There are already some open source alternatives to datomic. TerminusDB (https://github.com/terminusdb/terminusdb) for example is implemented in prolog (and Rust) so has the datalog variant query power that makes datomic so powerful. If you want free as in speech (thou I love free beer).
XTDB is also worth mentioning, especially since they’re on the HN front page with a v2 early access announcement. There are differences in how they do things. I can’t meaningfully comment on business usage of either or what the trade-offs between them are.
