← Databricks Interview Insights
The scope of this thing is brutal if you're not ready for it.
Start by clarifying requirements (scale, latency, consistency, features) and then walk through the design from client-server protocol to storage, fan-out, presence, and offline delivery. Use pseudocode to illustrate key components and justify trade-offs, especially around ordering, delivery guarantees, and group vs 1:1 chats.
Pro tip: Emphasize idempotency and message ordering using per-conversation sequence numbers, and discuss how to handle offline delivery with a pull-based sync mechanism to avoid overwhelming push notifications.
Ask about scale (users, messages per second), latency, consistency, and features like read receipts, typing indicators, and media. Define the core entities: users, conversations (1:1 and group), messages, and presence.
Choose a protocol (e.g., WebSocket for real-time, HTTP for history). Define endpoints for sending messages, fetching history, and managing conversations. Use pseudocode to show message format and authentication.
Describe how messages are ingested (e.g., via API gateway), assigned a sequence number per conversation, stored in a database (e.g., Cassandra for write-heavy), and fanned out to online recipients. For groups, use a fan-out-on-write or fan-out-on-read approach based on group size.
Design presence using heartbeats and a presence service. For push notifications, integrate with APNs/FCM. For offline delivery, use a pull-based sync where clients fetch missed messages upon reconnection, with a cursor based on last received sequence number.
Compare trade-offs: consistency vs availability, fan-out strategies, storage choices, and push vs pull for notifications. Address scaling with sharding, replication, and caching.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clarifying the system context and assumptions (e.g., at-least-once delivery, message ordering, failure model). Then present a concise pseudocode for the core send path, walking through each line to explain its purpose, invariants, and how it handles edge cases like retries, duplicates, and partial failures. Finally, discuss trade-offs and potential improvements.
Pro tip: Explicitly state your assumptions about the failure model and delivery guarantees upfront—this shows you understand distributed systems nuances and prevents the interviewer from derailing your answer with edge cases you haven't considered.
Ask clarifying questions about delivery semantics (at-least-once, exactly-once), ordering, durability, and failure scenarios. State your assumptions clearly to set the stage.
Write a concise pseudocode for the message-send path, focusing on the core steps: validation, persistence, replication, acknowledgment, and retry logic.
For each line, explain its purpose, the invariants it maintains, and how it contributes to correctness. Highlight any non-obvious design choices.
Discuss how the pseudocode handles edge cases such as network partitions, duplicate sends, message loss, and partial failures. Explain any retry or idempotency mechanisms.
Summarize the trade-offs made (e.g., latency vs. durability, complexity vs. guarantees) and mention alternative approaches or potential optimizations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clarifying the scope and assumptions (e.g., at-least-once delivery, message ordering, device types) before walking through the pseudocode. Then, present a high-level pseudocode for the message-deliver path, focusing on key steps like message reception, persistence, recipient lookup, device selection, and delivery attempts. Finally, systematically address edge cases such as offline recipients and multi-device scenarios, explaining how the design handles them and the trade-offs involved.
Pro tip: Proactively discuss trade-offs between consistency, latency, and cost—showing you understand that edge cases often require balancing competing priorities. Also, mention how you would monitor and test these edge cases in production, demonstrating operational maturity.
Ask clarifying questions to understand the expected delivery guarantees (at-least-once, exactly-once), ordering requirements, and device types. State your assumptions explicitly to set the context for your answer.
Present a structured pseudocode for the message-deliver path, covering message ingestion, persistence, recipient lookup, device selection, and delivery attempts. Keep it concise and focus on the main flow.
Explain how the system handles offline recipients: store messages durably, use push notifications or polling to detect when they come online, and implement retry logic with backoff. Discuss trade-offs like storage cost vs. delivery latency.
Describe how messages are delivered to multiple devices: fan-out to all active devices, maintain per-device delivery state, and handle synchronization (e.g., read receipts, message ordering). Mention potential issues like duplicate deliveries and how to deduplicate.
Systematically discuss other edge cases (e.g., device offline during delivery, message expiration, network partitions) and justify your design choices. Highlight trade-offs between consistency, availability, and latency.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.