How does a load balancer work and which algorithms would you choose?
Reported in Arm European engineering loops. Infrastructure design question covering L4/L7 balancers and health checks.
Interview scenario
Often asked in Arm loops at European offices (London, Berlin, Amsterdam, Paris, Stockholm, Dublin, and remote EU). Prepare a clear spoken answer plus key trade-offs.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Arm: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
A load balancer distributes incoming traffic across healthy backend instances to improve availability, throughput, and fault tolerance.
L4 (transport): routes by IP/port—fast, no HTTP awareness. L7 (application): routes by URL path, headers, cookies—enables canary releases and sticky sessions.
Algorithms: Round robin (simple), least connections (long-lived connections), weighted (heterogeneous hardware), consistent hashing (cache locality), random with two choices (power of two).
Health checks (HTTP /health, TCP probe) remove bad nodes. Discuss SSL termination, connection draining on deploy, global load balancing with GeoDNS, and autoscaling triggers from LB metrics.
Discussion
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