Versioned Key-Value Store with Nested Transactions (Begin, Commit, Rollback)
Company: xAI
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
Implement an in-memory versioned key-value database that supports transactional writes with three transaction-control operations: `begin` (start a transaction), `commit`, and `rollback`. Transactions may be **nested**: a `begin` issued while another transaction is open starts a new inner transaction layered on top of it.
Implement the class `VersionedKVStore`:
- `VersionedKVStore()` — initializes an empty store with no open transactions.
- `void set(String key, int value)` — sets `key` to `value` in the current context: the innermost open transaction if one exists, otherwise the permanent store.
- `int get(String key)` — returns the value of `key` as currently visible: the innermost open transaction is consulted first, then each enclosing transaction from inner to outer, then the permanent store. Returns `-1` if the key is absent, or if the most recent visible operation on it is a deletion.
- `void delete(String key)` — deletes `key` in the current context. Inside a transaction, the deletion shadows any value from outer transactions or the permanent store (so `get` returns `-1`), but takes permanent effect only if committed. Deleting a key that is not visible is a no-op in effect (subsequent `get` still returns `-1`).
- `void begin()` — opens a new transaction, nested inside the current innermost transaction if one is open.
- `boolean commit()` — if no transaction is open, returns `false`. Otherwise merges all changes (sets and deletions) of the **innermost** open transaction into its parent transaction — or into the permanent store if it is the outermost — closes that transaction, and returns `true`.
- `boolean rollback()` — if no transaction is open, returns `false`. Otherwise discards all changes made in the **innermost** open transaction, closes it, and returns `true`.
Reads and writes issued outside any transaction apply directly and permanently to the store.
### Example
```
Input:
["VersionedKVStore", "set", "get", "begin", "set", "get", "begin", "delete", "get", "rollback", "get", "commit", "get", "rollback", "get"]
[[], ["a", 1], ["a"], [], ["a", 2], ["a"], [], ["a"], ["a"], [], ["a"], [], ["a"], [], ["a"]]
Output:
[null, null, 1, null, null, 2, null, null, -1, true, 2, true, 2, false, 2]
```
Explanation:
1. `set("a", 1)` — no transaction open, written permanently. `get("a")` returns `1`.
2. `begin()` opens transaction T1; `set("a", 2)` inside T1; `get("a")` returns `2`.
3. `begin()` opens nested transaction T2; `delete("a")` inside T2; `get("a")` returns `-1` (the deletion in T2 shadows the outer value).
4. `rollback()` discards T2 and returns `true`; `get("a")` returns `2` again (T1's write is intact).
5. `commit()` merges T1 into the permanent store and returns `true`; `get("a")` returns `2`.
6. `rollback()` returns `false` — no transaction is open; `get("a")` still returns `2`.
### Constraints
- At most `10^5` operations in total.
- Keys are non-empty lowercase strings of length at most `20`.
- `0 <= value <= 10^9`
- Transaction nesting depth will not exceed `10^3`.
### Performance requirement
`set` and `delete` should run in `O(1)` average time. `get` may be `O(d)` in the worst case, where `d` is the current nesting depth (an `O(1)`-average `get` is possible with a more advanced design and is a nice discussion point). `commit` and `rollback` should cost time proportional to the number of keys modified in the innermost transaction — not the size of the whole store.
Quick Answer: This question evaluates understanding of transactional state management, nested transactions, visibility semantics for set/delete operations, and efficient in-memory data-structure manipulation.
Implement an in-memory versioned key-value database that supports transactional writes with three transaction-control operations: `begin`, `commit`, and `rollback`. Transactions may be **nested**: a `begin` issued while another transaction is open starts a new inner transaction layered on top of it.
Because the coding console runs a single function, drive the class through a command list. `solution(operations, args)` receives a list of method names and an aligned list of argument lists, replays them against one `VersionedKVStore` instance, and returns a list of results (one per operation).
Implement `VersionedKVStore`:
- `VersionedKVStore()` — empty store, no open transactions. (Result: `None`.)
- `set(key, value)` — set `key` in the current context: the innermost open transaction if one exists, otherwise the permanent store. (Result: `None`.)
- `get(key)` — return the value of `key` as currently visible: innermost transaction first, then each enclosing transaction inner-to-outer, then the permanent store. Return `-1` if the key is absent or if the most recent visible operation on it is a deletion.
- `delete(key)` — delete `key` in the current context. Inside a transaction the deletion shadows any outer/permanent value (so `get` returns `-1`) but only takes permanent effect if committed. (Result: `None`.)
- `begin()` — open a new transaction, nested inside the current innermost one if any. (Result: `None`.)
- `commit()` — if no transaction is open, return `False`. Otherwise merge the innermost transaction's changes (sets and deletions) into its parent transaction — or into the permanent store if it is the outermost — close it, and return `True`.
- `rollback()` — if no transaction is open, return `False`. Otherwise discard the innermost transaction's changes, close it, and return `True`.
Reads and writes issued outside any transaction apply directly and permanently.
### Example
```
operations = ["VersionedKVStore", "set", "get", "begin", "set", "get", "begin", "delete", "get", "rollback", "get", "commit", "get", "rollback", "get"]
args = [[], ["a", 1], ["a"], [], ["a", 2], ["a"], [], ["a"], ["a"], [], ["a"], [], ["a"], [], ["a"]]
returns [None, None, 1, None, None, 2, None, None, -1, True, 2, True, 2, False, 2]
```
set a=1 permanently (get->1). T1: set a=2 (get->2). Nested T2: delete a shadows the outer value (get->-1). rollback T2 restores T1's 2. commit T1 makes a=2 permanent. The final rollback has no open transaction, so it returns False and a stays 2.
Constraints
- At most 10^5 operations in total.
- Keys are non-empty lowercase strings of length at most 20.
- 0 <= value <= 10^9.
- Transaction nesting depth will not exceed 10^3.
- get must return -1 when a key is absent or when its most recent visible operation is a deletion.
- commit/rollback with no open transaction must return False (or the string "false" in the C++ template) and leave the store unchanged.
Examples
Input: (["VersionedKVStore", "set", "get", "begin", "set", "get", "begin", "delete", "get", "rollback", "get", "commit", "get", "rollback", "get"], [[], ["a", 1], ["a"], [], ["a", 2], ["a"], [], ["a"], ["a"], [], ["a"], [], ["a"], [], ["a"]])
Expected Output: [None, None, 1, None, None, 2, None, None, -1, True, 2, True, 2, False, 2]
Explanation: The problem's worked example. set a=1 permanently (get->1). T1: set a=2 (get->2). T2: delete a shadows the outer value (get->-1). rollback T2 restores T1's 2. commit T1 makes a=2 permanent. Final rollback with no open transaction returns False; a stays 2.
Input: (["VersionedKVStore", "set", "begin", "delete", "commit", "get"], [[], ["x", 5], [], ["x"], [], ["x"]])
Expected Output: [None, None, None, None, True, -1]
Explanation: A deletion made inside a transaction takes permanent effect when committed: after commit, x is truly removed from the store, so get returns -1.
Hints
- Model open transactions as a stack of dictionaries (layers). set/delete only ever touch the top layer; when no layer is open, they touch the permanent store directly.
- Represent a deletion inside a transaction as an explicit tombstone (map the key to a sentinel such as None) so it can shadow a value from an outer layer or the store. Absence of the key in a layer means 'defer to the layer below'.
- get walks the stack top-to-bottom, returning at the first layer that contains the key (yielding -1 if that entry is a tombstone), and falls back to the permanent store.
- commit pops the top layer and replays each of its entries onto the layer below (or the store if it was the outermost): a real value overwrites, a tombstone deletes. rollback just pops and discards. Both return False when the stack is empty.