At this point, the customers are notified that the upgrade is complete or, if the back-out plan was initiated, what was accomplished, what didn’t get accomplished, and the fact that the systems are usable again. This has three goals. First, it tells people that the services they have been denied access to are now usable. Second, it reminds the customers what has changed. Finally,
if they find problems that were not discovered during your own testing, it lets them know how to report problems they have found. If the back-out plan was initiated, customers should be informed that the system should be operating as it had before the upgrade attempt.
Just as there are many ways to announce the maintenance window, there are many ways to communicate the completion. There is a catch-22 here. Customers cannot read an email announcement if the email service is affected by the outage. However, if you keep to your maintenance window, then email, for example, will be working and customers can read the email announcement. If customers hear nothing, they will assume that at the end of the announced maintenance window, everything is complete.
Announcements should be short. Simply list which systems or service, are functioning again, and provide a URL that people can refer to for moreinformation and a phone number to call if a failed return to service might prevent people from being able to send email. One or two sentences should be fine.
The easiest way to keep the message short is to forward the original email that said that services were going down, and add a sentence to the top, saying that services are re-enabled and how to report problems. This gives people the context for what is being announced in a very efficient way.