This week we talk about the "Recent Activity" feature on CodePen. It's tricker than it looks. The beginnings of the idea was based on logging actions for support reasons. For instance, if we log the fact that a user deleted a Pen, there is information about that when we get a support request like "my Pen is gone and I don't know what happened."
But once that system was in place, we could add lots more actions. Like when other users interact with your content and when you interact with theirs. It gets pretty complicated though, and not the least of which of that complication is how you store the data and display it in a friendly readable way.