How many transactors should I have on Datomic?

Question

Or why would I want more than a first transactor and a second for availability?

Answer 1

In general you would want exactly one transactor at all times.

the second one prevents it from being unavailable, if the master is unavailable becuase someone trips over the power cord or deletes the instance.

Datomic is primarily designed for systems that are not throttled on writes, and preferably where many clients are doing interesting local queries across possibly more than one database. Having a single transactor is Datomic's main method of maintaining ACID semantics, and separating queries from transactions allows it to scale on the query side without resorting to eventual consistency on the writing side.