# SAA-C03 Certification: All About Elastic Load Balancer Explained

# ELB - Elastic Load Balancer

## Types of Load Balancers on AWS

* AWS has 4 kinds of managed Load Balancer
    
* Classic Load Balancer - 2009 (deprecated)
    
    * HTTP, HTTPS, TCP, SSL
        
* Application Load Balancer - 2016
    
    * HTTP, HTTPS, WebSocket
        
* Network Load Balancer - 2017
    
    * TCP, TLS, UDP
        
* Gateway Load Balancer - 2020
    
    * Operators at layer 3 (Network layer) - IP Protocol
        
* Some load balancers can be set as ***private or public*** ELBs
    

### 1. Application Load Balancer

* Layer 7 (HTTP)
    
* Support for HTTP/2 and WebSocket
    
* Support redirects (from HTTP to HTTPS)
    
* Use-cases:
    
    * Micro Services
        
    * Container-bases application (Docker & Amazon ECS)
        
* Has a port mapping feature to redirect to a dynamic port in ECS

### 2. Network Load Balancer

- Layer 4 (Forward  TCP & UDP traffic to instances)
  + Handle millions of requests per second
  + Ultra-low latency
- NLB has one static IP per AZ and supports assigning Elastic IP
- Not included in the AWS free tier
- Supports HTTP health checks as well as TCP and HTTPS

### 3. Gateway Load Balancer

- Deploy, scale, and manage a fleet of 3rd party network appliances in AWS
- Example: firewalls,...
- Operates at Layer 3 (Network Layer) - IP Packets
- Uses the GENEVE protocol on port 6081
- Combines the following functions:
  + Transparent Network Gateway - single entry/exit for all traffic
  + Load Balancer: distributes traffic to your EC2 instances

## Sticky Sessions (Session Affinity)

- It is possible to implement stickiness so that the same client is always redirected to the same instance behind a load balancer
- This works for Class, Application, Network Load Balancer
- The "cookie" used for stickiness has an expiration date you control

## Cross-AZ Load Balancing

- Application Load Balancer
  + Enable by default (can be disabled at the Target Group level)
  + No charges for inter-AZ data
- Network & Gateway Load Balancer
  + Disable by default
  + You pay charges for inter-AZ data if enable

## ELB - SSL Certificates

- The load balancer uses an X.509 certificate (SSL/TLS server certificate)
- You can manage certificates by using ***ACM*** (AWS Certificate Manager)
- You can upload your certificate alternatively
- HTTPS listener:
  + You must specify a default certificate
  + You can add an optional list of certs for multiple domains
  + Clients can use SNI (Server Name Indication) to specify the hostname they reach

### Server Name Indication

- SNI solves the problem of loading multiple SSL certificates onto one web server (to serve multiple websites)
- It is a newer protocol and requires the client to indicate the hostname of the target server in the initial SSL handshake.
- The server will then find the correct certificate, or return the default one.
- Note:
  + Only works for ***ALB & NLB, and CloudFront***
  + Does not work for CLB (older gen)

## Auto Scaling Groups (ASG)

### Scaling Policies

- Dynamic Scaling
  + Target Tracking Scaling, ex: I want the average ASG CPU to stay at around 40%
  + Simple / Step Scaling, ex: When a CloudWatch alarm is triggered
- Scheduled Scaling, ex: increase the minimum capacity to 10 at 5 pm on Fridays
- Predictive scaling: forecast load and schedule scaling ahead