You can use load balancers to ensure the functional availability of each tier in your architecture, providing high availability of the entire end-to-end system. Load balancers can be either a dedicated hardware appliance or a strictly software solution.
Load balancing is the best way to avoid a single application instance, server, or network as a single point of failure while at the same time improving the performance of the service. One of the primary goals of load balancing is to increase horizontal capacity of a service. For example, with a directory service, load balancers increase the aggregate number of simultaneous LDAP connections and LDAP operations per second that the directory service can handle.