**When:** During a pre-defined DBA maintenance windows
** Prerequisites **: https://wikitech.wikimedia.org/wiki/MariaDB/Primary_switchover
[] Team calendar invite
**Affected wikis:**: https://noc.wikimedia.org/conf/highlight.php?file=dblists/s4.dblist
**Checklist:**
NEW primary: db1138
OLD primary: db1160
[] Check configuration differences between new and old primary:
```
sudo pt-config-diff --defaults-file /root/.my.cnf h=db1160.eqiad.wmnet h=db1138.eqiad.wmnet
```
**Failover prep:**
[] Silence alerts on all hosts:
```
sudo cookbook sre.hosts.downtime --hours 1 -r "Primary switchover s4 T311611" 'A:db-section-s4'
```
[] Set NEW primary with weight 0 (and depool it from API or vslow/dump groups if it is present).
```
sudo dbctl instance db1138 set-weight 0
sudo dbctl config commit -m "Set db1138 with weight 0 T311611"
```
[] Topology changes, move all replicas under NEW primary
```
sudo db-switchover --timeout=25 --only-slave-move db1160 db1138
```
[] Disable puppet on both nodes
```
sudo cumin 'db1160* or db1138*' 'disable-puppet "primary switchover T311611"'
```
[] Merge gerrit puppet change to promote NEW primary: FIXME
**Failover:**
[] Log the failover:
```
!log Starting s4 eqiad failover from db1160 to db1138 - T311611
```
[] Set section read-only:
```
sudo dbctl --scope eqiad section s4 ro "Maintenance until 06:15 UTC - T311611"
sudo dbctl config commit -m "Set s4 eqiad as read-only for maintenance - T311611"
```
[] Check s4 is indeed read-only
[] Switch primaries:
```
sudo db-switchover --skip-slave-move db1160 db1138
echo "===== db1160 (OLD)"; sudo db-mysql db1160 -e 'show slave status\G'
echo "===== db1138 (NEW)"; sudo db-mysql db1138 -e 'show slave status\G'
```
[] Promote NEW primary in dbctl, and remove read-only
```
sudo dbctl --scope eqiad section s4 set-master db1138
sudo dbctl --scope eqiad section s4 rw
sudo dbctl config commit -m "Promote db1138 to s4 primary and set section read-write T311611"
```
[] Restart puppet on both hosts:
```
sudo cumin 'db1160* or db1138*' 'run-puppet-agent -e "primary switchover T311611"'
```
**Clean up tasks:**
[] Clean up heartbeat table(s).
```
sudo db-mysql db1138 heartbeat -e "delete from heartbeat where file like 'db1160%';"
```
[] change events for query killer:
```
events_coredb_master.sql on the new primary db1138
events_coredb_slave.sql on the new slave db1160
```
[] Update DNS: FIXME
[] Update candidate primary dbctl and orchestrator notes
```
sudo dbctl instance db1160 set-candidate-master --section s4 true
sudo dbctl instance db1138 set-candidate-master --section s4 false
(dborch1001): sudo orchestrator-client -c untag -i db1138 --tag name=candidate
(dborch1001): sudo orchestrator-client -c tag -i db1160 --tag name=candidate
```
[] Check zarcillo was updated
** db-switchover should do this. If it fails, do it manually: https://phabricator.wikimedia.org/P13956
```
sudo db-mysql db1115 zarcillo -e "select * from masters where section = 's4';"
```
[] (If needed): Depool db1160 for maintenance.
```
sudo dbctl instance db1160 depool
sudo dbctl config commit -m "Depool db1160 T311611"
```
[] Apply outstanding schema changes to db1160 (if any)
[] Update/resolve this ticket.