[Cassandra Wiki] Update of "Counters" by JorisvanderWel

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The "Counters" page has been changed by JorisvanderWel:
http://wiki.apache.org/cassandra/Counters?action=diff&rev1=14&rev2=15

Comment:
Added a small clarification to the Counter limitations (checked with rev 1197311)

  
    * If a write fails unexpectedly (timeout or loss of connection to the coordinator node) the client will not know if the operation has been performed. A retry can result in an over count [[https://issues.apache.org/jira/browse/CASSANDRA-2495|CASSANDRA-2495]].
    * Counter removal is intrinsically limited. For instance, if you issue very quickly the sequence "increment, remove, increment" it is possible for the removal to be lost (if for some reason the remove happens to be the last received messages). Hence, removal of counters is provided for definitive removal only, that is when the deleted counter is not increment afterwards. Note that if you need to reset a counter, you can read its ''value'' and insert ''-value''. 
+   * `CounterColumnType` may only be set in the `default_validation_class`. A column family either contains only counters, or no counters at all.
  
  == Further reading ==
  See [[https://issues.apache.org/jira/browse/CASSANDRA-1072|CASSANDRA-1072]] and especially the [[https://issues.apache.org/jira/secure/attachment/12459754/Partitionedcountersdesigndoc.pdf|design doc]] for further information about how this works internally (but note that some of the limitation fixed in these technical documents have been fixed since then, for instance all consistency level '''are''' supported, for both reads and writes).