In the newest 2.2 version of the Forum plugin, a subscription system was added. This system allows for users to subscribe to topics and forums and then receive email notifications at specific intervals. This feature required minor database changes so a new upgrade shell was added to handle this. You will only need to run the upgrade shell if you are upgrading from 2.0 or 2.1. If you are doing a fresh install, you can ignore this post and the upgrade shell.
Begin by downloading the latest 2.2 tag and uploading and overwriting the older version. Your forum will be broken until you run the upgrade shell and the following command. Once the command is executed, simply follow the on-screen instructions.
cake -app /path/to/app upgrade
Setting up the subscription notification cron
For the subscription emails to be sent out, the subscription shell will need to be triggered. The best way to accomplish this is to setup a cron job to be triggered every 24 hours (or another time of your liking). If you are unable to setup a cron job, you will need to disable the subscription system via the forum admin settings.
0 0 * * * /path/to/cake/console/cake -app /path/to/app subscription
By default the shell will find all new activity within the past 24 hours via the database timestamp (which is separate from the cron interval time). You can change this using the timeframe parameter. For example, if you want to send emails every hour instead of every 24 hours (quotes are required).
0 * * * * /path/to/cake/console/cake -app /path/to/app subscription -timeframe "1 hour"
Hopefully this small guide helps you upgrade to 2.2, enjoy!