Problem Statement #1
Once a new Debian package has been built it needs testing, at the moment this can be done for example by manually copying the package to labs and/or to canary hosts in production. While this works it is suboptimal, especially when dependencies are involved as well. A better option would be to have a separate distribution where to upload packages, this distribution would be installed only on particular hosts (e.g. canaries) and can be upgraded without affecting production. Once testing is complete the package can be copied to "$distribution-wikimedia" (in other words, promoted) using https://wikitech.wikimedia.org/wiki/Reprepro#Copying_between_distributions
see also https://etherpad.wikimedia.org/p/debian-packaging-automation
Problem Statement #2
Some clustered services (Cassandra for example), require identical software versions on constituent nodes. Having a new node provisioned with a newer version of software, simply because the repository has since been updated, is a recipe for disaster; Package {up,down}grades should only occur on an explicit basis. It is also not practical to assume that all of the clusters are able to move in version lock-step with one another. Ideally, our repository would be able to host multiple software versions, and Puppet could be utilized to ensure that the nodes of a cluster remained pinned to the apropos version. The approach cited above could be used here as well, but it would require the creation of a distribution for each new version added. Another approach would be to generate a packages manifest from a pool containing multiple versions, (and in the absence of pinning, hosts would simply get the most recent).