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add motivation for references

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Valentin Gagarin 2022-04-29 01:15:33 +02:00
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doc/manual/src/architecture/store/objects.md
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@ -37,9 +37,20 @@ Symlinks pointing outside of their own root, or to a store object without a matc
## Reference {#reference} ## Reference {#reference}
A store object can refer to other store objects or itself. A store object can reference other store objects.
Nix collects these references by scanning file contents for [store paths](./paths.md) when a new store object is created. Nix stores have the *closure property*: for each store object in the store, all the store objects it references must also be in the store.
Building, copying and deleting store objects must be done in a way that obeys this property:
- We can only safely delete unreferenced objects.
- When copying, to maintain correctness, either the result must be "revealed" atomically to the destination store, or objects must be copied in reference-dependency order.
- Newly built store objects must only refer to store objects in the closure of the build inputs.
This ensures the purity of the build.
### Reference scanning
While references could be arbitrary paths, Nix requires them to be store paths to ensure correctness. While references could be arbitrary paths, Nix requires them to be store paths to ensure correctness.
Anything outside a given store is not under control of Nix, and therefore cannot be guaranteed to be present when needed. Anything outside a given store is not under control of Nix, and therefore cannot be guaranteed to be present when needed.