What free realtime backends have you worked with in prod (not BAAS)?

Question

Database

Good afternoon,
please tell us about your experience of using any ready-made open-source backends in realtime production. To be understanding, the author (i.e. me) of the question, very superficially represents the modern back. I do most of my projects on FB or self-written (flask) creepy rest api, so I will be grateful for any of your thoughts and considerations on this matter.
PS At the moment I'm trying to write a small pet-project (messenger with CRM functionality) and collecting information about a possible subject area.