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docs/blog: add post on EESSI in OpenHPC 4.2
OpenHPC 4.2 ships an optional EESSI section in its install guide. It installs the CernVM-FS client on the head node and in the compute image, makes software.eessi.io available, and adds a module that exposes the EESSI software stack through the usual module commands. The post covers what the section does, the constraints that come with it (Lmod is required, and a local Squid proxy is recommended in production), and the longer-standing connection between OpenHPC and the EasyBuild community that it grew out of. Also add an .authors.yml entry for the author.
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‎docs/blog/.authors.yml‎

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description: MattX
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avatar: https://avatars.githubusercontent.com/u/574106
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slug: https://github.com/krisbuytaert
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adrianreber:
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name: Adrian Reber
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description: Amutable
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avatar: https://avatars.githubusercontent.com/u/8724298?v=4
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slug: https://github.com/adrianreber

‎docs/blog/posts/2026/11/eessi-in-openhpc-4.2.md‎

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---
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authors: [adrianreber]
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date: 2026-11-01
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slug: eessi-in-openhpc-4.2
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---
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# EESSI in OpenHPC 4.2
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<figure markdown="span">
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![OpenHPC](ohpc_logo.png){width=50%}
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</figure>
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[OpenHPC](https://openhpc.community/) 4.2 was
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[released on 1 November 2026](https://openhpc.community/news/2026/openhpc-v4-2-release-additional-provisioners-resource-managers-and-module-systems/).
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Among the new material in the install guide is an optional section that sets up
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[EESSI](../../../../index.md) on the cluster being built: the CernVM-FS client
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on the head node and in the compute image, the `software.eessi.io` repository,
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and a module that makes the EESSI software stack available through the usual
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`module` commands.
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<!-- more -->
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## A long-standing connection
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OpenHPC and the EasyBuild community have been in contact for a long time.
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EasyBuild has been packaged in OpenHPC since late 2016 and is still part of
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OpenHPC 4.2, alongside Spack, for sites that prefer to build their own
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software. EESSI, in turn, grew out of that same community: its
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[software layer](../../../../software_layer.md) is built with EasyBuild, and
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the installations are exposed through Lmod.
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The integration of EESSI in OpenHPC 4.2 is therefore not a first encounter
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between two unrelated projects, but an existing connection becoming visible in
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the install guide, where site administrators are likely to come across it.
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It followed from several conversations over the past few years about bringing
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the two projects closer together. While working on OpenHPC 4.2 I returned to
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the idea, and integrating EESSI proved easier than expected. The section was
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written on the OpenHPC side and reviewed by the EESSI project.
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## What it does
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Administrators who want EESSI on their cluster can enable the section when
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preparing the installation. The recipe then installs the CernVM-FS client and
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the EESSI configuration on the head node and in the compute image, and adds an
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`eessi` module to the OpenHPC module hierarchy. From that point on, the
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standard EESSI workflow applies:
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```console
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[test@sms ~]$ module load eessi
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[test@sms ~]$ module avail EESSI
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-------------------- /cvmfs/software.eessi.io/init/modules ---------------------
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EESSI/2023.06 (D) EESSI/2025.06 EESSI/2026.06
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Where:
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D: Default Module
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[test@sms ~]$ module load EESSI/2026.06
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[test@sms ~]$ module avail
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```
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The final `module avail` deserves attention. A freshly installed OpenHPC
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cluster already provides a substantial set of packages, but EESSI adds
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[more than 900 software projects](../../../../available_software/index.md) on
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top of it, built for the CPU architecture of the node on which the job runs,
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none of which has to be compiled at the site. For a cluster being set up today,
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this removes a considerable amount of work before users can start.
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The EESSI section is part of the recipe templates like every other section, and
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the test matrix includes configurations with `enable_eessi` set, so it is
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covered by the same integration testing as the rest of the guide. OpenHPC 4.2
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was validated with 172,794 tests over 306 hours.
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## Points to consider before enabling it
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The section documents two constraints that are worth repeating here:
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- **Lmod is required.** EESSI ships its initialization modules as Lua
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modulefiles, which only Lmod reads. OpenHPC 4.2 also offers Environment
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Modules 5.7 as an alternative module system, but with that selected the
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repository still mounts and its files remain usable directly, while the EESSI
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modules cannot be loaded. Support for Environment Modules is so far a
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[proposal](https://modules.readthedocs.io/en/latest/design/eessi.html) in the
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Environment Modules documentation rather than something EESSI provides.
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- **A proxy is recommended in production.** The recipe configures clients with
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`CVMFS_USE_CDN=yes`, so that content is fetched through a CDN rather than
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directly from the EESSI mirror servers. This is adequate for a test cluster
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and keeps the load on those servers low, but a production cluster should run
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a local Squid proxy and point the clients at it with `CVMFS_HTTP_PROXY`. See
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the EESSI documentation on
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[native installation on clusters](../../../../getting_access/native_installation.md#native-install-on-clusters).
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## Reading the section
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OpenHPC 4.2 includes a
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[Recipe Explorer](https://openhpc.github.io/ohpc/4.2/explorer/): after
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selecting a base OS, architecture, provisioner and resource manager, and
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enabling the optional features, it renders the corresponding install guide
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together with a `recipe.sh` and an `input.local`. The EESSI section for an
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AlmaLinux 10 / x86_64 / Warewulf / Slurm cluster is available
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[here](https://openhpc.github.io/ohpc/4.2/explorer/#almalinux10-x86_64-warewulf-slurm?chapter=eessi);
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for a different combination, enable EESSI in the options on the left and the
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section appears in the table of contents.
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## Questions
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Questions about OpenHPC itself are best raised on the
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[OpenHPC](https://github.com/openhpc/ohpc) issue tracker. Questions about EESSI,
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such as missing software or problems with the CernVM-FS repository, should be
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directed to the EESSI project through the channels listed on the
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[support page](../../../../support.md).

‎docs/blog/posts/2026/11/ohpc_logo.png‎

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