Saving and Restoring a Run

A snapshot saves a running simulation so you can continue it later from exactly the same point: same simulated time, same parts, same pending events and same random numbers. Use it to branch several what-if runs from one warmed-up state, or to keep a run you want to come back to.

A snapshot only continues the identical model. It is not a way to carry a run over to an edited layout.

Saving and loading

Save snapshot and Load snapshot… are in the DES snapshot menu of the side toolbar. Saving downloads a file named des-snapshot.json.
Snapshot now in the Experiments window stores a snapshot inside the running run. Checkpoint autosave stores them at regular simulated times; see Checkpoints.

What a snapshot contains

the simulated time and all pending events
every part (MU) with its position and state
the state of stations, storages, conveyors, sources and other components, drives and signal values
the position of the random number sequence
the state of scripts that declare it, including their running timers (see Scripts)
the state of fleet jobs, orders and zones for the supported fleet components

A snapshot does not contain the model itself. It is meant to be loaded into the model it was taken from.

When a snapshot is refused

A snapshot is checked completely before anything changes. If it is refused, the running simulation stays as it was. Save snapshot and Load snapshot… show a message with the reason and what to do, and the Problems panel lists the same warning with its technical code in square brackets, for example [des-snapshot-fingerprint].

Message (Problems panel title)	What it means and what to do
Model changed (`des-snapshot-fingerprint`)	The model changed since this snapshot was taken. A snapshot matches the model exactly as it was when saved: model files, node positions and settings, components, script text and the path network; any edit makes it fail, including a planner move. A different realvirtual release and a different random generator version count as a changed model. Load the original model or start a new run. The same title appears when you try to save after the model changed since the run started: restart the run, then save. Right after loading a model, saving is refused until its content has been checked in the background; try again after a moment.
Older snapshot format (`des-snapshot-legacy`)	The snapshot comes from an older format and cannot be converted. Start a new run.
Missing component ID (`des-snapshot-unaddressable`)	A component, drive or script has no stable ID for saving. The technical detail names it. Assign stable IDs, then start a new run.
Invalid snapshot data (`des-snapshot-contract`)	The snapshot contains events, parts or component states that the model cannot accept (a damaged snapshot). Take a new snapshot.
Logic cannot be saved (`des-snapshot-unsupported-component`)	LogicSteps, PLC programs (ST), IK paths or a replay are running. Models with them cannot be saved or loaded in this version. Stop or remove them, or continue without saving.
AGV is busy, Movement control is busy (`des-snapshot-agv-busy`, `des-snapshot-authority-busy`)	A fleet callback is running. Wait for it to finish, then try again.
Invalid AGV state, Invalid movement state, AGV task cannot be saved (`des-snapshot-agv-state`, `des-snapshot-authority-state`, `des-snapshot-agv-task-callbacks`)	Fleet state refers to missing paths or zones, or a task uses callbacks that cannot be restored. Check the path configuration or start without saving.
Placements changed (`des-restore-scene-changed`)	Placements were added or removed after the run was bound. Reset the simulation, then take a new snapshot.
Component state not declared, Script state not declared (`des-state-not-declared`, `des-script-undeclared-state`)	A component or script has state that cannot be restored safely. Declare persistent state, for scripts with `self.persist(...)`, then start a new run.

These do not count: the DES manager’s End Time, Statistics Reset Time (warm-up) and Master Seed. You can change them and still restore a snapshot, for example to run a saved state longer.

If a snapshot passes the check but cannot be applied, the Problems panel shows Snapshot restore failed (des-restore-commit-failed) and the simulation is reset. There is no rollback at that point: load the snapshot again or start a new run.

Scripts: keeping state across a restore

Keep changing script values in a single self.persist(...) object so they survive a restore. Pending script timers and the random sequence are restored too; do not restart timers after loading. See Scripting in DES for the complete example and code-check rules.

Known limits

Limit	Detail
Logic, PLC, IK, replay	Models with LogicSteps, PLC programs (ST), IK paths or a replay cannot be saved in V1. The simulation reports this visibly (a problem entry) and refuses to save or load instead of continuing with wrong state.
Script statistics	The state-time statistics of scripts (KPIs) are not saved in a snapshot.
Overhead conveyor	It is not saved, by design. Its rigid circulating chain has no DES events and does not advance in a DES run, so there is no state to continue.
Script callbacks	Callbacks that a script registers after `setup` are not saved. Register them in `setup`.
Timing and size	On the reference machine, saving takes 4 to 40 ms, loading 6 to 103 ms, and files are 32 to 396 KB (n=1 to n=16; measured 2026-11-02). See Model size and limits.