The clustering chapter of the guide must explain those elements a bit better:

• Shared HTTP cache (using Varnish)
• Shared Persistence cache and Sessions (using Memcached, or experimentally also Redis)
• Shared Database (using MySQL/MariaDB)
• Shared Filesystem (using NFS, or experimentally also S3)

Suggestions

Using varnish

Should at least link to our Using Varnish documentation.

Shared persistence cache and sessions

This is a critical part, and how to achieve that is burried in the Repository guide.

What we could do is move the memcache(d) chapter out of Repository, into Clustering. It is a very specialized scope for the repository, and better belongs out there. One sentence that mentions that custom configuration of persistence cache is required for multi-server, linking to the Clustering chapter, would work brilliantly.

We have the exact same situation with sessions, where [memcache(d) setup|Sessions-Clustersetup] is part of the Sessions guide. Given that it is more about Clustering than about sessions, I suggest we do the same movee-linkee-job stuff than for persistence cache.

An alternative to both would be to link from the clustering guide to the Repository / Sessions from the Clustering guide.

Shared database

This shouldn't require much extra doc, imho

Shared filesystem

This could be rephrased to be about binaries. Furthermore, it is what is actually covered in the "Binary files clustering" below, but not linked.