Question

I’ve outgrown the Sql Server custom actions available in WiX, so I’m taking the bold step of creating my own using Deployment Tools Foundation. I want to be a good citizen and make sure that mine support rollback. But what’s the best way of doing it?

I need to support SQL Server 2005 and later, all editions.

The problem, as I see it, is that Windows Installer works in two phases: it does the work, storing undo information as it goes. Then, when all the pieces are in place it either commits (deleting the undo information) or does a rollback.

This means that standard transactions won’t do the job. They would have to be completed inside my Execute custom action, and I wouldn’t get a chance to roll them back later.

I’ve considered taking a copy-only backup of the database that I can restore in the rollback action if necessary but I think this approach, whilst simple has shortcomings. I don’t know how big our databases will get, for example – so I can’t guarantee that there will be space available to hold the backup on the target machine. Also, backup and restore can take a while to complete, and I don’t want typical installs (where rollback doesn’t happen) to be unnecessarily slow.

So that brings me to my current favoured idea: make sure the Distributed Transaction Coordinator is started up, then initialise a Distributed Transaction before making changes, then either committing it or rolling it back in the appropriate custom actions.

It seems I can uses the members of the TransactionInterop class to export a cookie that will enable me to share the transaction between my different custom actions.

Can anyone with experience of this kind of thing say if it is likely to work?

Answer 1

Some database/instance operations cannot be done inside a transaction (eg. CREATE/ALTER/DROP ENDPOINT), and other operations cannot be done inside a distributed transaction (eg. SAVE TRANSACTION). So you won’t be able to do them at all in your proposed plan. Also your DB upgrade scripts will have to all work correctly when run inside an uncommitted transaction.

I would say that there are fewer risks of going down the backup/restore path (or alternatively creating a database snapshot and restoring from the snapshot on rollback, with the drawback of requiring EE).

Also an option is to have an undo script for every do script run during upgrade, and have the undo script run during rollback and remove the effects of the installation. I understand that this is a hard problem, probably doubles the amount of scripts that have to be developed (and tested…) and requires some serious developer discipline.