update percona cluster from 5.6 to 5.7

Based on percona update procedure (https://www.percona.com/doc/percona-server/5.7/upgrading_guide_56_57.html) you need to do :

sudo service mysql stop
sudo apt-get install percona-server-server-5.7
sudo mysql_upgrade
sudo service mysql restart

In my case I have a cluster with 3 nodes (multi-master):

My questions are :

1. Do I need to run mysql_upgrade on all nodes or only on first one upgraded ?
2. Is there any problem if until I complete update on all nodes some of them will run 5.6 and some 5.7 ?

Kind regards, Silviu

I found the answer in the Percona XtraDB documentation: (https://www.percona.com/doc/percona-xtradb-cluster/LATEST/howtos/upgrade_guide.html)

Basically you have to do for each node in the cluster:

First open 2 ssh sessions

on session 1

sudo service mysql stop
sudo apt-get remove percona-xtrabackup* percona-xtradb-cluster*
sudo apt-get install percona-xtradb-cluster-57
sudo service mysql stop
cp /var/lib/mysql/grastate.dat /etc/www/grastate.dat
sudo mysqld --skip-grant-tables --user=mysql --wsrep-provider='none'

then go to sesison #2

open session 2

mysql_upgrade
sudo mysqld stop
sudo service mysql restart