Multiple elevator coordination in LLD - Scalability & System Analysis

Scalability Analysis - Multiple elevator coordination

Growth Table: Multiple Elevator Coordination System

Users (Building Occupants)	Elevators	Requests per Minute	System Changes
100	2-3	~50	Simple scheduling, single controller, minimal queue management
10,000	10-20	~5,000	Distributed control, request queueing, basic load balancing
1,000,000	100+	~500,000	Hierarchical control, partitioned zones, advanced scheduling algorithms
100,000,000	1000+	~50,000,000	Multi-layer distributed system, real-time analytics, predictive scheduling, fault tolerance

First Bottleneck

The first bottleneck is the centralized scheduler/controller that manages elevator requests and assigns elevators. As user requests grow, a single controller struggles to process and assign requests quickly, causing delays and inefficient elevator usage.

Scaling Solutions

• Horizontal scaling: Add multiple controllers managing subsets of elevators or building zones to distribute load.
• Partitioning: Divide the building into zones, each with its own scheduler to reduce coordination overhead.
• Caching: Cache recent requests and elevator states to reduce repeated computations.
• Load balancing: Use algorithms to evenly distribute elevator assignments and avoid congestion.
• Predictive scheduling: Use historical data to anticipate demand and pre-position elevators.
• Fault tolerance: Implement fallback controllers and health checks to maintain service during failures.

Back-of-Envelope Cost Analysis

• At 10,000 users with 10 elevators, expect ~5,000 requests per minute (~83 requests/sec).
• Each controller can handle ~1000-5000 requests per second; a single controller suffices up to ~1000 users.
• Network bandwidth is minimal since messages are small (elevator commands and status updates).
• Storage needs are low, mainly for logs and historical data (few MBs per day).
• CPU and memory scale with number of requests and complexity of scheduling algorithms.

Interview Tip

Start by defining system scale and user load. Identify the main bottleneck (usually the scheduler). Discuss how to partition the problem (zones, controllers). Explain trade-offs between centralized and distributed control. Mention caching and predictive techniques. Always justify your choices with scalability and latency in mind.

Self Check

Your database handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Add read replicas and implement caching to reduce load on the primary database before considering sharding or more complex solutions.

Key Result

The centralized scheduler is the first bottleneck as user requests grow; scaling requires distributing control across zones and elevators with predictive and fault-tolerant scheduling.

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