This post will go into the modes used in the SAP HANA system replication but first, what is system replication in SAP HANA.

SAP HANA replication is the process of copying or replicating an SAP HANA database to another location which can be either an external data center or an on-premises database. The SAP database can simultaneously and continually synchronize with the primary or source system besides replication. Hence, when registering a secondary database to replicate a HANA database, a replication mode has to be selected to carry out the system replication. The goal of system replication is mainly to support high availability and data recovery.

Here are the four modes in SAP Hana replication.

· Synchronous In-Memory Mode: This is the default replication mode where the primary node waits for a confirmation from the secondary node that the log has been successfully received. Till such time no transactions take place in the primary node system. In a nutshell, the secondary system sends an acknowledgment to the primary system that its in-memory has received the data. Both the nodes are in the same data center or close proximity and must be in ACTIVE status. This SAP Hana replication mode is ideal for disaster recovery.

· Synchronous Mode: In this form of system replication, the primary mode will not execute its transactions and halts them till such time an acknowledgment is received from the secondary node. It indicates the persistence of log volumes on its disk as well as the acceptance of the data log. The main benefit of this mode in SAP Hana replication is that it makes sure of data consistency between the two nodes. The primary node ensures that it will not commit any transactions before the secondary node completes the replication.

· Synchronous full sync mode: The full sync mode ensures complete protection of data. All transactions on the primary node are blocked until the secondary node completes the replication and sends an acknowledgment that it has received the data logs and persisted the log volumes on the disk. This node is ideal for multi-tier system replication setups as the main concern is optimized data security and protection.

· Asynchronous mode: Here, the primary and the secondary nodes work asynchronously and the primary mode does not stop its transactions if an acknowledgment or confirmation is not received from the secondary mode. Instead, the primary node commits a transaction that has been written on the log file of the primary system which is sent to the secondary node. Later, the redo log buffers are sent to the secondary system asynchronously. This is the best SAP Hana replication mode in performance as there are no transaction delays and a high level of data consistency is maintained throughout the process. However, this mode is more vulnerable to data loss because of unexpected failovers and unplanned takeovers.

These are the four modes in SAP Hana replication.