You can use the standalone enqueue server as part of a high availability (HA) solution.
The HA enqueue server consists of the standalone enqueue server and an enqueue replication server. The replication enqueue server runs on another host and contains a replica of the lock table (replication table).
All clients and the replication server too are connected to the standalone enqueue server.
In the case of AS ABAP each work process has a separate connection to the standalone enqueue server. With AS Java each server process in the JEE cluster has a separate connection to the standalone enqueue server.
The standalone enqueue server communicates with its clients through port sapdpXX (XX is the instance number of the SCS/ASCS instance). Since an AS ABAP instance also communicates with its clients via this port, the standalone enqueue server must be running in a separate instance, whose instance number is not yet assigned on the host.
Connection Between Enqueue Server and Replication Server
The enqueue server opens the port sapgwXX (or enque/encni/repl_port) to wait for the connection from the replication server. If there is already a connection from the replication server, the enqueue server rejects any further connections, the message "Connect received, but already connected" is written to dev_enqrepl. The message "Connect from host lw1006.wdf.sap.corp" indicates which host the problem is coming from.
To write this message, the IP address must be converted into the host name. While the host name is being resolved the enqueue server is idle. If the name change is taking too long due to an incorrect network configuration, you can specify in parameter enque/encni/no_addr_resolve that the IP address is used instead of the host name.
Enqueue Server Failure
If the standalone enqueue server fails, it is started by the HA software on the host on which the replication server is running. The replication table stored on the replication server is transferred to the standalone enqueue server and the new lock table is created from it. The HA software also ensures that while the standalone enqueue server is out of action, clients' connection attempts go through host B instead of host A.
Replication Server Failure
If the replication server fails, it can be restarted on a different host. It retrieves the replication table from the standalone enqueue server when it restarts. When the replication server is running it is supplied only with delta information arising from each request to the enqueue server.
Installation and Configuration
The enqueue server is installed on two physical hosts (host1 andhost2, which have the same virtual host name (hahost). The SID (SAP system ID) of the system is called PLL in the graphic.
The enqueue server is installed with the installation tool SAPinst. After installation there are checks and configuration options, which are described in this documentation. The documentation for your HA software provided by your hardware partner is very important.
The sections below contain more details: