docs/blog: add post on EESSI in OpenHPC 4.2

docs/blog/.authors.yml

@@ -139,5 +139,8 @@ authors:
     description: MattX
     avatar: https://avatars.githubusercontent.com/u/574106
     slug: https://github.com/krisbuytaert
-
-
+  adrianreber:
+    name: Adrian Reber
+    description: Amutable
+    avatar: https://avatars.githubusercontent.com/u/8724298?v=4
+    slug: https://github.com/adrianreber

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

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+---
+authors: [adrianreber]
+date: 2026-11-01
+slug: eessi-in-openhpc-4.2
+---
+
+# EESSI in OpenHPC 4.2
+
+<figure markdown="span">
+![OpenHPC](ohpc_logo.png){width=50%}
+</figure>
+
+[OpenHPC](https://openhpc.community/) 4.2 was
+[released on 1 November 2026](https://openhpc.community/news/2026/openhpc-v4-2-release-additional-provisioners-resource-managers-and-module-systems/).
+Among the new material in the install guide is an optional section that sets up
+[EESSI](../../../../index.md) 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.
+
+<!-- more -->
+
+## 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](../../../../software_layer.md) 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:
+
+```console
+[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](../../../../available_software/index.md) 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](../../../../using_eessi/setting_up_environment.md);
+  without additional configuration, however, this would clobber the
+  Environment Modules environment. Explicit support for Environment Modules is
+  so far a
+  [proposal](https://modules.readthedocs.io/en/latest/design/eessi.html) 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](../../../../getting_access/native_installation.md#native-install-on-clusters).
+
+## Reading the section
+
+OpenHPC 4.2 includes a
+[Recipe Explorer](https://openhpc.github.io/ohpc/4.2/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](https://openhpc.github.io/ohpc/4.2/explorer/#almalinux10-x86_64-warewulf-slurm?chapter=eessi);
+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](https://github.com/openhpc/ohpc) 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](../../../../support.md).