# Taking a checkpoint

**🟠 Advanced · Lesson 17**

TAKING A CHECKPOINT - and settling which configuration ladder is the house rule.

YOU HAVE NOW SEEN THREE SUBSYSTEMS COME UP.

lesson 07 Aether you finalize, you activate the config, THEN aether.activate() - and handing it a merely-frozen config raises lesson 08 Nexus you hand over a config and activate() finalizes it FOR you lesson 17 Crystallizer ...follows AETHER.

The Crystallizer contract says it in the same capitals Aether used:

"ORDERING RULE: THE CONFIGURATION MUST BE ACTIVATED FIRST... Two distinct failure modes: 'not configured' and 'configuration not activated'."

So it is two against one. Caller-driven activation is the house rule; Nexus is the exception. That is worth knowing before you meet a fourth subsystem - guess Aether's ladder and you will be right more often than not.

AND THE BUILDER HERE IS THE MOST GENEROUS IN THE LIBRARY. CrystallizerConfigurationBuilder offers THREE terminators, one per rung, so you choose where to get off:

build() hand me the configuration finalize() ...frozen activate() ...and in force

Compare AetherConfigurationBuilder, which offers only build() and leaves rung 2 to you. Same pattern, different generosity - which is exactly the kind of divergence the configuration-uniformity program exists to catalogue.

THE CHECKPOINT DOOR create_checkpoint(profile_name=None, description=None) -> str It hands back an ID. Everything else keys off that ID: list_checkpoint_ids() what exists describe_checkpoint(id) what is in one analyze_checkpoint(id) a deeper read verify_checkpoint_chain(...) is the lineage intact Lesson 18 does the loading half.

TWO BITS, FIFTH APPEARANCE: activated / is_activated on the Crystallizer, and `activated` on its configuration. By now you should expect it.

## Before you run

Use the [Advanced guide](../../advanced/index.md) for prerequisite concepts. Run from a checkout with Melder installed and Python 3.14 free-threading selected. The collection download includes the level's local helper modules.

## Run the saved script

```bash
python UX_and_AIX_experiences/03_advanced/17_taking_a_checkpoint.py
```

```powershell
py -3.14t UX_and_AIX_experiences/03_advanced/17_taking_a_checkpoint.py
```

{download}`Download this collection <../../downloads/advanced-examples.zip>` · [Source on GitHub](https://github.com/Synaptic724/melder/blob/4b364a338debcb1c010d0f6e9dcbbabb62945ba0/UX_and_AIX_experiences/03_advanced/17_taking_a_checkpoint.py)

## Public surface

md.Crystallizer, md.CrystallizerConfiguration,
                   md.CrystallizerConfigurationBuilder, create_checkpoint,
                   list_checkpoint_ids, describe_checkpoint

## Code

```{literalinclude} ../../downloads/03_advanced/17_taking_a_checkpoint.py
:language: python
:linenos:
```

## Check the outcome

The script contains its own assertions or demonstrated refusal paths. Run it to evaluate those checks against your installed version. The code above is taken directly from the saved file; no run output is invented here.

[More advanced examples](index.md) · [Level guide](../../advanced/index.md)

## Related guides

- [Checkpoint and cache entry points](../../advanced/checkpoints.md)

## API contracts

- [Crystallizer](../../reference/api/persistence/crystallizer.md)
- [CrystallizerConfiguration](../../reference/api/persistence/crystallizerconfiguration.md)
- [CrystallizerConfigurationBuilder](../../reference/api/persistence/crystallizerconfigurationbuilder.md)