It was a fault-tolerant, fully distributed relational database which was compatible with MySQL's variant of SQL. There were no key-value stores involved.
Tables (and indexes) were automatically partitioned and replicated as needed, completely under the covers.
Queries (reads and writes) were distributed to the nodes where the data resided, in parallel.
Scaling the system was as simple as adding new nodes. Data was automatically rebalanced to take advantage of the new capacity.
Failure recovery was automatic too. If a disk or node failed, the data involved would be reconstructed from replicas and moved elsewhere with no interruption in service and no failed transactions.
It was a pretty impressive system, which predated Google Spanner. But, in the early days, you had to run their custom hardware to get it. There was no cloud version.
Alright, for your education startup employees, let's run through some numbers I have as a stockholder in Clustrix.
In 2010, they raised $12M in their Series B at ~$100M post-money valuation. Things were looking alright.
In 2013, they raised $16.5M in their Series C, and then shortly thereafter $10M in an unusual series D. That funding round reverse-split the outstanding stock 26-to-1 and converted all existing shares, preferred or otherwise, to common stock. What was left was $10M in new preferred stock, and $20M in existing common stock! New post-money valuation: $30M. This down round ended up being a 30x dilution for existing shareholders. If you had a tenth of a percent of $100M before, now you had a hundredth of a percent of $30M. Yowza!
After that bath, the board amended the charter so they stopped mailing out these notices. I don't know what's happened since, but I'll find out soon enough.
I feel bad that the company wasn't successful. It really was a great team and an impressive technical feat.
Early Clustrix employee here. Holy cow that took a long time! 12 years since the company was founded and finally they exit. I can't wait to exchange my illiquid startup stock for... stock in a _different_ private company.