Databases Reference
In-Depth Information
Writeback data is permanently stored on Analysis Services in a special partition, called
the
writeback partition
. This partition contains the delta values for each cell changed using
an
UPDATE CUBE
statement. When you issue a
COMMIT TRANSACTION
statement, Analysis
Services stores the delta values for updated cells in this partition.
When calculating the results of an MDX query, Analysis Services queries and aggregates
the results from all relevant partitions to return cell values. Analysis Services treats the
writeback partition as any other partition on the server, and when changes are made they
automatically become effective and visible to all users for that cube. This architecture
allows the server to answer any query immediately after a writeback is committed without
the need to recalculate the cube or its aggregations.
Immediately after a permanent writeback has been committed to the server, it should
become visible to all the users of the cube, including users who might have already used
temporary writebacks. Therefore, before it can commit the transaction, Analysis Services
needs to perform the operations shown in Figure 16.8:
1.
Move delta values from the writeback cache into the writeback partition.
2.
Delete the writeback cache on the session that committed the transaction.
3.
Iterate over other sessions that have a writeback cache and recalculate the delta val-
ues for cells that are already in the cache by subtracting the delta value stored in
the writeback partition from the delta value stored in the writeback caches of the
other sessions.
Temporary Writeback Cache 1
Temporary Writeback Cache 2
Delta 1-Delta2
Old Value
Old Value
…
Delta 1
Delta 1'
…
Old Value
Old Value
…
Delta 2
Delta 2'
…
2
1
3
Writeback Partition
Regular Partition
Delta 1
Delta 1'
…
Fact Value
Fact Value
…
+
FIGURE 16.8
Operations that Analysis Services performs after the user commits an update.
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