[Hadoop Wiki] Update of "Hbase/ColumnCaching" by Misty

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The "Hbase/ColumnCaching" page has been changed by Misty:
https://wiki.apache.org/hadoop/Hbase/ColumnCaching?action=diff&rev1=2&rev2=3

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- Overall goal: 
- 	To early out at as many places as possible.
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- Overall structure:
- 	The cache is implemented in Memcache.java as an extra step to find the requested columns
- 	Add early outs in HStore.java to check if the data needed is returned from memCache.
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- 	Add logger for stats, to see hits and misses in the cache.
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- Structure of cache:
- 	cacheKey(row+column) -> Value
- 	//and maybe row:family -> fullFlag(to indicate that all columns of the latest timestamp is in cache)
- 	and a hashset with the rows:families that are full
- 	cacheKey class with byte[] row, byte[] column
- 	and compareTo functions, to sort the inputs, should look like HStoreKey, but minimized since there is
- 	no need for timestamp and versions and so on, to save memory.
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- 	//Need a way to remove data from cache, when it is full, easiest probably to have a seperate list(FIFO) with just the
- 	//keys. 
- 	Start with soft references
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- Input From:
- 	get - memCache
- 		Check if key is in the cache, otherwise get it from mapfiles, put in cache on the way out
- 		Need to check the the number of versions wanted is 1, to use the k/V cache
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- 	getFull(multiple columns) - memCache
- 		Check if keys are in cache, otherwise get them from mapfiles, put in cache in way out
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- 	getFull(all columns) - memCache
- 		Check if fullFlag is set, get all columns from cache, if set abort, otherwise fetch remaining from mapfiles, put them in cache and set flag on way out 
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- 	deleteAll - done via add in memCache
- 		Delete row from cache, return ?
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- 	deleteFamily - done via add in memCache
- 		Delete row from cache, return ?	
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- 	add - memCache
- 		Check fullFlag, if set apply all the changes to row, otherwise do nothing to cache.	
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- 	batchUpdate - done via add in memCache
- 		Check fullFlag, if set apply all the changes to row, otherwise do nothing to cache.
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- Things to change: 
- 	In MemCache.getFull(), add early out if everything was found in memCache, do not need to check snapShot?	
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