HW15: Virtual server rental

Problem statement

Implement a smart contract for virtual server rental.

• The contract should have exactly one owner (the server's owner).
• Clients can buy access to virtual servers for time slots of 30 minutes.
• The owner sets the price per time slot.
• Access to time slots can be transferred between clients.
• Anyone can check if a client has access to the machine at any given time.
• The owner can withdraw the profits.

Example scenario

Alice owns a server park. She creates an instance of this contract to manage access to one of her servers, setting the price per time slot to 0.01 ether.

Bob reserves the time slot between 11:00am and 11:30am this Sunday using the time slot's number since contract creation. Later, he decides to transfer his reservation to Claire. After 11:00am, Claire sends her address and a packet signed with her private key to the server. The server validates the packet, checks is she has access during the current time slot, and grants access to Claire.

Contract interface

• constructor: function ReserveVS(uint256 pricePerSlot)

  • The creator of the contract (msg.sender) is the owner.
  • pricePerSlot is the price for renting the server for a single time slot.

• reserve: function reserve(uint24 slotId) payable

  • Reserve the slotId'th time slot since contract creation. For example, if the contract was created today at 00:00, then slot number 16 would be the one from 07:30 to 08:00, while slot number 64 would be the same time slot on the next day.
  • This transaction succeeds only if the time slot has not yet been reserved, and the sender transfers the correct amount of ether.

• transfer: function transfer(uint24 slotId, address to)

  • Transfer ownership of the slotId'th time slot to address to.
  • Succeeds only if the given time slot originally belonged to the sender of the transaction (msg.sender).

• validate: function validate(address client) view returns (bool)

  • Returns true if client is entitled to use the machine at the time of the validation.
  • Can be called by anyone. (In our use case, this will be called by the server itself.)

• withdraw: function withdraw()

  • Transfer profit to the sender.
  • Only the owner of the contract can call it.

Contract skeleton

pragma solidity ^0.4.21;

contract ReserveVS {

    // <contract_variables>

    // </contract_variables>

    function ReserveVS(uint256 pricePerSlot) public {
        // TODO
    }

    function reserve(uint24 slotId) public payable {
        // TODO
    }

    function transfer(uint24 slotId, address to) public {
        // TODO
    }

    function validate(address client) public view returns (bool) {
        // TODO
    }

    function withdraw() public {
        // TODO
    }

}