The OSI stack is a great tool when working with complex designs. Every layer in the OSI stack must be considered within the design and have a purposeful answer. Designs always start at the physical layer, working up the stack from the bottom to the top. See the following diagram. Many load balancers today work at all layers of the OSI stack. Back to the question: how are multiple load balancers physically connected to multiple servers creating multiple ingress and egress paths? Multiple switches may also be required. Today many load balancers combine the port density of switches, the routing capability of routers, and the logical functions of load balancers, all in a single device simplifying designs and saving a bit of budget money.

The web layer and application layers can often be collapsed into the same server. From a security perspective, this can be an issue. If the server is compromised, both services are potentially compromised. Many designs collapse these two layers, as they are tightly integrated, and performance can significantly increase using system bus speeds instead of slower network connections and additional devices.

From single server designs to single site to single site redundant, each design builds on the one previous. The following figure adds the additional components, servers, and load balancers to illustrate a baseline architecture for single site designs with redundancy. The following redundant design collapses both web and app onto the same virtual or physical server. Load balancers are added to the design to delegate the load across multiple servers. Database servers are shown as primary-backup with replication between them. This redundant architecture can protect against issues with applications due to system unavailability and downtime. Resiliency considerations may include RAID configurations for database drives, how databases are backed up and restored, how applications and devices handle state and session information, and how databases rebuild after data or drive loss.