Comments inline:
> -----Original Message-----
> From: John Ryan [mailto:[EMAIL PROTECTED]
> Sent: Monday, August 18, 2008 4:49 PM
> To: hbase-user@hadoop.apache.org
> Subject: Deletes in HBase
>
> How do deletes work in HBase? Suppose I have 2 Column Families and a key
> that has entries for both column families. Now I want to delete this key.
You mean you want to delete all the values for that row key?
See HTable.deleteAll({byte[]|String)
> Is a major compaction absolutely essential for this key to be deleted?
No. Essentially a record is written that indicates that a cell, row, or column
family has been deleted.
> Where I can I follow the code this operation?
All the following paths are prefixed with org.apache.hadoop.hbase:
client.HTable - eventually creates a Java Proxy which has the api specified by
ipc.HRegionInterface
which figures out which region server to send the message to. This call will be
answered by regionserver.HRegionServer.deleteAll which calles
regionserver.HRegion.deleteAll for the appropriate region, calling
HRegion.deleteMultiple, HRegion.update which first appends the change to the
HLog by calling regionserver.HLog.append, and then stores the information in
the HStore(s) for the appropriate families by calling regionserver.HStore.add,
which in turn stores it in the memcache for the HStore by calling
regionserver.Memcache.add, which calls regionserver.Memcache.add
Now the change has been persisted to the redo log (HLog) and is cached. When
the cache fills, a cache flush will write the contents of the cache out to disk
and may result in a minor compaction.
> Now I am assuming that major compaction doesn't take place all the time
> since it may be an expensive operation. Having said that how are the reads
> for this key supressed? Please explain.
Reads are suppressed at the level of HStore, and Memcache. They come across the
deleted markers and suppress the results that would otherwise have been
returned.
You would have to follow the call tree for get, getRow, and the various
scanner.next methods to see how this works. It is very complicated.
