# Persistence mesh your code their lanes

**🔵 Expert · Lesson 02**

THE ASYMMETRY AT THE EDGE OF YOUR WORLD, demonstrated rather than described. Melder never speaks to your database - it calls YOUR callables. So the interesting question is what happens when one of them is missing or broken, and the answer is deliberately different for writes and reads.

WRITES DEGRADE AND COUNT. A missing write lane is a silent no-op. A write lane that RAISES is swallowed and TALLIED. Your local seal never fails because a network you do not own did. READS REFUSE. A missing read lane raises. "A missing lane means UNAVAILABLE, NOT EMPTY" - because an empty answer would be indistinguishable from "your remote has nothing", and that is the one reply that must never be guessed.

LENIENCY WITHOUT ACCOUNTING IS JUST QUIET DATA LOSS. That is why the manager carries `upload_failure_count` and `store_failure_count` and puts them in `describe_external_persistence_manager()`. Your handler is allowed to fail; you are not allowed to be unaware of it.

AND CALLABLES NEVER ENTER THE RECORD. Handlers live in a separate configuration and the description "expose[s] handler PRESENCE flags, never callable objects", because executable code cannot be serialized into a world record. A recorded world stays code-free and portable, which is what lets it be restored somewhere your storage code has never run.

LOCAL CUSTODY IS NEVER HOSTAGE TO A NETWORK YOU DO NOT OWN. Expert 09 shows the same rule from the wiring side; this lesson is the failure side of it.

## Before you run

Use the [Expert guide](../../expert/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/04_expert/02_persistence_mesh_your_code_their_lanes.py
```

```powershell
py -3.14t UX_and_AIX_experiences/04_expert/02_persistence_mesh_your_code_their_lanes.py
```

{download}`Download this collection <../../downloads/expert-examples.zip>` · [Source on GitHub](https://github.com/Synaptic724/melder/blob/0895ce795453cd87e35d70665957b4e89aad3317/UX_and_AIX_experiences/04_expert/02_persistence_mesh_your_code_their_lanes.py)

## Public surface

ExternalPersistenceManagerConfiguration store/fetch/
                   list handlers, configure_external_persistence_manager,
                   flush_checkpoint through a working and then a FAILING
                   handler, reload_profile_from_external with no read
                   lane, and the failure counters in the describe payload

## Code

```{literalinclude} ../../downloads/04_expert/02_persistence_mesh_your_code_their_lanes.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 expert examples](index.md) · [Level guide](../../expert/index.md)

## Related guides

- [External storage integration](../../expert/external-storage.md)

## API contracts

- [ExternalPersistenceManagerConfiguration](../../reference/api/persistence/externalpersistencemanagerconfiguration.md)