Gautham KAMATH

BI Developer specializing in SSAS and MDX

Posts Tagged ‘Snapshot

Processing using Snapshot Isolation in Analysis Services (SSAS)

with 7 comments

Just a few months ago I was hit with an issue of dimension processing failure. The error read

Errors in the OLAP storage engine: The attribute key cannot be found when processing: Table: ‘<<TableName>>’, Column: ‘<<ColumnName>>’, Value: ‘XXXXXXX’. The attribute is ‘<<AttributeName>>’.

On investigation I found that constant updates to the dimension were the main cause. One of my dimensions had as many as 15 attributes and the processing failed when any update occurred during dimension processing. Generally speaking, the first attribute’s SELECT DISTINCT statement would conflict with the SELECT DISTINCTs of other attributes because of the constant update.

One way to get around this is was to change the ProcessingGroup property of the dimension to be ByTable (instead of the default value of ByAttribute). As the ByTable option takes the entire table data for that dimension into memory, it would have to process the same data without having to worry about the constant updates. The new updates would then get incorporated in the next processing cycle.

This option did not work as the ByTable option works only when you have only a solitary table to draw the data from. Multiple tables in star schema or snow-flake schema or a RDBMS view that encapsulates multiple tables will issue multiple queries against the data source.

P/S Try processing the Product and Geography dimensions using ByAttribute and ByTable options. The profile trace for ByTable ProcessingGroup mode for Product dimension will show you multiple SELECT DISTINCT statements being fired whereas for the Geography dimension, because of its single table data source, it would fire only one SELECT DISTINCT statement.

As I am using RDBMS database views that uses multiple table joins, this option for me, also went to pits. The only feasible solution was to use the Snapshot Isolation.

Snapshot Isolation

The Snapshot Isolation works on the Row Versioning framework.

Row versioning based isolation levels reduce the number of locks acquired by transaction by eliminating the use of shared locks on read operations.

For the Analysis Services database to work on Snapshot Isolation mode the source RDBMS database needs to have the READ_COMMITTED_SNAPSHOT option and ALLOW_SNAPSHOT_ISOLATION to be ON.

  • When the READ_COMMITTED_SNAPSHOT is ON, READ_COMMITTED transactions provide statement level read consistency using row versioning.
  • When ALLOW_SNAPSHOT_ISOLATION is ON, SNAPSHOT transactions provide transaction-level read consistent using row versioning.

The following is the SQL command that can enable the mentioned options in your RDBMS database.

        USE <<DatabaseName>>
        GO
        ALTER DATABASE <<DatabaseName>>
        SET READ_COMMITTED_SNAPSHOT ON
        GO
        ALTER DATABASE <<DatabaseName>>
        SET ALLOW_SNAPSHOT_ISOLATION ON
        GO

Enabling these options causes all data modifications for the database to be versioned. A copy of the data before update (or add) is copied and stored in tempdb. The data for processing is thereafter used from the tempdb. After the dimension processing operation (under Snapshot Isolation) is finished the tempdb frees the space occupied by the versioned rows.

Please note, the tempdb must be enabled to have enough space to hold the row versioned data generated for the source RDBMS database.

Another important thing is to enable the Multiple Active Result Sets (MARS) connection property in the DataSource in the Analysis Services database.

Not enabling MARS will give you the following error

OLE DB error: OLE DB or ODBC error: Cannot create new connection because in manual or distributed transaction mode.

By enabling the Snapshot Isolation on my Analysis Services database the dimension processing has not had any errors due to constant updates at source. The tempdb size also has been well within acceptable limits.

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Written by Gautham KAMATH

24/02/2012 at 16:34