# Elastic Load Balancer (ELB) Architecture / Types ### **What is ELB?** * Elastic Load Balancer (ELB) is like a traffic police for your AWS applications. * Imagine you have 3 servers running your website. If 1,000 users come at once, ELB makes sure the load is spread across the servers evenly, so no single server crashes. * If one server goes down, ELB automatically stops sending traffic to it β€” this is high availability. πŸ‘‰ In short: ELB = traffic manager + health checker + auto-scaler helper.
### **How ELB Works** 1. Client makes a request (e.g., visiting your website). 2. DNS points to the ELB (not directly to servers). 3. ELB checks which servers are healthy (via health checks). 4. ELB forwards request to a healthy server. 5. Server responds β†’ ELB sends back the response to the client. ### **ELB Architecture** Think of ELB as a middle layer between clients (users) and your backend (EC2, containers, etc.). The main components are: * Clients β†’ users sending requests * Load Balancer (LB) β†’ the managed AWS service (has listeners, nodes, DNS name) * Target Group(s) β†’ where the traffic goes (EC2, Lambda, IPs, containers) * Targets β†’ the actual servers/containers * Availability Zones (AZs) β†’ multiple data centers for redundancy ### **ELB Components** Listener 🎧 * The β€œentry point” for traffic into ELB. * Defines protocol + port (e.g., `HTTP:80`, `HTTPS:443`, `TCP:25`). * Has rules β†’ decides what to do with the request. πŸ‘‰ Example: * Rule: If URL = `/images/*` β†’ send to Image Target Group * Rule: If URL = `/api/*` β†’ send to API Target Group Listener Rules * Conditions + Actions. * Conditions β†’ Path, Hostname, HTTP headers, Query strings. * Actions β†’ Forward, Redirect, Fixed response, Authenticate. πŸ‘‰ Example: * `Host = api.example.com` β†’ Forward to API target group. Target Group 🎯 * A container for your servers (targets). * Defines how ELB routes traffic to your backend. * Can contain: * EC2 instances * Containers (ECS/EKS) * IP addresses * AWS Lambda functions * Each target group is associated with protocol, port, and health checks. πŸ‘‰ Example: * Target Group 1 β†’ EC2s on port 80 * Target Group 2 β†’ Lambdas for serverless requests Targets (The Servers) * The actual backend resources (EC2, Lambda, containers, IPs). * Each target is registered with a target group. * ELB distributes traffic across these targets. ### Load Balancer Nodes * AWS deploys LB nodes inside the Availability Zones (AZs) you enable. * Each AZ you add β†’ ELB spins up a node in that AZ. * Clients connect to the DNS name of ELB, which resolves to the closest node. * Nodes then route traffic to healthy targets in registered AZs. πŸ‘‰ Think of nodes as workers inside AWS data centers doing the balancing ### Availability Zones (AZs) * You can enable 2 or more AZs for your ELB. * Each AZ gets its own LB node. * With Cross-Zone Load Balancing, ELB can distribute traffic across all healthy targets in all enabled AZs. ### Health Checks 🩺 * ELB constantly checks if targets are alive. * You define: * Protocol (HTTP, TCP, HTTPS) * Path (`/health`) * Interval (time between checks) * Threshold (how many failures before marking unhealthy) πŸ‘‰ Example: * Health Check = `HTTP:80 /health` * If EC2 doesn’t respond β†’ ELB stops sending traffic there. ### DNS Name 🌐 * Each ELB gets an AWS-provided DNS name (e.g., `myapp-123456.elb.amazonaws.com`). * Clients connect to this DNS name. * Never point clients directly to IPs (they change). ### Security with ELB * Security Groups β†’ control inbound/outbound traffic for ELB. * AWS WAF β†’ can be attached to ALB for advanced filtering. * IAM Certificates β†’ for SSL/TLS on HTTPS listeners. ### **ELB Traffic Flow (Step-by-Step)** 1. Client Request β†’ User opens `myapp.com`. 2. DNS Resolution β†’ Domain resolves to ELB’s DNS name. 3. LB Node Receives Request (closest AZ). 4. Listener Evaluates protocol & rules. 5. Forward to Target Group based on rule. 6. Target Selected using load balancing algorithm (Round Robin, Least Connections, etc.). 7. Health Check Verified β†’ only healthy targets get traffic. 8. Response Returned β†’ Target replies β†’ LB node β†’ Client. Advanced * Sticky Sessions β†’ Keeps user bound to same target. * SSL Offloading β†’ ELB handles encryption. * Connection Draining (Deregistration Delay) β†’ Lets existing connections finish before shutting down a target. * X-Forwarded-For Header β†’ Preserves client IP (since traffic comes via ELB). * Idle Timeout β†’ Configurable for connections (default 60s). * Access Logs β†’ Can store request logs in S3. * Deletion Protection β†’ Prevents accidental deletion of ELB. ### Quick Real-Life Analogy Imagine an airport system: * Listener = Check-in desk (decides who goes where). * Rules = Airline desks (if ticket says "Delta" β†’ go to Delta gate). * Target Group = Boarding gates (each gate handles passengers for one flight). * Targets = Airplanes (actual backend servers taking the load). * Nodes = Airport staff (manage passenger flow). * Health Checks = Maintenance check on planes (if plane is broken, don’t board passengers). * DNS Name = Airport address (people just know β€œHeathrow”, not every gate number). Internet-facing LB
Internal LB
Cross-Zone LB
Key Points
``` /27 Subnet is recommended by AWS but /28 Also works Fine ``` ### **Load Balancer Consolidation**
Types of ELB 1. Application Load Balancer (ALB) * Best for websites & apps. * Works at Layer 7 (Application layer) β†’ understands HTTP/HTTPS. * Can do smart routing: * `/images` β†’ server A * `/api` β†’ server B * Supports modern apps: containers (ECS/EKS), microservices.

2\. Network Load Balancer (NLB) * Works at Layer 4 (Transport layer) β†’ understands TCP, UDP, TLS. * Super fast ⚑ β†’ best for millions of requests per second. * Used for gaming servers, VoIP, financial apps where performance is key.
ALB Vs NLB
3. Gateway Load Balancer (GWLB) * Special one for third-party appliances (firewalls, IDS/IPS, packet inspection). * Routes traffic to security tools before reaching your servers. 4. Classic Load Balancer (CLB) * The old version (Layer 4 & 7). * Mostly outdated β†’ use ALB or NLB instead.