Challenges/Requirements for Upgrade/Rollback

Based on the discussion above, a solution to the upgrade/rollback problem requires specific capabilities:

1. An upgrade/rollback solution should not require excessive change from existing processes for the distribution and installation of software.
2. A solution must enable software rollback with minimal operator intervention. Ideally, when an operator initiates the rollback procedure, the system automatically reverts all necessary state, such as program files, configuration files, etc.
3. The upgrade and rollback procedures should operate quickly.
4. The solution should minimize the down time due to upgrades and rollbacks. Note that this requirement can be relaxed for applications such as a web server cluster, because the application architecture itself can compensate for the unavailability of individual service nodes.

Note that the third and fourth requirements are not equivalent. It might be possible, for example, to minimize down time by running the upgrade procedure ``in the background''. But this might conflict with the goal of upgrading the software quickly.