At the time of writing, there are 3 types of Elastic Load Balancer: CLB (Classic Load Balancer), ALB (Application Load Balancer) and NLB (Network Load Balancer). AWS releases CLB first, then ALB, thats why CLB sometimes is referred as ELB-V1, and ALB is referred as ELB-V2. Then NLB comes as the latest release.
From my point of view, the reason to have 3 types of ELB is that AWS was initially trying to do both TCP (L4) and HTTP (L7) load balancing in CLB/ELB-V1. But for some technical limits, it could not do it well – both L4 and L7 comes with limited functions. Then AWS decided to split it into two types: http/https only and tcp only (as Jobs said less is more!). That’s why it ends up having 3 types ELB.
Here is my guess: CLB will be end of life sooner or later as there are increasing demands of http/2. ALB and NLB will be the two types of ELB eventually. ALB will be the product for http/https (L7) load balancing. It works sort of like Nginx or HAProxy. NLB is focus on tcp (L4) load balancing, technically it should be something like LVS.
One good use case of ALB is to consolidate CLB, here is a real example that I have done.
|Feature||Application Load Balancer||Network Load Balancer||Classic Load Balancer|
|Protocols||HTTP, HTTPS||TCP||TCP, SSL, HTTP, HTTPS|
|Connection draining (deregistration delay)||✔||✔||✔|
|Load Balancing to multiple ports on the same instance||✔||✔|
|IP addresses as targets||✔||✔|
|Load balancer deletion protection||✔||✔|
|Configurable idle connection timeout||✔||✔|
|Cross-zone load balancing||✔||✔|
|Server Name Indication (SNI)||✔|
|Back-end server encryption||✔||✔|
|Elastic IP address||✔|
|Preserve Source IP address||✔|