Presumably exchange server is not a single node?

Then it does whatever is needed to make it safe. For example, it might use a hash ring to assign each meeting room to a single node, and that node processes one request at a time. Most distributed systems are like this.

A traditional database funnels all your data changes down to one leader node which then accepts or rejects them, and (if A+C in the case of single node failure is desired) makes sure the data is replicated to follower nodes before accepting.

A distributed database is similar but different pieces of data can be on different leaders and different follower sets.

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