Overview of Resilience Options

This topic gives an overview of the resilience software that is available with Reality, providing a high-level description of the features covered by this section: Transaction Handling, Transaction Logging, Rapid Recovery, Shadow Database and FailSafe.

Introducing Reality Resilience

Reality resilience is the ability of Reality to recover normal database operation after an application or system crash, without data corruption and with minimum disruption in service to database users.

The features described in this section handle different aspects of recovery following a failure. You can configure one or more resilience options to:

• Ensure data integrity.
• Guard against loss of data.
• Minimise the amount of time when the database is not available.

The features provided are:

• Transaction Handling, which keeps sets of related database updates (transactions) together. Transaction Handling ensures that the updates defined as belonging within a transaction are maintained together as a set. If a transaction is not completed, the updates made since the start of the transaction are deleted from the database and the pre-updated items are restored. This prevents a database from becoming inconsistent due to a process failing in mid-transaction.
• Transaction Logging, which logs all database changes to a dedicated log disk so that if a database is corrupted by an application or system failure, the logged changes can be re-applied to the database and data is not lost.
• Rapid Recovery, which logs recent database structural changes to disk, so that in the event of a system failure, these can be rolled back and the database restored to a valid structure within minutes.
• Shadow Database, which provides a greater level of resilience by maintaining a copy of the “live” database on separate disks, or disk partitions.
• FailSafe, which provides the greatest level of resilience by maintaining two identical databases on different systems; one operating as the live database and the other as the standby.

Two other system-based resilience features which are not described in this section are:

• Resilient disk sub-system which duplicates data to two disks, for example, mirrored disks or Raid disks. Refer to the relevant system documentation.
• Heartbeat, only available on a UNIX-based Reality system, which augments FailSafe resilience by monitoring system health and executing an automatic switch-over to the secondary in the event of a primary failure. Separate documentation is provided with this feature.