Central Services Instance 
Central services form the basis of communication and synchronization for the AS Java cluster. They are responsible for lock administration, message exchange, and load balancing within the cluster.
Central services run on one physical machine and constitute a separate instance. This SAP Central Services Instance (SCS) comprises the message server and the enqueue server.
Note
The central services are always required when an AS Java cluster is installed. The central services instance has its own instance number.
The SAP start service (sapstartsrv(.exe)) starts the SCS instance first. When the central services are running, the other Java instances are started.
Message Server
The message server keeps a list of all server processes in the AS Java cluster and provides information about their availability to Internet Communication Manager (ICM). It also represents the infrastructure for data exchange between the participating server processes.
The message server is responsible for the following tasks in the AS Java cluster:
Notification of events that arise in the cluster, for example when a server process is started or stopped
Communication between different server processes
Forwarding of messages and requests to all participants
Preparation of logon information for the SAP Web Dispatcher
Exchange of cache information in the cluster
The message server is addressed through the Cluster Manager of one of the server processes in the Java cluster, as shown in the figure below.
Message Server Connection
More information: Cluster Communication in AS Java: Architecture
Enqueue Server
The enqueue server manages logical locks. The enqueue server runs on the Central Services instance of the Java cluster. It manages the lock table in the main memory and receives requests for setting or releasing locks. It maps the logical locks to the database.
The enqueue server has the following tasks:
Internal synchronization within the AS Java cluster.
The applications can lock objects and release locks again. The enqueue server processes these requests and manages the lock table with the existing locks.
Note
The enqueue server can be configured for high availability by setting it up with the replication server and a platform-independent high availability solution.
More information: