Working with customers migrating their SAP systems to Db2 12, I have seen something that has caused a few customers issues and though I would clarify here.
Remember that the Db2 Function Level is how development is now introducing new functionality into Db2 and APPLCOMPAT is being used to make functionality available to the applications - that is my simple explanation.
A description of the various levels (code, catalog, function application compatibility) can be found here. You can see the various Function Levels which are available here. As will all Db2 related maintenance for SAP, certified Function Levels can be found in SAP note 81737.
When Db2 12 was first introduced, things were easy. You would initially migrate the code, catalog, function and application compatibility levels all to V12R1M500. But at this point, Function Levels M501, M502 and M503 have been released (SAP has certified M501 and M502 as of this posting). As part of the migration, a customer has the choice of which Function Level to migrate to. If you want the LISTAGG built-in function which was introduced in M501, you can perform the Db2 12 migration and take Db2 directly to FL M501. There is no need to stop at M500 and then perform a second activation to get to M501. You could jump right to it (assuming the code and catalog levels are at M501).
In the Best Practice guide attached to SAP note 2303029, we see this diagram:
What has seemed to cause trouble for some customers is the idea that regardless of which Db2 Function Level you go to, APPLCOMPAT will need to be set at V12R1M500. Why? Because, as of today, SAP does not need anything higher than M500. Section 3.2.7.2 in the Best Practice guide describes setting APPLCOMPAT. The section describes how SAP, as part of an upgrade, will change APPLCOMPAT using SWPM and SUM when it is needed.
In summary, at a high level, you would take the following steps:
1. Choose the Db2 12 Function Level you want to migrate to (M500, M501, M502 are all possible today in SAP environments)
2. Apply the code to the libraries
3. Run CATMAINT to update the catalog level
4. Activate the desired Function Level
5. Set APPLCOMPAT to M500, regardless of which Db2 Function Level was activated (don't forget to BIND the Db2 Connect packages)
The other side of this discussion is about when SAP needs a particular Function Level. Check out SAP note 2302997: Release of Db2 12 for SAP Components for those details.
For a more in-depth overview of continuous delivery in Db2 12, see the IBM Redpaper publication Exploring IBM DB2 for z/OS Continuous Delivery by Chris Crone.
Richard Ruppel is a Client Technical Specialist with the SAP on IBM Z Solutions team at IBM and can be reached at rmruppel@us.ibm.com.