cassandra: Add documentation on how to upgrade a cassandra cluster

With the current static cluster.

Refs. swh/infra/sysadm-environment#5415

docs/sysadm/data-silos/cassandra/index.rst

@@ -8,3 +8,4 @@ Cassandra
 
 .. toctree::
    installation
+   upgrade

docs/sysadm/data-silos/cassandra/upgrade.rst

+.. _cassandra_upgrade_cluster:
+
+How to upgrade a cassandra cluster
+==================================
+
+.. admonition:: Intended audience
+   :class: important
+
+   sysadm staff members
+
+
+This page document the actions to upgrade a cassandra cluster.  The overall
+plan is to upgrade each node of the cluster one at a time.
+
+Our (static) cassandra clusters are managed through puppet.
+
+As we need to upgrade the nodes in a rolling upgrade fashion, we will want to
+stop the puppet agent from running.
+
+So first, connect to pergamon and trigger a puppet agent test (just in case
+some pending actions need to be applied) then stop the puppet agent.
+
+
+.. code-block:: shell
+
+   $ clush @staging-nodes 'puppet agent --test && \
+     puppet agent --disable "Upgrade to cassandra"'
+
+Then identify the desired new version and retrieve its sha512 hash.
+
+https://archive.apache.org/dist/cassandra/4.0.15/apache-cassandra-4.0.15-bin.tar.gz
+https://archive.apache.org/dist/cassandra/4.0.15/apache-cassandra-4.0.15-bin.tar.gz.sha512
+
+In the swh-site repository, adapt the environment's common.yaml file with
+those values:
+
+.. code-block:: yaml
+
+    $ echo $environment
+    staging
+    $ grep "cassandra::" .../swh-site/data/deployments/$environment/common.yaml
+    cassandra::version: 4.0.15
+    cassandra::version_checksum: 9368639fe07613995fec2d50de13ba5b4a2d02e3da628daa1a3165aa009e356295d7f7aefde0dedaab385e9752755af8385679dd5f919902454df29114a3fcc0
+
+Commit and push the changes.
+
+Connect to pergamon and deploy those changes.
+
+Then connect on each machine of the cluster in any order (lexicographic order
+is fine though).
+
+We'll need the nodetool access, so here is a simple alias to simplify the
+commands (used for the remaining part of the doc).
+
+.. code-block:: shell
+
+   $ USER=$(awk '{print $1}' /etc/cassandra/jmxremote.password)
+   $ PASS=$(awk '{print $2}' /etc/cassandra/jmxremote.password)
+   $ alias nodetool="/opt/cassandra/bin/nodetool --username $USER --password $PASS"
+
+
+From another node in the cluster, connect and check the status of the cluster
+is fine during the migration.
+
+.. code-block:: shell
+
+   $ period=10; while true; do \
+       date; nodetool status -r; echo; nodetool netstats; sleep $period; \
+     done
+
+
+Now, we can remove the node from the cluster (in terms of writing). It will
+stop the writing in that node so we can stop it without losing data.
+
+.. code-block:: shell
+
+   $ nodetool drain
+
+
+Lookup for the '- DRAINED' pattern in the service log to know it's done.
+
+.. code-block:: log
+
+   $ journalctl -e cassandra@instance1 | grep DRAINED
+   Nov 27 14:09:06 cassandra01 cassandra[769383]: INFO  [RMI TCP Connection(20949)-192.168.100.181] 2024-11-27 14:09:06,084 StorageService.java:1635 - DRAINED
+
+
+We stop the cassandra service.
+
+.. code-block:: shell
+
+    $ systemctl stop cassandra@instance1
+
+
+In the output of the `nodetool status`, the node whose service is stopped
+should be marked as DN (Down and Normal):
+
+   $ nodetool -h cassandra02 status -r | grep DN
+   DN  cassandra01.internal.softwareheritage.org  8.63 TiB  16      22.7%             cb0695ee-b7f1-4b31-ba5e-9ed7a068d993  rack1
+
+
+Finally we upgrade cassandra version in the node (through puppet):
+
+.. code-block:: shell
+
+    $ puppet agent --enable && puppet agent --test
+
+Let's check the correct version is installed in /opt
+
+.. code-block:: shell
+
+   $ ls -lah /opt/ | grep cassandra-$version
+   lrwxrwxrwx  1 root root   21 Nov 27 14:13 cassandra -> /opt/cassandra-$version
+   drwxr-xr-x  8 root root 4.0K Nov 27 14:13 cassandra-$version
+
+
+Now start back the cassandra service.
+
+.. code-block:: shell
+
+    $ systemctl start cassandra@instance1
+
+Once the service is started again, the `nodetool status` should display an
+`UN` (Up and Normal) status again for the node upgraded.
+
+   $ nodetool status -r
+   ...
+   UN  cassandra01.internal.softwareheritage.org  8.63 TiB  16      22.7%             cb0695ee-b7f1-4b31-ba5e-9ed7a068d993  rack1
+
+Finally, check the version is the expected one.
+
+.. code-block:: shell
+
+   $ nodetool version
+   ReleaseVersion: $version