A couple years ago, Google released an embeddedable key-value database called LevelDB. There was much rejoicing. Recently, Facebook released their fork of LevelDB, RocksDB. There’s a slide deck comparing the two, but the amusing part is the “our way is better!” subtext. A little bit of Valley rivalry there. You can also learn a lot of interesting tidbits about the design of modern, high-performance database systems from the Architecture Guide and Table Format documentation of RocksDB.
Say what you will about tiny, highly-targeted ads. To some extent they are subsidizing a lot of interesting technology development and the open sourcing thereof.