# Nix development and build environment

With Nix it's possible to quickly and reliably recreate the full environments
for developing, testing and building PostgREST.

## Getting started with Nix

You'll need to [get Nix](https://nixos.org/download.html). Follow the recommended installation for your operating system from the official download website.

## Building PostgREST

To build PostgREST from your local checkout of the repository, run:

```bash
$ nix-build --attr postgrestPackage

```

This will create a `result` directory that contains the PostgREST binary at
`result/bin/postgrest`. The `--attr` parameter (or short: `-A`) tells Nix to
build the `postgrestPackage` attribute from the Nix expression it finds in our
`default.nix` (see below for details). Nix will take care of getting the right
GHC version and all the build dependencies.

You can also build a statically linked binary with:

```bash
$ nix-build --attr postgrestStatic

$ ldd result/bin/postgrest
$       not a dynamic executable
```

## Binary cache

We recommend that you use the PostgREST binary cache on
[cachix](https://cachix.org/):

```bash
# Install cachix:
$ nix-env -iA cachix -f https://cachix.org/api/v1/install

# Set cachix up to use the PostgREST binary cache:
$ cachix use postgrest

```

Without cachix, your machine will have to rebuild all the dependencies that are
derived on top of `Musl` for the static builds, which can take a very long time.

## Developing

A development environment for PostgREST is available with `nix-shell`. The
following command will put you into a new shell that has GHC and Cabal on the
PATH:

```bash
$ nix-shell

```

Within `nix-shell`, you can run Cabal commands as usual. You can also run
stack with the `--nix` option, which causes stack to pick up the non-Haskell
dependencies from the same pinned Nixpkgs version that the Nix builds use.

## Working with `nix-shell` and the PostgREST utility scripts

The PostgREST utilities available in `nix-shell` all have names that begin with
`postgrest-`, so you can use tab completion (typing `postgrest-` and pressing
`<tab>`) in `nix-shell` to see all that are available:

```bash
# Note: The utilities listed here might not be up to date.
[nix-shell]$ postgrest-<tab>
postgrest-build                   postgrest-parallel-curl
postgrest-check                   postgrest-profiled-run
postgrest-clean                   postgrest-push-cachix
postgrest-commitlint              postgrest-release
postgrest-coverage                postgrest-repl
postgrest-coverage-draft-overlay  postgrest-run
postgrest-docs-build              postgrest-style
postgrest-docs-check              postgrest-style-check
postgrest-docs-dictcheck          postgrest-test-big-schema
postgrest-docs-linkcheck          postgrest-test-doctests
postgrest-docs-render             postgrest-test-io
postgrest-docs-serve              postgrest-test-memory
postgrest-docs-spellcheck         postgrest-test-replica
postgrest-dump-minimal-imports    postgrest-test-spec
postgrest-dump-schema             postgrest-test-spec-idempotence
postgrest-gen-ctags               postgrest-watch
postgrest-gen-jwt                 postgrest-with-all
postgrest-gen-secret              postgrest-with-git
postgrest-git-hooks               postgrest-with-pgrst
postgrest-hsie-graph-modules      postgrest-with-pg-13
postgrest-hsie-graph-symbols      postgrest-with-pg-14
postgrest-hsie-minimal-imports    postgrest-with-pg-15
postgrest-lint                    postgrest-with-pg-16
postgrest-loadtest                postgrest-with-pg-17
postgrest-loadtest-against        postgrest-with-slow-pg
postgrest-loadtest-report         postgrest-with-slow-postgrest
postgrest-nixpkgs-upgrade
...

[nix-shell]$

```

The `docker` module has large dependencies to be build before the shell becomes
available, which could take an especially long time if the cachix binary cache
is not used. You can activate it by passing a flag to `nix-shell` with
`nix-shell --arg docker true`. This will make the respective utilities available:

```bash
$ nix-shell --arg docker true
[nix-shell]$ postgrest-docker-<tab>
postgrest-docker-load
...

```

Note that `postgrest-docker-load` is now also available.

To run one-off commands, you can also use `nix-shell --run <command>`, which
will launch the Nix shell, run that one command and exit. Note that the tab
completion will not work with `nix-shell --run`, as Nix has yet to evaluate
our Nix expressions to see which utilities are available.

```bash
$ nix-shell --run postgrest-style

# Note that you need to quote any arguments that you would like to pass to
# the command to be run in nix-shell:
$ nix-shell --run "postgrest-foo --bar"

```

A third option is to install utilities that you use very often locally:

```bash
$ nix-env -f default.nix -iA devTools

# `postgrest-style` can now be run directly:
$ postgrest-style

```

If you use `nix-shell` very often, you might like to use
https://github.com/xzfc/cached-nix-shell, which skips evaluating all our Nix
expressions if nothing changed, reducing startup time for the shell
considerably.

Note: Once inside nix-shell, the utilities work from any directory inside
the PostgREST repo. Paths are resolved relative to the repo root:

```bash
[nix-shell]$ cd src
# Even though the current directory is ./src, the config path must still start
# from the repo root:
[nix-shell]$ postgrest-run test/io/configs/simple.conf
```

## Testing

In nix-shell, you'll find utility scripts that make it very easy to run our
test suite, including setting up all required dependencies and
temporary test databases:

```bash
# Run the tests against the most recent version of PostgreSQL:
$ nix-shell --run postgrest-test-spec

# Run the tests against all supported versions of PostgreSQL:
$ nix-shell --run "postgrest-with-all postgrest-test-spec"

# Run the tests against a specific version of PostgreSQL (use tab-completion in
# nix-shell to see all available versions):
$ nix-shell --run "postgrest-with-pg-13 postgrest-test-spec"

```

The io-test that test PostgREST as a black box with inputs and outputs can be
run with `postgrest-test-io`. The test runner under the hood is
[pytest](https://docs.pytest.org/) and you can pass it the usual options:

```bash
# Filter the tests to run by name, including all that contain 'config':
[nix-shell]$ postgrest-test-io -k config

# Run tests in parallel using xdist, specifying the number of processes:
[nix-shell]$ postgrest-test-io -n auto
[nix-shell]$ postgrest-test-io -n 8
```

The memory tests check that we don't surpass a memory threshold for big request bodies.

```bash
# Build the dependencies needed for the memory test
$ nix-shell --arg memory true

# Run the memory test
[nix-shell]$ postgrest-test-memory
```

The loadtests ensure that performance doesn't drop on a change. Underlyingly they use
[vegeta](https://github.com/tsenart/vegeta).

```bash
# Run the loadtests on the latest commit(HEAD)
[nix-shell]$ postgrest-loadtest

# You can loadtest comparing to a different branch
[nix-shell]$ postgrest-loadtest-against master

# You can simulate latency client/postgrest and postgrest/database
[nix-shell]$ PGRST_DELAY=5ms PGDELAY=5ms postgrest-loadtest

# You can build postgrest directly with cabal for faster iteration
[nix-shell]$ PGRST_BUILD_CABAL=1 postgrest-loadtest

# Produce a markdown report to be used on CI
[nix-shell]$ postgrest-loadtest-report
```

doctests for some of our modules are also available:

```bash
[nix-shell]$ postgrest-test-doctest
```

## Code coverage

Code coverage is available under the `postgrest-coverage` command. This will produce a `./coverage` directory that can be visualized on a browser.

```bash
# Will run all the tests and produce a coverage dir
[nix-shell]$ postgrest-coverage
...

postgrest-coverage: To see the results, visit file://$(pwd)/coverage/check/hpc_index.html
```

## Linting and styling code

The nix-shell also contains scripts for linting and styling the PostgREST
source code:

```bash
# Linting
$ nix-shell --run postgrest-lint

# Styling / auto-formatting code
$ nix-shell --run postgrest-style

```

There is also `postgrest-style-check` that exits with a non-zero exit code if
the check resulted in any uncommitted changes. It's mostly useful for CI.

## Documentation

The following commands can help you when working on the PostgREST docs:

```bash
# Build the docs
[nix-shell]$ postgrest-docs-build

# Build the docs and start a livereload server on `http://localhost:5500`
[nix-shell]$ postgrest-docs-serve

# Run aspell, to verify spelling mistakes
[nix-shell]$ postgrest-docs-spellcheck

# Detect obsolete entries in postgrest.dict
[nix-shell]$ postgrest-docs-dictcheck

# Build and run all the validation scripts
[nix-shell]$ postgrest-docs-check
```

## General development tools

Tools like `postgrest-build`, `postgrest-run`, `postgrest-repl` etc. are simple wrappers around
`cabal` and should do what you expect. `postgrest-check` runs most checks that will
also run in CI, with the exception of the IO and Memory checks that need to be run
separately.

`postgrest-with-pg-*` take a command as an argument and will run it
with a temporary database. `postgrest-with-all` will run the command against
all supported PostgreSQL versions. Tests run without `postgrest-with-*` are
run against the latest PostgreSQL version by default.

`postgrest-watch` takes a command as an argument that it will re-run if any source
file is changed. For example, `postgrest-watch postgrest-with-all postgrest-test-spec`
will re-run the full spec test suite against all PostgreSQL versions on every change.

## REPL

You can use `postgrest-repl` to manually inspect the PostgREST modules.

```bash
$ postgrest-repl

ghci> import PostgREST.<tab>
PostgREST.Admin                        PostgREST.Config.Database              PostgREST.Plan.MutatePlan              PostgREST.Response.OpenAPI
PostgREST.ApiRequest                   PostgREST.Config.JSPath                PostgREST.Plan.ReadPlan                PostgREST.SchemaCache
...

ghci> import PostgREST.MediaType
ghci> decodeMediaType "application/json"
MTApplicationJSON
```

## Working with locally modified Haskell packages

Sometimes, we need to modify Haskell libraries in order to debug them or enhance them.
For example, if you want to debug the [`hasql-pool`](https://hackage.haskell.org/package/hasql-pool)
library:

First, copy the package to the repo root. We'll use GitHub in this example.

```bash
$ git clone --depth=1 --branch=0.10.1 https://github.com/nikita-volkov/hasql-pool.git
$ rm -rf ./hasql-pool/.git
```

Then, pin the local package to the [`haskell-packages.nix`](./overlays/haskell-packages.nix) file.

```nix
  overrides =
    # ...
    rec {

      # Different subpath may be needed if the cabal file is not in the library's base directory
      hasql-pool = lib.dontCheck
        (prev.callCabal2nixWithOptions "hasql-pool" ../../hasql-pool "--subpath=." {} );

    };
```

Next, both [`cabal.project`](/cabal.project) and [`stack.yaml`](/stack.yaml) need to be updated
with the local library:

```cabal
-- cabal.project
packages:
  ./hasql-pool/hasql-pool.cabal
```

```yaml
# stack.yaml
extra-deps:
  - ./hasql-pool/hasql-pool.cabal
```

Lastly, run `nix-shell` to build the local package. You don't need to exit and
enter the Nix shell every time you modify the library's code, re-executing
`postgrest-run` should be enough.

This is done for development purposes only. Local libraries must not be left
in production ready code.

## Tour

The following is not required for working on PostgREST with Nix, but it will
give you some more background and details on how it works.

### `default.nix`

[`default.nix`](../default.nix) is our 'repository expression' that pulls all
the pieces that we define with Nix together. It returns a set (like a dict in
other programming languages), where each attribute is a derivation that Nix
knows how to build, like the `postgrest` attribute from earlier.

Internally, our `default.nix` uses the `pkgs.callPackage` function to import
the modules that we defined in the `nix` directory. It automatically passes the
arguments those modules require if they are available in `pkgs` (this means
that `pkgs` is defined in terms of itself, better not to think too much about
that).

We also use `default.nix` to load our pinned version of the `nixpkgs`
repository. This set of packages will always be the same, independently from
where or when you use it. The pinned version is taken from `flake.lock` and
can be updated with `postgrest-nixpkgs-upgrade`.

### `shell.nix`

[`shell.nix`](../shell.nix) defines an environment in which PostgREST can be
built and developed. It extends the build environment from our `postgrest`
attribute with useful utilities that will be put on the PATH in `nix-shell`.

### `nix/overlays`

Our overlays to the Nix package set are defined here. They allow us to tweak our
`pkgs` in `default.nix` by adding new packages or overriding existing ones.

## Upgrading dependencies

See the [upgrading checklist](UPGRADE.md) for how to upgrade the PostgREST
dependencies.
