Cassandra

I was reading instagram system design question

He said he will be using Cassandra for tables storing user, photo metadata, user follows tables... I didn't understand why he is using Cassandra and why not regular rdms like PostgreSQL or nosql like MongoDB.

Why should I consider DataStax Enterprise instead of vanilla Cassandra?

Developing a solution that collects Telemetry Data from different devices, nearly 1000 devices minimum and maximum 12000. Each device is sending 2 packets in 1 second. This is time-series data, and this data definition and different reports are saved on PostgreSQL. Like Building information, maintenance records, etc. I want to know about the best solution. This data is required for Math and ML to run different algorithms. Also, data is raw without definitions and information stored in PostgreSQL. Initially, I went with TimescaleDB due to PostgreSQL support, but to increase in sites, I started facing many issues with timescale DB in terms of flexibility of storing data.

My major requirement is also the replication of the database for reporting and different purposes. You may also suggest other options other than Druid and Cassandra. But an open source solution is appreciated.