# Public payments company Canonical URL: Domain: Pipeline Design ยท Difficulty: medium ## Problem Public payments company. The financial-close streaming aggregator was blamed last week for a duplicate-revenue incident during a Kafka rebalance; reports for two days had to be reissued. The canvas shows the dual-write antipattern, a Flink aggregator whose engine-level exactly-once does not extend to its sink, and a plain Snowflake destination. The CFO needs exact reconciliation against a bank settlement file at quarter close. Apply the section's six-step synthesis so every boundary is effectively-once and the contract is documented. ## Related - [All practice problems](https://datadriven.io/problems) - [Mock interview mode](https://datadriven.io/interview/public-payments-company-the-financial-close-streaming-aggre-531525be) - [System Design Interview Questions](https://datadriven.io/data-engineering-system-design) - [Data Engineering Interview Prep Guide](https://datadriven.io/data-engineer-interview-prep) - [Daily Challenge](https://datadriven.io/daily) --- Source: DataDriven (https://datadriven.io). 100% free data engineering interview prep. Live code execution against Postgres 16, Python 3.11, and Spark sandboxes. No paywall, no premium tier, no signup gate.