@drewverlee: See http://www.onyxplatform.org/docs/user-guide/latest/aggregation-state-management.html

Also see the papers listed at the bottom of this blog post http://www.onyxplatform.org/jekyll/update/2015/11/12/Onyx-0.8.0-Automatic-State-Management.html

We use an implementation of WID to store the window contents in memory (https://github.com/onyx-platform/onyx/blob/0.9.x/src/onyx/windowing/window_id.clj#L3) and replicate them onto disk using the techniques described above. The whole window contents need to fit in memory. We'll be implementing an alternate, pluggable windowing technique that pessimistically spills the entire window to disk.