I'm developing a new type (AFAIK) of database that makes many types of real-time multi-player use-cases really easy to implement. The original idea came from implementing something called "rollback netcode" in multi-player video games, but the technique works for all types of applications.
Traditional databases (e.g. sql databases and key-value stores) treat the database as a blob of data. This makes multi-player updates very difficult, as reconciling differences in real-time is a hard problem, hence the invention of CRDTs. My new database, however, treats the data as a series of events. In the example of a text editor, the stream of events might be something like: User 1 connected, User 1 pressed key a, User 2 connected, User 2 pressed b, User 1 disconnected, etc.
In addition to the stream of events, application developers also provide an "engine" that converts the stream of events into a blob of data. The engine runs locally on each connected client. When a client connects to the database, they fetch the engine from the database server. The client then receives the stream of events from the database server in real-time. When a client generates an event, it gets inserted into the local engine immediately and also gets broadcasted to the server. The server then broadcasts the event to other connected clients.
The most important part that makes this work comes from a technique called rollback netcode. Events in the stream are ordered and undo-able. When a connected client generates an event, they insert it into their local engine immediately, but other clients are also generating events simultaneously. If all clients optimistically inserted events locally, the event order would differ across clients. To solve this, the engine automatically will undo events when new events come in from the server. The engine then applies the received event in its proper place and reapplies all the undone events on top. The result is that all users have a consistent view of the data.
The logic of an engine looks something like this:
1) When User X connected, set X's cursor position to offset 0.
2) When User X types a letter, insert text at User X's cursor offset and increase User X's cursor offset by 1. Also increment the cursor offset of all other connected users if their cursor offset is > User X's.
And that's it for a basic insert-only text editor. You can see that applying all the events in order yields the correct state. Adding a moveable cursor and the ability to remove text is also trivial. Basically just the opposite of 1 and 2 above. The whole engine can be implemented in like 100 lines of code and is very easy to reason about.