--- title: "The simplest way to count 100 billion unique IDs: Part 1" excerpt: "How to build a simpler, real-time version of Reddit's complex system for counting unique IDs, involving Kafka, Redis, and Cassandra." authors: "Ariel Perez" categories: "I Built This!" createdOn: "2025-03-11 00:00:00" publishedOn: "2025-03-25 00:00:00" updatedOn: "2025-04-24 00:00:00" status: "published" ---

I recently came across an interesting LinkedIn post about how in 2017 Reddit designed the system to count post views and unique viewers:

Using Kafka, Redis, and Cassandra, they built a system to count 100 billion unique 64-bit IDs with only 12KB of storage. Impressive!

However, as someone who's spent quite some time helping companies solve these exact problems, I couldn't help but think: "There's definitely a simpler way to do this."

The Reddit approach

Here's what Reddit had built:

  1. Kafka ingests view events and checks if they should be counted.
  2. They're written to another Kafka topic for counting.
  3. A consumer updates Redis which maintains unique counts.
  4. Cassandra pulls the values from Redis every 10 seconds in case they're evicted.
  5. HyperLogLog for approximation (to save massive amounts of space).

And hey, it works! But, that's a lot of moving parts just to count things.

A simpler solution

What if you could:

Turns out, you can. Here's how.

The implementation

First, I created a Tinybird project to store and count post views. To do this, I used the tb create command in the Tinybird CLI and supplied a prompt to bootstrap a first version. If you want to follow along and don’t have Tinybird installed, you can catch up to me in just 3 steps, detailed here.

I find that it’s best to be as specific as possible with requirements when prompting a project generator to get the best results. 

This is what Tinybird came up with on the first pass:

datasources/post_views.datasource

endpoints/unique_post_viewers.pipe

This endpoint pipe automatically becomes a REST API, for now deployed on localhost:

Tinybird also generates some fake data to match the schema of the data source, so I was able to test that the endpoint did accurately count the unique views stored in the event log.

I can almost hear Javi Santana in the back of my head saying, “BOOM!”

So far, so good. I had a working API, but I did want to make one minor tweak to enforce that post_id is provided in the request:

You can see that as soon as I saved those changes, Tinybird hot reloads. and validates the build.

I then deployed it to the cloud with tb --cloud deploy, and I had a hosted, production-ready API to count unique post viewers.

But does it scale?

The simplicity of this solution is nice, but it's not a solution if it doesn't scale.

So, I tested it using Mockingbird to send 1 post with 10M views and ~1M unique viewers, which is about ~1GB of raw data. These are the results: 

While this isn't the 100B views in the post, it gives me an idea of how quickly I can read the data for a single post. Given how it's partitioned and sorted, I’d expect consistent performance even as the number of posts grows (hint: it’s a binary search on the primary key so, O(log2 n)).

As I add more views and unique viewers per post, it scales linearly. This is where the date filters really shine. Extrapolating from this, I’d expect the following for 10K posts with 10M views each (for a total of 100B events):

Of course, I’d load test this before going to Production to make sure I didn’t make a mistake in my napkin math. 

And yes, this includes all the raw data – not just pre-aggregated counts. Want to slice by hour? Add a dimension? Filter differently? Just modify the SQL.

The trade-offs

Nothing's perfect, so let's be upfront about the limitations:

  1. I'm storing more raw data (but modern columnar compression makes this surprisingly efficient and you can do other things with this data)
  2. Queries do more work than just fetching a pre-computed number
  3. At extreme scales, I might need some optimization (more on this in the next post)

But consider what I'm NOT dealing with:

Getting started

Want to try this yourself? Check out the Tinybird documentation to get started. You'll be up and running in minutes, not days.

What's next?

This solution works great for most use cases. But what happens when you hit real scale? When you need to count trillions of events? When uniqExact starts eating too much memory?

That's what I'll cover in the next post. Stay tuned.