Count Unique Active Users Design Deep Dive with Google SWE! | Systems Design Interview Question 26 

Excellent content as usual man. Keep these going! These videos are absolute masterpieces in their own league :)

ngl the best I came up with was the indexing approach, learning a lot from these vids! thanks.

Love your efforts! 😎

hey Jordan, is there a particular reason why we are using RDBMS here to capture the status update instead of lets say Cassandra? If we are just doing writes I would assume NoSQL db would be a better option? Also why are we streaming data change in DB if we could send the data straight from the client whenever a user logs in/ logs out? Thank you 🙏

"I took a hyperloglog". Saw that coming from a mile away, but still funny as shit. Literally

Plz let me know how the algo would work. suppose there are 10 active users, all of whose ids would map to rightmost bit of 1( i.e there is no streak of 0s on rightmost bits on any user id)

Great video Jordan. I don’t quite understand how to keep track of users going from active to inactive. (using the Hyperloglog path). You mentioned that each server needs to store a singleUserId at a time. And we want to randomize which server clients hit (so no consistent hashing here). So how do you handle users going offline so we stop keeping track of them in the approximation? I know it will depend on how you define what is an active user, but I’m curious about your thoughts on this. Thanks

hi jordon, please do post Data structures and Algorithm content as well if possible. Loved your session

I had a virtual onsite a few days ago (AWS) where the interviewer laid out a scenario where there was a massively popular mobile app and then asked how I would get a real-time count of active users, stored/grouped by minute, to be displayed on an internal dashboard that would update automatically over time. The data needed to persist so that other services (outside of scope) could query the data. I worked out a plan where the application's login and logout APIs would write login/logout events to a queue, events would be aggregated and compared (logins minus logouts, minus logins with no heartbeat/activity) using Flink or even a simple consumer service, then writing time-sequenced data to a database. Dashboard users would view the dashboard and receive updated data at whatever frequency we wanted over a WebSocket connection... It did seem like overkill but he had really emphasized that it was a massively popular app. After all of that, but before I even talked about caching or what database to use, he said, "Why do all of that? Haven't you heard of Amplify or Firebase? They're real-time and and push the data for you." That caught me by surprise because I had only really heard of Firebase in relation to mobile development and this was a backend SDE (aka SWE) position. He admitted that "firebase or amplify" was really what he was expecting for me to use and that he had been trying to hint at it, sort of implying that I got the wrong answer. Blew me away.

Man ur content is top notch. Funny thing is the kind of algorithms(for exa. hyperloglog) u r talking about many of them I never heard from any other books/videos. I'm just wondering how u keep track of all these novel algorithms.

Very nice video Jordan ! Could you please also do a video on designing cloud managed services like AWS S3

Couple of confusions: 1. how do you define an active user? a user who made a request (in some time frame that we are looking)? or an online user(checking heartbeats of a persistent connection from user's client devices)? 2. When I call API getUniqueUsers(): You mentioned to return unique users currently, what does that mean? Does it mean unique users who made a request in some specific timeframe/window, like last 1hr, last 1 min, last 1 day?

Awesome video Jordan 🙂, i would like to request you for designing a tagging based system like we do it on Facebook/ twitter i.e we tag our friends in a comment or on the picture or hash tags as well . Didn't find any good resources on it

1:46 "webiste" :)

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Noti gang, whats good.