The next step is a test of your technical and nontechnical skills. You must come to agreement with your customers on a maintenance window, that is, when the upgrade will happen. To do that, you must know how long the process will take and have a plan if the upgrade fails. That is more of a technical issue.
• When? Your SLA should include provisions for when maintenance can be done. Customers usually have a good idea of when they can withstand an outage. Most business systems are not needed at night or on the weekend. However, SAs might not want to work those hours, and the vendor support might not be available at certain times.Abalance must be found. Sites that are required to be up 24/7 have a maintenance plan engineered into the entire operation, perhaps including fall-back systems.
• How long? The length of the maintenance window equals the time the upgrade should take, plus the time testing should take, plus the time it will take to fix problems, plus the time it takes to execute the back-out plan, plus the time it takes to ensure that the back-out worked. Initially, it is best to double or triple your estimates to adjust for hubris. As time goes on, your estimates will become more accurate. Whatever length of time you have calculated, announce the window to be much longer. Sometimes, you may get started late. Sometimes, things take longer than you expect for technical reasons (hardware, software, or unrelated or unexpected events) or nontechnical reasons (weather or car problems). The flip side to calling a longer time window is that if you complete the upgrade and testing early, you should always notify the customers.
• What time is the back-out plan initiated? It is a good idea to clearly document the exact time that the back-out plan will be initiated for reasons described in step 4.