🔁 Load Balancer Fundamentals

Abstract

🌐 📋 Load balancers spread incoming traffic across multiple backends to maximize availability, throughput, and resilience.

Overview

• Prevents single-node overload and removes single points of failure.
• Improves latency by routing to healthy, nearby, or least-loaded instances.
• Enables rolling deploys, blue/green releases, and maintenance without downtime.

Why Use a Load Balancer?

• Scalability: horizontally add or remove compute without impacting clients.
• Availability: continue serving traffic when instances fail.
• Performance: route requests using health, latency, or customized policies.

Note

🔵 ✏ Without a load balancer, a surge of traffic can overload a single server, and any failure becomes service-wide.

Core Algorithms ⚙️

Strategy	Mechanism	Best For	Drawbacks
Round robin	Rotate through backend list	Homogeneous servers	Ignores load differences
Weighted round robin	Prefer nodes with higher capacity weights	Mixed instance sizes	Static weights; no live feedback
Least connections	Pick node with fewest active sessions	Long-lived connections	Needs accurate connection tracking
Least response time	Choose node with fastest current latency	Dynamic response environments	Requires continuous measurements
IP hash	Hash client IP to select backend	Session affinity	Can create uneven load distribution
Random	Randomly pick servers	Large pools with similar health	No awareness of hot nodes
Adaptive/dynamic	External metrics (CPU, queue depth) inform routing	Highly variable workloads	Complex to implement and tune

Stateful vs. Stateless Balancing 🧭

Type	How It Works	Pros	Cons
Stateful	Balancer tracks session/user state	Seamless affinity	Harder to scale/replicate balancer
Stateless	Each request routed independently	Easy to scale horizontally	Needs external store for session data

Tip

🌐 🔥 Prefer stateless load balancers with sticky-session cookies or consistent hashing when affinity is required.

Layer 4 vs. Layer 7

🔒 Layer 4 Load Balancing

• Definition: Operates at the transport layer of the OSI model.
• Function: Directs traffic based on IP addresses and ports.
• Advantages: Simple and fast due to minimal data inspection.
• Disadvantages: Lacks ability to make decisions based on application-specific data.

🌐 Layer 7 Load Balancing

• Definition: Operates at the application layer of the OSI model.
• Function: Directs traffic based on HTTP headers, cookies, and request content.
• Advantages: Supports sophisticated traffic management such as routing by URLs or user agents.
• Disadvantages: More complex and slower due to deep packet inspection.