Enqueue Server (also known as Lock Server) is the SAP system component that manages lock tables.
Even in a distributed SAP system, there is only one Enqueue server in the system. The server has several installation options:
- Configured as an Enqueue Work Process in an ABAP Application Server instance. The instance is called
Central instance.
This installation option is primarily intended for older SAP systems that contain only the ABAP technology stack and ABAP application servers that contain only a single instance.
- Installed as part of a separate instance. In this deployment model, the Enqueue server is installed as a separate physical server.
Together with the message server, this server becomes an SCS instance (SAP, Central Services - AS Java) or an ASCS instance (ABAP Central Services - AS ABAP). This means that both the SCS and the ASCS are single points of failure and must be highly available.
The Enqueu server receives the lock request and checks the lock table to determine if the lock request conflicts with an existing lock. If yes, the Enqueue server will reject it. If there are no conflicts, the Enqueu server sets the lock and creates an entry in the lock table.
When the Enqueu server is restarted, the lock will be lost if it is not saved in a backup file on the hard disk. During COMMIT WORK after a CALL FUNCTION … IN UPDATE TASK, those locks passed to the update task are stored in this file. The backup file is saved while the update request is active (in COMMIT WORK). Saved lock entries are reloaded into the lock table each time the queuing server is restarted.
If Enqueue Replication Service is used as part of a high availability solution, locks are not lost even if the Enqueu server fails or is restarted.
When a standalone Enqueue server is used for an SCS or ASCS instance, the ABAP worker process and the Java server process communicate directly with the Enqueue server.
In a traditional ABAP system with a central instance and multiple dialog instances, lock requests are passed through the Dispatcher and the message server to the central instance's Dispatcher, which forwards the request to the enqueue worker process. Worker processes on the central instance have direct access to lock table functions. This means they don't have to send lock requests through the scheduler and message server.
The above diagram shows the communication paths in a distributed SAP system with one central instance and other instances.