Classic ELB:

  • makes routing decisions at either the transport layer (TCP/SSL) or the application layer (HTTP/HTTPs), and supports either EC2-Classic or a VPC.
  • A "Classic" ELB is designed for simple balancing of traffic to multiple EC2 instances.
  • There are no granular routing "rules" - all instances get routed to evenly, and no special routing request can be made based on specific content request from the user.

  • Classic load balancing is best used when all instances (that are being served traffic) contain the same data.

Application ELB:

  • makes routing decisions at the application layer (HTTP/HTTPs), supports path-based routing, and can route requests to one or more ports on each EC2 instance or container instance in your VPC.

  • An "application" ELB is designed for complex balancing of traffic to multiple EC2 instances using Content-based "rules".

  • Content-based rules (setup on the listener) can be configured using:

    • Host-based rules: Route traffic based on the host field of the HTTP header.

    • Path-based rules: Route traffic based on the URL path of the HTTP header.

    • This allows you to structure your application as smaller services, and even monitor/auto-scale based on traffic to specific "target-groups".

  • An application ELB also supports ECS container, HTTPS, HTTP/2, Web Sockets, Access Logs, Sticky Sessions, and AWS AWF (Web Application Firewall).

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