Behind the scenes Nextjournal uses CRIU (Checkpoint/Restore In Userspace) to save and restore the in-memory state of your software. You might know "checkpointing" already from Virtual Machines (VMs), where you can restore the Operating System state from a checkpoint instead of booting it again from scratch.
CRIU does the same, but not for the whole Operating System, but for a single Process, the REPL process in your runtime. While in a VM checkpoint the whole state of the machine is saved, including the state of all (emulated) hardware, a CRIU checkpoint contains only the process state and CRIU must ensure during restoring, that the environment is still the same or must do some adjustments to handle the changed environment.
E.g. it must ensure that no opened files are deleted, that same mount points exist, it must restore shared memory areas, file handles, restore it with a different PID etc.
Because Nextjournal runs your environments in isolated Docker containers, we can restore your runtime without any conflicts with other software and can ensure, that the filesystem state is exactly the same when restoring the checkpoint as it was during checkpointing.