NoSQL refers to non-relational data stores that break with the history of relational databases and ACID guarantees. Popular open source NoSQL data stores include:

* &lt;a href=&quot;http://incubator.apache.org/cassandra/&quot;&gt;Cassandra&lt;/a&gt; (tabular, written in Java, used by Cisco, WebEx, Digg, Facebook, IBM, Mahalo, Rackspace, Reddit and Twitter)
* &lt;a href=&quot;http://couchdb.apache.org/&quot;&gt;CouchDB&lt;/a&gt; (document, written in Erlang, used by BBC and Engine Yard)
* &lt;a href=&quot;http://github.com/cliffmoon/dynomite&quot;&gt;Dynomite&lt;/a&gt; (key-value, written in Erlang, used by Powerset)
* &lt;a href=&quot;http://hadoop.apache.org/hbase/&quot;&gt;HBase&lt;/a&gt; (key-value, written in Java, used by Bing)
* &lt;a href=&quot;http://www.hypertable.org/&quot;&gt;Hypertable&lt;/a&gt; (tabular, written in C++, used by Baidu)
* &lt;a href=&quot;http://sourceforge.net/projects/kai/&quot;&gt;Kai&lt;/a&gt; (key-value, written in Erlang)
* &lt;a href=&quot;http://memcachedb.org/&quot;&gt;MemcacheDB&lt;/a&gt; (key-value, written in C, used by Reddit)
* &lt;a href=&quot;http://www.mongodb.org/&quot;&gt;MongoDB&lt;/a&gt; (document, written in C++, used by Electronic Arts, Github, NY Times and Sourceforge)
* &lt;a href=&quot;http://neo4j.org/&quot;&gt;Neo4j&lt;/a&gt; (graph, written in Java, used by some Swedish universities)
* &lt;a href=&quot;http://project-voldemort.com/&quot;&gt;Project Voldemort&lt;/a&gt; (key-value, written in Java, used by LinkedIn)
* &lt;a href=&quot;http://code.google.com/p/redis/&quot;&gt;Redis&lt;/a&gt; (key-value, written in C, used by Craigslist, Engine Yard and Github)
* &lt;a href=&quot;http://riak.basho.com/&quot;&gt;Riak&lt;/a&gt; (key-value, written in Erlang, used by Comcast and Mochi Media)
* &lt;a href=&quot;http://github.com/tuulos/ringo&quot;&gt;Ringo&lt;/a&gt; (key-value, written in Erlang, used by Nokia)
* &lt;a href=&quot;http://code.google.com/p/scalaris/&quot;&gt;Scalaris&lt;/a&gt; (key-value, written in Erlang, used by OnScale)
* &lt;a href=&quot;http://code.google.com/p/terrastore/&quot;&gt;Terrastore&lt;/a&gt; (document, written in Java)
* &lt;a href=&quot;http://code.google.com/p/thrudb/&quot;&gt;ThruDB&lt;/a&gt; (document, written in C++, used by JunkDepot.com)
* &lt;a href=&quot;http://1978th.net/tokyocabinet/&quot;&gt;Tokyo Cabinet/Tokyo Tyrant&lt;/a&gt; (key-value, written in C, used by Mixi.jp (Japanese social networking site))

I&#39;d like to know about specific problems you - the SO reader - have solved using data stores and what NoSQL data store you used.

Questions:

* What scalability problems have you used NoSQL data stores to solve?
* What NoSQL data store did you use? 
* What database did you use prior to switching to a NoSQL data store?

&lt;b&gt;I&#39;m looking for first-hand experiences, so please do not answer unless you have that.&lt;/b&gt;


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What scalability problems have you encountered using a NoSQL data store?