External Replication: Overview
In this blog, I am going to give you a conceptual overview of the new external replication feature for your High Availability and Disaster Recovery (HADR) system. The external replication feature is available from the SAP Adaptive Server Enterprise 16.0 SP02 PL05 version and onwards.
An HADR system contains two SAP ASE servers: a primary server and a standby server (DR mode) or companion (HA mode). The former is where all transaction processing takes place. The latter acts as a warm standby for the primary server, and contains copies of the primary server’s designated databases.
The HADR system includes an embedded SAP Replication Server, which synchronizes the databases between the primary and companion servers. SAP ASE uses the Replication Management Agent (RMA) to communicate with Replication Server, and SAP Replication Server uses Open Client connectivity to communicate with the companion SAP ASE.
Since the HADR system is a closed system, it can’t replicate the data in and out of the system. However, the new external replication feature lets you replicate data from an existing HADR system to an external system, and vice versa.
Why do you need External Replication?
With the external replication feature you can upgrade your existing single ASE instances to HADR clusters.
For example, here are 3 separate ASE instances in Singapore, New York, and London connected by 3 Replication Servers:
In this setup:
- Transactions entered in NY get replicated to LDN & SGP + NY_IQ
- Transactions entered in LDN get replicated to NY & SGP + NY_IQ
- Transactions entered in SGP get replicated to NY & LDN + NY_IQ
However, these single ASE instances can’t deal with data loss in a disaster. That’s where an HADR system comes in.
As replication in and out of an HADR system is possible with the external replication feature, you can now upgrade your single ASE instances by adding an HADR cluster:
The replication transactions between the 3 sites continue to work as before, but now with the added protections provided by the HADR system.
Architecture of an External Replication System
An external replication system:
- Includes all components in a replication system except the current HADR system with which you are working.
- Is an SAP Replication Server that contacts either an HADR system or an SAP ASE server.
- Treats a HADR system as a single ASE system to which it replicates data.
The external replication feature supports replication in and replication out:
|Replication In||Replication Out|
|Data flow direction||From an external system to the HADR system.||
From an HADR system to the external system by using the embedded Simple Persistent Queue (SPQ) Agent, a new component in the HADR companion server.
You can manage it using new Replication Server and Replication Management Agent (RMA) commands.
|Role of the HADR system||A target database for replication. The failover mechanism inside remains the same.||A primary database for replication. The SPQ Agent reads from the simple persistent queue and forwards messages to the external replication system.|
|Role of the external replication system||Applies any data changes to the primary ASE server in the HADR system. Details of the HADR system like routes and subscriptions are hidden from the external system.||Receives data and replicates it to the target data server.|
|Differences in a bi-directional system||Changes replicated to the HADR system are not replicated out to avoid circular replication.||Changes made inside the HADR system are replicated to the external replication system.|
For more information about some of these concepts in this blog, see: