Is it possible to use the drone migration tool to migrate these piecemeal or is it a big bang only?
The automated migration tool is big bang. You would need to migrate the full database in order to use the migration utility.
Ideally I would like to switch over some projects to the new 1.x instance and keep the others on 0.8 so we can see what issues we have, before migrating the rest.
This is the approach we recommend. Instead of an automated migration you can leave the 0.8 instance in place and setup a 1.x instance, and allow teams to move projects over manually.
I’d like to keep the build numbers as these are used as part of the build pipeline for image tags etc.
If you do a periodic, manual migration as suggested above, you can manually override the build counter to match the previous value. For example, if the build number for repository octocat/hello-world is 42 in version 0.8, you could execute the following command in 1.x
drone repo update octocat/hello-world --build-counter=42
This would ensure subsequent builds in 1.x use the previous build number.
If this partial migration isn’t possible, and we have to do an all-at-once approach, what sort of issues are we likely to encounter with incompatible pipelines etc.?
Hopefully very few. You can continue to user 0.8 syntax with 1.x. If you encounter an edge case where a configuration file does not work with 1.x it may require the team to update the yaml to 1.0 syntax. We can help advise if you encounter such an edge case.
The biggest issue you will encounter when migrating to 1.x is that some manual database cleanup may be necessary. This is documented in the readme, but we can also advise on how to resolve.
Please note the migration utility may require manual database cleanup. For example, in 0.8 the same repository can be listed in the database twice if it has been renamed, however, in 1.0 this will cause unique key violations. These edge cases require manual intervention. You should therefore be comfortable with sql and database troubleshooting before you proceed.