System requirements

The employees have a schedule of shifts they need to honor.
The employee can request shift swap for 1 day for other employees from same company.
Other users can accept the shift swap if they have almost the same position within the company. They can trade the swift or they can accept to work if they are on their free time.
The administrator or the employee manager accepts the trade.
The system should notify the employees about accepted trades.

The system should be scalable since will be used by 1000 large companies world wide which can own 10K
The swaps should be durable, once they are registered and accepted.
The system should be consistent, not allowing employee from other company to accept trade, or employee on other qualifications
The system should be available to for the users to see shift details (list of employees)
The system should be secured since the users from other companies cannot access the trades of other companies.