Xai·Software Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Got a coding question at xAI for a software engineer role that looked deceptively straightforward until the nested transaction part hit. The constraints basically ruled out the naive copy-on-begin approach, so you had to actually think through the data structure.

Questions Asked (1)

Build an in-memory key-value store that supports SET, GET, and DELETE commands, plus nested transactions with BEGIN, ROLLBACK, and COMMIT. ROLLBACK should only undo the most recent open transaction block, and COMMIT applies everything permanently. The catch: copying the whole database on every BEGIN is explicitly not allowed.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The basic SET/GET/DELETE part took me about two minutes.

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AI HintsAI Generated

Suggested Approach

Design the store with a main data map and a stack of transaction layers, where each layer records only the changes made since BEGIN. For SET, write to the main map and log the previous value in the current transaction; for GET, read from the main map; for DELETE, remove from the main map and log the previous value. ROLLBACK pops the top layer and undoes its changes in reverse order, while COMMIT merges the top layer's undo log into the parent transaction or discards it if it's the outermost.

Pro tip: Emphasize that the undo log approach is O(1) per operation and avoids copying the database, and mention that nested transactions can be handled by chaining undo logs or using a stack of logs. Also, clarify that COMMIT doesn't need to do anything except discard the undo log for the outermost transaction, but for nested transactions it should merge the undo log into the parent.

1. Clarify requirements and constraints

Confirm that transactions can be nested, ROLLBACK only affects the most recent BEGIN, and COMMIT makes changes permanent. Explicitly rule out copying the entire database on BEGIN.

2. Choose data structures

Use a hash map for the main key-value store and a stack (or linked list) of transaction frames. Each frame contains an undo log mapping keys to their previous values (or a sentinel for non-existence).

3. Implement SET, GET, DELETE with transaction logging

For SET, record the old value in the current transaction's undo log before updating the main map. For DELETE, record the old value and remove the key. GET simply reads from the main map.

4. Implement BEGIN, ROLLBACK, COMMIT

BEGIN pushes a new empty undo log onto the stack. ROLLBACK pops the top undo log and applies its entries in reverse order to restore previous values. COMMIT pops the top undo log; if there is a parent transaction, merge the undo log into it (preserving order), otherwise discard it.

5. Analyze complexity and edge cases

Discuss time complexity: O(1) per operation except ROLLBACK/COMMIT which are O(k) where k is the number of changes in the transaction. Handle edge cases like ROLLBACK with no active transaction, nested COMMIT, and keys deleted then re-added.

Key Points to Mention

Undo log per transaction to record previous values without copying the whole database
Stack of transaction frames to support nested transactions
ROLLBACK undoes changes in reverse order to handle multiple updates to the same key
COMMIT merges undo log into parent transaction or discards if outermost
Time complexity: O(1) for SET/GET/DELETE, O(k) for ROLLBACK/COMMIT where k is number of changes
Edge cases: empty transactions, rollback without BEGIN, nested commits

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