Release 10.1A: OpenEdge Data Management:
Database Administration

> ## Introduction to recovery mechanisms

> The OpenEdge RDBMS has three kinds of recovery mechanisms:

> - Crash recovery — Uses primary recovery (BI) data to recover from system failures.

> - Roll-forward recovery — Uses backups and after-image data to recover from media failures.

> - Two-phase commit — Ensures that transactions occur consistently across multiple databases.

> Depending on your site requirements, you might choose not to implement all three of these recovery mechanisms. Figure 6–1 shows the order of precedence of these mechanisms. Crash recovery requires use of a recovery (BI) log and occurs without any interaction. Roll-forward recovery requires use of an after-image (AI) log. Two-phase commit requires use of a transaction log (TL). If you use two-phase commit, be sure to also use after-imaging.

> Figure 6–1: OpenEdge recovery mechanisms

> Each mechanism relies on notes that are written to a file to record database changes. A note is a record of the smallest unit of change in a database. For example, a record of one change made to one block in the database. The database engine automatically records database changes as notes in the primary recovery (BI) log. If after-imaging is enabled, it also records notes to the after-image (AI) log. If two-phase commit is enabled, it also records transactions and notes to the transaction log (TL).

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