## 2.1 Introduction
SAP HANA system replication is a mechanism for ensuring the high availability of your SAP HANA system.
## What is system replication?
System replication is SAP's recommended configuration for addressing SAP HANA outage reduction due to planned maintenance, faults, and disasters. It supports a recovery point objective (RPO) of 0 seconds and a recovery time objective (RTO) measured in minutes.
System replication is set up so that a secondary system is configured as an exact copy of the active primary system, with the same number of active hosts in each system. The number of standby hosts need not be identical. Furthermore, it requires a reliable link between the primary and secondary systems.
Each service of the primary system communicates pairwise with a counterpart in the secondary system. The main difference to the primary system is that the secondary system does not accept requests or queries. The secondary system can accept queries only in an Active/Active (read enabled) configuration. For more information, see SAP HANA System Replication with Active/Active (Read Enabled) .
The secondary system can be located near the primary system to serve as a rapid failover solution for planned downtime, or to handle storage corruption or other local faults. Alternatively or additionally, a secondary system can be installed in a remote data center for disaster recovery. The instances in the secondary system operate in live replication mode. In this mode all secondary system services constantly communicate with their primary counterparts, replicate and persist data and logs, and typically load data to memory. The log and data can be compressed before shipping. For more information, see Data and Log Compression .
## How does system replication work?
Once SAP HANA system replication is enabled, each server process on the secondary system establishes a connection with its primary system counterpart and requests a snapshot of the data. From then on, all logged changes in the primary system are replicated continuously. Whenever logs are persisted (meaning they are written to the log volumes of each service) in the primary system, they are also sent to the secondary system.
The following graphic illustrates the general system replication processes:
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##  Note
To prevent unauthorized access to the SAP HANA database, the internal communication channels between the primary site and the secondary site in a system replication scenario need to be protected. This may include filtering access to the relevant ports and channels by firewalls, implementing network separation, or applying additional protection at the network level (for example, VPN, IPSec). We recommend routing the connection between the two sites over a special site-to-site high-speed network, which typically already implements security measures such as separation from other network access and encryption or authentication between sites. The details of security measures and implementation of additional network security measures depend on your specific environment. For more information about network and security aspects, see the SAP HANA Master Guide and the SAP HANA Security Guide .
A transaction in the primary system is not committed before the redo logs are replicated. This is determined by the selected replication mode when setting up system replication. For a detailed description of each replication mode, see Replication Modes for SAP HANA System Replication .
If the connection to the secondary system is lost or if the secondary system crashes, the primary system (after a brief, configurable timeout) resumes operations. The way the received logs on the secondary system are handled depends on the selected operation mode. For a detailed description of each operation mode, see Operation Modes for SAP HANA System Replication .
While system replication is running, the secondary system configured identically to the primray system is on standy until a takeover takes place.
In the event of a failure that justifies a full system takeover, you switch the secondary system from the live replication mode to a full operation mode. The secondary system, which already preloaded the same column data as the primary system and possibly is already read enabled, becomes the primary system by replaying the last transaction logs and then starts to accept queries.When the original system can be restored to service, it can be configured as the new secondary system or reverted to the original configuration.
## Which other setups are possible?
Besides the above presented standard setup, in which a primary system ships all the data to the secondary system, you can also configure a multitier or a multitarget system replication.
In a multititer system replication, a tier 2 system replication setup can be used as the source for replication in a chained setup of primary system, tier 2 secondary system, and tier 3 secondary system. The primary system is always on tier 1. The replication source for the tier 2 secondary system is the primary system, while the replication source for the tier 3 secondary system is the tier 2 secondary. For more information, see SAP HANA Multitier System Replication .
In a multitarget system replication, the primary system can replicate data changes to more than one secondary system. For more information, see SAP HANA Multitarget System Replication .
## What about supported endianness?
There is no support for running one site on a little-endian hardware and the other site on a big-endian system in a system replication landscape. In SAP HANA the following endianness is supported in the corresponding SAP HANA and OS versions:
Supported Endianness
| SAP HANA Version   | Linux OS Version                        | Endianness    |
|--------------------|-----------------------------------------|---------------|
| SPS11 &SPS12       | Linux Intel SLES 11                     | little-endian |
| SAP HANA2.0 SPS00  | Linux Intel SLES 12 Linux Power SLES 12 | little-endian |
Furthermore, system replication is supported between Intel little-endian (SAP HANA SPS12 or SAP HANA 2.0 SPS 00) and Power little-endian (SAP HANA 2.0 SPS 00).
## What about license validity?
The primary system automatically replicates relevant license information to the secondary system. No additional license needs to be installed, since the primary and secondary system have the same SID. For more information about licensing in SAP HANA system replication, see SAP Note 2211663 .
When using an Active/Active (read enabled) system replication configuration, the secondary system is subject to licensing. For more information, see SAP HANA Feature Scope Description .
## Related Information
| SAP HANASystem Replication with Active/Active (Read Enabled) [page 113]   |
|---------------------------------------------------------------------------|
| Replication Modes for SAP HANASystem Replication [page 12]                |
| Operation Modes for SAP HANASystem Replication [page 14]                  |
| Data and Log Compression [page 30]                                        |
| SAP HANAMultitier System Replication [page 128]                           |
| SAP HANAMultitarget System Replication [page 142]                         |
| SAP Note 2211663                                                          |