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96 lines
4.3 KiB
Markdown
96 lines
4.3 KiB
Markdown
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# Content-Addressing Store Objects
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Just [like][fso-ca] [File System Objects][File System Object],
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[Store Objects][Store Object] can also be [content-addressed](@docroot@/glossary.md#gloss-content-addressed),
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unless they are [input-addressed](@docroot@/glossary.md#gloss-input-addressed-store-object).
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For store objects, the content address we produce will take the form of a [Store Path] rather than regular hash.
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In particular, the content-addressing scheme will ensure that the digest of the store path is solely computed from the
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- file system object graph (the root one and its children, if it has any)
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- references
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- [store directory](../store-path.md#store-directory)
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- name
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of the store object, and not any other information, which would not be an intrinsic property of that store object.
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For the full specification of the algorithms involved, see the [specification of store path digests][sp-spec].
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[File System Object]: ../file-system-object.md
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[Store Object]: ../store-object.md
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[Store Path]: ../store-path.md
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## Content addressing each part of a store object
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### File System Objects
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With all currently supported store object content addressing methods, the file system object is always [content-addressed][fso-ca] first, and then that hash is incorporated into content address computation for the store object.
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### References
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With all currently supported store object content addressing methods,
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other objects are referred to by their regular (string-encoded-) [store paths][Store Path].
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Self-references however cannot be referred to by their path, because we are in the midst of describing how to compute that path!
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> The alternative would require finding as hash function fixed point, i.e. the solution to an equation in the form
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> ```
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> digest = hash(..... || digest || ....)
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> ```
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> which is computationally infeasible.
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> As far as we know, this is equivalent to finding a hash collision.
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Instead we just have a "has self reference" boolean, which will end up affecting the digest.
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### Name and Store Directory
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These two items affect the digest in a way that is standard for store path digest computations and not specific to content-addressing.
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Consult the [specification of store path digests][sp-spec] for further details.
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## Content addressing Methods
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For historical reasons, we don't support all features in all combinations.
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Each currently supported method of content addressing chooses a single method of file system object hashing, and may offer some restrictions on references.
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The names and store directories are unrestricted however.
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### Flat { #method-flat }
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This uses the corresponding [Flat](../file-system-object/content-address.md#serial-flat) method of file system object content addressing.
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References are not supported: store objects with flat hashing *and* references can not be created.
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### Text { #method-text }
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This also uses the corresponding [Flat](../file-system-object/content-address.md#serial-flat) method of file system object content addressing.
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References to other store objects are supported, but self references are not.
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This is the only store-object content-addressing method that is not named identically with a corresponding file system object method.
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It is somewhat obscure, mainly used for "drv files"
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(derivations serialized as store objects in their ["ATerm" file format](@docroot@/protocols/derivation-aterm.md)).
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Prefer another method if possible.
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### Nix Archive { #method-nix-archive }
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This uses the corresponding [Nix Archive](../file-system-object/content-address.md#serial-nix-archive) method of file system object content addressing.
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References (to other store objects and self references alike) are supported so long as the hash algorithm is SHA-256, but not (neither kind) otherwise.
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### Git { #method-git }
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> **Warning**
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>
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> This method is part of the [`git-hashing`][xp-feature-git-hashing] experimental feature.
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This uses the corresponding [Git](../file-system-object/content-address.md#serial-git) method of file system object content addressing.
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References are not supported.
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Only SHA-1 is supported at this time.
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If [SHA-256-based Git](https://git-scm.com/docs/hash-function-transition)
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becomes more widespread, this restriction will be revisited.
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[fso-ca]: ../file-system-object/content-address.md
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[sp-spec]: @docroot@/protocols/store-path.md
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[xp-feature-git-hashing]: @docroot@/contributing/experimental-features.md#xp-feature-git-hashing
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