Ref: https://learn.cantrill.io/courses/1820301/lectures/41301438 and https://learn.cantrill.io/courses/1820301/lectures/41301441

• Summary:
• Contents:

AWS Elastic Load Balancer (ELB) - Evolution and Types

1. v1 (legacy)
   • Classic Load Balancer (CLB) → avoid its use, migrate to v2 if still using it
2. v2
   • Application Load Balancer (ALB)
   • Network Load Balancer (NLB)

• 💡 NOTE: There's a fourth type of ELB, the Gateway Load Balancer (GWLB), but it's a bit of an odd member in the ELB family, check its own dedicated section

ELBv1 (Legacy) - Classic Load Balancer (CLB)

Diagram: https://github.com/acantril/aws-sa-associate-saac03/blob/main/1500-HA_and_SCALING/00_LEARNINGAIDS/ALBvsNLB-1.png

• 🔧 Legacy ELB (introduced in 2009)
  • Balances HTTP(S) and other low-level protocols
    • 💡 But can't really be considered an OSI Layer7 ELB…
  • ‼️ 👎 Lacks modern features!! → AVOID it's use nowadays, migrate from it!
    • CLB connects directly to a single backend pool, made of backend instances/ASGs (Auto Scaling Groups). Can only distribute traffic evenly to that pool.
• Disadvantages/Limitations:
  • 👎 No SNI support → Limit of 1 SSL certificate per CLB → Scales very badly
    • If you have many domains, you will need many SSL certificates, and hence many CLBs → many CLBs can get very expensive
    • 💡 Server Name Indication (SNI) = extension to the Transport Layer Security (TLS) protocol, by which a client indicates which hostname it is attempting to connect to during TLS handshake, at Layer 4 (before encryption) → server can understand which SSL certificate the client needs → allows a server to host many SSL certificates, each with a different IP
  • 👎 NO support for target groups (just one single backend pool)
  • 👎 Only single-protocol listeners, whereas ELBv2 supports multi-protocol listeners
  • 👎 Even though it can balance HTTP(S), CLB can NOT take routing decisions based on HTTP protocol features
    • Unlike ALBs, NO support for rules (path-based routing, host-based routing, HTTP-content routing…)
    • Unlike ALBs, NO per-rule health checks

ELBv2

Diagram: https://github.com/acantril/aws-sa-associate-saac03/blob/main/1500-HA_and_SCALING/00_LEARNINGAIDS/ALBvsNLB-1.png

• 🔧 Modern ELB (introduced in 2016)
  • Two types:
    • Application Load Balancer (ALB) → L7: HTTP(S) & WebSocket protocols
    • Network Load Balancer (NLB) → L4: TCP, TLS & UDP protocols
• 👍 Faster, cheaper, support target groups
• 👍 ALBs support rules → 1 SSL certificate per rule, instead of per ELB → a single ALB can be used for multiple things (unlike CLB)

Application Load Balancer (ALB)