KV Store with Rollback
Reported by candidates from Tesla's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
You're looking at Tesla's April 2026 OA question on a versioned key-value store. This is a design problem that tests your ability to snapshot state and reverse time. You'll need to track versions, manage mutable state carefully, and handle rollbacks without losing data. The trap is thinking you can just store references. You need actual copies. If you blank on the implementation during the OA, StealthCoder will show you the exact snapshot-and-restore pattern in real time.
The problem
Design and implement an in-memory key-value store that supports version checkpoints and rollback. Supported operations are: CHECKPOINT creates a new snapshot version. ROLLBACK version restores the store to the exact state at that checkpoint. Return the lines printed by the command sequence. Only GET outputs are required in the examples below. Function Description Complete the function runVersionedKvStore in the editor below. runVersionedKvStore has the following parameter: Returns The first GET sees the updated value 2. After rolling back to the first checkpoint, a returns to 1. Rolling back to version 1 restores the snapshot created after y was added but before x was changed to 7.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The core trick is that CHECKPOINT must create a full copy of the current store state, not a reference. Most candidates try to be clever and store deltas or pointers, then fail on rollback. The pattern is straightforward: maintain a list of store snapshots indexed by version number. On CHECKPOINT, deep copy the current store. On ROLLBACK, retrieve the snapshot and replace the working store entirely. On GET, just read from the current store. The challenge is efficiency: if your store is large, copying it every checkpoint is expensive. But for the OA, correctness beats optimization. StealthCoder will flag whether you're doing shallow or deep copies when you implement.
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KV Store with Rollback FAQ
Do I need to handle version IDs that don't exist?+
The problem doesn't specify, but assume all ROLLBACK calls reference valid versions. Focus on the happy path. If you have time, add error handling, but it won't make or break the OA.
Can I use a dictionary for the store itself?+
Yes. Use a dict for the current state and a list of dicts for snapshots. On CHECKPOINT, append a copy of the dict. On ROLLBACK, retrieve the snapshot and replace your working dict with it.
What if the same key is set twice before a checkpoint?+
Only the latest value matters. When you snapshot, you snapshot the current state of the dict. Earlier values are irrelevant. No delta tracking needed.
How do I copy a dict in Python without breaking the snapshot?+
Use dict.copy() or copy.deepcopy(). Since values are likely strings or ints, dict.copy() is fine. Deepcopy is safer if values are nested structures.
Is this really a 'design' problem or just implement-the-obvious?+
It's implement-the-obvious dressed as design. No fancy architecture needed. Snapshot on CHECKPOINT, restore on ROLLBACK, read on GET. Clean code and correct behavior beat elegance.