I'm making website and I have different entities: users, comments, forum threads. It's something like wordpress with simple forum but I built it m...
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The only approach that I can think of right now is:
Then, you can apply a S3 lifecycle policy to the temp directory and have the non-posted images expire after X days, with an expiration action that deletes them. That would make the temp directory self cleaning and all the in-use files would be in the perm directory.
Now, if you also wanted to clean up the old image rows in the database, you could add a DateCreated and IsPosted columns to the image table and set those when the user posts the comment/thread.
Then create a computed column IsDeleted which uses both the IsPosted flag and DateCreated datetime column to return true when IsPosted is false and X days (from the S3 lifecycle policy) have past.
Now you'd only need a cron job to clean up the unused image rows. Or if you didn't care about cleaning up the image rows, use the IsDeleted flag in your SQL queries, so that you could ignore the deleted images when querying the images table.
Just make sure to add an index on IsPosted and DateCreated, so that the cron job is super fast in finding the image rows it needs to delete.
If you want additional suggestions on this, maybe add the #webdev tag to the post. This seems like something web developers would encounter frequently. You would need to remove a tag to add one since limit is 4. Maybe remove #algorithms since this isn't really an algorithms thing, and then add #webdev. That might help get your question into feeds of more people likely to have relevant suggestions on how to solve this issue.
I agree with the suggestion in mykezero's comment. I was about to suggest the same. Put images in a temporary location while user drafting post. If they abandon the post then remove them. If they post it, move images to a more long term location.