nix-eval-jobs/README.md

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# nix-eval-jobs
This project evaluates nix attributes sets in parallel with streamable json
output. This is useful for time and memory intensive evaluations such as NixOS
machines, i.e. in a CI context. The evaluation is done with a controllable
number of threads that are restarted when their memory consumption exceeds a
certain threshold.
To facilitate integration, nix-eval-jobs creates garbage collection roots for
each evaluated derivation (drv file, not the build) within the provided
attribute. This prevents race conditions between the nix garbage collection
service and user-started nix builds processes.
## Why using nix-eval-jobs?
- Faster evaluation by using threads
- Memory used for evaluation is reclaimed after nix-eval-jobs finish, so that the build can use it.
- Evaluation of jobs can fail individually
## Example
In the following example we evaluate the hydraJobs attribute of the [patchelf](https://github.com/NixOS/patchelf) flake:
```console
$ nix-eval-jobs --gc-roots-dir $(pwd)/gcroot --flake 'github:NixOS/patchelf#hydraJobs'
{"attr":"build-sanitized-clang.x86_64-linux","drvPath":"/nix/store/igmkq61cwys8nj34yqvnpdg921h0i0mp-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/nwwgff1fwkws4wxv7k7cfvvin8ab9gbh-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"build-sanitized.aarch64-linux","drvPath":"/nix/store/d8ma8d7gjwx6ix4ibs910z9fkm3hwdvz-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/6j26m4sznwdyfk4sbmnls3sk0lxm38ih-patchelf-0.14.3"},"system":"aarch64-linux"}
{"attr":"build-sanitized.i686-linux","drvPath":"/nix/store/87rwijvfqqs7dw9lbmckmz4nbryvjaq3-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/za5w0gzf97na44fza9sdys15qnjqayd7-patchelf-0.14.3"},"system":"i686-linux"}
{"attr":"build-sanitized.x86_64-linux","drvPath":"/nix/store/nmx50wly2qvd00svx0vqsjfh0jv7q3kl-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/38d6bhz3a5jq48gm1diji0rjfcm5vi9n-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"build.aarch64-linux","drvPath":"/nix/store/yjz9msbr6pl8mj7im5kiyhk7wwkvxywa-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/as9xhcfwnhfy5x30kxh7lfgla1qrk182-patchelf-0.14.3"},"system":"aarch64-linux"}
{"attr":"build.i686-linux","drvPath":"/nix/store/nwcmdcimnaci0knri5ga019lgbvc4am4-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/64x12dmbscnnl42r4y2av52y55ksphhk-patchelf-0.14.3"},"system":"i686-linux"}
{"attr":"build.x86_64-linux","drvPath":"/nix/store/k6p4qnjryr2l1lz31pf085ay9bd7j8gj-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/h9a779ghpibfqkkdchx6s08bb3v3i8vy-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"coverage","drvPath":"/nix/store/lsrg05dx3hyi5b6ak99pn9g1rn8xwx39-patchelf-coverage-0.14.3.drv","name":"patchelf-coverage-0.14.3","outputs":{"out":"/nix/store/6h4l5axy5lvxzq662yw47y9r60mxw3zz-patchelf-coverage-0.14.3"},"system":"x86_64-linux"}
{"attr":"release","drvPath":"/nix/store/dgn5gy64pjskfnv7vqh0s86nb998f8sq-patchelf-0.14.3.drv","name":"patchelf-0.14.3","outputs":{"out":"/nix/store/nn05yaznr5af8g8mpgd82yx16pvfzjcy-patchelf-0.14.3"},"system":"x86_64-linux"}
{"attr":"tarball","drvPath":"/nix/store/5ajrgfd5nx29ykgg942k154mcaqfbhxd-patchelf-tarball-0.14.3.drv","name":"patchelf-tarball-0.14.3","outputs":{"out":"/nix/store/5cli6rh0h32yhfcgjkgbplcc73cqvplv-patchelf-tarball-0.14.3"},"system":"x86_64-linux"}
```
The output here is newline-seperated json according to https://jsonlines.org.
The code is derived from [hydra's](https://github.com/nixos/hydra) eval-jobs executable.
## Further options
``` console
$ nix-eval-jobs --help
USAGE: nix-eval-jobs [options] expr
--arg Pass the value *expr* as the argument *name* to Nix functions.
--argstr Pass the string *string* as the argument *name* to Nix functions.
--debug Set the logging verbosity level to 'debug'.
--eval-store The Nix store to use for evaluations.
--flake build a flake
--gc-roots-dir garbage collector roots directory
--help show usage information
--impure set evaluation mode
--include Add *path* to the list of locations used to look up `<...>` file names.
--log-format Set the format of log output; one of `raw`, `internal-json`, `bar` or `bar-with-logs`.
--max-memory-size maximum evaluation memory size
--meta include derivation meta field in output
--option Set the Nix configuration setting *name* to *value* (overriding `nix.conf`).
--override-flake Override the flake registries, redirecting *original-ref* to *resolved-ref*.
--quiet Decrease the logging verbosity level.
--verbose Increase the logging verbosity level.
--workers number of evaluate workers
```
## Potential use-cases for the tool
**Faster evaluator in deployment tools.** When evaluating NixOS machines,
evaluation can take several minutes when run on a single core. This limits
scalability for large deployments with deployment tools such as
[NixOps](https://github.com/NixOS/nixops).
**Faster evaluator in CIs.** In addition to evaluation speed for CIs, it is also
useful if evaluation of individual jobs in CIs can fail, as opposed to failing
the entire jobset. For CIs that allow dynamic build steps to be created, one
can also take advantage of the fact that nix-eval-jobs outputs the derivation
path separately. This allows separate logs and success status per job instead
of a single large log file.