nix
What is nix?
nix is a package manager that aims to avoid the dependency hell that you some times enter with conventional package
managers such as rpm or pip. This nix-pills
post explains nix advantages in more detail,
but basically, it stores each package and its version at /nix/store/hash-name (eg,
/nix/store/s4zia7hhqkin1di0f187b79sa2srhv6k-bash-4.2-p45/). Everything in this store is immutable, and packages are
built against absolute paths whose code never changes. Per nix-pills,
What does all this mean? It means that you could run mysql 5.2 with glibc-2.18, and mysql 5.5 with glibc-2.19. You could use your python module with python 2.7 compiled with gcc 4.6 and the same python module with python 3 compiled with gcc 4.8, all in the same system.
For our purposes, we configure the dependencies needed in some of our repos with flake.nix files such as the one in
lhcb-ntuples-gen. You then type nix develop, et
voila, you have all the packages you need (after some compilation if it is the first time you do it).
ROOT in nix and our documentation
Importantly, nix allows us to run various versions of CERN's ROOT simultaneously, which is needed because in some
cases, old code like the uBDT classifier relies on ROOT versions too old to be compiled in modern systems.
We set up various ROOT versions at root-curated: 5.34/38, 6.16/00, and
6.24/02. These can then be picked up in the various repos, see
here
an example. Note that various repos may use the same version of ROOT. You will only need to compile it once for all of
them.
The README in root-curated also provides our basic
nix documentation, for instance installation in macOS or
linux or automatic nix
develop with
direnv.
nixpkgs
nixpkgs is the main registry that contains all official packages provided by Nix/NixOS project (nix is a package in
nixpkgs). When you install nix, a system-wide version of nixpkgs is specified in the installer (let's call it
nixpkgs-ver_sys). In the root-curated flake, we pin a version of nixpkgs (say, nixpkgs-ver_root_curated).
In general, nixpkgs-ver_sys != nixpkgs-ver_root_curated. This nixpkgs-ver_root_curated is only effective in the
root-curated project. It is completely isolated from your system.
In lhcb-ntuples-gen flake, we pin a version of root-curated. We also configure this flake in a way such that we use
the nixpkgs pinned in the pinned root-curated flake. Denote this nixpkgs as nixpkgs-ver_ntuples_gen Similarly,
nixpkgs-ver_ntuples_gen != nixpkgs-ver_root_curated, unless you manually update root-curated pointer in
lhcb-ntuples-gen. Another possibility of nixpkgs-ver_ntuples_gen == nixpkgs-ver_root_curated is that we don't update
the nixpkgs pointer in root-curated.
The diagram below gives a visual example of the nixpkgs version changing, and only being picked up by root-curated
with the pinning changes.
