Working Hours Register Program - Level 4: Real-Time Clock-In/Out and Concurrent Access

Question

## Round 1 - Coding

## Problem

Extend your register to handle real-time clock-in/out events and concurrent modifications:

1. `clock_in(emp_id, timestamp)` — record start of a session; raise if the employee is already clocked in.
2. `clock_out(emp_id, timestamp)` — close the open session; raise if not clocked in.
3. `get_current_status(emp_id)` -> `"clocked_in" | "clocked_out"` with the time of last event.
4. Thread-safety: two employees can clock in simultaneously without corrupting state.

```python
import threading
from datetime import datetime

class WorkHoursRegister:
    def __init__(self):
        self._lock = threading.Lock()
        self._open_sessions: dict[str, datetime] = {}
        self._sessions: dict[str, list] = {}

def clock_in(self, emp_id: str, ts: datetime) -> None: ...
    def clock_out(self, emp_id: str, ts: datetime) -> None: ...
    def get_current_status(self, emp_id: str) -> dict: ...
```

```
clock_in("E1", 09:00) -> OK
clock_in("E1", 09:30) -> raises AlreadyClockedIn
clock_out("E1", 17:00) -> OK
get_current_status("E1") -> {"status": "clocked_out", "since": "17:00"}
```

## Follow-ups

1. Replace the global lock with per-employee locks. What does that buy you and what does it cost?
2. If the service crashes between clock_in and clock_out, how do you recover open sessions on restart?
3. How would you design an audit log of all clock events for compliance purposes?
4. Describe a race condition that could still occur even with your locking strategy.

Working Hours Register Program - Level 4: Real-Time Clock-In/Out and Concurrent Access

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