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introduce mapping to Unix files and processes

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Valentin Gagarin 2022-05-11 14:20:56 +02:00
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1
doc/manual/src/architecture/architecture.md
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@ -37,7 +37,6 @@ The command line and Nix language are what users interact with most.
::: {.note}
The Nix language itself does not have a notion of *packages* or *configurations*.
As far as we are concerned here, the inputs and results of a derivation are just data.
In practice this amounts to a set of files in a file system.
:::
Underlying these is the [Nix store](./store/store.md), a mechanism to keep track of build plans, data, and references between them.

51
doc/manual/src/architecture/store/store.md
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@ -39,7 +39,7 @@ A reference will always point to exactly one store object.
An added store object cannot have references, unless it is a build task.
Building a store object will add appropriate references, according to provided build instructions.
These references can only come from declared build inputs, and are not known by build instructions a priori.
These references can only come from declared build inputs, and are not known to build instructions a priori.
```haskell
data Data = Data | Task BuildTask
@ -55,14 +55,59 @@ Garbage collection will delete all store objects that cannot be reached from any
<!-- more details in section on garbage collection, link to it once it exists -->
## Files and Processes
Nix provides a mapping between its store model and the [Unix paradigm](https://en.m.wikipedia.org/wiki/Everything_is_a_file) on the interplay of [files and processes](https://en.m.wikipedia.org/wiki/File_descriptor).
Nix encodes immutable store objects and opaque identifiers as file system primitives: files, directories, and paths.
That allows processes to resolve references contained in files and thus access the contents of store objects.
```
+-----------------------------------------------------------------+
| Nix |
| [ commmand line interface ]------, |
| | | |
| evaluates | |
| | manages |
| V | |
| [ configuration language ] | |
| | | |
| +-----------------------------|-------------------V-----------+ |
| | store evaluates to | |
| | | | |
| | referenced by V builds | |
| | [ build input ] ---> [ build plan ] ---> [ build result ] | |
| | ^ | | |
| +---------|----------------------------------------|----------+ |
+-----------|----------------------------------------|------------+
| |
file system object store path
| |
+-----------|----------------------------------------|------------+
| operating system +------------+ | |
| '------------ | | <-----------' |
| | file | |
| ,-- | | <-, |
| | +------------+ | |
| execute as | | read, write, execute |
| | +------------+ | |
| '-> | process | --' |
| +------------+ |
+-----------------------------------------------------------------+
```
Store objects are therefore implemented as the pair of
- a *file system object* for data
- a set of *store paths* for references.
There exist different types of stores, which all follow this model.
Examples:
- store on the local file system
- remote store accessible via SSH
- binary cache store accessible via HTTP
Every store with a file system representation has a *store directory*, which contains that store’s objects accessible through [store paths](paths.md).
The store directory defaults to `/nix/store`, but is in principle arbitrary.
Every store ultimately has to make store objects accessible to processes through the file system.
## A [Rosetta stone][rosetta-stone] for build system terminology