High availability
Quick answer
HA handles component failure within a site (keeping the system up); disaster recovery handles loss of a whole site.
Key takeaways
- In an SAP system, the components whose loss would stop everything are the database and the central services (message and enqueue…
- A subtlety unique to SAP: the enqueue (lock) service must fail over without losing locks, or in-flight transactions break.
- Clustered central services (ASCS/ERS): with an enqueue replication server so locks survive a failover.
- Watch out: Protecting app servers but not central services/database.
The single points of failure
In an SAP system, the components whose loss would stop everything are the database and the central services (message and enqueue services, ASCS). HA design focuses on making these redundant, since ordinary application-server instances are already horizontally redundant.
HA building blocks
- Clustered central services (ASCS/ERS): with an enqueue replication server so locks survive a failover.
- Database HA: clustering or replication (e.g. HANA System Replication).
- Multiple application servers: load-balanced, so one failing is tolerated.
- Redundant infrastructure: network, storage, power.
Enqueue replication
A subtlety unique to SAP: the enqueue (lock) service must fail over without losing locks, or in-flight transactions break. The Enqueue Replication Server (ERS) exists precisely to preserve locks across a central-services failover.
HA vs DR
HA handles component failure within a site (keeping the system up); disaster recovery handles loss of a whole site. A complete strategy needs both.
Common pitfalls
- Protecting app servers but not central services/database.
- Ignoring enqueue replication, losing locks on failover.
- Never testing failover until a real outage.
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Check your understanding
Which statement is true of High availability?
- A. HA design focuses on making these redundant, since ordinary application-server instances are already…
- B. Migration in SAP usually means moving a system to a new database or platform, most importantly migrating to…
- C. HANA performance is usually excellent by design, but realising it requires good modeling and SQL: pushing…
Show answer
A. HA design focuses on making these redundant, since ordinary application-server instances are already…
Covered in the “The single points of failure” section of this lesson.
Which of these also applies to High availability?
- A. A subtlety unique to SAP: the enqueue (lock) service must fail over without losing locks, or in-flight…
- B. Tuning without measuring (ST03 first).
- C. Inactive OData service, the classic "app not working".
Show answer
A. A subtlety unique to SAP: the enqueue (lock) service must fail over without losing locks, or in-flight…
Covered in the “Enqueue replication” section of this lesson.
Which part of the Learn SAP curriculum covers High availability?
- A. Getting started
- B. SAP architecture
- C. SAP interview questions
Show answer
B. SAP architecture
This lesson sits in the SAP architecture section of the Learn SAP course.