Since matrix project is about running a lot of similar things, it'd be nice to support some notion of exclusion, such as "these builds share the single cluster-wide resource (like external DB), so only one build at once per cluseter" or "these builds share a single system-wide resource (like a TCP port), so only one build at once per node", and so on.

Note that port-allocator plugin and locks-and-latches plugin can be used for similar purpose.