# Design a feature flag platform: system design interview question

> Fast flag decisions with stable rollout groups, clear rule precedence, and continued service during configuration outages.

Source: https://sysround.com/questions/design-feature-flag-platform
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a feature flag platform that enables features for selected users, tenants, or a rollout percentage. Applications need fast flag decisions, even during a platform outage."

## What to clarify first

- Flag rules
- Rollout identity
- Scale
- Read and write mix
- Latency
- Change visibility
- Consistency and availability
- Durability and startup
- Access control
- Scope limits

## Scope of a strong answer

Core requirements:

- Create and change boolean flags within a project and an environment
- Apply user overrides, tenant overrides, and percentage rules
- Evaluate flags with low latency during a control-plane outage
- Deliver flag changes to healthy applications within five seconds

Usually out of scope:

- Browser and mobile SDKs
- Experiment analysis
- Arbitrary audience queries
- Billing
- Real-time evaluation analytics

## What interviewers probe

### Who receives a feature

Services must agree on rollout membership for the same identity and configuration. Rule precedence must also remain clear across users and tenants.

### How edits reach applications

An admin edit and its delivery span separate systems. Lost notices, old updates, and incomplete snapshots can leave applications with incorrect configuration.

### Decisions during an outage

A fast decision cannot wait for an unavailable service. Existing processes, restarted processes, and new processes have different access to prior configuration.

## Common mistakes

- The design calls the flag service for every application request.
- The percentage rule chooses a random value for each request.
- The SDK changes its hash seed when the flag version changes.
- The design treats an admin write as proof that every application uses the new value.
- The SDK waits for a network timeout before each flag decision.
- The design promises an immediate global shutdown during a network partition.

## The bar by level

- **Senior:** Separates administration, distribution, and evaluation. Handles lost updates, version order, atomic snapshot replacement, restart behavior, and cross-language rollout consistency.
- **Staff+:** Defines edit, publication, and adoption guarantees separately. Resolves offline shutdown limits. Addresses credential isolation, SDK compatibility, reconnect bursts, and delivery cost.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=feature-flag-platform


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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.