Transparent checkpointing. When a flow wishes to wait for a message from another party (or input from a human being) the underlying stack frames are suspended onto the heap, then crawled and serialized into the node’s underlying relational database using an object serialization framework. The written objects are prefixed with small schema definitions that allow some measure of portability across changes to the layout of objects, although portability across changes to the stack layout is left for future work. Flows are resumed and suspended on demand, meaning it is feasible to have far more flows active at once than would fit in memory. The checkpointing process is atomic with changes to local storage and acknowledgement of network messages.