Collaborative Playlist · Requirements

Source-backed behavior

Create and update playlists.
View ordered playlist contents.
Add, remove, replace, and reorder entries.
Paginate large playlists.
Record who added an item and when.
Support public, private, and collaborative status.
Return a version identifier and apply mutations to create a new snapshot.

Interview contract

Authorized collaborators can edit the same playlist concurrently.
Clients receive accepted mutations in playlist order.
Retries preserve idempotency.
Reconnect retrieves mutations after a known version.
Duplicate tracks remain valid, distinct entries.
Every accepted mutation is durable before acknowledgement.

Hard invariants

One operation_id produces at most one accepted mutation.
Every accepted mutation receives exactly one playlist version.
Versions increase monotonically within a playlist.
A removed entry stays removed when a delayed move arrives.
Every materialized playlist equals its snapshot plus subsequent mutations.

Quality targets to clarify

Dimension	Interview question
Playlist size	Typical and maximum entries?
Editors	How many simultaneous collaborators?
Latency	Target for local feedback and remote propagation?
Offline mode	Read-only cache or queued offline mutations?
History	Audit only, or user-visible undo and restore?
Permissions	Owner, editor, viewer, and link-sharing rules?

Variable sizing

mutation QPS = active editors × edits/editor/secondfanout events/s = accepted mutations/s × connected collaboratorshistory bytes/day = mutations/day × average event bytes

Source material provides behavior and version semantics while leaving traffic figures open. State assumptions aloud and size from the chosen interview numbers.

Scope boundaries

Audio delivery, licensing, recommendations, and playback queue behavior form separate systems.
The playlist stores media references and ordered metadata.
Presence, selections, and drag previews remain ephemeral.
Mutation history can power audit, repair, and optional undo.