# AWS ELB (Elastic Load Balancer) # **AWS ELB (Elastic Load Balancer) Complete Guide for Beginners** When users open a website, they expect it to be fast and always available. But if all users hit only one server, that server can crash. This is where **AWS ELB (Elastic Load Balancer)** helps. ELB automatically distributes traffic across multiple EC2 instances so your application stays **fast, stable, and highly available**. --- ## **What is ELB in AWS** **Elastic Load Balancer (ELB)** is a service that sits between users and your EC2 servers. Its job is simple: > It receives traffic from users and sends it to healthy servers. Instead of sending all traffic to one EC2, ELB spreads it across many EC2 instances. --- ## **Why ELB is Needed** Imagine you have: * One EC2 running a website * 1,000 users visit at the same time The server will slow down or crash. With ELB: * You can have 5 or 10 EC2 instances * Traffic is divided among them * If one EC2 fails, ELB sends traffic to others This makes your application reliable. --- ## **How ELB Works** 1. User sends request to your website 2. Request goes to ELB 3. ELB checks which EC2 is healthy 4. ELB forwards the request to one of them 5. EC2 sends response back to user Users never directly talk to EC2. They talk to ELB. --- ## **Types of Load Balancers in AWS** AWS provides four types of load balancers. --- ### **1\. Application Load Balancer (ALB)** Used for: * Web apps * APIs * Microservices It works at **Layer 7 (HTTP/HTTPS)**. It can: * Route traffic based on URL * Route based on hostname * Support containers and Kubernetes --- ### **2\. Network Load Balancer (NLB)** Used for: * Very high traffic * TCP and UDP apps * Low latency systems It works at **Layer 4** and is extremely fast. --- ### **3\. Gateway Load Balancer** Used for: * Firewalls * Security tools * Traffic inspection Mostly used in advanced networking. --- ### **4\. Classic Load Balancer** Old version. Not recommended for new applications. --- ## **What is Target Group** ELB does not directly send traffic to EC2. It sends traffic to a **Target Group**. A Target Group can contain: * EC2 instances * IP addresses * Containers ELB checks health of targets before sending traffic. --- ## **Health Checks** ELB constantly checks if your servers are working. If an EC2: * Stops responding * Returns error ELB removes it from traffic. When it becomes healthy again, ELB adds it back. This gives automatic failure handling. --- ## **Real Example** You have: * 3 EC2 running a website * One ALB in front Users hit: ```plaintext www.myapp.com → ALB → EC2 ``` If one EC2 crashes: * ALB sends traffic only to the other two * Users never notice the failure --- ## **Creating an ELB (Simple Flow)** 1. Open EC2 dashboard 2. Click **Load Balancers → Create Load Balancer** 3. Choose **Application Load Balancer** 4. Select VPC and subnets 5. Create a **Target Group** 6. Register EC2 instances 7. Create the load balancer Now your app is load balanced. --- ## **ELB with Auto Scaling** ELB works perfectly with **Auto Scaling**. When traffic increases: * Auto Scaling launches new EC2 * ELB starts sending traffic to them When traffic decreases: * EC2 are removed * ELB updates automatically This gives **fully automatic scaling**. --- ## **Security with ELB** ELB supports: * HTTPS * SSL certificates * AWS Certificate Manager This means: * You can secure your website easily * ELB handles encryption --- ## **ELB vs EC2 Public IP** Without ELB: * Users connect to one EC2 IP With ELB: * Users connect to one stable DNS name * EC2 can be replaced anytime * No downtime --- ## **Benefits of ELB** * High availability * Automatic failover * Better performance * Easy scaling * Works with Auto Scaling --- ## **Final Thoughts** ELB is one of the most important services in AWS. If you are building: * Websites * APIs * SaaS products You will almost always use ELB. It makes your application **professional, scalable, and reliable**. # All About Load Balancer ## **What is a Load Balancer** A **Load Balancer** is a middle layer between users and your servers. Users never directly connect to EC2. They connect to the **Load Balancer**, and the load balancer forwards the request to one of the servers. Think of it like a traffic police. User → Load Balancer → EC2 Servers Its job is: * Distribute traffic * Avoid overload * Send users only to healthy servers --- ## **What is a Target Group** A **Target Group** is a group of backend servers. These can be: * EC2 instances * IP addresses * Containers The load balancer does not send traffic directly to EC2. It sends traffic to a **Target Group**, and the target group contains your EC2s. So the flow is: User → Load Balancer → Target Group → EC2 --- ## **What is Health Check** Load balancer keeps checking every target. It sends a request like: ```plaintext GET /health ``` If a server responds correctly, it is healthy. If it fails, load balancer stops sending traffic to it. This is how AWS automatically avoids broken servers. --- ## **Listener** A **Listener** tells the load balancer: * Which port to listen on * Which protocol to use Example: * HTTP on port 80 * HTTPS on port 443 When traffic comes to port 443, the listener sends it to a target group. --- ## **Rules** Rules define how traffic is routed. Example: * /api → API servers * /images → image servers * /login → auth servers This is called path based routing and is done using **Application Load Balancer**. --- ## **What is Trust Store** Trust store is used when you want **mutual TLS (mTLS)**. This means: * Client also sends a certificate * Load balancer verifies it Trust store is a place where: * You upload trusted CA certificates * Load balancer checks if client certificate is valid It is used for: * Banking apps * Internal company systems * Secure APIs --- ## **SSL Certificate in ELB** ELB supports HTTPS. You attach: * SSL certificate from AWS Certificate Manager ELB does: * Encryption * Decryption Your EC2 can run normal HTTP but users get HTTPS. --- ## **What is Sticky Session** Sticky session means: * One user always goes to same server Used when: * App stores session in memory Enabled at target group level. --- ## **Load Balancer with Auto Scaling** When traffic increases: * Auto Scaling adds EC2 * Target group updates automatically * Load balancer sends traffic to new EC2 No manual work needed. --- ## **Simple Real Flow** User opens website → Request hits Load Balancer → Listener receives it → Rule checks URL → Sends to Target Group → Target Group selects healthy EC2 → EC2 responds Everything is automatic. --- ## **Why Load Balancer is Critical** Without it: * One server crash = website down With it: * No single point of failure * High availability * Smooth traffic handling