in 2011 I wrote about the Bus Factor, i.e. the minimum number of people on your project (or in your support team) whose loss would cause serious issues for your project/support team. The name of this factor derives from the possibility of one or more team members getting hit by a bus. An alternative (less tragic) name - highlighted by Angela Hall at the time - is "lottery factor", i.e. we assume that one or more people got a big win on the lottery and immediately left work, never to return. Either way, it's a serious factor and must be managed.
At the time, I offered a number of techniques to help increase your team's bus factor (a good thing). Here are a few more that I use, all focused on the greater sharing of knowledge. If you ingrain the techniques of active and deliberate knowledge sharing into your team members then you need worry less about your bus factor, but don't completely take your eye off the ball - remember to manage it.
Push-Based Knowledge Sharing. The person who holds the knowledge about something asks a person who does not know about it to join them to learn about it. They thereby PUSH the information towards the other person.
Pull-Based Knowledge Sharing. The person who does not have knowledge about something asks another person who knows about it to teach them about it in some way. In this way, they establish a PULL of the information from the other person.
Pairing is hard to achieve in organisations where pairing was never a “thing” people do. If you cannot get enough people to pair, or the bus factor is happening when a person from a different team knows something that your team replies on, it’s time to start encouraging implicit knowledge gathering, or implicit learning.