title: What are the most mature and high performance options for attribute-value databases?
body: <p>I am working in electronic health records domain, and the standard I'm using (http://www.openehr.org) creates a serious mismatch in many use cases if one attempts to use relational databases. </p>
  
  <p>I've managed to develop some fairly successful methods of using relational databases to handle mostly tree structures, but I could have done a lot better if I have used a key-value type of storage. </p>
  
  <p>The problem is, the RDMS concept has become so dominant in market that maturity is almost exclusively associated with relational databases. Whenever someone considers moving out of the relational space, especially these days, the NOSQL song begins to play. 
  Most NOSQL options are way too young, and I would have a hard time investing in them to handle sensitive healthcare data. </p>
  
  <p>So I'm looking for mature, open source, high performance options in the non relational space, especially ones convenient for key-value type of operations. BerkeleyDB was one such option for example, but Oracle's current licensing terms do not work for me. </p>
  
  <p>I don't need SQL, I will have to implement a custom query language anyway (which is defined already as part of openEHR specifications). I don't need tables, since my data is all tree structures. I need maturity, stability and performance, I need ACID compliance, scalability, and I need open source. I've even considered bringing together various mature Java frameworks to achieve these goals, and asked a question about it here, but it appears it was not a realistic approach. </p>
  
  <p>Are there any hidden or maybe obvious gems I'm missing?</p>