Log shipping involves four jobs, which are handled by dedicated SQL Server Agent jobs. These jobs include the backup job, copy job, restore job, and alert job.
The user controls how frequently log backups are taken, how frequently they are copied to each secondary server, and how frequently they are applied to the secondary database. To reduce the work required to bring a secondary server online, for example after the production system fails, you can copy and restore each transaction log backup soon after it is created. Alternatively, perhaps on a second secondary server, you can delay applying transaction log backups to the secondary database. This delay provides an interval during which you can notice and respond to a failure on the primary, such as accidental deletion of critical data.
Backup Job
A backup job is created on the primary server instance for each primary database. It performs the backup operation, logs history to the local server and the monitor server, and deletes old backup files and history information. By default, this job will run every 15 minutes, but the interval is customizable.
When log shipping is enabled, the SQL Server Agent job category "Log Shipping Backup" is created on the primary server instance.
SQL Server 2008 Enterprise and later versions support backup compression. When creating a log shipping configuration, you can control the backup compression behavior of log backups. For more information, see Backup Compression (SQL Server).
Copy Job
A copy job is created on each secondary server instance in a log shipping configuration. This job copies the backup files from the primary server to a configurable destination on the secondary server and logs history on the secondary server and the monitor server. The copy job schedule, which is customizable, should approximate the backup schedule.
When log shipping is enabled, the SQL Server Agent job category "Log Shipping Copy" is created on the secondary server instance.
Restore Job
A restore job is created on the secondary server instance for each log shipping configuration. This job restores the copied backup files to the secondary databases. It logs history on the local server and the monitor server, and deletes old files and old history information. The SQL Server job category "Log Shipping Restore" is created on the secondary server instance when log shipping is enabled.
On a given secondary server instance, the restore job can be scheduled as frequently as the copy job, or the restore job can delayed. Scheduling these jobs with the same frequency keeps the secondary database as closely aligned with the primary database as possible to create a warm standby database.
In contrast, delaying restore jobs, perhaps by several hours, can be useful in the event of a serious user error, such as a dropped table or inappropriately deleted table row. If the time of the error is known, you can move that secondary database forward to a time soon before the error. Then you can export the lost data and import it back into the primary database.
Alert Job
If a monitor server is used, an alert job is created on the monitor server instance. This alert job is shared by the primary and secondary databases of all log shipping configurations using this monitor server instance. Any change to the alert job (such as rescheduling, disabling, or enabling the job) affects all databases using that monitor server. This job raises alerts (for which you must specify alert numbers) for primary and secondary databases when backup and restore operations have not completed successfully within specified thresholds. You must configure these alerts to have an operator receive notification of the log shipping failure. The SQL Server Agent job category "Log Shipping Alert" is created on the monitor server instance when log shipping is enabled.
If a monitor server is not used, alert jobs are created locally on the primary server instance and each secondary server instance. The alert job on the primary server instance raises errors when backup operations have not completed successfully within a specified threshold. The alert job on the secondary server instance raises errors when local copy and restore operations have not completed successfully within a specified threshold.
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