---
title: "Chapter 1: Packages"
description: "Here we show where packages come from"
date: 2023-06-13T19:36:57+02:00
weight: 1
---

In this chapter we describe where the packages available in the cluster come
from and how to load them.

## Where packages come from

The packages in the jungle cluster are constructed by *layers*. Each layer
applies some changes over the previous one:

1. The first layer is [nixpkgs][1], a large repository of packages maintained by
   the NixOS community. It provides packages like gcc, bash, gcc or the linux
   kernel.

[1]: https://github.com/NixOS/nixpkgs/

2. The second layer is [bscpkgs][2], it takes the nixpkgs set of packages and
   expands it by adding custom packages from the BSC such as Nanos6, nOS-V,
   NODES, ovni or wxparaver.

[2]: https://pm.bsc.es/gitlab/rarias/bscpkgs

3. The third layer is [jungle][3], it takes the extended packages from bscpkgs
   and configures them for the jungle cluster. For example, we configure MPICH
   to use the OmniPath network and set it as the default implementation.

[3]: https://pm.bsc.es/gitlab/rarias/jungle

These layers are called *overlays* in Nix and they are the default mechanism
used to modify the packages. Generally you will use the packages defined in the
last layer (jungle) but you can define your own additional layer to specify
custom changes. For example, instead of choosing MPICH, you may want to use
Intel MPI instead by default.

## Loading packages in an ephemeral shell

You can manually load packages in a *new* shell with `nix shell jungle#<pkg>`,
for example:

```
hut% which ovniemu
ovniemu not found
hut% nix shell jungle#bsc.ovni
hut% which ovniemu
/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0/bin/ovniemu
hut% exit
hut%
```

You can also specify multiple packages by listing them as parameters of `nix
shell`:

```
hut% nix shell jungle#bsc.ovni jungle#bsc.osumb
hut% which osu_bw
/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1/bin/osu_bw
hut% exit
```

Or make the bash (zsh in this case) shell expand them:

```
hut% echo nix shell jungle#bsc.{ovni,osumb}
nix shell jungle#bsc.ovni jungle#bsc.osumb
hut% nix shell jungle#bsc.{ovni,osumb}
hut% which osu_bw
/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1/bin/osu_bw
hut% exit
```

You can use TAB to see which packages are available:

```
hut% nix shell jungle#bsc.n<TAB>
jungle\#bsc.nanos6              jungle\#bsc.nixtools
jungle\#bsc.nanos6Debug         jungle\#bsc.nix-wrap
jungle\#bsc.nanos6Git           jungle\#bsc.nodes
jungle\#bsc.nanos6GlibcxxDebug  jungle\#bsc.nodesGit
jungle\#bsc.nanos6-icc          jungle\#bsc.nodesRelease
jungle\#bsc.nanos6-icx          jungle\#bsc.nodesWithOvni
jungle\#bsc.nanos6Release       jungle\#bsc.nosv
jungle\#bsc.nix-mn4
```

Notice that these packages are evaluated at the moment the command is invoked.
So if you come back a month later and run the same command, you may find that
the packages have been updated and that could be problematic.

In the next section we will create a new flake that defines the packages of the
shell and also records the exact version of the packages that we used at the
evaluation time for future use.

In the [next chapter](../ch2) we will see how to create a permanent shell that
will retain the same packages even if they are upgraded in the cluster, until we
decide to upgrade them.