zalora nix expressions library
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README.md

deployix

The Zalora public nix library.

deployix is home to packages and functions that we want to share with the community but don't fit into upstream nixpkgs for whatever reason. Additionally, it is the repository for defnixos, our service-oriented deployment specification library.

Deployix uses nix-exec. Its default.nix requires you to pass in the nix-exec lib and returns a nix-exec IO value. See <deployix/defnixos/nixos-wrappers/ipsec-wrapper.nix> for an example of how you can use deployix when not using nix-exec for evaluation.

Organization

deployix currently has the following components:

  • lib: Pure nix library functions
  • nixpkgs: Upstream dependencies from nixpkgs
  • eval-support: Functions for calculation at evaluation time that cannot be done in pure nix (i.e. require import from derivations)
  • nix-exec: Native nix-exec functions
  • build-support: Functions and packages expected to be used directly only at build time (e.g. compilers)
  • pkgs: Functions and packages expected to be used at runtime
  • defnixos: The deployment specification library

See the individual subdirectories for more information.

Filesystem layout

deployix uses the directory hierarchy to automatically provide structure. In the normal case, adding a new package or function requires simply creating a subdirectory in the relevant category and writing a default.nix. The default.nix should take a deployix argument, which contains the entire set of values in deployix. Adding nested subdirectories before the default.nix will result in a nested set being added to the top-level deployix value.

In some cases, having a separate directory for a given function may be overkill. If a directory contains an uncomposed.nix file, it will be imported and passed the deployix lib and the normal recursion is stopped there. See <deployix/build-support/uncomposed.nix> for a concrete example, but as a convention uncomposed.nix should recursively import its current directory and add any trivial functions to the set that results from that.

Composition and overriding

The top-level uncomposed.nix imports the entire deployix tree as described above. The result is expected to be a tree represented as an attribute set, where the leaf nodes are all functions taking a deployix argument. The top-level default.nix ties this all together by calling each such function with the composed deployix set. This means that any leaf values that don't make sense as a function taking a deployix argument (e.g. the deployix lib) need to be handled manually.

The top-level default.nix takes a set of overrides as an argument. As each leaf function is reached, default.nix first checks to see if the overrides set contains a value at that path, and uses it if so. For example, if the overrides set is { build-support.compile-c = something; }, then deployix.build-support.compile-c will refer to something instead of to the result of calling <deployix/build-support/compile-c/default.nix> with the full deployix set. An overrides set of { build-support = {}; }, however, will not affect any leaf values as no full path to any leaf is present.

Some examples:

  • (import <deployix> { build-support.cc = "${pkgs.klibc}/bin/klcc"; }).pkgs.notify-readiness will result in a version of notify-readiness linked against klibc instead of glibc.
  • import <deployix> { nixpkgs.haskellPackages.hakyll = import <my-hakyll> { ??? }; } will override the hakyll package.

Configuration values

In some cases, leaf values may be configurable in some way that doesn't make sense to express as a function argument. For example, importing nixpkgs requires a system argument, but we don't want to have the set of packages from nixpkgs all be functions and have to pass system around every time we want to use one. Instead, deployix.config is used to hold configuration values in a global set that all packages can access. For example, deployix.config.system is used by nixpkgs to determine which system to import the nixpkgs checkout with, it is set to builtins.currentSystem by default but can be overridden in the call to default.nix (see <deployix/defnixos/nixos-wrappers/ipsec-wrapper.nix> for an example).

Some configuration values affect many different components, such as, system, and these should be set with default values in the top-level default.nix. Others are component-specific, and should be put into a component-specific namespace in deployix.config with default values handled by the component. For example, if we ever want to make the set of cflags used globally for compile-c to be configurable, we should use something like deployix.config.build-support.compile-c.cflags or default-flags in <deployix/build-support/compile-c/default.nix>.

Native import

Because some functions in deployix (such as those in nix-exec) are meant to run native code on the evaluating machine, the deployix.native attribute points to an import of deployix with config.system set to builtins.currentSystem. Functions that need it should use deployix.native where relevant, so that the end user can just import deployix with config.system set to the target system and still evaluate everything on their native host.