Develop an Ads Management and Display System for a Social Feed — System Design Interview Practice

Interview prompt

Design Design an advertising platform for targeting, serving, and billing ads in social media feeds. so users can Create and manage ad campaigns reliably at scale.

• Define the source of truth for Create and manage ad campaigns; Target ads by demographics and interests and make retries idempotent.
• Use bounded, partitioned state to meet Handle billions of ad impressions per day and Low latency for ad serving (<100ms).
• Separate the critical request path from Ad ranking and auction algorithms, User profiling for targeting, Real-time bidding infrastructure.
• Explain consistency, failure recovery, authorization, observability, and a degraded mode.

Requirements and scale assumptions

• Support the core workflow to Create and manage ad campaigns.
• Expose status, results, and freshness appropriate to Design an advertising platform for targeting, serving, and billing ads in social media feeds..
• Support authorization, validation, updates, deletion, and recovery semantics.
• Meet Low latency for ad serving (<100ms) under normal load.
• Scale to Handle billions of ad impressions per day without a single hot key or unbounded synchronous work.
• Do not lose committed state; make retries and duplicate events safe.
• Degrade safely when downstream workers, caches, or external dependencies fail.
• Handle billions of ad impressions per day
• Partition by the primary tenant, user, item, or geographic key and isolate hot partitions.
• Keep serving state bounded; retain raw events or durable records for replay and auditing.

• Peak scale: Handle billions of ad impressions per day — Capacity assumption that drives partitioning and backpressure.
• Latency target: Low latency for ad serving (<100ms) — User-facing budget for the primary request or read path.
• Durable boundary: Committed before async — The source of truth is Create and manage ad campaigns; Target ads by demographics and interests.
• Async boundary: At-least-once workers — Keep Ad ranking and auction algorithms, User profiling for targeting, Real-time bidding infrastructure off the synchronous path.

Key entities

• InteractioninteractionId, actorId, objectId, type, version, occurredAt

  Canonical ads management and display system interaction with an idempotency key and ordering version.

• ConnectionSessionsessionId, userId, deviceId, roomKey, lastHeartbeat, status

  Ephemeral but observable ads management and display system connection registration used for routing and presence.

• FanoutCursorstreamKey, shard, offset, consumerGroup, updatedAt

  Durable progress marker for ads management and display system fan-out and replay.

• DeliveryReceiptinteractionId, recipientId, channel, attempt, status, deliveredAt

  Deduplicated ads management and display system delivery state for reconnects, retries, or acknowledgements.

Data flow

1. 1. Accept and commit the interactionThe ads management and display system gateway authenticates the actor, validates room or object membership, applies rate limits, and conditionally commits the interaction.
2. 2. Publish an ordered eventAn outbox emits the committed ads management and display system transition with an event ID, partition key, sequence, and replay retention.
3. 3. Fan out by partitionConsumers route ads management and display system events to connected recipients, durable inboxes, or notification channels without making the origin write wait for every recipient.
4. 4. Resume and reconcile connectionsClients reconnect with a cursor; the ads management and display system service replays missed events, deduplicates delivery, and exposes stale or degraded state.
5. 5. Measure latency and recoverOperations tracks ads management and display system publish-to-deliver latency, hot partitions, reconnect storms, dropped events, and consumer lag for replay or repair.

Deep dives and trade-offs

• Ordering, idempotency, and hot keysChoose a ads management and display system partition key that preserves required order while distributing high-volume rooms, users, or objects. Use event IDs, inboxes, consumer offsets, and conditional state transitions for at-least-once delivery. Split or isolate hot partitions without changing the client-visible sequence contract.
• Reconnect and replay semanticsIssue resumable ads management and display system cursors with an expiry and a clear snapshot-plus-delta fallback. Bound replay windows and rebuild from durable state when a cursor is too old. Expose version and freshness so a client can distinguish current, catching up, and degraded state.
• Backpressure and presenceKeep connection heartbeats and ephemeral presence separate from durable ads management and display system interactions. Coalesce safe updates, shed low-value work, and protect critical events during reconnect storms. Measure end-to-end delivery, not only broker publish latency.
• Direct fan-out versus pull-based readsUse push for latency-sensitive ads management and display system deltas and pull or replay for reconnect, history, and recovery. A push-only design loses state when clients disconnect and a pull-only design wastes latency and bandwidth.
• Per-recipient queues versus shared streamsUse shared partitioned streams with per-recipient cursors where fan-out is large, and isolate exceptional high-fanout objects. A queue per recipient becomes expensive and hard to inspect at large scale.
• Strong ordering versus availabilityGuarantee ordering only within the scope the product needs, such as a room, object, or conversation. Global ordering introduces a bottleneck and still does not solve duplicate delivery or reconnect recovery.