Installing the Standalone Enqueue Server 
Procedure
Install (A)SCS Instance
To install the (A)SCS instance (standalone enqueue server and message server), use installation program SAPinst. For more information see the SAP Service Marketplace under http://service.sap.com/instguides in the installation guide:
Here you choose your operating system, your database, and the installation option (ABAP only, ABAP+Java, Java only).
ABAP only: If you choose the HA installation, SAPinst installs an ASCS instance (ABAP Central Services) that contains the standalone enqueue server and the message server.
NoteNormally, a classical central instance with the enqueue work process is installed (no standalone enqueue server).
End of the note.ABAP+Java: With AS Java an SCS instance is installed by default (SAP Central Services). With AS ABAP, the same applies as with ABAP only.
Java only: With AS Java an SCS instance is installed by default.
Changing the Existing Installation
You can also later change the configuration of a classical ABAP central system (with enqueue work process) into a system with an ASCS instance. Here you use the option SAPInst Reinstall Central Instance. You can then make this a high availability system.
Installing the Enqueue Replication Server
If you want your system to be a high availability solution, you also have to install and configure the replication server. You also use SAPinst for this. You can find the description in the installation guide mentioned above.
Post-Installation Checks
After you have installed the replication server, you should carry out some tests to ensure that it is running correctly.
The procedure for UNIX platforms is described in the section, Replication Server: Check Installation. For Windows you can find the post-installation checks in the installation guide.
Optional Configuration
Depending on your HA solution there are configuration options that are not selected automatically during installation.
More information: Configuring the Replication Server
Start and Stop Order
Caution
If the enqueue server and the replication server are running on the same host, a problem may arise if they are shut down normally and restarted.
The shutdown and restart must occur in the correct order.
Stop Order
Stop the enqueue server
Stop the replication server
This ensures that the enqueue server does not receive any more requests if the replication server is no longer running, and therefore no requests can get lost.
Start sequence
Start replication server
Start enqueue server
This ensures that the replication server deletes its old replica first (that is, it cleans up its shared memory), and therefore unwanted replications are avoided.
Note
The problem only occurs on Unix/Linux, since here the shared memory still exists after the processes belonging to it have finished.