( RTO) This means the acceptable delay between the incident and the return to a normal production.
It is thus essential to define the highly available systems which guarantee the delay for return in production expected by the company.
| Replication in Synchronous Mode | This mode guarantees that all writing operations performed on the site where they were generated are recorded on the backup sites. As long as the writing has not been acknowledged by the replication service, the application can send no other record. Performances are based on the slowest writing speed of each site. The delays of data propagation including on long distances should be properly evaluated. |
| Replication in Asynchronous Mode | Local and remote writing operations are separate in this mode. The replication service does not wait for the acknowledge of the remote writing to validate it in the application and accept the writing operations which follow. This mode more or less penalizes the application for writing, but is inconvenient as it has different datasets at a given time and does not guarantee data consistency. |
The traditional term of redundancy is to create an architecture called No Single Point of Failure(NSPoF). The availability of the system is guaranteed by the fact that there is no single breakdown which can cause a service unavailability. Thus, a redundancy system to support a simultaneous breakdown of the N component does not work at N+1. Therefore, this system cannot be defined as highly available as there is a subtle or permanent service unavailability.
The objective of defining a highly available information system is to allow users to work beyond the usual usage frequency of the information system. It is essential to define preventive and protection measures to limit the downtime and to implement rapid methods of restoration.
The aim of online data replication is not only to guarantee a second copy of data identical to that of production, but to guarantee the possible reconstruction and exploitation of copies which have survived the disaster. Files system or database replication tools should reflect the integrity of the data replicated so that these can be used in case of disaster. These tools should integrate mechanisms called Write Ordering or of Crash Consistency. Replication technologies can be classified in two categories:
