High availability is crucial for ensuring that problems with an SAP server do not affect client machines or users. A sure way to achieve this is by incorporating clustering, load balancing and failover services in your network mix. Clustering increases the number of SAP servers in your network, load balancing spreads the workload among the available servers and failover services transfer the workload from a failed to a working server in the event of a problem. SAP Central Services, or SCS, is an essential component in an SAP clustered environment.
SCS, which resides on a separate server within the cluster environment, is the communications and synchronization director in an SAP cluster. The Message Server component in Central Services handles communications while the Enqueue Server component is in charge of synchronization tasks as well as traffic direction in the event an SAP server fails. SCS is of such great importance that when a computer on the network opens an SAP instance with the “start services” command, SCS always starts first.
The Message Server in SCS facilitates communication between SAP instances, supplies information to load-balancing components in the SAP cluster and performs some low-level load balancing tasks. As a communication facilitator, the Message Server connects, interacts and communicates directly with the Cluster Manager “middleman” on each SAP server, which then forwards information to its appropriate location. Message Server also keeps track of the tasks -- also called processes -- that each SAP server is performing and communicates availability information to load-balancing components such as the Web Dispatcher and Internet Communication Manager. Finally, Message Server helps balance the load by redirecting log-on requests to SAP servers with the greatest availability.
The Enqueue Server achieves synchronization of tasks and activities within the cluster via its lock management capabilities. The Lock concept is essential to an SAP database as it prevents request collisions and determines the order in which SAP places user requests. Essentially, the Enqueue Server manages the lock table, which is not a physical database table but rather a table that exists in main memory, receives requests for setting or releasing differing lock modes and maps the logical locks to the database.
High Availability Options
SCS plays a greater role in ensuring high availability when the Enqueue Server becomes a standalone server. This requires not only separating the Enqueue Server from the Message Server and making it a standalone component but also connecting it to client machines and a server -- called an Enqueue Replication Server -- that contains a copy, or replication, of the lock table. Then, if an intermittent or long-term problem causes the Enqueue Server to stop processing lock requests, failover transfers the workload to the replication server and work continues as normal.
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