A Java Program, a COBOL Program, and a DB2 Update - An Interesting Case Study

Not long ago, an IBM colleague of mine who is a WebSphere Application Server (WAS) for z/OS specialist sent to me and to a few other IBMers (including some of our CICS and DB2 for z/OS developers) some information about an application scenario, and asked for our ideas and feedback. The scenario involved a Java program that accessed a local DB2 for z/OS subsystem (i.e., a subsystem in the same z/OS LPAR as the WAS instance) and selected a row to be updated, and then invoked a COBOL program (also in the same z/OS LPAR) that performed the actual row update operation. The IBM WAS for z/OS specialist had some particular questions about DB2 locking in this situation, and was looking for an approach that would be favorable from a concurrency perspective. Everyone chimed in with their thoughts on the matter, and in this blog entry I'll synthesize and summarize that information. I think it makes for an interesting case study.

The client with which my WAS for z/OS specialist colleague (named Edward) was working had an application requirement whereby, as I mentioned, a Java program was to select a DB2 row for update and then was to invoke a COBOL program which would perform the prescribed update operation. The COBOL program existed already in the form of a CICS transaction, and as CICS has advanced capabilities regarding interaction with Java applications, Edward wanted to assess the options available for invoking the COBOL program in its present, CICS transactional form. Edward's chief concerns were locking and data integrity (locking, of course, is a mechanism for preserving data integrity). Edward was thinking that he wanted the Java program to acquire and retain a lock on the DB2 row to be updated, so as to prevent some other program from altering the record between its selection by the Java program and its subsequent update via the Java-invoked CICS transaction; he was concerned, however, that the Java-acquired lock on the row would end up blocking the update action of the called CICS transaction program.

The impact of the concurrency issue raised by Edward hinged on whether or not the selecting Java program and the updating CICS program would be seen by DB2 for z/OS as being associated with the same "global" transaction: if "no" then a lock (row- or page-level, depending on the target table space's LOCKSIZE attribute) acquired by the Java program would indeed block the update attempt by the invoked CICS transaction; if "yes" then a Java-acquired lock would be owned by the global transaction and would therefore be inherited by the CICS program. In that latter case, then, even an X-lock acquired on the row (or the row's page) by the Java program would not impede the CICS program's subsequent update operation. [Sometimes, the choice is indeed to X-lock a selected row, to prevent it form even being read by another program prior to its "downstream" modification by a called process -- such a "hands-off" lock is sometimes acquired by way of an update statement that sets the value of a column in a row equal to its current value, thereby X-locking the row without actually changing its contents.]

To this question our CICS and DB2 for z/OS developers responded with some enlightening information. First, if the row-selecting Java program were to run in WAS for z/OS and then link into CICS to invoke the row-updating program, the two processes (row-select and row-update) would be seen by DB2 as two different units of work, and the result would be a lock conflict. This conflict could be avoided by having the Java program execute in the CICS Liberty environment (the Liberty Profile is a lightweight Java application development and runtime environment, one form of which is available within CICS); however, CICS Liberty itself is only one part of the solution -- there is also the matter of the JDBC driver used to access DB2 for z/OS from the Java program. If the type 4 JDBC driver is utilized, the global nature of the overall transaction will not be recognized by DB2, and the lock conflict challenge will remain. If the type 2 JDBC driver is used (and that is an option when the target DB2 subsystem is in the same z/OS LPAR as the CICS region), and if the link to the COBOL program is local (i.e., not a distributed program link to a program in another CICS region), access to DB2 from the Java program running in the CICS Liberty environment and from the COBOL program will both be associated with the same DB2 thread; therefore, any lock acquired on DB2 data by way of the row-selecting Java program will belong as well to the row-updating COBOL program.

So, a CICS-based solution satisfying the application requirement was definitely available to the organization with which Andrew was working. As an alternative approach -- a plan B, if you will -- I suggested to Andrew the following: have the Java program invoke the row-updating COBOL program as a DB2 stored procedure. In that case, the same DB2 thread should be used for the Java-based SELECT and the COBOL-based UPDATE. Going the stored procedure route could in some cases be quite straightforward: generally speaking, if a COBOL program that executes in a CICS region issues SQL statements but no CICS commands, it can be invoked in the form of a stored procedure with little, if any, modification.

Yet another approach would not depend on the Java and COBOL programs sharing locks. Suppose that the Java program did NOT acquire and retain a lock on the target DB2 row prior to invoking the updating COBOL program (assuming that the typical cursor stability isolation level is in effect, an S lock acquired on a target row will be released following completion of a SELECT statement). If a lock on the target row is not acquired and retained by the Java program, how can one avoid the situation in which some other program comes in and changes the row before the Java-invoked COBOL program gets to it, potentially leading to an undesired "double update" of the target row? The answer, in a nutshell, is that you don't avoid that situation. Instead, you expect that it won't happen, and you detect it if it does. This is what so-called optimistic locking is about.

Optimistic locking was introduced with DB2 9 for z/OS, and there is a good description of this feature in section 7.3 of the IBM "redbook" titled DB2 9 for z/OS Technical Overview. Basically, it works like this: you alter the target table to add what is called a row change timestamp. That timestamp value is maintained by DB2 and it does what you'd expect: it records the time at which a row is changed. With that done (and continuing with the application requirement of which I've written), the Java program would select a row (without retaining a lock on the data) and then invoke the COBOL program to update the row. In doing that, the Java program would pass to the COBOL program the value of the row's last-changed timestamp. The COBOL program would then issue an UPDATE that would search for the target row using the appropriate qualifying criteria (e.g., an account number) AND the value of the row-change timestamp retrieved by the Java program's SELECT. If the target row has already been changed by an intervening statement (i.e., a data-changing SQL statement that gets to the row between the Java SELECT and the COBOL UPDATE), the row-change timestamp value will have changed (again, DB2 maintains that value), and the COBOL UPDATE won't find the target row -- that's how you avoid the aforementioned "double-update" scenario. If you want, you could have the COBOL program in that situation select the target row using its unique key but NOT with a predicate referencing the row-change timestamp -- that would show you that the row is there (assuming the intervening data-change statement wasn't a DELETE), but that it was updated between the Java SELECT and the COBOL UPDATE. The extra SELECT to check on this occurrence might strike you as being a little expensive, but keep in mind that it's something your program will rarely have to do -- the term "optimistic locking" suggests an expectation that the target row will NOT be changed between the Java SELECT and the COBOL UPDATE. Something that's a little expensive but seldom done is usually not a big deal.

There you have it: at least three ways to address the stated application requirement:

• Use the CICS Liberty environment for the Java program, and the type 2 JDBC driver and a local link to the COBOL program in the form of a CICS transaction, or
• Invoke the COBOL program as a DB2 stored procedure, or
• Use the optimistic locking approach.

The first two of these options involve locking the target row via the selecting Java program (as mentioned, that could be done by updating the row to set a column equal to itself), and then invoking the updating COBOL program which will not be blocked by the Java-acquired lock because it will share ownership of that lock. The third approach does not depend on such shared lock ownership.

Faced with a similar application requirement, the approach you should take is the one that best suits your needs and your environment. Always nice to have options, I say.