# Design a reverse ETL platform: system design interview question

> Copy warehouse results into operational tools. Detect changes, respect shared destination limits, and restore current values after partial or uncertain writes.

Source: https://sysround.com/questions/design-reverse-etl-platform
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a platform that copies customer data from a data warehouse into tools such as a CRM. Keep destination data current as warehouse results change."

## What to clarify first

- Scale
- Traffic and data size
- Schedule and latency
- Admission and freshness
- Warehouse query
- Destination contract
- Destination failures
- Consistency and durability
- Fields and scope
- Tenant boundaries

## Scope of a strong answer

Core requirements:

- Configure a warehouse query, a customer key, and a field map for each job.
- Copy the initial result and send later changes to the destination.
- Respect destination limits across jobs that share credentials.
- Recover failed writes and report progress for each row.

Usually out of scope:

- Load source data into the warehouse.
- Build a visual tool for data transformation.
- Copy destination changes into the warehouse.
- Delete customers when they leave a query result.
- Survive a regional failure.

## What interviewers probe

### Changes in warehouse results

A join can change a customer result without changing that customer's source timestamp. Repeated full queries can also dominate cost.

### Incomplete destination writes

An old request can overwrite a newer success. A paused worker can also send after recovery assumes its request lifetime has ended.

### Destination capacity

Jobs share a fixed external quota. Freshness depends on destination completion as well as source work and dispatch capacity.

## Common mistakes

- The design assumes a customer timestamp captures changes from every join and computed field.
- The design treats a completed warehouse read as proof of successful destination delivery.
- The design retries batches without tracking results for each row.
- The design promises freshness without admission limits or a budget for destination completion.
- The design starts the remote lifetime at attempt creation and ignores a paused sender.
- The design forgets uncertain writes after a newer success and waits for warehouse hashes to change.

## The bar by level

- **Senior:** Separates source progress from delivery progress. Calculates admission and freshness through destination commit. Handles partial batches, worker failures, and fairness. Preserves uncertain attempts and independent repair work.
- **Staff+:** Derives source and destination admission bounds. Proves repair safety for paused senders and late commits. States weaker connector guarantees. Defines regional ownership transfer only if the interviewer asks the regional failure follow-up.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=reverse-etl


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SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.