Bitcoin blockchain in mongodb
On the subject of why LevelDB is used, core developer Greg Maxwell stated the following to the ref:


  I think people are falling into a trap of thinking "It's a &lt;database&gt;,
  I know a &lt;black box&gt; for that!"; but the application and needs are
  very specialized here. . . It just so happens that on the back of the
  very bitcoin specific cryptographic consensus algorithim there was a
  slot where a pre-existing high performance key-value store fit; and so
  we're using one and saving ourselves some effort...