We have a few different roles in Azure. Currently these are each deployed to separate instances so they can scale separately (and in production this is what we want), but for testing this seems wasteful and we would like to be able to deploy all the roles to a single instance to minimise costs.
Can we do this?
Roles are essentially definitions for what will run inside a set of Windows Azure VM instances. By definition, they have their own instances, so they cannot be targeted toward a single set of instances.
That said: there's nothing stopping you from combining code from different roles into one single role. You'd need to make sure your OnStart() and Run() take care of all needed tasks, as well as combining startup script items.
The upside (which you already surmised): cost savings, especially when running at low volume (where the entire app might be able to run in two instances, vs. several more near-idle instances split up by role).
One potential downside: Everything combined into a single role will now scale together. This may or may not be an issue for you.
Also, think about sizing. Let's say your website is perfectly happy in a Small, yet some background task you have requires XL (maybe it's a renderer needing 10GB RAM or something). And let's say you always run 2 instances of your website, for SLA purposes. Now, even at very low volume, your app consists of two XL instances instead of 2 Small (web) and one XL (background). Now, your near-idle system could cost more as one combined role than as separate roles. This might not apply to you - just giving an example where it might not make sense to combine...
Adding on to David's great explanation, adding things together and gluing them via the OnStart or Run overrides will work, but are you really testing things properly? Configuration values merged together, potential issues with memory usage, concurrency, etc. You would not be testing the same product as you deploy to production.
Better way, would be to deploy extra-small instances to your QA environment. They cost a fraction of the price of say, Medium or Large servers and provide meaningful testing platform.
