I can see 2 big companies like Klarna and Whatsapp are using Mnesia as their in memory database (not sure how they persist data with Mnesia with 2GB limit). My question is: why companies like those, and may be more I don't know, use Mnesia instead of Riak or couchDB, both are Erlang, where both databases support faster in memory databases, better painless persistence, and much more features. Do I miss something here?
You are missing a number of important points:
First of all, mnesia has no 2 gigabyte limit. It is limited on a 32bit architecture, but hardly any are present anymore for real work. And on 64bit, you are not limited to 2 gigabyte. I have seen databases on the order of several hundred gigabytes. The only problem is the initial start-up time for those.
Mnesia is built to handle:
The design is such that you avoid a separate process since data is in the Erlang system already. You have QLC for datalog-like queries. And you have the ability to store any Erlang term.
Mnesia fares well if the above is what you need. Its limits are:
My normal goto-trick is to start out with Mnesia in Erlang-systems and then switch over to another system as the data size grows. If data sizes grows slowly, then you can keep everything in memory in Mnesia and get up and running extremely quickly.
very attentively. There are no actual limits for mnesia disk table size.
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