[GitHub] keith-turner commented on issue #527: Add set if non-existant operation

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keith-turner commented on issue #527: Add set if non-existant operation
URL: https://github.com/apache/fluo/issues/527#issuecomment-340588227
 
 
   This is a very rough outline of what I think needs to be done for this feature.
   
    * Add `setIfAbsent(Bytes row, Column col, Value val)` method to TransactionBase interface.
    * Implement `setIfAbsent(Bytes row, Column col, Value val)` in TransactionImpl.  This method should call `set(Bytes row, Column col, Value val)` and remember the row+col in special map that tracks row+cols that expected to be absent.  
    * In the [prewrite phase](https://github.com/apache/fluo/blob/rel/fluo-1.1.0-incubating/modules/core/src/main/java/org/apache/fluo/core/impl/TransactionImpl.java#L340) of `commit()` when a transaction sets its locks, I think a condition can be added to the conditional mutation used for locks.  This condition could configure the snapshot iterator and expect the result to be empty.
   
   Some background on conditional mutations :  http://rleary.com/blog/accumulo-conditional-mutations/
   
   Need to determine the correct behavior for the case when a transactions calls `set()` and then `setIfAbsent()` (or visa-versa) for the same row+col.  Currently if `set()` is called twice for the same row+col an exception is thrown.

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