I really want to build a raspberry pi cluster but I don't know what to do with it...
Has anyone of you ever built a raspi cluster?
What could I do with it?

You can set up a home automation cluster using Home Assistant. Or you can set up a local PaaS using kubernetes, or maybe bitcoin mining :) good luck with your project

Bitcoin mining? On a Pi? Raaaaight.

@JohnDoesNot I guess a cluster means more than one

*Free web server

@raze Bitcoin mining is done on GPU power. Which a Pi hasn’t. Bitcoin mining is already nearly impossible on a heavy personal server stack.

@JohnDoesNot ok, mining can be performed on any CPU, the difference with a GPU is the parallelization performed as a hardware level, is for sure more slow to mine or perform cryptographic operations in ARM processors but still possible. Here's an example https://qz.com/1117836/...
And Here's a tutorial http:///www.instructables.com/id/...

You CAN mine with ASICs and just use the Pis for control etc.

@raze the point is not possibility, but profitability. Dedicated ASICs are faster more efficient by many orders of magnitude, mining on an RPi cluster would just fry the cpus needlessly.
It's like trying to win a F1 race by walking.

osmark's idea would be much more reasonable though

I can recommend a small Pi cluster if you want to experiment with scaling your backend - be it node, Java, C# or whatever, a Raspberry can likely run it.

While ARM processors will perform a bit different than enterprise level x64 or ia64 Xeon servers, you will still run into the same problems/challenges of a multithreading, multi-server architecture which you would on way more expensive dedicated servers.

I use 4 Pis to test my clustered node backend and can run automated tests on all 16 cores that test nicely if my changes break the assumption that the backend scales "infinitely".

The production servers may cost multiple 100 euros per month, the compete cluster was less than 150€.

(btw: Viennese devs, assemble!!!)