Chapter 1. Introduction to SAP NetWeaver or SAP S/4HANA High Availability

An SAP NetWeaver or S/4HANA environment as a high-availability system consists of a set of different instances. The central services instance (SCS or ASCS) and the database instances are single points of failure. Therefore, it is important that you configure an HA solution to protect these instances to avoid data loss or corruption and unnecessary outages of the SAP system.

For the application servers, the enqueue lock table, which is managed by the Standalone Enqueue Server in the ASCS instance, is the most critical component. To enhance resilience, the Enqueue Replication Server (ERS) keeps a backup copy of the Standalone Enqueue Server’s lock table. In an HA setup the ASCS instance runs on a different server than the ERS instance to keep the lock table and its backup separate.

The Standalone Enqueue Server 2 and Enqueue Replicator 2 are improved versions of the classic enqueue replication components.

Standalone Enqueue Server (ENSA1)

SAP NetWeaver and SAP S/4HANA older than ABAP Platform 2020 support enqueue replication as ENSA1. When there is an issue with the ASCS instance in an ENSA1 setup, it is required that the ASCS instance recover next to the ERS instance. That means an HA cluster must start the ASCS instance on the host where the ERS instance is currently running. Using the classic enqueue replication components, this is necessary to restore the enqueue lock table from the backup managed by the ERS instance into the ASCS instance. See The SAP Lock Concept and Standalone Enqueue Server for more information on how the Standalone Enqueue Server (ENSA1) works.

Standalone Enqueue Server 2 (ENSA2)

Starting with SAP NetWeaver 7.52 and S/4HANA, the ENSA2 setup is supported. Starting with S/4HANA 1809, ENSA2 is the default installation. Contrary to the ENSA1 setup, the new Standalone Enqueue Server 2 does not have to follow the Enqueue Replicator 2 anymore. This means that the HA cluster can start the ASCS instance on any available cluster node, no matter on which node the ERS instance is running. The new version is able to restore the lock table through the network connection between the ASCS and ERS instances. Using the ENSA2 configuration enhances the flexibility and allows the configuration of more than two HA cluster nodes for even higher resiliency. For more information on ENSA2, see SAP Note 2630416 - Support for Standalone Enqueue Server 2. Using SAP S/4HANA, you can also configure a cost-optimized HA cluster. In such a setup you configure the cluster for managing the HANA system replication as well as managing the ASCS and ERS instances. For more information, see Configuring a Cost-Optimized SAP S/4HANA HA cluster (HANA System Replication + ENSA2) using the RHEL HA Add-On.

Multi-SID support

You can manage the ASCS/ERS instances for multiple SAP environments (Multi-SID) within the same HA cluster. In this case you must take additional considerations into account.

To avoid conflicts, you must install each pair of ASCS/ERS instances with a different SID. Additionally, each instance must have a unique instance number, even if the instances belong to different SIDs.

Ensure that each HA cluster node meets the SAP requirements for sizing to support multiple instances. You can check SAP resources like Hardware Requirements or Sizing - Helping our Customers Determine Their Hardware Requirements.

The following two packages provide the components for managing SAP application server instances in a HA cluster:

The listed packages provide the following resource agents and additional tools for your setup:

Red Hat supports the following components of the solution: