(This is the third part of a series on deployment. See part 1 and part 2.)
The deployment process I've outlined in the previous two posts works really well in a continuous deployment environment, where changesets that are merged to master go up quickly to production. It also works really well for mainline development, where changes track in one direction only and all changesets start from master. When there are no bugs in production that have to be fixed right away, before what's in master can be promoted to production.
You and I don't work on projects or teams that run continuous deployment. (Very few teams do, for good reasons.) There will come a time, possibly often, where you will need to fix a bug in production and cannot run the change through the master branch first. You need a hotfix.
The hotfix process is very similar to the mainline process outlined in part 1. The primary difference are the branch and merge points. Mainline development always branches from and merges to master. (You never develop directly in master.) Hotfixes, on the other hand, branch from and merge to the version of master which was used to build what is currently in production. They go through the same process of building a package, promoting to a hotfix-test environment (separate from the mainline test environment), then promoting to production. (This requires a separate hotfix-test package repository.)
At this point, we have successfully promoted our hotfix to production. One last item remains - we need to merge our hotfix into the current mainline development. If you've done everything right, this is one of the only two places where merge conflicts can occur. (The other is when pulling master into your development branch.) Both git and mercurial will apply the new diff (of the hotfix) to the sequence of changes right after the production diff, then apply the subsequent changes made to master on top of it. If any of the diffs conflict, the merging developer will need to fix the conflicts.
After the conflicts (if any) have been fixed, all that's left is to pull the hotfix changes into any existing development branches. And, we're done!