Feb 3 ’12

CICS Transaction Server for z/OS V4.2: Scalability Enhancements

by John Tilling 

IBM released CICS Transaction Server for z/OS Version 4.2 on June 24, 2011. One of the themes of the new release is scalability; enhancements allow more work to occur faster in a single CICS system. This allows increased vertical scaling and may reduce the need to scale horizontally, decreasing the number of regions required to run the production business applications.

Scalability enhancements in CICS Transaction Server V4.2 fall into two broad areas: increased exploitation of Open Transaction Environment (OTE) and increased exploitation of 64-bit storage. 

OTE is an architecture introduced for three purposes:

To benefit from OTE capabilities, customers must ensure their applications are threadsafe. If the mainframe has many CPUs and many processes are running in parallel, an application that’s threadsafe runs correctly and generates the right result. CICS makes sure its code runs correctly, but customers must ensure their COBOL code, for example, runs correctly. If an application is threadsafe, it can be defined to CICS via a CONCURRENCY keyword so it uses OTE. If an application isn’t threadsafe, CICS runs it without using OTE.

Applications that can’t use OTE must run on the main CICS Task Control Block (TCB), the Quasi-Reentrant (QR) TCB, while applications that use OTE can run on a CICS open TCB. A CICS system has only one QR TCB, and the CICS dispatcher shares use of the QR TCB between all the tasks. However, a single CICS system can have hundreds of open TCBs. Exploiting OTE effectively keeps an application running on an open TCB as long as possible and minimizes the number of times it must switch back to the QR TCB. This provides CPU savings and improves throughput; the open TCBs can run in parallel and take advantage of the multi-processor mainframe. 

OTE enhancements in CICS TS 4.2 fall into three areas:

Concurrency Settings

Prior to CICS TS 4.2, an application program can be defined as CONCURRENCY(QUASIRENT) or CONCURRENCY(THREADSAFE):

CICS TS 4.2 provides a new CONCURRENCY(REQUIRED) setting (see Figure 1). As with CONCURRENCY(THREADSAFE), the new setting specifies that the program was coded to threadsafe standards and contains threadsafe logic, but the program also must run on an open TCB. So the program runs on an open TCB from the start, and if CICS has to switch to the QR TCB to process a non-threadsafe CICS command, CICS returns to the open TCB when it returns control to the application program.

The CONCURRENCY(REQUIRED) option lets the user define that the program must start on an open TCB, independently of defining what APIs it uses.

If the program uses only CICS-supported APIs (including access to external resource managers such as DB2, IMS, and WebSphere MQ), it should be defined with program attribute API(CICSAPI). In this case, CICS always uses an L8 open TCB, regardless of the program execution key because CICS commands don’t rely on the TCB key.

If the program uses other non-CICS APIs, it must be defined with program attribute API(OPENAPI). In this case, CICS uses an L9 TCB or an L8 TCB, depending on the program execution key. This allows non-CICS APIs to operate correctly. This OPENAPI behavior is the same as in previous releases.

Existing threadsafe applications, which have taken advantage of the performance gains of being able to run on the same TCB as the call to an external resource manager, may be able to gain further throughput advantages by being defined as CONCURRENCY(REQUIRED) API(CICSAPI). Throughput gains accrue when an application can run for longer periods of time on an open TCB.

However, not all applications are suitable. For example, a threadsafe application that issues many EXEC SQL requests and then issues many EXEC CICS commands that aren’t threadsafe is best left as CONCURRENCY(THREADSAFE). Defining the application as CONCURRENCY(REQUIRED) would mean two TCB switches for each non-threadsafe CICS command because control always returns to the application on the open TCB (see Figure 2).


This situation demonstrates the importance of knowing what the application does. To help you find out, tools such as CICS Interdependency Analyzer for z/OS (CICS IA) enable you to discover application execution paths. In particular, its command flow feature shows the order in which CICS, IMS, WebSphere MQ and DB2 commands run, and what TCB each command ran on. Other tools, such as CICS Performance Analyzer for z/OS (CICS PA), analyze CICS performance System Management Facility (SMF) data and show, for example, how much CPU has been consumed on which TCBs and how many TCB switches have occurred. Tools such as CICS IA and CICS PA are invaluable aids to have in your toolbox when embarking on a threadsafe project (see Figure 3). 

Threadsafe Mirrors

The CICS-supplied mirror program, DFHMIRS, used by all mirror transactions, is now defined as threadsafe. In addition, the IPIC transformers are threadsafe. For IPIC connections only, CICS runs the mirror program on an L8 open TCB when possible. For threadsafe applications that function ship commands to other CICS regions using IPIC, the resulting reduction in TCB switching improves application performance compared to other intercommunication methods. To gain the performance improvement for remote files, you must specify the system initialization parameter FCQRONLY=NO in the File Owning Region (FOR).

For remote file control or temporary storage requests shipped over IPIC connections, CICS will no longer force a switch to the QR TCB in the Application Owning Region (AOR) if it’s currently running on an open TCB. The requests will be shipped running on the open TCB.

In the FOR or Queue Owning Region (QOR), the mirror decides when to switch to an open TCB. It does so for the first file control or temporary storage request received over an IPIC connection. The idea is for long-running mirrors to keep the mirror running on an open TCB.

A new option, MIRRORLIFE, was added to the IPCONN attributes for function-shipped file control and temporary storage requests using an IPIC connection. MIRRORLIFE improves efficiency and provides performance benefits by specifying the lifetime of mirror tasks and the amount of time a session is held.

Threadsafe CICS-DBCTL Interface

CICS provides a CICS-IMS Database Control (CICS-DBCTL) interface to support CALL DLI and EXEC DLI requests issued by applications running in a CICS region. In CICS TS 4.2, the CICS-DBCTL interface was enhanced to exploit OTE; CICS can run the CICS-DBCTL Task Related User Exit (TRUE) on an L8 open TCB.

OTE is supported from IMS Version 12 with Program Temporary Fixes (PTFs) for APARs PM31420, PM45414, and PM47327 applied. IMS indicates to CICS during the connection process that the OTE is supported. Consequently, CICS defines the CICS-DBCTL TRUE as an OPENAPI TRUE. For IMS Version 11 and earlier, OTE isn’t supported and CICS runs the CICS-DBCTL TRUE on the QR TCB and the IMS code switches to an IMS thread TCB.

Running an application on an open TCB improves throughput and performance by reducing the use of the QR TCB. Threadsafe CICS applications that run on an L8 open TCB and issue CALL DLI or EXEC DLI commands can avoid two TCB switches for each call to IMS.

For a non-threadsafe application, there’s no reduction in the amount of switching. Instead of switching from the QR TCB to an IMS thread TCB and back again for each IMS request, the application switches from QR to L8 and back again.

For a threadsafe application, if it’s running on the QR TCB, it switches to L8 and then stays on L8 when control is returned to the application.

For a threadsafe application that’s already running on an L8 TCB, or for a CONCURRENCY(REQUIRED) application that’s running on an L8 TCB, no TCB switching occurs for the IMS request.

Threadsafe SYNCPOINT and Other Commands

CICS commands that were made threadsafe in CICS TS 4.2 include named counter server commands, QUERY SECURITY, SIGNON, SIGNOFF, VERIFY PASSWORD, CHANGE PASSWORD, EXTRACT TCPIP and EXTRACT CERTIFICATE, along with several new SPI commands. Most significant, however, is the SYNCPOINT command.

The CICS Recovery Manager domain now processes a SYNCPOINT command on an open TCB where possible to minimize TCB switching. Syncpoint processing can occur on an open TCB for all resource types declared as threadsafe that were accessed in the unit of work. If resource types not declared as threadsafe were accessed in the unit of work, the Recovery Manager switches to the QR TCB for those resource types. Prior to CICS TS 4.2, CICS would switch to the QR TCB prior to the end of task sync point. In CICS TS 4.2, the application remains on an open TCB, if it’s running on one, until end of task sync point is called. Afterward, CICS switches to QR for the task detach logic.

Prior to CICS TS 4.2, a threadsafe application running on an open TCB that had, for example, updated DB2 and WebSphere MQ and then issued a sync point, would require nine TCB switches:

In CICS TS 4.2, if a transaction is terminal-driven, one TCB switch to QR will occur. For a non-terminal-driven transaction (and assuming no other non-threadsafe resources were touched), no TCB switches occur. 

64-Bit Exploitation

CICS TS 4.2 contains major changes to provide a CICS domain architecture environment that exploits the underlying z/Architecture for 64-bit addressing, and to provide the infrastructure for the future. This has allowed exploitation by CICS services in CICS TS 4.2, and lays the foundation for future CICS applications to be able to use and exploit 64-bit addressing mode. The exploitation of the 64-bit addressing provided by the z/Architecture enables CICS to remove some of the previous limitations that affect scalability and availability by delivering large address spaces.

CICS can use z/OS 64-bit virtual storage to increase capacity by supporting more concurrent users and concurrent transactions. CICS can also keep up with the virtual storage demands of increased workloads for existing applications and the larger memory requirements of new applications and new technologies.

64-Bit Domains

CICS domains can use stack storage and domain anchor storage, and can allocate domain control blocks in virtual storage above the bar. Kernel, Monitoring, Storage Manager, Lock Manager, Trace, Message and Temporary Storage are all CICS domains that now run AMODE 64 and keep their data above the bar.


The z/OS MEMLIMIT parameter limits the amount of 64-bit (above-the-bar) storage that the CICS address space can use. This storage includes the CICS dynamic storage areas above the bar (collectively called the GDSA) and MVS storage in the CICS region outside the GDSA.

A CICS region requires at least 4GB of 64-bit storage. You can’t start a CICS region with a MEMLIMIT value lower than 4GB. If you attempt to do so, CICS issues a message and terminates.

Note: CICS doesn’t try to obtain the MEMLIMIT amount of storage when initializing; 64-bit storage is obtained as required.

CICS Storage Manager domain was greatly enhanced to manage 64-bit storage and provide additional statistical information about 64-bit storage consumption. In addition, for 31-bit storage, the minimum and default EDSALIM values have changed to 48MB to ensure there’s sufficient storage for CICS initialization.

Exploiting 64-Bit Storage

CICS temporary storage is one of the major exploiters of 64-bit storage in CICS TS 4.2. TS main temporary storage queues can now use 64-bit storage. CICS provides new facilities so you can check the storage use of main temporary storage queues and limit that storage use. Auxiliary temporary storage queues and shared temporary storage queues continue to use 31-bit storage.

Main temporary storage is in 64-bit storage rather than 31-bit (above-the-line) storage, depending on the version of z/OS and whether the CICS region operates with transaction isolation. If your CICS applications use large amounts of main temporary storage, the move to 64-bit storage can increase the available storage elsewhere in your CICS regions. An additional new capability is provided to clean up unwanted queues after a specified time interval.

CICS trace domain exploits 64-bit storage (depending on the version of z/OS being used and whether the CICS region operates with transaction isolation) by allocating the CICS internal trace table above the bar. This provides virtual storage constraint relief for 31-bit storage and allows for much larger trace tables to aid problem determination. Trace control blocks and transaction dump trace tables also move above the bar, as do message tables used by the message domain.

For CICS Java applications, all Java Virtual Machines (JVMs) now run in AMODE 64 instead of AMODE 31, increasing the capacity for running more JVMs in a CICS region. JVM servers and pooled JVMs use 64-bit storage, significantly reducing the storage constraints in a CICS region for running Java applications. You can therefore reduce the number of CICS regions that run Java to simplify system management and reduce infrastructure costs. You can also use System z Application Assist Processors (zAAPs) to run eligible Java workloads.


The scalability enhancements provided by CICS Transaction Server for z/OS for V4.2 provide, via the OTE enhancements, the ability for more workload to exploit the power of the mainframe. The 64-bit enhancements provide the ability to scale vertically and do more work in a single CICS region, establishing a foundation for even greater capacity in the future.

Additional Resources