derived views, notifications, analytics, and cleanup behind a durable boundary. | Async work needs idempotency, lag monitoring, replay, and a product definition for partial completion. |
| Simple vs. scaled | Begin with one logical owner and a clear API. Partition or replicate only the resource proven to be the first bottleneck. | Migration requires stable identities, versioned contracts, backfill, and a rollback path. |
Failure review
Design the recovery path.
Topic-specific risk
Missing idempotency, authorization, pagination, or lifecycle states makes APIs unsafe.
ResponsePersist enough identity and state to distinguish retry, resume, compensation, and operator repair.
Dependency timeout
A timeout is ambiguous: the remote side may have failed, succeeded, or still be running.
ResponseUse deadlines, bounded backoff with jitter, idempotency keys, and a status or reconciliation path.
Overload or skew
Average capacity can look healthy while a tenant, key, partition, region, or expensive request saturates one owner.
ResponseExpose queue depth and hot-key share, apply backpressure, isolate tenants, and degrade optional work before correctness.
Evidence + level bar
Prove the design can be operated.
Health of the promise
Measure user-visible latency or freshness, correctness drift, saturation, retry volume, and time to recover. Alert on the failed promise—not only CPU.
Complete and clear
Finish the happy path, identify the state owner, choose reasonable building blocks, and explain one scale mechanism.
Trade-offs and failure
Separate read and write paths, define consistency, explain partitioning, and make duplicate or partial failure safe.
Evolution and operations
Discuss multi-region boundaries, migration, tenant isolation, capacity, observability, and how the architecture changes over time.
Interview language
Open the deep dive with a claim.
“For Entities and APIs, the decision I want to make explicit is this: Use domain operations when they protect an invariant; do not force every workflow into raw CRUD. I’ll trace the state-changing path first, show where the result becomes durable, then test the design against the highest-risk failure and our target scale.”
08 · Retrieval check
Can you defend it without the page?
- For Entities and APIs, where is the correctness boundary and which failure would you test first?
- Which component owns committed truth, and what event or response proves the commit?
- Where is the first scaling or coordination bottleneck under the stated envelope?
- What happens after an ambiguous timeout or duplicate operation?
- Which complexity would you remove at one hundredth of the scale?