The APPLCOMPAT option of the Db2 for z/OS BIND and REBIND PACKAGE commands is really important - that's why I posted part 1 and part 2 blog entries on the topic back in 2019. The APPLCOMPAT parameter in ZPARM, on the other hand (referring to DSNZPARM, the data set that contains a Db2 subsystem's configuration parameter settings), is less important. I pointed this out in part 1 of the aforementioned two-part blog entry on APPLCOMPAT, but I still find that plenty of Db2 for z/OS people ascribe significance to the ZPARM parameter APPLCOMPAT that just doesn't jibe with reality. That being the case, I am writing this blog entry in the hope that it will help to drive home the point that the ZPARM parameter called APPLCOMPAT should (generally speaking) not be the main focus of your APPLCOMPAT-related concerns.
To illustrate the point that plenty of people continue to over-inflate the importance of the APPLCOMPAT parameter in ZPARM, I'll share with you a question that a Db2 for z/OS person sent to me by way of one of my colleagues. The question was, basically, "We are getting ready to activate Db2 12 function level V12R1M510 (a prerequisite for migration from Db2 12 to Db2 13 for z/OS). Can we be pretty much assured that doing this will not cause SQL behavioral changes if we leave the value of APPLCOMPAT in ZPARM unchanged (they had this ZPARM parameter set to V10R1)?" In responding to this question, I explained that in advancing a Db2 system's active function level, one can indeed protect application programs from the risk of SQL behavioral changes (I'll explain what that means in a moment), but, I noted, this SQL behavioral change protection is provided by the APPLCOMPAT package bind specification, NOT by the APPLCOMPAT parameter in ZPARM. You can take a Db2 system's active function level as high as you want, and that will not lead to application-affecting SQL behavioral changes as long as you don't change the APPLCOMPAT value of your applications' Db2 packages. The value of the APPLCOMPAT parameter in ZPARM is only somewhat relevant to this discussion.
OK, what's all this about "SQL behavioral changes?" The term refers to this situation: same SQL statement, same data, different result. You might think, "How could that happen?" Well, every now and then, the Db2 for z/OS development team decides that the behavior of a given SQL statement should change, for one reason or another (and it's well-considered - these changes are not effected lightly). That change can be introduced with a new version or a new function level of Db2. My favorite example of a Db2 for z/OS SQL behavioral change is one that happened with Db2 11. In a Db2 10 environment, you could use a SQL statement to cast an eight-byte store clock value (a time value that a program can obtain from the z/OS operating system) as a Db2 timestamp value. In a Db2 11 system, that same SQL statement - cast an eight-byte store clock value as a Db2 timestamp - would fail with a -180 SQL error code. Same SQL statement, same data, different result in a Db2 11 versus a Db2 10 environment.
Here's one reason I really like this example: how many programs do you have that need to cast an eight-byte store clock value as a Db2 timestamp? Probably none - and this is typically the case for Db2 SQL behavioral changes - they usually affect either zero or very few of an organization's Db2-accessing programs. Alright, but what if you did have programs that needed the Db2 10 behavior of the Db2 TIMESTAMP function? Would you have been up a creek when your Db2 system went from Db2 10 to Db2 11? No - you would have been fine in that case, because you could just bind the Db2 packages used by those programs with APPLCOMPAT(V10R1), and that would mean that the programs would execute with Db2 10 SQL behavior, and that would mean that those programs could cast an eight-byte store clock value as a Db2 timestamp. See - it's the APPLCOMPAT package bind specification that provides protection (when needed) from Db2 SQL behavioral changes.
[By the way, in the Db2 for z/OS documentation, what I have been calling "SQL behavioral changes" are referred to as "SQL incompatibilities." These are documented for each Db2 application compatibility level, going back to V10R1 (that's as far back as Db2 application compatibility goes).]
So, I said up front that the APPLCOMPAT parameter is ZPARM is not as important as the APPLCOMPAT specification for your Db2 packages. Does that mean that the ZPARM has no significance? No. What is the purpose of the APPLCOMPAT parameter in ZPARM? It's this: the ZPARM parameter provides the default value for a package's APPLCOMPAT setting when the BIND PACKAGE command is issued without an APPLCOMPAT specification. That's it. I tell people to think of APPLCOMPAT in ZPARM as being like a cubbyhole. A BIND PACKAGE command may be issued without an APPLCOMPAT specification. The package in question needs an APPLCOMPAT value. Where is Db2 going to get that value, when the value was not provided via the BIND PACKAGE command? Db2 in that case is going to look in the cubbyhole labeled APPLCOMPAT in ZPARM. In that cubbyhole is a piece of paper (figuratively speaking) on which (for example) V12R1M509 is written. OK, that will be the package's APPLCOMPAT value.
[You might wonder: what if REBIND PACKAGE is issued without an APPLCOMPAT specification? Will the rebound package in that case get the APPLCOMPAT value to which the ZPARM parameter APPLCOMPAT has been set? Probably not. Why not? Because it is very likely that a package being rebound already has an APPLCOMPAT value, and in that case if the REBIND PACKAGE command is issued without an APPLCOMPAT specification then the package's current APPLCOMPAT value will be retained. For REBIND PACKAGE, then, the APPLCOMPAT parameter in ZPARM is relevant only when the REBIND PACKAGE command is issued without an APPLCOMPAT specification and the package in question does not already have an APPLCOMPAT value (again, unlikely, though not impossible - you can check on this via a query of the SYSIBM.SYSPACKAGE catalog table, which has a column named APPLCOMPAT).]
Given that APPLCOMPAT in ZPARM simply provides the default value for APPLCOMPAT when BIND PACKAGE is issued without an APPLCOMPAT specification, what should the value of this ZPARM parameter be? There isn't a right or wrong answer to this question - it's up to you. Personally, I'd lean towards making the value of APPLCOMPAT in ZPARM as high as it can be, which would be equal to the currently active function level in a Db2 system. Why would I want that? Because APPLCOMPAT, in addition to providing protection (when needed) from Db2 SQL incompatibilities, also enables use of newer SQL syntax and functionality. If I have APPLCOMPAT in ZPARM set to, for example, V10R1, and BIND PACKAGE at my site is typically issued without an APPLCOMPAT specification, I am limiting application programmers to SQL syntax and functionality only up to the Db2 10 level - can't use newer built-in functions such as LISTAGG and PERCENTILE_CONT, can't use Db2 global variables, can't use Db2 arrays, can't use newer special registers such as CURRENT LOCK TIMEOUT, etc. Is that what you want? Sure, if a program using one of those newer SQL capabilities fails at bind time because of the default V10R1 APPLCOMPAT value, you can fix that problem by issuing BIND PACKAGE a second time with an APPLCOMPAT specification that is current enough to support the desired functionality, but again, is that what you want?
At some Db2 for z/OS sites, APPLCOMPAT in ZPARM is indeed set at V10R1. Why so low? One reason maybe the misunderstanding (to which I've referred) of the purpose of the ZPARM parameter. Alternatively, maybe APPLCOMPAT in ZPARM is set at V10R1 because of concern about BIND PACKAGE issued for programs that aren't net new but rather have had a bit of an SQL change (which would then require a new precompile and BIND in the case of a static SQL-issuing program, as opposed to a REBIND). A person might think, "What if there's an existing program with 20 static SQL statements, and a programmer changes just one of those statements? When there is a BIND PACKAGE (with ADD or REPLACE, as the case may be) for that program's Db2 package, and the BIND PACKAGE is issued without an APPLCOMPAT specification, I want a from-the-ZPARM-parameter default APPLCOMPAT value that will have the 19 SQL statements that weren't changed behaving as they always have." OK. I get that. Like I said, it's up to you. Just keep in mind that the risk of adverse impact on your programs from Db2 SQL incompatibilities is usually very low - these incompatibilities are relatively few and far between, and they tend to affect few if any of your Db2-accessing programs.
The main point I want to make is this: when you change the value of the APPLCOMPAT parameter in ZPARM, that action is not, in and of itself, going to cause Db2-accessing programs in your environment to suddenly start behaving differently. All you've done with the ZPARM parameter update is change the APPLCOMPAT value that a package will get if a BIND PACKAGE command is issued without an APPLCOMPAT specification. Consider the ZPARM value change in that light, and act accordingly.