How the interviewer opens it
“Design a service where users create, share, and edit ordered media playlists together. The service must support concurrent edits and very large playlists without losing updates.”
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
- Entry behavior
- Access and shares
- Scale and playlist size
- Traffic and read:write ratio
- Latency
- Pages and view behavior
- Consistency and availability
- Live changes and reconnect
- Durability and retries
- Excluded work
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
- Create playlists and add, remove, or move media entries.
- Read large playlists in small pages.
- Share playlists with view or edit access.
- Apply concurrent edits without silent data loss and show changes to other users.
Usually out of scope
- Media upload, storage, and playback
- Media search and recommendations
- Offline edits and later merge
- A fixed snapshot across a complete playlist export
What interviewers ask about
- Concurrent edits
- Two valid user actions can refer to the same entry or position. Retries must not create duplicate entries or overwrite another edit.
- Large playlists
- A million-entry playlist makes full reads and position rewrites expensive. Repeated edits at one position can exhaust gaps in an order representation.
- Shared views and access
- Clients read only part of a changing list. Missed events can corrupt their view. A disabled link must also stop existing live subscriptions.
Common mistakes
- Replace the complete playlist for each edit and silently overwrite concurrent changes.
- Use media identity instead of a distinct identity for each playlist entry.
- Assign consecutive positions and rewrite the rest of the playlist after each insertion.
- Use large offsets or complete reads for access near the end of a playlist.
- Treat socket notifications as durable history or allow unlimited replay.
- Omit retry protection or leave live subscriptions active after their link loses access.
The bar by level
- Senior (L5/E5)
- Defines conflict outcomes, missing-target behavior, and retry rules. Explains indexed order access and safe position maintenance. Provides bounded revision replay and atomic event records. Handles share revocation for requests and existing subscriptions.
- Staff (L6+/E6+)
- Quantifies contention for one playlist and sets throughput limits. Bounds maintenance, replay, and revocation work during failures. Explains a path to higher edit rates. If the interviewer asks about offline edits, defines a migration path for operation history and conflict rules.