Applied·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

Applied gave me a coding round that was basically a mini database engine. One problem, but it had enough edge cases to keep me busy for the whole session.

Questions Asked (1)

Build an in-memory key-value store that supports SET, GET, DELETE, and nested transactions with COMMIT and ROLLBACK semantics.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was a stack of hashmaps, one per transaction level.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: single-threaded, in-memory, nested transactions with commit/rollback. Then design a data structure that maintains a stack of transaction layers, each recording changes relative to the previous state. Implement operations to work on the top layer, and commit/rollback to merge or discard layers accordingly.

Pro tip: Discuss the trade-off between copying the entire store on each transaction (simple but memory-heavy) versus maintaining an undo log or change set per transaction (more efficient but complex). Showing awareness of this trade-off demonstrates maturity.

1. Clarify requirements and constraints

Ask about expected operations, transaction nesting depth, concurrency, and memory limits. Confirm that transactions are nested and that commit/rollback affect only the current transaction.

2. Choose a data structure for transaction layers

Decide between a stack of full copies, a stack of change sets (undo logs), or a persistent data structure. Consider the trade-offs in time and space complexity.

3. Implement core operations with transaction awareness

For SET, GET, DELETE, ensure they operate on the current transaction layer, falling back to parent layers if needed. For GET, search from top down; for SET/DELETE, record changes in the top layer.

4. Implement COMMIT and ROLLBACK

COMMIT merges the top layer into its parent (or base store if no parent). ROLLBACK discards the top layer. Handle edge cases like committing/rolling back with no active transaction.

5. Analyze complexity and discuss optimizations

Explain time and space complexity of each operation. Discuss potential optimizations like lazy copying, persistent trees, or batching changes.

Key Points to Mention

Nested transactions require a stack of transaction contexts, each recording changes relative to its parent.
GET must search from the innermost transaction outward to the base store.
SET and DELETE should only affect the current transaction layer, not parent layers.
COMMIT merges the current transaction's changes into its parent; ROLLBACK discards them.
Trade-off: full copy per transaction is simple but O(n) space per transaction; undo log is more efficient but requires careful handling of overwrites.
Edge cases: committing/rolling back with no active transaction, deleting a key that doesn't exist, nested rollback affecting only the innermost transaction.

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