EESSI in OpenHPC 4.2

OpenHPC 4.2 was released on 1 November 2026. Among the new material in the install guide is an optional section that sets up EESSI on the cluster being built: the CernVM-FS client on the head node and in the compute image, the software.eessi.io repository, and a module that makes the EESSI software stack available through the usual module commands.

A long-standing connection

OpenHPC and the EasyBuild community have been in contact for a long time. EasyBuild has been packaged in OpenHPC since late 2016 and is still part of OpenHPC 4.2, alongside Spack, for sites that prefer to build their own software. EESSI, in turn, grew out of that same community: its software layer is built with EasyBuild, and the installations are exposed through Lmod.

The integration of EESSI in OpenHPC 4.2 is therefore not a first encounter between two unrelated projects, but an existing connection becoming visible in the install guide, where site administrators are likely to come across it.

It followed from several conversations over the past few years about bringing the two projects closer together. While working on OpenHPC 4.2 I returned to the idea, and integrating EESSI proved easier than expected. The section was written on the OpenHPC side and reviewed by the EESSI project.

What it does

Administrators who want EESSI on their cluster can enable the section when preparing the installation. The recipe then installs the CernVM-FS client and the EESSI configuration on the head node and in the compute image, and adds an eessi module to the OpenHPC module hierarchy. From that point on, the standard EESSI workflow applies:

[test@sms ~]$ module load eessi
[test@sms ~]$ module avail EESSI

-------------------- /cvmfs/software.eessi.io/init/modules ---------------------
   EESSI/2023.06 (D)    EESSI/2025.06    EESSI/2026.06

  Where:
   D:  Default Module

[test@sms ~]$ module load EESSI/2026.06
[test@sms ~]$ module avail

The final module avail deserves attention. A freshly installed OpenHPC cluster already provides a substantial set of packages, but EESSI adds more than 900 software projects on top of it, built for the CPU architecture of the node on which the job runs, none of which has to be compiled at the site. For a cluster being set up today, this removes a considerable amount of work before users can start.

The EESSI section is part of the recipe templates like every other section, and the test matrix includes configurations with enable_eessi set, so it is covered by the same integration testing as the rest of the guide. OpenHPC 4.2 was validated with 172,794 tests over 306 hours.

Points to consider before enabling it

Two points are worth highlighting before enabling the section:

• The integration as implemented in OpenHPC 4.2 relies on Lmod. EESSI ships its initialization modules as Lua modulefiles, which only Lmod reads. OpenHPC 4.2 also offers Environment Modules 5.7 as an alternative module system, but with that selected the repository still mounts and its files remain usable directly, while the EESSI modules cannot be loaded by Environment Modules. EESSI itself does ship Lmod, so it remains possible to initialise and use EESSI as per the standard usage instructions; without additional configuration, however, this would clobber the Environment Modules environment. Explicit support for Environment Modules is so far a proposal in the Environment Modules documentation rather than something EESSI currently provides.
• A proxy is recommended in production. The recipe configures clients with CVMFS_USE_CDN=yes, so that content is fetched through a CDN rather than directly from the EESSI mirror servers. This is adequate for a test cluster and keeps the load on those servers low, but a production cluster should run a local Squid proxy and point the clients at it with CVMFS_HTTP_PROXY. See the EESSI documentation on native installation on clusters.

Reading the section

OpenHPC 4.2 includes a Recipe Explorer: after selecting a base OS, architecture, provisioner and resource manager, and enabling the optional features, it renders the corresponding install guide together with a recipe.sh and an input.local. The EESSI section for an AlmaLinux 10 / x86_64 / Warewulf / Slurm cluster is available here; for a different combination, enable EESSI in the options on the left and the section appears in the table of contents.

Questions

Questions about OpenHPC itself are best raised on the OpenHPC issue tracker. Questions about EESSI, such as missing software or problems with the CernVM-FS repository, should be directed to the EESSI project through the channels listed on the support page.