Elevator system design

Let's first start with the use cases or requirements of the elevator. There are multiple modules/ systems involved in complete elevator system. We will break down the uses cases as per the individual system.

High Level Requirements:

1. User should able to request the elevator from the floor.
2. Once elevator reaches the requested floor, User should get in and should able to choose destination floor.
3. Once destination is chosen by user, elevator should move to destination floor. 
4. Once elevator reaches destination floor, user leaves elevator.

User

• Presses the button on particular floor to request the elevator. 
• Inside elevator cabin (referred as Cabin here after) user presses the required floor button.
• Leaves the cabin once elevator reaches particular floor.

Floor

• Floor has ID which indicates which floor it is.
• Floor has buttons to request the elevator.
• Floor has a display which indicates current location of the lift.

Buttons

• Button get illuminated when pressed indicating the action is performed.
• Button should stop illuminating once action is complete. 
• When pressed, button places request for elevator for particular floor.

Cabin

• Cabin can move up or down as requested by controller.
• Cabin has set of buttons which tells user which all floors elevator can go.
• Cabin has button to close/open door.
• Cabin has door.
• Cabin commands door to open.
• Cabin commands door to close.
• Cabin has weight limit, which indicate how many passengers it can carry.

Door

• Door can be opened or closed as requested.
• Once door is open it closes automatically after certain delay.

Elevator Controller

• Elevator controller start up or stops the elevator.
• Elevator controller has status of the elevator.
• Elevator controller accepts request from floor and cabin buttons.
• Elevator controller commands cabin to move to particular floor.
• Elevator controller commands cabin to stop.
• Elevator controller updates status to each floor.

Elevator Request

• Stores all elevator requests.
• Has a scheduling algorithm which decides, which request should be served next.
• Each request has direction, floor id. 

Elevator Status

• Cabin State
• Current floor
• Directions

Cabin State

• Moving
• Idle
• Under Maintenance

Direction

• Up
• Down

Now we have got the requirements and also modeled in different artifacts, we can build the UML diagram of the same.