# A mobile SDK queued a checkout_completed event with event_time Tuesday 09:14 on a phone with patchy Canonical URL: Domain: Pipeline Design ยท Difficulty: medium ## Problem A mobile SDK queued a checkout_completed event with event_time Tuesday 09:14 on a phone with patchy reception. The phone reconnected Thursday 14:27 and the event arrived at the pipeline two days late. The section names three things a pipeline can do with a late event: include it in its event-time bucket and update history, include it in the bucket of when it arrived, or drop it. The first is correct but mutates yesterday; the second is wrong but cheap; the third is wrong but predictable. Place the late event on the canvas: replace the processing-time aggregation with one that buckets by event time, and add a node whose name states which of the three reactions the pipeline now uses. ## Related - [All practice problems](https://datadriven.io/problems) - [Mock interview mode](https://datadriven.io/interview/a-mobile-sdk-queued-a-checkoutcompleted-event-with-eventti-16e5f044) - [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.