Best Way to Track Read and Unread Messages in a Shared Inbox?

Your proposed approach of using a join table is the typical way to handle this 'many-to-many' relationship. One alternate method is to store an array of user ids in the message table to indicate who has read it, but that could complicate things. Stick with the join table—it'll be more effective in the long run.

Yep, you're right on track with your design. Another option might be to implement a 'last_read_at' timestamp for threads rather than individual messages if it's a conversation. When a user marks a thread as read, you update that timestamp. If they mark it unread, you can either delete that timestamp or set it to null, depending on how you want to handle it.

Just like others mentioned, using a relation table is necessary here. While there may be other solutions like websocket and long polling for real-time updates on status changes, managing the read/unread state efficiently still relies on your initial database design.

Storing read/unread status for each user and message combination is indeed standard practice if you expect different users to have different states. While 500,000 entries sound massive, modern databases handle this easily with appropriate indexing. You might want to test this setup in practice by inserting fake data and using some query analysis tools to ensure performance is acceptable. Remember, optimizing should happen based on actual performance metrics rather than assumptions!

Your strategy sounds solid. A lot of developers go with the read/state mapping method when user-read states differ. It's not too much data for a good database. One optimization I've seen is to only log entries for READ messages instead of marking every message. This means unread is the default, and you only add a row when a user reads a message, which can save a significant amount of space!