Best Ways to Manage Read and Unread Messages in a Shared Inbox

You might also want to consider tracking a 'last_read_at' timestamp per conversation. This way, you can show when the last time a user read a conversation. If a user wants to mark it as unread, you could update this timestamp to null—keeping it flexible for your users.

Your setup with a join table is pretty standard. It models the many-to-many relationship well. As an alternative, you could think about having an array of user IDs in the message table to track who has read it, but that might complicate things more than it's worth.

If you're looking for real-time updates, consider using websockets or long polling techniques. But honestly, the relational approach seems solid for managing read/unread states in your shared inbox.

It seems you’re on the right track with the join table approach. If user states can differ, like some marking messages as read while others mark the same as unread, then you’ll need a solution like yours. 500,000 rows is quite manageable with good indexing. One method people use is to store only when a user reads a message instead of every state. So by default, everything is marked as unread until a user reads it, which helps save on storage.

To track the read/unread state for N messages across M users, you're right that it typically requires storing N * M records. The good news is that most modern databases handle that size easily. Ensure you set up proper indexes so queries can run efficiently. I suggest testing it out; set up a Postgres instance and throw in some fake data to see how it performs. You might find that spending time optimizing may not be necessary since storage isn't that expensive these days.