It's pretty well known that getting the activated function level of a Db2 12 for z/OS system to V12R1M510 is required prior to migrating that Db2 12 system to Db2 13 (I blogged about that a couple of years ago). At present (June, 2024), there are still a good number of sites running Db2 12 for z/OS with an activated function level of V12R1M500. I sometimes get, from people at such sites, a question like this: "How should we go from Db2 12 function level 500 to function level 510?" Generally speaking, what these folks are wanting is for me to opine on going straight from Db2 12 function level 500 to function level 510, versus getting from FL500 to FL510 by first going to some "in-between" function level and then going to 510. In this blog entry, I'll tell you what I tell people who ask me this "getting from A to B" question.
Right off the bat, I'll say that I'd be comfortable with either approach - in other words, there is not a "right" and a "wrong" way to go from Db2 12 FL500 to FL510. Let's consider first the "one fell swoop" approach of going straight from Db2 12 FL500 to FL510. What is required, in a technical sense, to do this? The code level of the Db2 12 system (which you can check using the output of the Db2 command -DISPLAY GROUP) has to be 121510. The catalog level (also check-able with -DISPLAY GROUP) has to be V12R1M509 (there is no 510 catalog level; and, you can use the CATMAINT utility to take the catalog level to V12R1M509 if it is not already there). Additionally, you can't have any packages that have been executed within the past 1.5 years that were last bound or rebound prior to Db2 11 (this "no pre-Db2 11 still-in-use packages" requirement is explained in the aforementioned blog entry about preparing to migrate from Db2 12 to Db2 13).
Let's say those technical prerequisites are in-place. Given that you could, in that case, go straight from Db2 12 FL500 to FL510, why wouldn't you? What would hold you back? In my experience, the chief concern is often something like this: "If we make the big jump from Db2 12 FL500 to FL510, I'm afraid that will mess up some of our Db2-accessing applications."
That concern, though understandable, is essentially unfounded if you don't change package APPLCOMPAT values when making the big jump in the activated function level of the Db2 12 system. I've written plenty about APPLCOMPAT, including a part 1 and part 2 entry in this blog. The important thing to remember about APPLCOMPAT, in relation to a change in the activated function level of a Db2 system, is this: a package's APPLCOMPAT value can serve to insulate an associated application program from SQL behavioral changes that could be introduced via a change to a higher activated function level in a Db2 for z/OS environment. Let's unpack that statement. A SQL behavioral change - referred to in the Db2 for z/OS documentation as a "SQL incompatibility" - can be succinctly described thusly: same SQL, same data, different result. That might sound kind of alarming. Here's the two-pronged good news: 1) SQL incompatibilities are quite few and far between (they are documented whenever they are introduced by a new Db2 for z/OS version or function level), and 2) they tend to affect either very few or none of an organization's Db2-accessing applications (they are usually associated with highly specialized, often unusual scenarios).
Now, while it's highly unlikely that any of an organization's Db2-accessing programs would be negatively impacted by a SQL incompatibility associated with a newer Db2 function level, I can't say that the risk of application impact related to a change in activated Db2 function level is zero if applications are exposed to SQL behavioral changes. That's where APPLCOMPAT comes in. See, if the Db2 package used by an application program has an APPLCOMPAT value of (for example) V12R1M100, SQL behavior for that program will be that of a Db2 12 system with function level 100 activated, even if the Db2 environment in which the program is executing has an activated function level of V12R1M510 (or V13R1M100 or some other Db2 13 function level, after the system has been migrated to Db2 13). That is how APPLCOMPAT insulates a Db2 application program from SQL behavioral changes that might be introduced when a higher function level is activated for the Db2 system in which the program's SQL statements execute. So, make a jump in activated function level from 500 to 510 in your Db2 environment, and leave package APPLCOMPAT values as they are, and your Db2 application programs will be (conceptually speaking) unaware of the environmental change.
OK, so I'd be plenty comfortable in taking a Db2 12 system's activated function level from 500 right to 510, and I hope you'd be similarly comfortable with such a move. You might find, however, that "selling" the powers that be on a plan for getting to Db2 12 FL510 from FL500 would be easier if that plan included a "rest stop" at a function level between 500 and 510. If that were the case, you could go that route and not lose any honor in my eyes; and, yours wouldn't be the first Db2-using organization to take the 2-step approach for getting from Db2 12 FL500 to FL510. I can tell you that a pretty popular intermediate stop on the way from Db2 12 FL500 to FL510 is FL505. This is true for a couple of reasons: stopping for a while (for however long makes people comfortable: a few weeks, a couple of months - whatever) right between FL500 and FL510 appeals to some folks. Additionally, Db2 12 FL505 makes available to you one of my favorite Db2 12 features: rebind phase-in.
Alright, to sum up: if your organization has been sitting at Db2 12 FL500 for a long time, and you want to get to FL510 so you can go from there to Db2 13, consider taking one of two routes from A to B:
- Straight through - If your code level and catalog level and no-still-in-use-pre-Db2-11-packages ducks are all in a row, go from Db2 12 FL500 straight to FL510, keeping in mind that leaving package APPLCOMPAT values as they are is a way to insulate Db2-accessing programs from any impact related to the Db2 environmental change.
- Go part way, sit a spell, then go the rest of the way. As noted, Db2 12 FL505 is a popular rest stop en route from FL500 to FL510.
