Reliability

Two additional elements help ensure E-Maj reliability: internal checks are performed at key moments in the life of table groups, and event triggers can block some risky operations.

Internal Checks

When a function is executed to start a table group, set a mark, or roll back a table group, E-Maj performs checks to verify the integrity of the table groups being processed.

These table group integrity checks verify that:

  • The PostgreSQL version at table group creation time is compatible with the current version.

  • Each application sequence or table of the group still exists.

  • Each table of the group has its log table, log function, and triggers.

  • The log tables’ structure always reflects the related application tables’ structure and contains all required technical columns.

  • For each table, the generated columns list is unchanged.

  • For ROLLBACKABLE table groups, no table has been altered as UNLOGGED.

  • For ROLLBACKABLE table groups, application tables have their primary key, and their structure has not changed.

By using the emaj_verify_all() function, the administrator can perform the same checks on demand for all table groups.

Event Triggers

Installing E-Maj adds two event triggers of type sql_drop:

  • emaj_sql_drop_trg blocks drop attempts of:

    • Any E-Maj object (log schema, log table, log sequence, log function, and log trigger).

    • Any application table or sequence belonging to a table group in LOGGING state.

    • Any primary key of a table belonging to a ROLLBACKABLE table group.

    • Any schema containing at least one table or sequence belonging to a table group in LOGGING state.

  • emaj_protection_trg blocks drop attempts of the emaj extension itself and the main emaj schema.

Installing E-Maj also adds an event trigger of type table_rewrite:

  • emaj_table_rewrite_trg blocks any structure change of application or log tables.

It is possible to deactivate and reactivate these event triggers using two functions: emaj_disable_protection_by_event_triggers() and emaj_enable_protection_by_event_triggers().

However, the protections do not cover all risks. In particular, they do not prevent:

  • Any table or sequence renaming.

  • Any schema change.

Additionally, some other DDL statements that alter table structures do not fire any trigger.

Note that these event triggers may not exist when the E-Maj extension has been installed by a non-SUPERUSER role.