[diziet]

Yes, that was my idea as well. They must have some sort of cache system for serving basic user meta-data at scale when a page is loaded, and they could add a time-expiring list post ids of the posts viewed by a user to do detection on a per user basis on the backend.

I think they want to break it into different services for (whatever) reasons.

Running a counter across a sharded in memory cache implementation (like Redis) for something like post views should be ok, but they might have some weird rule-sets for spam detection that are slow.

[sethammons]

I'm not following your solution for how it handles the unique visitor counting. You have to know when to increment the counter. Thus why they used the HLL.

[diziet]

Ah, sorry if it wasn't clear. By storing an expiring list post IDs in the close cache for loading the user metadata, you could filter out the posts that were already visited by that user.