# Design social media privacy and audience controls: system design interview question

> Post access across cached and replicated content: blocks, audience changes, strict revocation, and popular posts.

Source: https://sysround.com/questions/design-social-media-privacy-and-audience-controls
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a service that controls who can view social posts. Support public, private, friends-only, and custom-audience posts, including blocks and later permission changes."

## What to clarify first

- Visibility and identity
- Custom audiences
- Blocks
- Protected content
- Scale
- Traffic and read:write ratio
- Latency and regions
- Change completion
- Availability and durability
- Existing systems and update ownership

## Scope of a strong answer

Core requirements:

- Set and change a post's audience
- Authorize access to post text, media, and metadata
- Apply friendship changes, blocks, and custom audience changes
- Enforce current permissions across content caches and replicas

Usually out of scope:

- Build the feed, search service, or media store
- Recommend friends or audiences
- Edit post content
- Remove content that a viewer already saved
- Control access to private messages

## What interviewers probe

### Relationships and audiences

One relationship change can affect thousands of old posts. Blocks must override every audience without a separate update to each post.

### Access through content copies

A correct authorization service cannot protect a post when another path serves the content without a check.

### Permissions after a change

Independent caches can retain an allow decision after a successful change. Separate reads can also combine facts from incompatible states.

### Popular posts and outages

One public post can receive most reads. Blocks still make access depend on the viewer, and failures cannot permit stale access.

## Common mistakes

- Claims immediate revocation while allow decisions remain valid until cache expiry.
- Checks a post policy but ignores changes to friendship, blocks, or custom membership.
- Protects the feed API but leaves media URLs or content origins public.
- Updates every affected post after a block or friendship removal.
- Uses a stale replica when the authoritative permission store fails.
- Claims that revocation removes bytes that a viewer already received.

## The bar by level

- **Senior:** Defines the revocation boundary. Uses atomic fact updates and current transactional reads. Covers custom audiences, media paths, retries, and denial during uncertain authorization.
- **Staff+:** Explains which coordination strict revocation requires. Designs safe replica reads, hot-post capacity, and failover. Rejects incompatible availability promises and avoids revocation work per post.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=social-content-privacy


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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.