Tuesday, October 02, 2007

Dirty Reads in PeopleTools 8.49

In PeopleTools 8.49 you can configure the application server to use Dirty Reads on SQL Server and DB2/OS390. Let's start with a quick review of database isolation levels. If you run PeopleSoft on an Oracle database, you hardly ever have to worry about isolation levels, because Oracle defaults to Read Committed, where all transactions are transactionally or read consistent, and that doesn't bring any scalability problems. (In writing this note, I have spent a fair amount of time reading various Oracle and SQL Server websites. It has been an excellent revision!) What is read consistency? Well, if I start a long-running query at say 09.00, and then I update some data at 09.01 and commit that update, and then at 09.02 the query comes to the data that was changed at 09.01, it will report the data as at 09.00. So the set of data that is retrieved by the long-running query is consistent with itself. On Oracle, this is done by keeping information to reverse the change, essentially the old data values, in the undo (or rollback) segments from which it can construct, in memory, a read consistent version of data blocks as the start of the transaction. So at 9.02, Oracle creates a copy of the block in the buffer cache and uses the undo information to roll it back to the state it was in when the query began at 9.00, and then it reads the data from that read consistent copy. Thus, Oracle is capable of maintaining read consistency without locking, so that writers never block readers, and readers never block writers. Oracle also supports two other isolation levels:
  • Serializable: queries cannot see changes committed after the transaction began, but before the current query began.
  • Read-Only: as Serializable but no INSERT, UPDATE or DELETE statements are permitted.
Oracle has not explicitly implemented Repeatable Read as an isolation level, but the same effect can be achieved if you use SELECT ... FOR UPDATE. However, the other database platforms that are certified for PeopleSoft have additional isolation levels that can be set at database and session-level (and hence in the application). In SQL Server the isolation modes are:
  • Read Uncommitted (also known as Dirty Read): Queries simply read whatever is in the data blocks, so it is possible to read changes in the database that have not been committed and that may potentially be rolled back!
  • Read Committed (SQL Server default): Uncommitted changes cannot be read, queries can be blocked by shared locks created by updates unless using Read Committed Snapshot, or other transaction in higher isolation levels.
  • Read Committed Snapshot (available SQL Server 2005): row versioning is used to produce a transactionally consistent snapshot of the data. This is effectively equivalent to Read Committed on Oracle.
  • Repeatable Read: Locks are placed on all data that is read.
  • Serializable: Locks are placed on all data that is read, and rows cannot be inserted ranges of data queried by other statements.
  • Snapshot: Changes made after the start of the transaction cannot be seen.
PeopleSoft running on Microsoft SQL Server 2000 and Sybase has always had scalability problems with high concurrency because read consistency is achieved by the use of locking. This is addressed in SQL Server 2005 with the READ_COMMITTED_SNAPSHOT option, but this version of the database is not certified below PeopleTools 8.47.
Dirty Reads in PeopleTools 8.49 
Dirty-read mode can be specified for the application server:
; Settings for PSQRYSRV
; Use dirty-read(uncommitted read) for PSQRYSRV only on DB2/OS390 or SQL Server
Use dirty-read=0
It can also be set for scheduled queries run by Application Engine in the Process Scheduler:
; Setting to control dirty read for Scheduled PSQueries.
ScheduledQuery-DirtyRead = 0
This is what Enterprise PeopleTools 8.49 PeopleBook: System and Server Administration says:

"Use Dirty-Read: Enter 1 to enable the application server to read uncommitted data from a table. Enter 0 to disable dirty reads.

This parameter can be used for general reporting and PeopleSoft Query. You can run dirty read queries through the application server, the Process Scheduler, and in a two-tier connection. The Use Dirty-Read setting in the application server configuration controls the behaviour of PSAPPSRV, PSQCKSRV, and PSQRYSRV.

Note. Dirty reads are not recommended if you are reading data and doing subsequent processing based on the disposition of the data at the time that it is read. Between the time the data is read by a subsequent process and the time the unit of work is completed by the first process, any activity affecting the table data at the time a subsequent process read could be rolled back, invalidating the accuracy of the data that a subsequent process read." However, the parameter only appears in the query server section (PSQRYSRV) of the application server configuration file (psappsrv.cfg), so I suspect that it only applies to PS/Query queries (and therefore Crystal and BI Publisher reports) and nVision reports. Although, I haven't yet found confirmation of that in PeopleBooks or other documentation. The delivered comment in the configuration file says that dirty-reads apply to DB2/OS390 and SQL Server, but the documentation doesn't mention which databases it applies to. Dirty-reads do not occur in Oracle, but this isolation level is also available on Sybase and Informix. 
Conclusion You can get around the locking problems in SQL Server 2000 and DB2, but at a price. Your reports may not show you entirely accurate data! But, if you are running PeopleTools 8.49 on SQL Server 2000, I would question why you are not running SQL Server 2005.