View source for CICS

{{short description|IBM mainframe transaction monitor}}
{{Other uses}}
{{Use dmy dates|date=April 2022}}
{{Infobox software
| name                   = 
| title                  = 
| logo                   = [[File:IBM_CICS_Transaction_Server_for_z-OS_app_icon.svg|200px]]
| other_names            = Customer Information Control System
| released               = {{Start date and age|1969|07|8}}
| latest release version = CICS Transaction Server V6.1
| latest release date    = {{Start date and age|2022|06|17}}<ref>{{cite web|url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&appname=gpateam&supplier=897&letternum=ENUS222-092|date=2022-04-05|website=IBM|access-date=2023-05-15|title=IBM CICS Transaction Server for z/OS, V5.6 delivers significant improvements to the developer experience, security, resilience, and management}}</ref>
| operating system       = [[z/OS]], [[z/VSE]]
| platform               = [[IBM Z]]
| genre                  = [[Teleprocessing monitor]]
| license                = [[Proprietary software|Proprietary]]
| qid                    = Q1146247
}}

'''IBM CICS''' ('''Customer Information Control System''') is a family of mixed-language [[application servers]] that provide online [[business transaction management|transaction management]] and connectivity for applications on [[IBM mainframe]] systems under  [[z/OS]] and [[z/VSE]].

CICS family products are designed as [[middleware]] and support rapid, high-volume [[online transaction processing]]. A CICS ''transaction'' is a unit of processing initiated by a single request that may affect one or more objects.<ref>{{cite web|last=IBM Corporation|title=CICS Transaction Server for z/OS Glossary:T|website=[[IBM]] |url=https://www.ibm.com/support/knowledgecenter/SSGMCP_latest/glossary/glossary.html#gloss_T|access-date=February 2, 2021|archive-date=June 15, 2021|archive-url=https://web.archive.org/web/20210615130507/https://www.ibm.com/docs/en/cics-ts/5.6?topic=glossary#gloss_T|url-status=live}}</ref>  This processing is usually interactive (screen-oriented), but background transactions are possible.

CICS Transaction Server (CICS TS) sits at the head of the CICS family and provides services that extend or replace the functions of the operating system. These services can be more efficient than the generalized operating system services and also simpler for programmers to use, particularly with respect to communication with diverse terminal devices.

Applications developed for CICS may be written in a variety of [[programming language]]s and use CICS-supplied language extensions to interact with resources such as files, [[database connection]]s, terminals, or to invoke functions such as web services.  CICS manages the entire transaction such that if for any reason a part of the transaction fails all recoverable changes can be backed out.

While CICS TS has its highest profile among large financial institutions, such as banks and insurance companies, many [[Fortune 500]] companies and government entities are reported to run CICS. Other, smaller enterprises can also run CICS TS and other CICS family products. CICS can regularly be found behind the scenes in, for example, bank-teller applications, [[automated teller machine|ATM]] systems, industrial production control systems, insurance applications, and many other types of interactive applications.

Recent CICS TS enhancements include new capabilities to improve the developer experience, including the choice of APIs, frameworks, editors, and build tools, while at the same time providing updates in the key areas of security, resilience, and management. In earlier, recent CICS TS releases, support was provided for [[Web service]]s and [[Java (programming language)|Java]], [[event processing]], [[Atom (Web standard)|Atom]] feeds, and [[representational state transfer|REST]]ful interfaces.

==History==
[[Image:CICS.svg|thumb|left|Chart depicting high-level architecture of CICS (in French)]]
CICS was preceded by an earlier, single-threaded [[transaction processing system]], [[IBM MTCS]]. An 'MTCS-CICS bridge' was later developed to allow these transactions to execute under CICS with no change to the original application programs.
IBM's Customer Information Control System (CICS) was first developed in conjunction with [[Michigan Bell]] in 1966.<ref name="IBM">{{cite web |title=IBM archives |date=23 January 2003 |url=https://www.ibm.com/ibm/history/history/year_1968.html |publisher=IBM |access-date=6 December 2022}}</ref> Ben Riggins was an IBM systems engineer at Virginia Electric Power Co. when he came up with the idea for the online system.<ref name="Esm">{{cite web |title=ESM Mainframe hall of fame |url=https://www.enterprisesystemsmedia.com/mainframehalloffame |publisher=ESM |access-date=6 December 2022}}</ref>

CICS was originally developed in the United States out of the IBM Development Center in [[Des Plaines, Illinois]], beginning in 1966 to address requirements from the public utility industry. The first CICS product was announced in 1968, named ''Public Utility Customer Information Control System'', or PU-CICS.  It became clear immediately that it had applicability to many other industries, so the Public Utility prefix was dropped with the introduction of the first release of the CICS [[Software as a Product|Program Product]] on July 8, 1969, not long after [[IBM Information Management System|IMS]] [[database management system]].

[[Image:C Block, IBM Hursley Laboratory - geograph.org.uk - 969045.jpg|thumb|right|IBM Hursley, where much of the CICS development has been done, 2008]]

For the next few years, CICS was developed in Palo Alto and was considered a less important "smaller" product than IMS which IBM then considered more strategic. Customer pressure kept it alive, however. When IBM decided to end development of CICS in 1974 to concentrate on IMS, the CICS development responsibility was picked up by the [[IBM Hursley]] site in the United Kingdom, which had just ceased work on the [[PL/I]] compiler and so knew many of the same customers as CICS. The core of the development work continues in Hursley today alongside contributions from labs in India, China, Russia, Australia, and the United States.

===Early evolution===
CICS originally only supported a few IBM-brand devices like the 1965 [[IBM 2741]] Selectric (golf ball) typewriter-based terminal. The 1964 [[IBM 2260]] and 1972 [[IBM 3270]] [[video display terminal]]s were widely used later.

In the early days of IBM mainframes, computer software was free{{snd}} bundled at no extra charge with [[computer hardware]]. The [[OS/360]] operating system and application support software like CICS were "open" to IBM customers long before the [[open-source software]] initiative. Corporations like Standard Oil of Indiana (Amoco) made major contributions to CICS.

The IBM Des Plaines team tried to add support for popular non-IBM terminals like the [[ASCII]] [[Teletype Model 33]] ASR, but the small low-budget software development team could not afford the $100-per-month hardware to test it. IBM executives incorrectly felt that the future would be like the past, with [[batch processing]] using traditional [[punch cards]].

IBM reluctantly provided only minimal funding when public utility companies, banks and credit-card companies demanded a cost-effective interactive system (similar to the 1965 [[IBM Airline Control Program]] used by the American Airlines [[Sabre (computer system)|Sabre]] [[computer reservation system]]) for high-speed data access-and-update to customer information for their telephone operators (without waiting for overnight batch processing punch card systems).

When CICS was delivered to Amoco with Teletype Model 33 ASR support, it caused the entire OS/360 operating system to crash (including non-CICS application programs). The majority of the CICS Terminal Control Program (TCP{{snd}} the heart of CICS) and part of OS/360 had to be laboriously redesigned and rewritten by Amoco Production Company in Tulsa Oklahoma. It was then given back to IBM for free distribution to others.

In a few years,{{when|date=November 2012}} CICS generated over $60 billion in new hardware revenue for IBM, and became their most-successful mainframe software product.

In 1972, CICS was available in three versions{{snd}} DOS-ENTRY (program number 5736-XX6) for [[DOS/360 and successors|DOS/360]] machines with very limited memory, DOS-STANDARD (program number 5736-XX7), for DOS/360 machines with more memory, and OS-STANDARD V2 (program number 5734-XX7) for the larger machines which ran OS/360.<ref>{{cite book |url=http://bitsavers.org/pdf/ibm/370/CICS/GH20-1028-3_CICS_General_Information_Manual_Dec72.pdf |title=Customer Information Control System (CICS) General Information Manual |location=White Plains, New York |publisher=[[IBM]] |date=December 1972 |id=GH20-1028-3 |access-date=2016-04-01 |archive-date=2019-05-29 |archive-url=https://web.archive.org/web/20190529212532/http://bitsavers.org/pdf/ibm/370/CICS/GH20-1028-3_CICS_General_Information_Manual_Dec72.pdf |url-status=live }}</ref>

In early 1970, a number of the original developers, including Ben Riggins (the principal architect of the early releases) relocated to California and continued CICS development at IBM's [[Palo Alto, California|Palo Alto]] Development Center. IBM executives did not recognize value in software as a revenue-generating product until after federal law required software [[unbundling]]. In 1980, IBM executives failed to heed Ben Riggins' strong suggestions that IBM should provide their own [[EBCDIC]]-based operating system and integrated-circuit [[microprocessor]] chip for use in the [[IBM Personal Computer]] as a CICS [[thin client|intelligent terminal]] (instead of the incompatible Intel chip and relatively immature [[MS-DOS]]).

[[Image:Librarian listing of CICSGEN module.jpg|thumb|right|Beginning of a CICSGEN stage one module, 1982]]

Because of the limited capacity of even large processors of that era every CICS installation was required to assemble the source code for all of the CICS system modules after completing a process similar to [[system generation]] (sysgen), called ''CICSGEN'', to establish values for conditional assembly-language statements. This process allowed each customer to exclude support from CICS itself for any feature they did not intend to use, such as device support for terminal types not in use.

CICS owes its early popularity to its relatively efficient implementation when hardware was very expensive, its multi-threaded processing architecture, its relative simplicity for developing terminal-based real-time transaction applications, and many open-source customer contributions, including both debugging and feature enhancement.

===Z notation===
Part of CICS was formalized using the [[Z notation]] in the 1980s and 1990s in collaboration with the [[Oxford University Computing Laboratory]], under the leadership of [[Tony Hoare]]. This work won a [[Queen's Awards for Enterprise|Queen's Award]] for Technological Achievement.<ref>{{Cite book|last=King|first=Steve|editor-last=Hayes|editor-first=Ian|title=Specification Case Studies|year=1993|publisher=Prentice Hall|location=New York|pages=202–213|chapter=The Use of Z in the Restructure of IBM CICS|edition=2nd|isbn=978-0-13-832544-2}}</ref>

===CICS as a distributed file server===
In 1986, IBM announced CICS support for the record-oriented file services defined by [[Distributed Data Management Architecture]] (DDM). This enabled programs on remote, network-connected computers to create, manage, and access files that had previously been available only within the CICS/MVS and CICS/VSE transaction processing environments.<ref>{{cite journal |title=IBM Gives PC Programs Direct Mainframe Access: PC Applications Can Alter Files |journal=[[InfoWorld]] |date=1987-02-23 |last=Warner |first=Edward |volume=9 |issue=8 |page=1 |url=https://books.google.com/books?id=1DAEAAAAMBAJ&pg=PA1 |access-date=2016-04-01 |archive-date=2016-12-24 |archive-url=https://web.archive.org/web/20161224182237/https://books.google.com/books?id=1DAEAAAAMBAJ&pg=PA1 |url-status=live }}</ref>

In newer versions of CICS, support for DDM has been removed. Support for the DDM component of CICS z/OS was discontinued at the end of 2003, and was removed from CICS for z/OS in version 5.2 onward.<ref>{{cite web |url=http://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS214-107 |title=IBM CICS Transaction Server for z/OS, V5.2 takes service agility, operational efficiency, and cloud enablement to a new level |publisher=[[IBM]] |date=2014-04-07 |access-date=2016-04-14 |quote="CICS DDM is no longer available from IBM and support was discontinued, as of December 31, 2003. CICS DDM is no longer available in CICS TS from Version 5.2 onwards." |archive-date=2021-06-15 |archive-url=https://web.archive.org/web/20210615130501/https://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS214-107 |url-status=live }}</ref> In CICS TS for z/VSE, support for DDM was stabilised at V1.1.1 level, with an announced intention to discontinue it in a future release.<ref>{{cite web |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS214-074 |title=IBM z/VSE Central Functions Version 9.2 - z/VSE Version 5.2 |publisher=[[IBM]] |date=April 7, 2014 |access-date=2016-04-14 |quote="Support for CICS Distributed Data Management (DDM) is stabilized in CICS TS for VSE/ESA V1.1.1. In a future release of CICS TS for z/VSE, IBM intends to discontinue support for CICS DDM." |archive-date=2016-03-24 |archive-url=https://web.archive.org/web/20160324192812/http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS214-074 |url-status=live }}</ref> In CICS for z/VSE 2.1 onward, CICS/DDM is not supported.<ref>{{cite web |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS215-362 |title=IBM CICS Transaction Server for z/VSE V2.1 delivers enhancements for future workloads |publisher=[[IBM]] |date=October 5, 2015 |access-date=2016-04-14 |quote="CICS Distributed Data Management (CICS/DDM) is not supported with CICS TS for z/VSE V2.1." |archive-date=2016-04-24 |archive-url=https://web.archive.org/web/20160424205210/http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS215-362 |url-status=live }}</ref>

===CICS and the World Wide Web===
CICS Transaction Server first introduced a native [[Hypertext Transfer Protocol|HTTP]] interface in version 1.2, together with a Web Bridge technology for wrapping green-screen terminal-based programs with an HTML facade. CICS Web and Document APIs were enhanced in CICS TS V1.3 to enable web-aware applications to be written to interact more effectively with web browsers.

CICS TS versions 2.1 through 2.3 focused on introducing [[Common Object Request Broker Architecture|CORBA]] and [[Enterprise JavaBeans|EJB]] technologies to CICS, offering new ways to integrate CICS assets into distributed application component models. These technologies relied on hosting [[Java (programming language)|Java]] applications in CICS. The Java hosting environment saw numerous improvements over many releases. A multi-threaded JVM resource called the JVMSERVER was introduced during the CICS TS version 4.1 release, this was further enhanced to use 64-bit JVM technology in version 5.1. Version 5.1 also saw the introduction of the WebSphere Liberty profile web-container. Ultimately [[IBM WebSphere Application Server|WebSphere Liberty]] was fully embedded into CICS Transaction Server in version 5.3. Numerous web facing technologies could be hosted in CICS using Java, this ultimately resulted in the removal of the native CORBA and EJB technologies.

CICS TS V3.1 added a native implementation of the [[Simple Object Access Protocol|SOAP]] and [[Web Services Description Language|WSDL]] technologies for CICS, together with client side HTTP APIs for outbound communication. These twin technologies enabled easier integration of CICS components with other Enterprise applications, and saw widespread adoption. Tools were included for taking traditional CICS programs written in languages such as [[COBOL]], and converting them into WSDL defined Web Services, with little or no program changes. This technology saw regular enhancements over successive releases of CICS.

CICS TS V4.1 and V4.2 saw further enhancements to web connectivity, including a native implementation of the [[Atom (Web standard)|Atom]] publishing protocol.

Many of the newer web facing technologies were made available for earlier releases of CICS using delivery models other than a traditional product release. This allowed early adopters to provide constructive feedback that could influence the final design of the integrated technology. Examples include the Soap for CICS technology preview SupportPac for TS V2.2, or the ATOM SupportPac for TS V3.1. This approach was used to introduce [[JSON]] support for CICS TS V4.2, a technology that went on to be integrated into CICS TS V5.2.

The [[JSON]] technology in CICS is similar to earlier [[SOAP]] technology, both of which allowed programs hosted in CICS to be wrapped with a modern facade. The JSON technology was in turn enhanced in z/OS Connect Enterprise Edition, an IBM product for composing JSON APIs that can leverage assets from several mainframe subsystems.

Many partner products have also been used to interact with CICS. Popular examples include using the CICS Transaction Gateway for connecting to CICS from [[Java EE Connector Architecture|JCA]] compliant Java application servers, and IBM DataPower appliances for filtering web traffic before it reaches CICS.

Modern versions of CICS provide many ways for both existing and new software assets to be integrated into distributed application flows. CICS assets can be accessed from remote systems, and can access remote systems; user identity and transactional context can be propagated; RESTful APIs can be composed and managed; devices, users and servers can interact with CICS using standards-based technologies; and the IBM WebSphere Liberty environment in CICS promotes the rapid adoption of new technologies.

===MicroCICS===
By January, 1985 a 1969-founded consulting company, having done "massive on-line systems" for Hilton Hotels, FTD Florists, Amtrak, and Budget Rent-a-Car, announced what became ''MicroCICS''.<ref name=MicroCICS.IW86>{{cite magazine |magazine=[[InformationWeek]]
   |title=Unicorn is Betting that CICS is easer and cheaper on a PC
   |author=Paul E. Schindler, Jr.
   |date=October 27, 1986 |pages=41–44}}</ref> The initial focus was the [[IBM XT/370]] and [[IBM AT/370]].<ref>{{cite magazine
|magazine=[[Computerworld]] |date=December 9, 1985 |page=98
|title=Unicorn MicroCICS/RT
|quote=IBM Personal Computer XT/370 family}}</ref>

===CICS Family===
Although when CICS is mentioned, people usually mean CICS Transaction Server, the ''CICS Family'' refers to a portfolio of transaction servers, connectors (called [[CICS Transaction Gateway]]) and CICS Tools.

CICS on distributed platforms—not mainframes—is called [[IBM TXSeries]]. TXSeries is distributed transaction processing middleware. It supports C, C++, COBOL, Java™ and PL/I applications in cloud environments and traditional data centers. TXSeries is available on [[AIX]], Linux x86, [[Microsoft Windows|Windows]], [[Solaris (operating system)|Solaris]], and [[HP-UX]] platforms.<ref>{{cite magazine |magazine=Midrange Systems |page=35
   |title=IBM Get its CICS
   |date=November 10, 1992}}</ref> CICS is also available on other operating systems, notably [[IBM i]] and [[OS/2]]. The z/OS implementation (i.e., CICS Transaction Server for z/OS) is by far the most popular and significant.

Two versions of CICS were previously available for [[VM/CMS]], but both have since been discontinued. In 1986, IBM released ''CICS/CMS'',<ref>"announced .. October of 1985 .. didn't start deliveries until July of this year."</ref><ref name=MicroCICS.IW86/> which was a single-user version of CICS designed for development use, the applications later being transferred to an [[MVS]] or [[DOS/VS]] system for production execution.<ref>{{cite web |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=OC&subtype=NA&htmlfid=897/ENUS5668-795 |title=CICS/CMS |publisher=[[IBM]] |access-date=2016-04-01 |archive-date=2016-04-02 |archive-url=https://web.archive.org/web/20160402054915/http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=OC&subtype=NA&htmlfid=897%2FENUS5668-795 |url-status=live }}</ref><ref>{{cite web |url=http://www-01.ibm.com/common/ssi/rep_ca/5/897/ENUS285-385/index.html |title=CUSTOMER INFORMATION CONTROL SYSTEM/ CONVERSATIONAL MONITOR SYSTEM (CICS/CMS) RELEASE 1 ANNOUNCED AND PLANNED TO BE AVAILABLE JUNE 1986 |publisher=[[IBM]] |date=October 15, 1985 |access-date=2016-04-02 |archive-date=2016-04-02 |archive-url=https://web.archive.org/web/20160402053817/http://www-01.ibm.com/common/ssi/rep_ca/5/897/ENUS285-385/index.html |url-status=live }}</ref> Later, in 1988, IBM released ''CICS/VM''.<ref>{{cite web |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=OC&subtype=NA&htmlfid=897/ENUS5684-011 |title=(CICS/VM) Customer Information Control System / Virtual Machine |publisher=[[IBM]] |access-date=2016-04-01 |archive-date=2016-04-13 |archive-url=https://web.archive.org/web/20160413110835/http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=OC&subtype=NA&htmlfid=897%2FENUS5684-011 |url-status=live }}</ref><ref>{{cite web |url=http://www-01.ibm.com/common/ssi/printableversion.wss?docURL=/common/ssi/rep_ca/9/897/ENUS287-459/index.html |title=CUSTOMER INFORMATION CONTROL SYSTEM/VIRTUAL MACHINE (CICS/VM) |publisher=[[IBM]] |date=October 20, 1987 |access-date=2016-04-02 |archive-date=2016-04-02 |archive-url=https://web.archive.org/web/20160402054851/http://www-01.ibm.com/common/ssi/printableversion.wss?docURL=%2Fcommon%2Fssi%2Frep_ca%2F9%2F897%2FENUS287-459%2Findex.html |url-status=live }}</ref> CICS/VM was intended for use on the [[IBM 9370]], a low-end mainframe targeted at departmental use; IBM positioned CICS/VM running on departmental or branch office mainframes for use in conjunction with a central mainframe running CICS for MVS.<ref name="Babcock1987">{{cite magazine
|magazine=[[Computerworld]]
|last=Babcock
|first=Charles
|title=VM/SP update eases migration
|url=https://books.google.com/books?id=mUSIMiurpfYC&pg=PP25
|volume=21
|date=2 November 1987
|publisher=IDG Enterprise
|pages=25, 31
|issn=0010-4841
|issue=44
|access-date=30 March 2017
|archive-date=31 March 2017
|archive-url=https://web.archive.org/web/20170331051731/https://books.google.com/books?id=mUSIMiurpfYC&pg=PP25
|url-status=live
}}</ref>

===CICS Tools===
Provisioning, management and analysis of CICS systems and applications is provided by CICS Tools. This includes performance management as well as deployment and management of CICS resources. In 2015, the four core foundational CICS tools (and the CICS Optimization Solution Pack for z/OS) were updated with the release of CICS Transaction Server for z/OS 5.3. The four core CICS Tools: CICS Interdependency Analyzer for z/OS, CICS Deployment Assistant for z/OS, CICS Performance Analyzer for z/OS and CICS Configuration Manager for z/OS.

===Releases and versions===
CICS Transaction Server for z/OS has used the following release numbers:

<!-- Template:Version - for version & release history. Documentation and examples: http://en.wikipedia.org/wiki/Template:Version -->
{| class="wikitable"
|-
! Version
! Announcement Date
! Release date
! End of Service Date
! Features
|-
| {{Version |o |CICS Transaction Server for OS/390 1.1}}
| {{date|1996-09-10|ISO}}<ref name="ibm.com">{{cite web |title=US - IBM CICS Transaction Server (CICS TS) for OS/390 |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=dd&subtype=sm&appname=ShopzSeries&htmlfid=897/ENUS5655-147 |archive-url=https://web.archive.org/web/20220507013915/https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=dd&subtype=sm&appname=ShopzSeries&htmlfid=897/ENUS5655-147 |archive-date=2022-05-07|website=www.ibm.com |access-date=7 May 2022 |date=3 February 2004}}</ref>
| {{date|1996-11-08|ISO}}
| {{date|2001-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for OS/390 1.2}}
| {{date|1997-09-09|ISO}}<ref name="ibm.com"/>
| {{date|1997-10-24|ISO}}
| {{date|2002-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for OS/390 1.3}}
| {{date|1998-09-08|ISO}}<ref name="ibm.com"/>
| {{date|1999-03-26|ISO}}
| {{date|2006-04-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 2.1}}
| 2001-03-13<ref>{{Cite web |date=2001-05-23 |title=CICS TS for z/OS V2 |url=https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/0/872/ENUSAP01-1070/index.html |access-date=2022-05-13 |website=www.ibm.com |language=en-US}}</ref>
|2001-03-30
| {{date|2002-06-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 2.2}}
| {{date|2001-12-04|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS V2.2 Delivers Major Value to All CICS Customers |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS201-354 |website=www.ibm.com |access-date=7 May 2022 |date=4 December 2001}}</ref>
| {{date|2002-01-25|ISO}}
| {{date|2008-04-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 2.3}}
| {{date|2003-10-28|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS V2.3 advances towards on demand business |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS203-296 |website=www.ibm.com |access-date=7 May 2022 |date=28 October 2003}}</ref>
| {{date|2003-12-19|ISO}}
| {{date|2009-09-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 3.1}}
| {{date|2004-11-30|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS V3.1 offers improved integration, application transformation |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS204-285 |website=www.ibm.com |access-date=7 May 2022 |date=30 November 2004}}</ref>
| {{date|2005-03-25|ISO}}
| {{date|2015-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 3.2}}
| {{date|2007-03-27|ISO}}<ref>{{cite web |title=CICS Transaction Server for z/OS V3.2 delivers significant innovation for application connectivity |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS207-051 |website=www.ibm.com |access-date=7 May 2022 |date=27 March 2007}}</ref>
| {{date|2007-06-29|ISO}}
| {{date|2015-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 4.1}}
| {{date|2009-04-28|ISO}}<ref>{{cite web |title=IBM US Announcement Letter |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?subtype=ca&infotype=an&appname=iSource&supplier=897&letternum=ENUS209-135 |website=www.ibm.com |access-date=7 May 2022 |date=28 April 2009}}</ref>
| {{date|2009-06-26|ISO}}
| {{date|2017-09-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 4.2}}
| {{date|2011-04-05|ISO}}<ref>{{cite web |title=IBM US Announcement Letter |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS211-080 |website=www.ibm.com |access-date=7 May 2022 |date=5 April 2011}}</ref>
| {{date|2011-06-24|ISO}}
| {{date|2018-09-30|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 5.1}}
| {{date|2012-10-03|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS V5.1 delivers operational efficiency and service agility with cloud enablement |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS212-325 |website=www.ibm.com |access-date=7 May 2022 |date=3 October 2012}}</ref>
| {{date|2012-12-14|ISO}}
| {{date|2019-07-01|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 5.2}}
| {{date|2014-04-07|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, V5.2 takes service agility, operational efficiency, and cloud enablement to a new level |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS214-107 |website=www.ibm.com |access-date=7 May 2022 |date=7 April 2014}}</ref>
| {{date|2014-06-13|ISO}}
| {{date|2020-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 5.3}}
| {{date|2015-10-05|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, V5.3 delivers advances in service agility, operational efficiency, and cloud enablement with DevOps |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS215-363 |website=www.ibm.com |access-date=7 May 2022 |date=5 October 2015}}</ref>
| {{date|2015-12-11|ISO}}
| {{date|2021-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 5.4}}
| {{date|2017-05-16|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, V5.4 delivers unparalleled mixed language application serving |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS217-113 |website=www.ibm.com |access-date=7 May 2022 |date=16 May 2017}}</ref>
| {{date|2017-06-16|ISO}}
| {{date|2023-12-31|ISO}}
| 
|-
| {{Version |o |CICS Transaction Server for z/OS 5.5}}
| {{date|2018-10-02|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, V5.5 delivers support for Node.js and further enhancements to CICS Explorer, systems management, and security |url=https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/2/897/ENUS218-352/index.html |website=www.ibm.com |access-date=7 May 2022 |date=2 October 2018}}</ref>
| {{date|2018-12-14|ISO}}
| {{date|2025-09-30|ISO}}
| 
|-
| {{Version |co |CICS Transaction Server for z/OS 5.6}}
| {{date|2020-04-07|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, V5.6 delivers significant improvements to the developer experience, security, resilience, and management |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&appname=gpateam&supplier=897&letternum=ENUS220-077 |website=www.ibm.com |access-date=6 May 2022 |date=7 April 2020}}</ref>
| {{date|2020-06-12|ISO}}
|
| Support for [[Spring Framework#Spring Boot|Spring Boot]], [[Jakarta EE]] 8, [[Node.js]] 12. New JCICSX API with remote development capability. Security, resilience and management enhancements.
|-
| {{Version |co |CICS Transaction Server for z/OS 6.1}}
| {{date|2022-04-05|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS, 6.1 delivers significant improvements in the areas of developer productivity, security, and management |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&appname=gpateam&supplier=897&letternum=ENUS222-092 |website=www.ibm.com |access-date=6 May 2022 |date=5 April 2022}}</ref>
| {{date|2022-06-17|ISO}}
|
| Support for [[Java (programming language)|Java]] 11, [[Jakarta EE]] 9.1, Eclipse MicroProfile 5, [[Node.js]] 12, [[Transport Layer Security|TLS]] 1.3. Security enhancements and simplifications. Region tagging.
|-
| {{Version |c |CICS Transaction Server for z/OS 6.2}}
| {{date|2024-04-09|ISO}}<ref>{{cite web |title=IBM CICS Transaction Server for z/OS 6.2 general availability |url=https://www.ibm.com/docs/en/announcements/cics-transaction-server-zos-620-ga-announcement?region=US |website=www.ibm.com |access-date=23 Sep 2024 |date=9 April 2024}}</ref>
| {{nowrap|{{date|2024-06-14|ISO}}}}
|
| 
|-
| colspan="5" | <small>{{Version |l}}</small>
|}
{{Incomplete table|date=May 2022}}

==Programming==

===Programming considerations===
Multiple-user interactive-transaction application programs were required to be [[wikt:quasi-|quasi]]-[[reentrant (subroutine)|reentrant]] in order to support multiple concurrent [[database transaction|transaction]] [[thread (computer science)|thread]]s. A software coding error in one application could block all users from the system. The modular design of CICS reentrant / reusable control programs meant that, with judicious "pruning," multiple users with multiple applications could be executed on a computer with just 32K of expensive [[magnetic core]] [[physical memory]] (including the [[operating system]]).

Considerable effort was required by CICS application programmers to make their transactions as efficient as possible. A common technique was to limit the size of individual programs to no more than 4,096 bytes, or 4K, so that CICS could easily reuse the memory occupied by any program not currently in use for another program or other application storage needs. When [[virtual memory]] was added to versions OS/360 in 1972, the 4K strategy became even more important to reduce [[paging]] and [[thrashing (computer science)|thrashing]] unproductive resource-contention overhead.

The efficiency of compiled high-level COBOL and PL/I language programs left much to be desired. Many CICS application programs continued to be written in assembler language, even after COBOL and PL/I support became available.

With 1960s-and-1970s hardware resources expensive and scarce, a competitive "game" developed among system optimization analysts. When [[critical path method|critical path]] code was identified, a code snippet was passed around from one analyst to another. Each person had to either (a) reduce the number of bytes of code required, or (b) reduce the number of [[CPU]] cycles required. Younger analysts learned from what more-experienced mentors did. Eventually, when no one could do (a) or (b), the code was considered optimized, and they moved on to other snippets. Small shops with only one analyst learned CICS optimization very slowly (or not at all).

Because application programs could be shared by many [[concurrency (computer science)|concurrent]] threads, the use of [[static variable]]s embedded within a program (or use of operating system memory) was restricted (by convention only).

[[Image:OLIVER advert.png|thumb|right|upright=0.7|Advertisement for CICS debugging product, 1978]]

Unfortunately, many of the "rules" were frequently broken, especially by COBOL programmers who might not understand the internals of their programs or fail to use the necessary restrictive [[compile time]] options. This resulted in "non-re-entrant" code that was often unreliable, leading to spurious [[storage violation]]s and entire CICS system crashes.

Originally, the entire [[memory management (operating systems)#Partitioned allocation|partition]], or [[Multiple Virtual Storage]] (MVS) region, operated with the same [[memory protection key]] including the CICS kernel code. Program corruption and CICS control block corruption was a frequent cause of system downtime. A software error in one application program could overwrite the memory (code or data) of one or all currently running application transactions. Locating the offending application code for complex transient timing errors could be a very-difficult operating-system analyst problem.

These shortcomings persisted for multiple new releases of CICS over a period of more than 20 years, in spite of their severity and the fact that top-quality CICS skills were in high demand and short supply. They were addressed in TS V3.3, V4.1 and V5.2 with the Storage Protection, Transaction Isolation and Subspace features respectively, which utilize operating system hardware features to protect the application code and the data within the same [[address space]] even though the applications were not written to be separated. CICS application transactions remain mission-critical for many public utility companies, large banks and other multibillion-dollar financial institutions.

===Macro-level programming===
When CICS was first released, it only supported application transaction programs written in [[IBM Basic Assembly Language|IBM 360 Assembler]]. [[COBOL]] and [[PL/I]] support were added years later. Because of the initial assembler orientation, requests for CICS services were made using assembler-language [[macro instruction|macros]]. For example, the request to read a record from a file were made by a macro call to the "File Control Program" of CICS might look like this:

 DFHFC TYPE=READ,DATASET=myfile,TYPOPER=UPDATE,....etc.

This gave rise to the later terminology "'''Macro-level''' CICS."

When high-level language support was added, the macros were retained and the code was converted by a pre-compiler that expanded the macros to their COBOL or PL/I CALL statement equivalents. Thus preparing a [[high-level programming language|HLL]] application was effectively a "two-stage" [[compiler|compile]]{{snd}} output from the preprocessor fed into the HLL compiler as input.

'''COBOL considerations''': unlike PL/I, IBM COBOL does not normally provide for the manipulation of pointers (addresses).  In order to allow COBOL programmers to access CICS control blocks and dynamic storage the designers resorted to what was essentially a hack.  The COBOL ''Linkage Section'' was normally used for inter-program communication, such as parameter passing. The compiler generates a list of addresses, each called a ''Base Locator for Linkage'' (BLL) which were set on entry to the called program. The first BLL corresponds to the first item in the Linkage Section and so on. CICS allows the programmer to access and manipulate these by passing the address of the list as the first argument to the program.  The BLLs can then be dynamically set, either by CICS or by the application to allow access to the corresponding structure in the Linkage Section.<ref>{{cite book|last1=IBM Corporation|title=Customer Information Control System (CICS) Application Programmer's Reference Manual|date=1972|url=http://www.bitsavers.org/pdf/ibm/370/CICS/SH20-1047-4_CICS_Application_Programmers_Reference_Manual_Dec72.pdf|access-date=Jan 4, 2016|archive-date=May 29, 2019|archive-url=https://web.archive.org/web/20190529212914/http://bitsavers.org/pdf/ibm/370/CICS/SH20-1047-4_CICS_Application_Programmers_Reference_Manual_Dec72.pdf|url-status=live}}</ref>

===Command-level programming===
During the 1980s, IBM at Hursley Park produced a version of CICS that supported what became known as "Command-level CICS" which still supported the older programs but introduced a new API style to application programs.

A typical Command-level call might look like the following:

<syntaxhighlight lang="cobolfree">
 EXEC CICS
     SEND MAPSET('LOSMATT') MAP('LOSATT')
 END-EXEC
</syntaxhighlight>

The values given in the SEND MAPSET command correspond to the names used on the first DFHMSD macro in the map definition given below for the MAPSET argument, and the DFHMSI macro for the MAP argument. This is pre-processed by a pre-compile batch translation stage, which converts the embedded commands (EXECs) into call statements to a stub subroutine. So, preparing application programs for later execution still required two stages. It was possible to write "''Mixed mode''" applications using both Macro-level and Command-level statements.

Initially, at execution time, the command-level commands were converted using a run-time translator, "The EXEC Interface Program", to the old Macro-level call, which was then executed by the mostly unchanged CICS nucleus programs.  But when the CICS Kernel was re-written for TS V3, EXEC CICS became the only way to program CICS applications, as many of the underlying interfaces had changed.

===Run-time conversion===
The '''Command-level-only''' CICS introduced in the early 1990s offered some advantages over earlier versions of CICS. However, IBM also dropped support for Macro-level application programs written for earlier versions. This meant that many application programs had to be converted or completely rewritten to use Command-level EXEC commands only.

By this time, there were perhaps millions of programs worldwide that had been in production for decades in many cases. Rewriting them often introduced new bugs without necessarily adding new features. There were a significant number of users who ran CICS V2 application-owning regions (AORs) to continue to run macro code for many years after the change to V3.

It was also possible to execute old Macro-level programs using conversion software such as APT International's ''[[Command CICS]]''.<ref>{{cite web|url=http://www.ibm.com/partnerworld/gsd/solutiondetails.do?solution=16478&expand=true&lc=en|work=[[IBM]]|title=Command/CICS|access-date=22 April 2018|archive-date=15 June 2021|archive-url=https://web.archive.org/web/20210615130438/https://www-356.ibm.com/partnerworld/gsd/solutiondetails.do?solution=16478&expand=true&lc=en|url-status=live}}</ref>

===New programming styles===
Recent CICS Transaction Server enhancements include support for a number of modern programming styles.

CICS Transaction Server Version 5.6<ref>{{Cite web |url=https://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS220-077 |title=IBM CICS Transaction Server for z/OS, V5.6 delivers significant improvements to the developer experience, security, resilience, and management |date=7 April 2020 |access-date=2020-07-09 |archive-date=2020-07-10 |archive-url=https://web.archive.org/web/20200710230551/https://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=an&subtype=ca&supplier=897&letternum=ENUS220-077 |url-status=live }}</ref> introduced enhanced support for Java to deliver a cloud-native experience for Java developers. For example, the new CICS Java API ([https://www.ibm.com/support/knowledgecenter/SSGMCP_5.6.0/applications/developing/java/jcicsx-api.html JCICSX]) allows easier unit testing using mocking and stubbing approaches, and can be run remotely on the developer's local workstation. A set of [https://search.maven.org/search?q=g:com.ibm.cics CICS artifacts on Maven Central] enable developers to resolve Java dependencies using popular dependency management tools such as [[Apache Maven]] and [[Gradle]]. Plug-ins for Maven ([https://github.com/IBM/cics-bundle-maven cics-bundle-maven]) and Gradle ([https://github.com/IBM/cics-bundle-gradle cics-bundle-gradle]) are also provided to simplify automated building of CICS bundles, using familiar IDEs like [[Eclipse (software)|Eclipse]], [[IntelliJ IDEA]], and [[Visual Studio Code]]. In addition, [[Node.js]] z/OS support is enhanced for version 12, providing faster startup, better default heap limits, updates to the V8 JavaScript engine, etc. Support for [[Jakarta EE]] 8 is also included.

CICS TS 5.5 introduced support for IBM SDK for Node.js, providing a full JavaScript runtime, server-side APIs, and libraries to efficiently build high-performance, highly scalable network applications for IBM Z.

CICS Transaction Server Version 2.1 introduced support for Java. CICS Transaction Server Version 2.2 supported the Software Developers Toolkit. CICS provides the same run-time container as IBM's WebSphere product family so Java EE applications are portable between CICS and Websphere and there is common tooling for the development and deployment of Java EE applications.

In addition, CICS placed an emphasis on "wrapping" existing application programs inside modern interfaces so that long-established business functions can be incorporated into more modern services. These include WSDL, SOAP and JSON interfaces that wrap legacy code so that a web or mobile application can obtain and update the core business objects without requiring a major rewrite of the back-end functions.

==Transactions==
A CICS transaction is a set of operations that perform a task together. Usually, the majority of transactions are relatively simple tasks such as requesting an inventory list or entering a debit or credit to an account. A primary characteristic of a transaction is that it should be [[atomicity (database systems)|atomic]]. On [[IBM Z]] servers, CICS easily supports thousands of transactions per second, making it a mainstay of enterprise computing.

CICS applications comprise transactions, which can be written in numerous [[programming language]]s, including COBOL, PL/I, C, C++, IBM Basic Assembly Language, [[Rexx]], and Java.

Each CICS program is initiated using a transaction identifier. CICS screens are usually sent as a construct called a map, a module created with [[Basic Mapping Support]] (BMS) assembler macros or third-party tools. CICS [[panel (computer software)|screens]] may contain text that is highlighted, has different colors, and/or blinks depending on the terminal type used. An example of how a map can be sent through COBOL is given below. The end user inputs data, which is made accessible to the program by receiving a map from CICS.

<syntaxhighlight lang="cobolfree">
 EXEC CICS
     RECEIVE MAPSET('LOSMATT') MAP('LOSATT') INTO(OUR-MAP)
 END-EXEC.
</syntaxhighlight>

For technical reasons, the arguments to some command parameters must be quoted and some must not be quoted, depending on what is being referenced. Most programmers will code out of a reference book until they get the "hang" or concept of which arguments are quoted, or they'll typically use a "canned template" where they have example code that they just copy and paste, then edit to change the values.

===Example of BMS Map Code===
Basic Mapping Support defines the screen format through assembler macros such as the following.  This was assembled to generate both the ''physical map set''{{snd}} a load module in a CICS load library{{snd}} and a ''symbolic map set''{{snd}} a structure definition or ''DSECT'' in PL/I, COBOL, assembler, etc. which was copied into the source program.<ref>{{cite web|last=IBM Corporation|title=Basic mapping support|url=http://publib.boulder.ibm.com/infocenter/cicsts/v2r3/index.jsp?topic=/com.ibm.cics.ts23.doc/dfhp3/dfhp370.htm|archive-url=https://archive.today/20130103084403/http://publib.boulder.ibm.com/infocenter/cicsts/v2r3/index.jsp?topic=/com.ibm.cics.ts23.doc/dfhp3/dfhp370.htm|url-status=dead|archive-date=2013-01-03|work=CICS Information Center}}</ref>

<syntaxhighlight lang="cobolfree">
 LOSMATT DFHMSD TYPE=MAP,                                               X
                MODE=INOUT,                                             X
                TIOAPFX=YES,                                            X
                TERM=3270-2,                                            X
                LANG=COBOL,                                             X
                MAPATTS=(COLOR,HILIGHT),                                X
                DSATTS=(COLOR,HILIGHT),                                 X
                STORAGE=AUTO,                                           X
                CTRL=(FREEKB,FRSET)
 *
 LOSATT  DFHMDI SIZE=(24,80),                                           X
                LINE=1,                                                 X
                COLUMN=1
 *
 LSSTDII DFHMDF POS=(1,01),                                             X
                LENGTH=04,                                              X
                COLOR=BLUE,                                             X
                INITIAL='MQCM',                                         X
                ATTRB=PROT
 *
         DFHMDF POS=(24,01),                                            X
                LENGTH=79,                                              X
                COLOR=BLUE                                              X
                ATTRB=ASKIP,                                            X
                INITIAL='PF7-          8-           9-          10-     X
                    11-            12-CANCEL'
 *
           DFHMSD   TYPE=FINAL
           END
</syntaxhighlight>

==Structure==
[[Image:WOLA-Link-Server.jpg|thumb|left|Chart showing a particular task invocation of CICS, 2010]]

<!-- [[Tor (disambiguation)]] links to this section -->
In the [[z/OS]] environment, a CICS installation comprises one or more "[[memory management (operating systems)#Partitioned allocation|regions]]" (generally referred to as a "CICS Region"),<ref>{{Cite web
  | last = IBM
  | title = CICS Transaction Server glossary
  | work = CICS Transaction Server for z/OS V3.2
  | publisher = IBM Information Center, Boulder, Colorado
  | date = September 13, 2010
  | url = http://publib.boulder.ibm.com/infocenter/cicsts/v3r2/index.jsp
  | access-date = December 12, 2010
  | archive-url = https://web.archive.org/web/20130901150521/http://publib.boulder.ibm.com/infocenter/cicsts/v3r2/index.jsp
  | archive-date = September 1, 2013
  | url-status = dead
}}</ref> spread across one or more z/OS system images. Although it processes interactive transactions, each CICS region is usually started as a batch processing|batch address space with standard [[Job Control Language|JCL]] statements: it's a job that runs indefinitely until shutdown. Alternatively, each CICS region may be started as a [[daemon (computing)|started task]]. Whether a batch job or a started task, CICS regions may run for days, weeks, or even months before shutting down for maintenance (MVS or CICS). Upon restart a parameter determines if the start should be "Cold" (no recovery) or "Warm"/"Emergency" (using a warm shutdown or restarting from the log after a crash). Cold starts of large CICS regions with many resources can take a long time as all the definitions are re-processed.

Installations are divided into multiple address spaces for a wide variety of reasons, such as:

* application separation,
* function separation,
* avoiding the workload capacity limitations of a single region, or address space, or mainframe instance in the case of a z/OS SysPlex.

A typical installation consists of a number of distinct applications that make up a service. Each service usually has a number of "Terminal-Owning Region" (TORs) that route transactions to multiple "Application-Owning Regions" (AORs), though other topologies are possible. For example, the AORs might not perform File I/O. Instead there would be a "File-Owning Region" (FOR) that performed the File I/O on behalf of transactions in the AOR{{snd}} given that, at the time, a VSAM file could only support recoverable write access from one address space at a time.

But not all CICS applications use VSAM as the primary data source (or historically other single-address-space-at-a-time datastores such as CA Datacom) − many use either IMS/DB or Db2 as the database, and/or MQ as a queue manager. For all these cases, TORs can load-balance transactions to sets of AORs which then directly use the shared databases/queues. CICS supports XA two-phase commit between data stores and so transactions that spanned MQ, VSAM/RLS and Db2, for example, are possible with ACID properties.

CICS supports distributed transactions using SNA LU6.2 protocol between the address spaces which can be running on the same or different clusters. This allows ACID updates of multiple datastores by cooperating distributed applications. In practice there are issues with this if a system or communications failure occurs because the transaction disposition (backout or commit) may be in-doubt if one of the communicating nodes has not recovered. Thus the use of these facilities has never been very widespread.

==Sysplex exploitation==
[[Image:CICS-TX-Sec.jpg|thumb|right|A diagram showing one site's relationship between z/OS and CICS, 2010]]

At the time of CICS ESA V3.2, in the early 1990s, IBM faced the challenge of how to get CICS to exploit the new z/OS [[IBM Parallel Sysplex|Sysplex]] mainframe line.

The Sysplex was to be based on [[CMOS]] (Complementary Metal Oxide Silicon) rather than the existing [[emitter-coupled logic|ECL]] (Emitter Coupled Logic) hardware. The cost of scaling the mainframe-unique ECL was much higher than CMOS which was being developed by a ''[[keiretsu]]'' with high-volume use cases such as Sony PlayStation to reduce the unit cost of each generation's CPUs. The ECL was also expensive for users to run because the gate drain current produced so much heat that the CPU had to packaged into a special module called a Thermal Conduction Module (TCM<ref>{{Cite web|url=https://www-03.ibm.com/ibm/history/exhibits/vintage/vintage_4506VV2137.html|title=IBM Archives: Thermal conduction module|date=2003-01-23|website=www-03.ibm.com|language=en-US|access-date=2018-06-01|archive-date=2016-07-20|archive-url=https://web.archive.org/web/20160720232359/http://www-03.ibm.com/ibm/history/exhibits/vintage/vintage_4506VV2137.html|url-status=dead}}</ref>) that had inert gas pistons and needed plumbed with high-volume chilled water to be cooled. However, the air-cooled CMOS technology's CPU speed initially was much slower than the ECL (notably the boxes available from the mainframe-clone makers [[Amdahl Corporation|Amdahl]] and [[Hitachi Data Systems|Hitachi]]). This was especially concerning to IBM in the CICS context as almost all the largest mainframe customers were running CICS and for many of them it was the primary mainframe workload.

To achieve the same total transaction throughput on a Sysplex, multiple boxes would need to be used in parallel for each workload. However, a CICS address space, due to its quasi-reentrant application programming model, could not exploit more than about 1.5 processors on one box at the time{{snd}} even with use of MVS sub-tasks. Without enhanced parallelism, customers would tend to move to IBM's competitors rather than use Sysplex as they scaled up the CICS workloads. There was considerable debate inside IBM as to whether the right approach would be to break upward compatibility for applications and move to a model like [[IMS/DC]] which was fully reentrant, or to extend the approach customers had adopted to more fully exploit a single mainframe's power{{snd}} using multi-region operation (MRO).

Eventually the second path was adopted after the CICS user community was consulted. The community vehemently opposed breaking upward compatibility given that they had the prospect of Y2K to contend with at that time and did not see the value in re-writing and testing millions of lines of mainly COBOL, PL/I, or assembler code.

The IBM-recommended structure for CICS on Sysplex was that at least one CICS Terminal Owning Region was placed on each Sysplex node which dispatched transactions to many Application Owning Regions (AORs) spread across the entire Sysplex. If these applications needed to access shared resources they either used a Sysplex-exploiting datastore (such as [[IBM Db2]] or [[IBM Information Management System|IMS/DB]]) or concentrated, by function-shipping, the resource requests into singular-per-resource Resource Owing Regions (RORs) including File Owning Regions (FORs) for [[Virtual Storage Access Method|VSAM]] and CICS Data Tables, Queue Owning Regions (QORs) for [[IBM WebSphere MQ|MQ]], CICS Transient Data (TD) and CICS Temporary Storage (TS). This preserved compatibility for legacy applications at the expense of operational complexity to configure and manage many CICS regions.

In subsequent releases and versions, CICS was able to exploit new Sysplex-exploiting facilities in VSAM/RLS,<ref>{{cite book |title=IMS |publisher=John Wiley & Sons, Ltd |isbn=9780470750001 |location=Chichester, UK |pages=1–39 |doi= 10.1002/9780470750001.ch1 |chapter= IMS Context |year= 2009}}</ref> MQ for zOS<ref>{{Cite web|url=https://www.ibm.com/support/knowledgecenter/en/SSFKSJ_7.0.1/com.ibm.mq.csqzal.doc/fg10370_.htm|title=IBM Knowledge Center MQ for zOS|website=www.ibm.com|date=11 March 2014 |language=en-US|access-date=2018-06-01|archive-date=2016-08-07|archive-url=https://web.archive.org/web/20160807050826/http://www.ibm.com/support/knowledgecenter/en/SSFKSJ_7.0.1/com.ibm.mq.csqzal.doc/fg10370_.htm|url-status=live}}</ref> and placed its own Data Tables, TD, and TS resources into the architected shared resource manager for the Sysplex: the [[Coupling Facility]] or CF, dispensing with the need for most RORs. The CF provides a mapped view of resources including a shared timebase, buffer pools, locks and counters with hardware messaging assists that made sharing resources across the Sysplex both more efficient than polling and reliable (utilizing a semi-synchronized backup CF for use in case of failure).

By this time, the CMOS line had individual boxes that exceeded the power available by the fastest ECL box with more processors per CPU. When these were coupled together, 32 or more nodes would be able to scale two orders of magnitude greater in total power for a single workload. For example, by 2002, Charles Schwab was running a "MetroPlex" consisting of a redundant pair of mainframe Sysplexes in two locations in Phoenix, AZ, each with 32 nodes driven by one shared CICS/DB/2 workload to support the vast volume of pre-[[dot-com bubble|dotcom-bubble]] web client inquiry requests.

This cheaper, much more scalable CMOS technology base, and the huge investment costs of having to both get to 64-bit addressing and independently produce cloned CF functionality drove the IBM-mainframe clone makers out of the business one by one.<ref>{{Cite news|url=https://www.computerworld.com/article/2588995/retail-it/amdahl-gives-up-on--mainframe-business.html|title=Amdahl gives up on mainframe business|last=Vijayan|first=Jaikumar|work=Computerworld|access-date=2018-06-01|language=en|archive-date=2018-11-03|archive-url=https://web.archive.org/web/20181103131245/https://www.computerworld.com/article/2588995/retail-it/amdahl-gives-up-on--mainframe-business.html|url-status=live}}</ref><ref>{{Cite news|url=https://www.theregister.co.uk/2017/05/24/hitachi_exits_mainframe_hardware/|title=Hitachi exits mainframe hardware but will collab with IBM on z Systems|access-date=2018-06-01|language=en|archive-date=2018-06-13|archive-url=https://web.archive.org/web/20180613060121/https://www.theregister.co.uk/2017/05/24/hitachi_exits_mainframe_hardware|url-status=live}}</ref>

==CICS Recovery/Restart==
The objective of recovery/restart in CICS is to minimize and if possible eliminate damage done to Online System when a failure occurs, so that system and data integrity is maintained.<ref>{{cite web|url=https://www.ibm.com/support/knowledgecenter/SSGMCP_latest/administering/recovery/dfht219.html|title=IBM Knowledge Center|website=publib.boulder.ibm.com|access-date=2 February 2021|archive-date=15 June 2021|archive-url=https://web.archive.org/web/20210615130445/https://www.ibm.com/docs/en/cics-ts/5.6?topic=recovery-cics-warm-restart|url-status=live}}</ref> If the CICS region was shut down instead of failing it will perform a "Warm" start exploiting the checkpoint written at shutdown. The CICS region can also be forced to "Cold" start which reloads all definitions and wipes out the log, leaving the resources in whatever state they are in.

Under CICS, following are some of the resources which are considered recoverable. If one wishes these resources to be recoverable then special options must be specified in relevant CICS definitions:

* VSAM files
* CMT CICS-maintained data tables
* Intrapartition TDQ
* Temporary Storage Queue in auxiliary storage
* I/O messages from/to transactions in a VTAM network
* Other database/queuing resources connected to CICS that support XA two-phase commit protocol (like IMS/DB, Db2, VSAM/RLS)

CICS also offers extensive recovery/restart facilities for users to establish their own recovery/restart capability in their CICS system. Commonly used recovery/restart facilities include:

* Dynamic Transaction Backout (DTB)
* Automatic Transaction Restart
* Resource Recovery using System Log
* Resource Recovery using Journal
* System Restart
* Extended Recovery Facility

==Components==
Each CICS region comprises one major task on which every transaction runs, although certain services such as access to [[IBM Db2]] data use other tasks (TCBs). Within a region, transactions are [[cooperative multitasking|cooperatively multitasked]]{{snd}} they are expected to be well-behaved and yield the CPU rather than wait. CICS services handle this automatically.

Each unique CICS "[[task (computing)|Task]]" or transaction is allocated its own dynamic [[computer storage|memory]] at start-up and subsequent requests for additional memory were handled by a call to the "Storage Control program" (part of the CICS '''nucleus''' or "[[kernel (operating system)|kernel]]"), which is analogous to an [[operating system]].

A CICS system consists of the online ''nucleus'', batch support programs, and applications services.<ref name=SysProgRef>{{cite book|last=IBM Corporation|title=Customer Information Control System (CICS) System Programmer's Reference Manual|year=1975|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/370/CICS/SC20-1043-6_CICS_System_Programmers_Reference_Manual_Mar75.pdf|access-date=2012-11-21|archive-date=2011-02-17|archive-url=https://web.archive.org/web/20110217070450/http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/370/CICS/SC20-1043-6_CICS_System_Programmers_Reference_Manual_Mar75.pdf|url-status=live}}</ref>

===Nucleus===
The original CICS nucleus consisted of a number of functional modules written in 370 assembler until V3:
* Task Control Program (KCP)
* Storage Control Program (SCP)
* Program Control Program (PCP)
* Program Interrupt Control Program (PIP)
* Interval Control Program (ICP)
* Dump Control Program (DCP)
* Terminal Control Program (TCP)
* File Control Program (FCP)
* Transient Data Control Program (TDP)
* Temporary Storage Control Program (TSP)

Starting in V3, the CICS nucleus was rewritten into a kernel-and-domain structure using IBM's [[PL/AS]] language{{snd}} which is compiled into assembler.

The prior structure did not enforce separation of concerns and so had many inter-program dependencies which led to bugs unless exhaustive code analysis was done. The new structure was more modular and so resilient because it was easier to change without impact. The first domains were often built with the name of the prior program but without the trailing "P". For example, Program Control Domain (DFHPC) or Transient Data Domain (DFHTD). The kernel operated as a switcher for inter-domain requests{{snd}} initially this proved expensive for frequently called domains (such as Trace) but by utilizing PL/AS macros these calls were in-lined without compromising on the separate domain design.

In later versions, completely redesigned domains were added like the Logging Domain DFHLG and Transaction Domain DFHTM that replaced the Journal Control Program (JCP).

===Support programs===
In addition to the online functions CICS has several support programs that run as batch jobs.<ref name=Logic>{{cite book|last=IBM Corporation|title=Customer Information Control System/Virtual Storage (CICS/VS) Version 1, Release 3 Introduction to Program Logic Manual|year=1977|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/370/CICS/SC33-0067-0_CICS_VS_Intro_To_Pgm_Logic_Feb77.pdf|access-date=2012-11-24|archive-url=https://web.archive.org/web/20110217070557/http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/370/CICS/SC33-0067-0_CICS_VS_Intro_To_Pgm_Logic_Feb77.pdf|archive-date=2011-02-17|url-status=dead}}</ref> {{rp|pp.34–35}}
* High-level language (macro) preprocessor
* Command language translator
* Dump utility &ndash; prints formatted dumps generated by CICS Dump Management
* Trace utility &ndash; formats and prints CICS trace output
* Journal formatting utility{{snd}} prints a formatted dump of the CICS region in case of error

===Applications services===
The following components of CICS support application development.<ref name=Logic />{{rp|pp.35–37}}
* Basic Mapping Support (BMS) provides device-independent terminal input and output
* APPC Support that provides LU6.1 and LU6.2 API support for collaborating distributed applications that support two-phase commit
* Data Interchange Program (DIP) provides support for [[IBM 2780/3780#2770.2C 3770|IBM 3770]] and [[IBM 3790]] programmable devices
* 2260 Compatibility allows programs written for [[IBM 2260]] display devices to run on 3270 displays
* EXEC Interface Program{{snd}} the stub program that converts calls generated by <code>EXEC CICS</code> commands to calls to CICS functions
* Built-in Functions &ndash; table search, phonetic conversion, field verify, field edit, bit checking, input formatting, weighted retrieval

==Pronunciation==
Different countries have differing pronunciations<ref>{{cite web
  | title = CICS - An Introduction
  | publisher = IBM Corporation
  | date = July 8, 2004
  | url = ftp://service.boulder.ibm.com/software/htp/cics/PDF/cics_introduction.pdf
  | archive-url = https://web.archive.org/web/20210829130916/ftp://service.boulder.ibm.com/software/htp/cics/PDF/cics_introduction.pdf
  | archive-date = 2021-08-29
  | url-status = dead
  | access-date = April 20, 2014
  }}</ref>
* Within IBM (specifically [[Tivoli Software|Tivoli]]) it is referred to as {{IPAc-en|ˈ|k|ɪ|k|s}}.
* In the US, it is more usually pronounced by reciting each letter {{IPAc-en|ˌ|s|iː|ˌ|aɪ|ˌ|s|iː|ˈ|ɛ|s}}.
* In Australia, Belgium, Canada, Hong Kong, the UK and some other countries, it is pronounced {{IPAc-en|ˈ|k|ɪ|k|s}}.
* In Denmark, it is pronounced ''kicks''.
* In Finland, it is pronounced {{IPA|fi|kiks|}}
* In France, it is pronounced {{IPA|fr|se.i.se.ɛs|}}.
* In Germany, Austria and Hungary, it is pronounced {{IPA|de|ˈtsɪks|}} and, less often, {{IPA|de|ˈkɪks|}}.
* In Greece, it is pronounced ''kiks''.
* In India, it is pronounced ''kicks''.
* In Iran, it is pronounced ''kicks''.
* In Italy, is pronounced {{IPA|it|ˈtʃiks|}}.
* In Poland, it is pronounced {{IPA|pl|ˈkʲiks|}}.
* In Portugal and Brazil, it is pronounced {{IPA|pt|ˈsiks|}}.
* In Russia, it is pronounced ''kiks''.
* In Slovenia, it is pronounced ''kiks''.
* In Spain, it is pronounced {{IPA|es|ˈθiks|}}.
* In Sweden, it is pronounced ''kicks''.
* In Uganda, it is pronounced ''kicks''.
* In Turkey, it is pronounced ''kiks''.

==See also==
* [[IBM TXSeries]] (CICS on distributed platforms)
* [[IBM WebSphere]]
* [[IBM 2741]]
* [[IBM 2260]]
* [[IBM 3270]]
* [[OS/360 and successors]]
* [[Source Program Maintenance Online II]] (early online editor running under CICS)
* [[Transaction Processing Facility]]
* [[Virtual Storage Access Method]] (VSAM)

==References==
{{Reflist|30em}}

==External links==
*{{Official website|https://www.ibm.com/it-infrastructure/z/cics}}
*[http://www-01.ibm.com/support/docview.wss?rs=1083&uid=swg27013865 Why to choose CICS Transaction Server for new IT projects]{{snd}} IBM CICS whitepaper
*{{webarchive |url=https://web.archive.org/web/20090204135052/http://www-01.ibm.com/software/htp/cics/35/ |date=February 4, 2009 |title=IBM Software - CICS - 35 year Anniversary (2004)}}
*[https://ibmmainframes.com/forum-10.html Support Forum for CICS Programming]
*[http://www.cicsworld.com/ CICS User Community website for CICS related news, announcements and discussions] {{Webarchive|url=https://web.archive.org/web/20080805190855/http://www.cicsworld.com/ |date=5 August 2008 }}
*{{webarchive |url=https://web.archive.org/web/20050205184628/http://www.yelavich.com/ |date=February 5, 2005 |title=Bob Yelavich's CICS focused website. (This site uses frames, but on high-resolution screens the left-hand frame, which contains the site index, may be hidden. Scroll right within the frame to see its content.)}}

[[Category:Assembly language software]]
[[Category:History of human–computer interaction]]
[[Category:IBM mainframe operating systems]]
[[Category:IBM mainframe software]]
[[Category:IBM software]]
[[Category:Middleware]]<!--as noted above-->
[[Category:Multimodal interaction]]
[[Category:Transaction processing]]
[[Category:Z notation]]