EasyExperience level: 0-2 years · 3-5 years1 min read

How does a load balancer distribute traffic, and what algorithms are common?

Reviewed by Gurusankar M.

Asked inAmazonInfosysCognizantWipro
#load balancing#round robin#least connections#l4 l7#health checks
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Short Answer

A load balancer spreads requests across healthy backends and removes failed ones via health checks. Common algorithms: round-robin, least-connections, and consistent-hashing (sticky by key). L4 balances by TCP/IP (fast), L7 by HTTP content (path/host routing).

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Concept Made Simple

Coffee Chat Question

“How does a load balancer distribute traffic, and what algorithms are common?”

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Mind Map Answer

Round-robin
even rotation
Least-connections
to the least-busy backend
Hash/sticky
same client/key → same backend
L4 vs L7
TCP fast / HTTP content routing

Think Beyond the Expected

What If?

Why are health checks essential to a load balancer?

Without them the LB keeps sending traffic to dead/unhealthy instances, so users get errors. Health checks let the LB detect failures and route only to healthy targets — the core of self-healing and zero-downtime deploys.

Real World

L7 load balancers (ALB/Nginx) do path/host routing and TLS termination; least-connections suits uneven request costs. Health checks + connection draining make deploys and failures invisible to users.

Interviewer's Expectation

Keywords they're listening for:

  • distribute + health check
  • round-robin/least-conn/hash
  • L4 vs L7
  • sticky sessions trade-off
  • removes unhealthy

Common Mistakes

  • No/weak health checks
  • Sticky sessions defeating even distribution
  • Single LB as a SPOF

Best Practices

  • Health checks + connection draining
  • Pick the algorithm to match request cost
  • Run the LB redundantly

Follow-up Questions

  1. When do you need sticky sessions (and why avoid them)?
  2. L4 vs L7 — trade-offs?
  3. How does the LB itself avoid being a SPOF?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Load Balancing (System Design)
Interview question: "How does a load balancer distribute traffic, and what algorithms are common?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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