19-Usage-Billing-Engine

Appearance

14.9 Reliability, Resilience & Reconciliation ​

1. What It Is in Plain Words ​

Reliability is the probability that a system performs its required functions under specified conditions. Resilience is the system's ability to absorb failures (network partitions, hardware crashes) and recover automatically.

2. How This Project Uses It ​

In aggregation/periods.py:L130:

  • Differential Reconciliation: End-of-cycle audits compare pre-aggregated rollups with raw event SQL sums (recompute_from_scratch).
  • Payment Retries with Backoff: Dunning retries at Day 0, 3, 7, and 14 with grace period tracking.

3. 30-Second Interview Answer ​

"We ensure financial resilience through automated differential reconciliation. Before closing any billing period, our worker computes a from-scratch SQL sum across all raw usage events and compares it against pre-aggregated rollups. If a discrepancy of even one cent is detected, the cycle halts and alerts engineering, preventing corrupt invoices from reaching customers."