There is no rollback plan applicable. All binaries/scripts are provided for fresh install. In the case of binaries, the rollback step would be to just delete those binaries and revert to the earlier version. In the case of database, rollback plan would be to just delete/drop the user from the database and restore the earlier version of the tables / schema from backup.
There are two possible sequences in the rollback plan:

If a new application is being built with this release of Canvas, no rollback is applicable. You just need to delete the JARs, CTRIAFramework.zip contents from your WAR file and drop the schema created for Canvas.
If an existing application is being built on earlier releases of Canvas, perform the following steps:
1. For Canvas release 17.x till 17.1.4.1 patch release: You must revert to the earlier version of the canvas JARs and CTRIAFramework.zip contents. The database upgrade does not impact the earlier version of Canvas.
2. For Canvas builds 16.x or earlier release: Direct migration to 18.1 is not supported. Instead you need to migrate to 17.1 first before applying the 18.1 changes. You must revert to the earlier version of the Canvas JARs and CTRIAFramework.zip contents. The database upgrade does not impact the earlier version of Canvas.

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