Why Multiple elevator coordination in LLD? - Purpose & Use Cases

The Big Idea

What if your elevator always knew exactly when and where to come, making your wait almost disappear?

The Scenario

Imagine a tall building with many floors and several elevators. Without a smart system, each elevator works on its own, picking random requests. People press buttons, but elevators might come late or go empty to floors where no one waits.

The Problem

Manually controlling elevators means slow response and confusion. Elevators may waste time going up and down unnecessarily. People get frustrated waiting too long. It's hard to keep track of all requests and assign elevators efficiently by hand.

The Solution

Multiple elevator coordination uses a smart system to assign elevators to requests. It tracks all calls and decides which elevator can serve best, reducing wait times and energy waste. This system balances loads and moves elevators smoothly like a well-organized team.

Before vs After

✗ Before

onButtonPress(floor):
  call nearest elevator
  elevator moves to floor
  no coordination between elevators

✓ After

onButtonPress(floor):
  assign elevator based on current positions and directions
  update elevator schedules
  elevators coordinate to serve requests efficiently

What It Enables

It enables fast, fair, and energy-saving elevator service that feels natural and reduces waiting frustration.

Real Life Example

In a busy office tower, multiple elevator coordination ensures people reach meetings on time by smartly directing elevators to floors with waiting passengers, avoiding empty rides and long waits.

Key Takeaways

Manual elevator control causes delays and inefficiency.

Coordinated systems assign elevators smartly to reduce wait times.

Better coordination improves user experience and saves energy.

Practice

1. What is the main goal of multiple elevator coordination in a building?

easy

A. To reduce wait and travel times for passengers

B. To increase the number of elevators in the building

C. To make elevators move randomly

D. To keep all elevators idle at the ground floor

Solution

1. Step 1: Understand elevator coordination purpose

   Multiple elevator coordination aims to improve efficiency by reducing passenger wait and travel times.

2. Step 2: Evaluate options based on goal

   Options B, C, and D do not focus on improving passenger experience or efficiency.

3. Final Answer:

   To reduce wait and travel times for passengers -> Option A

4. Quick Check:

   Goal of coordination = reduce wait/travel times [OK]

Hint: Focus on passenger experience improvement goals [OK]

Common Mistakes:

• Confusing coordination with adding more elevators
• Thinking elevators should stay idle
• Assuming random movement improves service

2. Which of the following is a correct way to assign an elevator to a new request in a multiple elevator system?

easy

A. Always assign the elevator on the ground floor

B. Assign the elevator farthest from the request floor regardless of direction

C. Assign the elevator closest to the request floor moving in the same direction

D. Assign elevators randomly to balance usage

Solution

1. Step 1: Understand assignment criteria

   Elevators should be assigned based on proximity and direction to minimize wait time.

2. Step 2: Analyze options

   Assign the elevator closest to the request floor moving in the same direction matches this logic. Options A, B, and C ignore direction or proximity, causing inefficiency.

3. Final Answer:

   Assign the elevator closest to the request floor moving in the same direction -> Option C

4. Quick Check:

   Closest elevator + direction match = correct assignment [OK]

Hint: Match elevator direction and proximity for assignment [OK]

Common Mistakes:

• Ignoring elevator direction when assigning
• Choosing elevators randomly
• Always picking ground floor elevator

3. Consider a system with 2 elevators: Elevator A at floor 3 moving up with destinations [5, 7], Elevator B at floor 6 moving down with destinations [4, 2]. A request comes from floor 4 to go up. Which elevator should be assigned?

medium

A. Neither elevator

B. Elevator B

C. Either elevator

D. Elevator A

Solution

1. Step 1: Analyze elevator positions and directions

   Elevator A is at floor 3 going up; Elevator B is at floor 6 going down.

2. Step 2: Match request direction and elevator direction

   Request is at floor 4 going up. Elevator A is below floor 4 and moving up, so it can pick up on the way. Elevator B is above floor 4 but moving down, so it cannot pick up going up.

3. Final Answer:

   Elevator A -> Option D

4. Quick Check:

   Elevator moving towards request floor in same direction = Elevator A [OK]

Hint: Pick elevator moving towards request floor in same direction [OK]

Common Mistakes:

• Choosing elevator moving away from request
• Ignoring elevator direction
• Assuming either elevator works

4. In a multiple elevator system, the controller assigns requests but sometimes an elevator gets stuck and does not update its position. What is the likely problem and how to fix it?

medium

A. Elevator state not updated; add regular position updates and health checks

B. Elevator hardware failure; replace elevator immediately

C. Controller assigns requests randomly; fix assignment logic

D. Elevator doors stuck open; fix door sensors

Solution

1. Step 1: Identify cause of stuck elevator in system

   If elevator position is not updated, controller cannot assign requests properly.

2. Step 2: Determine fix

   Adding regular position updates and health checks ensures controller has current elevator status to assign requests correctly.

3. Final Answer:

   Elevator state not updated; add regular position updates and health checks -> Option A

4. Quick Check:

   Missing updates cause stuck state; fix with health checks [OK]

Hint: Ensure elevator regularly reports position to controller [OK]

Common Mistakes:

• Assuming hardware failure without checking software updates
• Blaming random assignment logic
• Ignoring elevator state updates

5. You are designing a multiple elevator coordination system for a 20-floor building with 4 elevators. To minimize average wait time during peak hours, which strategy is best?

hard

A. Assign elevators randomly to requests to balance load

B. Divide floors into zones and assign elevators to zones dynamically

C. Let all elevators serve all floors equally without zoning

D. Keep all elevators idle at ground floor until called

Solution

1. Step 1: Understand peak hour challenges

   High traffic causes many requests; serving all floors equally can cause delays and conflicts.

2. Step 2: Evaluate zoning strategy

   Dividing floors into zones and assigning elevators reduces travel distance and wait time by localizing service.

3. Step 3: Compare other options

   Random assignment or no zoning causes inefficiency; keeping elevators idle wastes capacity.

4. Final Answer:

   Divide floors into zones and assign elevators to zones dynamically -> Option B

5. Quick Check:

   Zoning elevators reduces wait time in tall buildings [OK]

Hint: Use zoning to reduce travel distance and wait time [OK]

Common Mistakes:

• Ignoring zoning benefits in tall buildings
• Assuming random assignment balances load
• Keeping elevators idle wastes capacity