Klaviyo·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say

May 2026

Summary

Klaviyo SWE interview threw a pretty involved system design question at me, the kind where you realize halfway through that the edge cases are the whole point. One question, but it had enough moving parts to fill an entire session.

Questions Asked (1)

Design and implement an in-memory banking system supporting scheduled transfers with expiry, account merges, and historical balance queries. Define the data structures, APIs, and algorithms you'd use, then write a working single-threaded implementation.

System DesignAlgorithms & Data StructuresData Modeling

Author's notes

This one wrecked me a little.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the core entities (accounts, transfers, transactions) and their relationships. Then outline the data structures and algorithms for each feature, focusing on trade-offs and edge cases. Finally, present a clean, modular implementation with tests, explaining your design choices as you go.

Pro tip: Emphasize correctness and edge cases (e.g., insufficient funds, expiry handling, merge conflicts) before optimizing performance; interviewers value a robust, well-tested solution over a prematurely optimized one.

1. Clarify Requirements and Scope

Ask questions to pin down functional and non-functional requirements: account types, transfer scheduling (one-time vs recurring), expiry semantics, merge behavior, historical query granularity, and concurrency assumptions.

2. Define Data Model and APIs

Specify the core classes (Account, Transfer, Transaction) and their attributes, along with the public API methods (e.g., createAccount, scheduleTransfer, mergeAccounts, getBalanceAt).

3. Design Algorithms and Data Structures

Choose appropriate structures: e.g., a min-heap or priority queue for scheduled transfers, a map for account lookup, and a time-indexed log or versioned balances for historical queries. Explain how merges are handled (e.g., redirect pointers, transfer re-assignment).

4. Implement Core Logic

Write the single-threaded implementation, ensuring correct handling of edge cases: insufficient funds, expired transfers, merge conflicts, and historical balance reconstruction.

5. Test and Validate

Walk through test scenarios (e.g., scheduling, expiry, merge, historical query) and discuss potential pitfalls or extensions (e.g., concurrency, persistence).

Key Points to Mention

Use a priority queue (min-heap) for scheduled transfers to efficiently process due transfers.
Maintain a transaction log or versioned balance snapshots to support historical balance queries.
Handle account merges by redirecting references and consolidating balances/transfers, ensuring atomicity.
Define clear expiry semantics: when a transfer expires, it should be removed from the schedule and not executed.
Consider edge cases: insufficient funds at execution time, merging accounts with pending transfers, and querying balances before account creation.
Discuss trade-offs between time and space complexity for historical queries (e.g., full log vs periodic snapshots).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.