3 KiB
Store
A Nix store is a collection of store objects with associated operations.
These store objects can hold arbitrary data, and Nix makes no distinction if they are used as build inputs, build results, or build plans.
A Nix store allows adding and retrieving store objects. It can perform builds, that is, transform build inputs using instructions from the build plans into build outputs. It also keeps track of references between data and can therefore garbage-collect unused store objects.
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.
The store directory defaults to /nix/store, but is in principle arbitrary.
A Rosetta stone for build system terminology
The Nix store's design is comparable to other build systems. Usage of terms is, for historic reasons, not entirely consistent within the Nix ecosystem, and still subject to slow change.
The following translation table points out similarities and equivalent terms, to help clarify their meaning and inform consistent use in the future.
| generic build system | Nix | Bazel | Build Systems à la Carte | programming language |
|---|---|---|---|---|
| data (build input, build result) | component | artifact | value | value |
| build instructions | builder | (depends on action type) | Task |
function |
| build step | derivation | action | Task |
thunk |
| build plan | derivation graph | action graph, build graph | Tasks |
call graph |
| build | realisation | build | application of Build |
evaluation |
| persistence layer | store | action cache | Store |
heap |
All of these systems share features of declarative programming languages, a key insight first put forward by Eelco Dolstra et al. in Imposing a Memory Management Discipline on Software Deployment (2004), elaborated in his PhD thesis The Purely Functional Software Deployment Model (2006), and further refined by Andrey Mokhov et al. in Build Systems à la Carte (2018).