Photon is the production ready event store from the Muon project. It is scalable, federated etc. All the good things.
It is not, however, easy to embed in every language. If you are on the JVM, it can be done, but outside of that, it’s hard.
This project provides purely in memory, embedded implementations of the streaming features of Photon to give a good experience of using an event store during testing.
They do not currently provide implementations of the projection features.