Cassandra is open source and used in a large number of IoT applications. Most famous one being Nest.
Also as far as throughput Netflix is doing 1.5 trillion (yes trillion) transactions per day in production on Cassandra.
Also as far as throughput Netflix is doing 1.5 trillion (yes trillion) transactions per day in production on Cassandra.
I know of a production IoT system in the private sector that does 1.5 trillion (quasi-)transactions every 10 minutes, so almost three orders of magnitude higher throughput. Cassandra is an okay choice for storing IoT data but it isn't real-time in the sense that you can do immediate, fast queries about the relationships across those records as they are hitting the system.
That depends on how you are using Cassandra. Typically, you are expected to know your query patterns up front, and so you will lay your data out accordingly when ingesting. When done properly, this allows for ~1ms queries that return completely up-to-date results.
Also, to nitpick, these are not transactions; these are "operations" or some other word that doesn't imply what the word "transactions" implies.