My new favorite open source project
I have a very strong background in databases as I started my career as Oracle DBA then moved to Cloud and Big Data. Now, I am learning and sharing more on Blockchain. I did forget to mention that NoSql is also one of the areas of interest. Personally, I heart Apache Cassandra but now, there is more love for LevelDB.
LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values. Source: https://github.com/google/leveldb
Now, go to https://github.com/bitcoin/bitcoin and search for leveldb or click https://github.com/bitcoin/bitcoin/search?utf8=%E2%9C%93&q=leveldb&type=
You have to read this thread to learn more about Bitcoin and leveldb. LevelDB is being used to store the chain state. https://en.bitcoin.it/wiki/Bitcoin_Core_0.11_(ch_2):_Data_Storage#Use_of_LevelDB
Initially, it was started with SqlLite (Definitely not production ready database) but then (Glad) it moved to LevelDB
As Ethereum is fork of Bitcoin so by default it uses LevelDB too