Client asks for 100% uptime 
I don't understand what the issue is. The client wants you to plan for disaster, and they aren't math oriented, so asking for 100% probability sounds reasonable. The engineer, as engineers are prone to do, remembered his first day of prob&stat 101, without considering that the client might not.When they say this, they aren't thinking about nuclear winter, they are thinking about Fred dumping his coffee on the office server, a disk crashing, or an ISP going down.Furthermore, you can accomplish this. With geographically distinct, independent, self monitoring servers, you will basically have no downtime. With 3 servers operating at an independent[1] three 9 reliability, with good failover modes, your expected downtime is under a second per year [2]. Even if this happens all at once, you are still within a reasonable SLA for web connections, and therefore the downtime practically does not exist.The client still has to deal with doomsday scenarios, but Godzilla excluded, he will have a service that is "always" up.[1] A server in LA is reasonably independent from the server in Boston, but yes, I understand that there is some intersection involving nuclear war, Chinese hackers crashing the power grid, etc. I don't think your client will be upset by this.[2] DNS failover may add a few seconds. You are still in a scenario where the client has to retry a request once a year, which is, again, within a reasonable SLA, and not typically considered in the same vein as "downtime". With an application that automatically reroutes to an available node on failure, this can be unnoticeable.