Glossary

Key Points

Introduction	Full CMSSW release containers are very big. It is more practical to use light-weight containers and obtain CMSSW via CVMFS. The centrally supported way to run CMSSW in a container is using Apptainer.
Using full CMSSW containers	Standalone CMSSW containers are currently not routinely built due to their size. They need to be built/requested when needed.
Containers based on CVMFS	The light-weight CMS containers need to mount CVMFS. They will only work with CVMFS available.
Accessing CVMFS from Docker locally	You can install CVMFS on your local computer. The `cvmfs-automounter` allows you to provide CVMFS to other containers on Linux. Privileged containers can be dangerous. You can mount CVMFS from within a container on container startup.
Using Singularity	Singularity needs to be used on LXPLUS. CMS Computing provides a wrapper script to run CMSSW in different Linux environments (SLC5, SLC6, CC7, CC8). To run your own container, you need to run Singularity manually.
Using unpacked.cern.ch	The `unpacked.cern.ch` CVMFS area provides a very fast way of distributing unpacked docker images for access via Singularity. Using this approach you can run versioned and reusable stages of your analysis.
Finding container images	Container images are available from different registries. The images can be found in different ways. There are most likely already images available that work for your analysis. You can build on top of those images.

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