Revealing the differences between HANA Host Auto-Failover and System Replication

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After a short break, I would like to continue SAP HANA High Availability and Disaster Recovery article series with a high level overview between the two most used high availability options in SAP HANA: System Replication and Host Auto-Failover.

Host Auto-Failover is basically an automated failover from crashed host to a standby host in the same system. In this method, SAP HANA regularly checks if all the cluster members are still active; and when an active host fails, a standby host “automatically” takes over its place. An internal cluster manager “nameserver” manages this entire failover process. Host Auto-Failover is a very reliable HA solution and provides absolute zero data loss.

System Replication is the continuous update of secondary system(s) by primary system, combined with different replication and operation modes which also provides in-memory table loading. It is a standard SAP HANA feature and can be used as an HA or DR solution. Because all data is replicated to the secondary site and data is pre-loaded into memory, the recovery time objective is reduced significantly.

The previous articles about Host Auto-Failover and System Replication contain useful detailed information you need to know about both and I believe below table would be more suitable to provide a high-level overview between two methods.

 

Syncmem: Primary node waits until data is received by secondarySync: Primary node waits until data is received and persisted by secondaryFull sync: transaction processing on the primary node is blocked until secondary system becomes availableAsync: primary node does not wait any acknowledgement or confirmation from the secondary node

	Host Auto-Failover	System Replication
Description	Cluster-like solution in the same data center	Similar to classical shadow database solution
Can be used for	High Availability	High Availability and Disaster Recovery
RPO	Zero	Zero with synchronous replication
Architecture	"N+M" HA/fault recovery solution. One or more Standby nodes, data is not preloaded.	N+N HA/DR solution. Secondary has the same number of active nodes, data is pre-loaded.
Automated fail-over	Available on the host level (the failure of a single service does not trigger the failover). Failover is internally managed by SAP HANA nameserver.	Available with external cluster management software provided by the hardware vendors.
Data pool	One data pool, shared NFS or distributed filesystem.	Multiple data pools, single or multiple data centers support.
Data consistency	Heartbeat (regular TCP communication) and I/O (isolating the failed node) fencing options	Continuous replication with different replication and operation modes
Multitenant Database Container Support	Available on the host level	Replication of the whole system, single tenant replication is not possible.
Replication	Not required as disk is shared and data consistency is already achieved	Syncmem: Primary node waits until data is received by secondary Sync: Primary node waits until data is received and persisted by secondary Full sync: transaction processing on the primary node is blocked until secondary system becomes available Async: primary node does not wait any acknowledgement or confirmation from the secondary node
Scalability	Better suited for scale-out	Better suited for scale-up
Flexibility	Rigid	Secondary node can be used for QA with additional disk

 

As you can see, both methods are quite different from architecture to failover process. I believe the best approach for HA/DR solution should start with determining business requirements properly, followed by the effective sizing of SAP HANA and purchasing the correct hardware if you are implementing on premise. During the sizing phase, you should decide which high availability, fault recovery and disaster recovery methods would meet the business requirements without breaking the bank. Detailed analysis of each method would significantly reduce the risk of wrong architecture.

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Have any questions about SAP HANA System Replication or Host Auto-Failover?

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If you liked this post, you might like these relevant posts:

Choosing the right HANA Database Architecture

The Ultimate Guide to Effective Sizing of SAP HANA

SAP HANA High Availability and Disaster Recovery Series #1

SAP HANA HA and DR Series #2: Redundancy and Fault Recovery Support

SAP HANA HA and DR Series #3: Host Auto-Failover

SAP HANA HA and DR Series #4: Storage Replication

SAP HANA HA and DR Series #5: System Replication

SAP HANA HA and DR Series #6: System Replication Operation Modes & Parameters

SAP HANA HA and DR Series #7: Log Replication Modes