What is the difference between Layer 4 and Layer 7 load balancing in Envoy?

Layer 4 load balancing operates at the transport layer of the OSI model, which means it makes routing decisions based on the transport layer protocol information, such as IP address and TCP/UDP port numbers. This approach is primarily concerned with forwarding packets based on the connection without inspecting the content of the packets. As a result, it is generally more efficient and can handle more connections since it doesn’t require deep content inspection.

On the other hand, Layer 7 load balancing is application-aware, operating at the application layer. This means it can make routing decisions based on the content of the application data within the packets, such as HTTP headers, cookies, or the request path. This capability allows for more advanced routing strategies, such as directing traffic based on user sessions, types of requests, or the characteristics of the content being requested.

Because Layer 7 is able to inspect and interpret application data, it enables more granular control over how traffic is directed compared to Layer 4. This difference in operation is fundamental to how load balancing works at these two layers, making the distinction very important in the context of load balancing strategies used in systems like Envoy.