Coding (Platform)

NestJS: Enforce idempotent operations for billing module command handling

A service boundary accepts valid traffic but mishandles duplicate, stale, or unauthorized records. The affected area is billing module.

Repair the billing module command handling implementation so it accepts first operations, recognizes safe repeats, and rejects key reuse with a different payload.

Evidence

Affected surface: NestJS / billing module.
Observed failure family: enforce idempotent operations.
Scope policy: tenantId must equal tenant-primary.

What this task practices

This coding (platform) exercise trains the same sequence expected in an interview: understand the prompt, make assumptions explicit, produce a concrete answer, and explain the decisions behind it. The task is marked medium difficulty. The private workspace adds the tools required by this round, such as recording, code execution, diagrams, evidence panels, or structured notes. It also preserves the attempt so later feedback can be compared with previous work. Evaluation is based on task-specific criteria and the candidate seniority selected in the preparation path. Public pages never expose the reference solution, hidden tests, evaluator instructions, or another candidate’s work.