Service Availability

Service availability means that a service is available, even when a service provider fails. Service availability is generally achieved using multiple identically configured service instances (redundancy). Redundancy eliminates single points of failure (assuming that simultaneous failure of all instances is extremely unlikely). If one instance providing a service fails, another instance is available to take over. This mechanism is known as service failover.

Service failover is supported in the reference configuration through two mechanisms:

• Load balancing. Load balancing uses redundant hardware and software components to distribute requests for a service among multiple component instances that provide the service. This redundancy provides greater capacity than would be possible with a single instance. This redundancy also means that if any one instance of a components fails, other instances are available to assume a heavier load. Depending on the latent capacity that is built into the deployment, a failure might not result in significant degradation of performance. Load balancing is used in all of the service modules in the reference configuration.

• Directory Server multimaster replication. The preferred solution for Directory Server, this mechanism provides data that is crucial to the operation of the entire deployment. Multimaster replication is specifically designed to synchronize data between the two (or more) Directory Server instances shown in the deployment architecture. Multimaster replication is the simplest directory service failover implementation and is suitable for all but the highest-end deployments that need to support millions of users.