What is WAL Fallback? Describe the architecture.

WAL Fallback is a data recovery mechanism that uses Write-Ahead Logging (WAL). In systems with WAL architecture, all changes are first recorded in a log, ensuring reliability and rollback capability in case of failures.

The architecture typically includes:

• Main data storage (e.g., a database).
• WAL — a sequential log of changes recorded before applying changes to the main storage.
• Recovery mechanism that reads the WAL upon failure and restores the database to the last consistent state.

Fallback in this context means that in case of a primary system failure, data is rolled back or restored from the WAL, preventing data loss and maintaining integrity.

Here's how it generally works:

1. Operation is written to the WAL.
2. Operation is applied to the database.
3. In case of failure — the WAL is read and operations are reapplied for recovery.

This ensures reliability and fault tolerance.