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From Apache Wiki <>
Subject [Cassandra Wiki] Update of "API" by JoeStump
Date Tue, 25 Aug 2009 00:15:01 GMT
Dear Wiki user,

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The following page has been changed by JoeStump:

The comment on the change is:
Added notes on ConsistencyLevel.

   UnavailableException:: Not all the replicas required could be created and/or read.
   TApplicationException:: Internal server error.
+ == Structures ==
+ === ConsistencyLevel ===
+ The `ConsistencyLevel` is an `enum` that controls both read and write behavior based on
`<ReplicationFactor>` in your `storage-conf.xml`. The different consistency levels have
different meanings, depending on if you're doing a write or read operation.
+ ==== Write ====
+ ||'''Level'''||'''Behavior'''||
+ ||`ZERO`||Only ensure that the write has been written to the commit log.||
+ ||`ONE`||Ensure that the write has been written to at least 1 node's commit log and memory
+ ||`QUORUM`||Ensure that the write has been written to `<ReplicationFactor> / 2 + 1`
+ ||`ALL`||Ensure that the write is written to `<ReplicationFactor>` nodes.||
+ ==== Read ====
+ ||'''Level'''||'''Behavior'''||
+ ||`ZERO`||Not supported.||
+ ||`ONE`||Will return the record returned by the first node to respond. A background thread
is always fired off to fix any consistency issues when `ConsistencyLevel.ONE` is used. This
means subsequent calls will have correct data while the initial read may not.||
+ ||`QUORUM`||Will query all storage nodes and return the record that is prevailing in consistency.
For instance, if `foo = 1` on nodes A and B, while `foo = 2` on node C then the prevailing
consistency is `foo = 1`. A background thread will be fired off to fix consistency issues.||
+ ||`ALL`||Not supported.||

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