How the interviewer opens it
“Design a photo and video service like Snapchat. Users send media that recipients can view briefly before it disappears.”
The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.
What to clarify first
- Recipients
- Scale
- Media and uploads
- Read and write mix
- View rules
- Expiration and deletion
- Latency
- Consistency and availability
- Regional scope and durability
- Security limits and exclusions
The interviewer has the answers. In a SysRound round, Alex gives these answers, as a real interviewer does.
Scope of a strong answer
Core requirements
- Send a photo or short video to selected recipients.
- Show each recipient an inbox.
- Allow one short view session per recipient.
- Expire access and remove media by fixed deadlines.
Usually out of scope
- Public stories and discovery
- Text chat, calls, and media filters
- Prevention of screenshots or saved copies on hostile devices
- Legal retention and moderation systems
- Recovery from a full regional outage
What interviewers ask about
- Concurrent opens
- Two devices can open one snap at once. A lost response must not create another session or reset its deadline.
- Media delivery latency
- Fast delivery must preserve recipient access rules. A cached copy must not become a permanent public copy.
- Media expiration and deletion
- Access denial and media removal use different deadlines. Upload retries and delayed cleanup can retain media or delete the wrong object.
Common mistakes
- Treats the client timer as the only expiration control.
- Reads the viewed flag and writes it in separate operations.
- Exposes permanent media URLs or checks access only at the origin.
- Confuses access expiration with physical media deletion.
- Lets more than one snap consume the same upload.
- Assumes high cache reuse despite the small recipient count.
The bar by level
- Senior (L5/E5)
- Handles concurrent opens, lost responses, byte ranges, and upload failures. Enforces access rules at the edge. Defines durable cleanup and atomic upload consumption.
- Staff (L6+/E6+)
- Proves view safety during zone failover within one region. Tests upload reuse and cleanup races. Defines provider deletion limits. Quantifies transfer costs and cache value.