# Three operational questions show up in a 3am incident: 'what specifically happened during last night Canonical URL: Domain: Pipeline Design ยท Difficulty: medium ## Problem Three operational questions show up in a 3am incident: 'what specifically happened during last night's run' (read individual events), 'is rows_written following its normal trend' (aggregate over time and alert), and 'where in the pipeline did the request slow down' (see one logical run as a path). The section names three signal classes that answer each: logs (timestamped human-readable events), metrics (low-cardinality numerics aggregated and alerted), and traces (a path tying together the operations of one run). Pick logs, metrics, or traces by adding three destinations downstream of the transform, one per signal class, each with a tech that fits its role. ## Related - [All practice problems](https://datadriven.io/problems) - [Mock interview mode](https://datadriven.io/interview/three-operational-questions-show-up-in-a-3am-incident-what-add4d9fe) - [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.