“Keep multiple copies of a service running in different locations, accept modifications everywhere, and disseminate all modifications among them.”
Given a group of people, we have a basic understanding of what consensus means: within the group, all members agree on a proposal and acknowledge that this agreement is unanimous. From personal experience, we know that reaching consensus is a process that can take quite a bit of time and coordination effort, especially if the matter starts out as being contentious—in other words, if initially there are multiple competing proposals, and the group must decide which single one to support.