View source for Continuous integration

{{short description|Software development practice of building and testing frequently}}
{{more footnotes|date=July 2016}}
{{Use dmy dates|date=April 2022}}
{{EngvarB|date=April 2022}}
{{Software development process}}
[[File:Continuous Integration.jpg|thumb|right|Sketch of [[flow diagram]] for continuous integration]]

'''Continuous integration''' ('''CI''') is the practice of integrating [[source code]] changes frequently and ensuring that the integrated codebase is in a workable state.

Typically, developers [[Merge (version control)|merge]] changes to an [[Branching (revision control)|integration branch]], and an automated system [[Software build|builds]] and [[software testing|tests]] the [[software system]].<ref name="martinfowler" /> 
Often, the automated process runs on each [[Commit (version control)|commit]] or runs on a schedule such as once a day. 

[[Grady Booch]] first proposed the term CI in [[Booch method|1991]],<ref>{{Cite book |last=Booch |first=Grady |url=https://books.google.com/books?id=w5VQAAAAMAAJ&q=continuous+integration+inauthor:grady+inauthor:booch |title=Object Oriented Design: With Applications |publisher=[[Benjamin Cummings]] |year=1991 |isbn=9780805300918 |page=209 |author-link=Grady Booch |access-date=18 August 2014}}</ref> although he did not advocate integrating multiple times a day, but later, CI came to include that aspect.<ref>{{Cite journal |last=Beck |first=K. |date=1999 |title=Embracing change with extreme programming |journal=Computer |volume=32 |issue=10 |pages=70–77 |doi=10.1109/2.796139 |issn=0018-9162}}</ref>

== History ==
{{expand section|date=August 2014}}

The earliest known work (1989) on continuous integration was the Infuse environment developed by G. E. Kaiser, D. E. Perry, and W. M. Schell.<ref>{{Cite conference |last1=Kaiser |first1=G. E. |last2=Perry |first2=D. E. |last3=Schell |first3=W. M. |year=1989 |title=Infuse: fusing integration test management with change management |conference=Proceedings of the Thirteenth Annual International Computer Software & Applications Conference |location=Orlando, Florida |pages=552–558 |doi=10.1109/CMPSAC.1989.65147|citeseerx=10.1.1.101.3770 }}</ref>

In 1994, Grady Booch used the phrase continuous integration in ''Object-Oriented Analysis and Design with Applications'' (2nd edition)<ref>{{Cite book |last=Booch |first=Grady |url=http://www.cvauni.edu.vn/imgupload_dinhkem/file/pttkht/object-oriented-analysis-and-design-with-applications-2nd-edition.pdf |title=Object-Oriented Analysis and Design with applications |date=December 1998 |edition=2nd |access-date=2 December 2014 |archive-date=19 August 2019 |archive-url=https://web.archive.org/web/20190819035559/http://www.cvauni.edu.vn/imgupload_dinhkem/file/pttkht/object-oriented-analysis-and-design-with-applications-2nd-edition.pdf |url-status=dead }}</ref> to explain how, when developing using micro processes, "internal releases represent a sort of continuous integration of the system, and exist to force closure of the micro process".

In 1997, [[Kent Beck]] and [[Ron Jeffries]] invented [[extreme programming]] (XP) while on the [[Chrysler Comprehensive Compensation System]] project, including continuous integration.<ref name="martinfowler">{{Cite web |title=Continuous Integration |url=http://martinfowler.com/articles/continuousIntegration.html |last=Fowler |first=Martin |date=1 May 2006 |access-date=9 January 2014}}</ref>{{self-published source|date=May 2020}} Beck published about continuous integration in 1998, emphasising the importance of face-to-face communication over technological support.<ref>{{Cite conference |last=Beck |first=Kent |date=28 March 1998 |title=Extreme Programming: A Humanistic Discipline of Software Development |url=https://books.google.com/books?id=YBC5xD08NREC&q=%22Extreme+Programming%3A+A+Humanistic+Discipline+of+Software+Development%22&pg=PA4 |location=Lisbon, Portugal |publisher=[[Springer Science+Business Media|Springer]] |volume=1 |pages=4 |isbn=9783540643036 |book-title=Fundamental Approaches to Software Engineering: First International Conference}}</ref> In 1999, Beck elaborated more in his first full book on Extreme Programming.<ref name="Beck, Extreme Programming Explained">{{Cite book |last=Beck |first=Kent |url=https://archive.org/details/extremeprogrammi00beck |title=Extreme Programming Explained |publisher=Addison-Wesley Professional |year=1999 |isbn=978-0-201-61641-5 |page=[https://archive.org/details/extremeprogrammi00beck/page/97 97] |ref=Beck, Extreme Programming Explained |author-link=Kent Beck |url-access=registration}}</ref> [[CruiseControl]], one of the first open-source CI tools,<ref>{{Cite news |date=1 February 2018 |title=A Brief History of DevOps, Part III: Automated Testing and Continuous Integration |work=CircleCI |url=https://circleci.com/blog/a-brief-history-of-devops-part-iii-automated-testing-and-continuous-integration/ |access-date=19 May 2018}}</ref>{{self-published source|date=May 2020}} was released in 2001.

In 2010, Timothy Fitz published an article detailing how [[IMVU]]'s engineering team had built and been using the first practical CD system. While his post was originally met with skepticism, it quickly caught on and found widespread adoption<ref>{{Citation | chapter=A Brief Survey of Current Software Engineering Practices in Continuous Integration and Automated Accessibility Testing | doi=10.1109/WiSPNET51692.2021.9419464| arxiv=2103.00097| s2cid=232076320| chapter-url=https://ieeexplore.ieee.org/document/9419464| title=2021 Sixth International Conference on Wireless Communications, Signal Processing and Networking (WiSPNET)| year=2021| last1=Sane| first1=Parth| pages=130–134| isbn=978-1-6654-4086-8}}</ref> as part of the [[lean software development]] methodology, also based on IMVU.

== Practices ==

The core activities of CI are developers co-locate code changes in a shared, integration area frequently and that the resulting integrated codebase is verified for correctness. The first part generally involves merging changes to a common version control branch. The second part generally involves automated processes including: building, testing and many other processes.

Typically, a [[Server (computing)|server]] builds from the integration area frequently; i.e. after each commit or periodically like once a day. The server may perform [[quality control]] checks such as running unit tests<ref>{{Cite web |title=Continuous integration |url=https://www.atlassian.com/agile/continuous-integration |last=Radigan |first=Dan |website=Atlassian Agile Coach}}</ref> and collect [[software quality]] metrics via processes such as static analysis and performance testing.

== Related practices ==
{{howto|section|date=May 2015}}

This section lists [[best practice]]s from practitioners for other practices that enhance CI.

=== Build automation ===

[[Build automation]] is a best practice.<ref name="Brauneis, [OSLC] Possible new Working Group - Automation">{{Cite mailing list |url=http://open-services.net/pipermail/community_open-services.net/2010-January/000214.html |title=[OSLC] Possible new Working Group – Automation |date=1 January 2010 |last=Brauneis |first=David |mailing-list=open-services.net Community |access-date=16 February 2010 |archive-url=https://web.archive.org/web/20180901173720/http://open-services.net/pipermail/community_open-services.net/2010-January/000214.html |archive-date=1 September 2018 |url-status=dead }}</ref><ref name="Taylor, Rails Deployment and Automation with ShadowPuppet and Capistrano">{{Cite web |title=Rails Deployment and Automation with ShadowPuppet and Capistrano |url=http://blog.railsmachine.com/articles/2009/02/10/rails-deployment-and-automation-with-shadowpuppet-and-capistrano/ |last=Taylor |first=Bradley |website=Rails machine |type=[[World Wide Web|blog]] |url-status=dead |archive-url=https://archive.today/20121202054105/http://blog.railsmachine.com/articles/2009/02/10/rails-deployment-and-automation-with-shadowpuppet-and-capistrano/ |archive-date=2 December 2012 |access-date=16 February 2010 |ref=Taylor, Rails Deployment and Automation with ShadowPuppet and Capistrano}}</ref>

=== Atomic commits ===

CI requires the version control system to support [[atomic commit]]s; i.e., all of a developer's changes are handled as a single commit.

=== Committing changes ===

When making a code change, a [[software developer|developer]] creates a branch that is a copy of the current [[codebase]]. As other changes are committed to the [[source code repository|repository]], this copy diverges from the latest version.

The longer development continues on a branch without merging to the integration branch, the greater the risk of multiple integration conflicts<ref>{{Cite book |last=Duvall |first=Paul M. |title=Continuous Integration. Improving Software Quality and Reducing Risk |publisher=Addison-Wesley |year=2007 |isbn=978-0-321-33638-5}}</ref> and failures when the developer branch is eventually merged back. When developers submit code to the repository they must first update their code to reflect the changes in the repository since they took their copy. The more changes the repository contains, the more work developers must do before submitting their own changes.

Eventually, the repository may become so different from the developers' baselines that they enter what is sometimes referred to as "merge hell", or "integration hell",<ref name="Cunningham, Integration Hell">{{Cite web |title=Integration Hell |url=http://c2.com/cgi/wiki?IntegrationHell |last=Cunningham |first=Ward |author-link=Ward Cunningham |date=5 August 2009 |website=WikiWikiWeb |access-date=19 September 2009}}</ref> where the time it takes to integrate exceeds the time it took to make their original changes.<ref>{{Cite web |title=What is Continuous Integration? |url=https://aws.amazon.com/devops/continuous-integration/ |website=Amazon Web Services}}</ref>

=== Testing locally ===

Proponents of CI suggest that developers should 
use [[test-driven development]] and to
ensure that all [[unit testing|unit tests]] pass locally before committing to the integration branch so that one developer's work does not break another developer's copy. 

Incomplete features can be disabled before committing, using [[feature toggle]]s.

=== Continuous delivery and continuous deployment ===

[[Continuous delivery]] ensures the software checked in on an integration branch is always in a state that can be deployed to users, and [[continuous deployment]] automates the deployment process.

''Continuous delivery'' and ''continuous deployment'' are often performed in conjunction with CI and together form a CI/CD pipeline. 

=== Version control ===
{{Main|Version control}}

Proponents of CI recommend storing all files and information needed for building in [[version control]], (for [[git]] a ''repository''); that the system should be buildable from a fresh checkout and not require additional dependencies. 

[[Martin Fowler (software engineer)|Martin Fowler]] recommends that all developers commit to the same integration branch.<ref name="Fowler, Continuous Integration practices">{{Cite web |title=Practices |url=http://martinfowler.com/articles/continuousIntegration.html#PracticesOfContinuousIntegration |last=Fowler |first=Martin |author-link=Martin Fowler (software engineer) |website=Continuous Integration |type=article |access-date=29 November 2015}}</ref> 

=== Automate the build ===
{{Main|Build automation}}

[[List of build automation software|Build automation tools]] automate building. 

Proponents of CI recommend that a single command should have the capability of building the system. 

Automation often includes automating the integration, which often includes [[software deployment|deployment]] into a production-like [[Deployment environment|environment]]. In many cases, the build script not only compiles binaries but also generates documentation, website pages, statistics and distribution media (such as Debian [[Deb (file format)|DEB]], Red Hat [[RPM Package Manager|RPM]] or Windows [[Microsoft Installer|MSI]] files).

=== Commit frequently ===

Developers can reduce the effort of resolving conflicting changes by synchronizing changes with each other frequently; at least daily. Checking in a week's worth of work risks conflict both in likelihood of occurrence and complexity to resolve. Relatively small conflicts are significantly easier to resolve than larger ones. Integrating (committing) changes at least once a day is considered good practice, and more often better.<ref>{{cite book
|title=Continuous Integration: Improving Software Quality and Reducing Risk
|author1=Paul M. Duvall
|author2=Steve Matyas
|author3=[[Andrew Glover]]
|isbn=978-0-321-33638-5
|publisher=[[Addison-Wesley Professional]]
|date=2007
}}</ref>

===Daily build===

[[daily build|Building daily]], if not more often, is generally recommended.{{Citation needed|date =April 2012}}

=== Every commit should be built ===

The system should build commits to the current working version to verify that they integrate correctly. A common practice is to use Automated Continuous Integration, although this may be done manually. Automated Continuous Integration employs a continuous integration server or [[Daemon (computer software)|daemon]] to monitor the [[revision control|revision control system]] for changes, then automatically run the build process.

=== Every bug-fix commit should come with a test case ===

When fixing a bug, it is a good practice to push a test case that reproduces the bug. This avoids the fix to be reverted, and the bug to reappear, which is known as a [[Regression testing|regression]].

=== Keep the build fast ===

The build needs to complete rapidly so that if there is a problem with integration, it is quickly identified.

=== Test in a clone of the production environment ===
{{main|Test environment}}
Having a [[test environment]] can lead to failures in tested systems when they deploy in the [[production environment]] because the production environment may differ from the test environment in a significant way. However, building a replica of a production environment is cost-prohibitive. Instead, the test environment or a separate [[pre-production environment]] ("staging") should be built to be a scalable version of the production environment to alleviate costs while maintaining [[technology stack]] composition and nuances. Within these test environments, [[service virtualisation]] is commonly used to obtain on-demand access to dependencies (e.g., APIs, third-party applications, services, [[Mainframe computer|mainframes]], etc.) that are beyond the team's control, still evolving, or too complex to configure in a virtual test lab.

=== Make it easy to get the latest deliverables ===

Making builds readily available to stakeholders and testers can reduce the amount of rework necessary when rebuilding a feature that doesn't meet requirements. Additionally, early testing reduces the chances that defects survive until deployment. Finding errors earlier can reduce the amount of work necessary to resolve them.

All programmers should start the day by updating the project from the repository. That way, they will all stay up to date.

=== Everyone can see the results of the latest build ===

It should be easy to find out whether the build breaks and, if so, who made the relevant change and what that change was.

=== Automate deployment ===

Most CI systems allow the running of scripts after a build finishes. In most situations, it is possible to write a script to deploy the application to a live test server that everyone can look at. A further advance in this way of thinking is [[continuous deployment]], which calls for the software to be deployed directly into production, often with additional automation to prevent defects or regressions.<ref>{{Cite web |title=Continuous deployment in 5 easy steps |url=http://radar.oreilly.com/2009/03/continuous-deployment-5-eas.html |last=Ries |first=Eric |date=30 March 2009 |website=Radar |publisher=O’Reilly |access-date=10 January 2013}}</ref><ref>{{Cite web |title=Continuous Deployment at IMVU: Doing the impossible fifty times a day |url=http://timothyfitz.wordpress.com/2009/02/10/continuous-deployment-at-imvu-doing-the-impossible-fifty-times-a-day/ |last=Fitz |first=Timothy |date=10 February 2009 |publisher=Wordpress |access-date=10 January 2013}}</ref>

== Benefits ==
{{More citations needed section|date=May 2016}}

CI benefits include:

* Facilitates detecting [[software bug|bugs]] earlier
* Reduces effort to find cause of bugs; if a CI test fails then changes since last good build contain causing change; if build after each change then exactly one change is the cause<ref name="martinfowler" />
* Avoids the chaos of integrating many changes
* When a test fails or a bug is found, reverting the codebase to a good state results in fewer lost changes
* Frequent availability of a known-good build for testing, demo, and release
* Frequent code commit encourages modular, less complex code<ref>{{Cite journal |last1=Junpeng |first1=Jiang |last2=Zhu |first2=Can |last3=Zhang |first3=Xiaofang |date=July 2020 |title=An Empirical Study on the Impact of Code Contributor on Code Smell |url=https://qrs20.techconf.org/download/QRS-IJPE/12_An%20Empirical%20Study%20on%20the%20Impact%20of%20Code%20Contributor%20on%20Code%20Smell.pdf |journal=International Journal of Performability Engineering |volume=16 |issue=7 |pages=1067–1077 |doi=10.23940/ijpe.20.07.p9.10671077|s2cid=222588815 }}</ref>
* Quick feedback on system-wide impact of code changes
* Supports collection of [[software metrics]] such as [[code coverage]], [[code complexity]]

== Risks ==

Risks of CI include:

* [[Build system]] setup requires effort<ref>{{Cite journal |last=Laukkanen |first=Eero |year=2016 |title=Problems, causes and solutions when adopting continuous delivery—A systematic literature review |journal=Information and Software Technology |volume=82 |pages=55–79 |doi=10.1016/j.infsof.2016.10.001 |doi-access=free}}</ref>
* Writing and maintaining an automated test suite requires effort
* Value added depends on the quality of tests<ref name=":1">{{Cite web |title=Assessing challenges of continuous integration in the context of software requirements breakdown: a case study |url=http://publications.lib.chalmers.se/records/fulltext/220573/220573.pdf |last=Debbiche |first=Adam}}</ref>
* High build latency (sitting in queue) limits value<ref name=":1" />
* Implies that incomplete code should not be integrated which is counter to some developer's preferred practice<ref name=":1" />
* Safety and mission-critical development assurance (e.g., [[DO-178C]], [[ISO 26262]]) require documentation and review which may be difficult to achieve

== Best practices for cloud systems ==
The following practices can enhance productivity of [[CI/CD|pipelines]], especially in systems hosted in the [[Cloud computing|cloud]]: <ref>{{Cite book |title=Serverless Architectures on AWS |date=29 March 2022 |publisher=Manning |isbn=978-1617295423}}</ref><ref>{{Cite book |title=Pipeline as Code Continuous Delivery with Jenkins, Kubernetes, and Terraform |date=23 November 2021 |publisher=Manning |isbn=9781638350378}}</ref><ref>{{Cite book |title=Continuous Delivery Reliable Software Releases Through Build, Test, and Deployment Automation |isbn=9780321670229 |last1=Humble |first1=Jez |last2=Farley |first2=David |date=27 July 2010 |publisher=Pearson Education }}</ref>

* '''Number of Pipelines''': Small teams can be more productive by having one [[Repository (version control)|repository]] and one pipeline. In contrast, larger organizations may have separate repositories and pipelines for each team or even separate repositories and pipelines for each service within a team.
* '''Permissions''': In the context of [[CI/CD|pipeline-related]] permissions, adhering to the [[principle of least privilege]] can be challenging due to the dynamic nature of [[Software architecture|architecture]]. Administrators may opt for more permissive permissions while implementing compensating [[security controls]] to minimize the blast radius.

== See also ==
* {{Annotated link|Application release automation}}
* {{Annotated link|Build light indicator}}
* {{Annotated link|Comparison of continuous integration software}}
* {{Annotated link|Continuous design}}
* {{Annotated link|Continuous testing}}
* {{Annotated link|Multi-stage continuous integration}}
* {{Annotated link|Rapid application development}}

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

== External links ==
* {{Cite journal |title=Continuous Integration |url=http://www.c2.com/cgi/wiki?ContinuousIntegration |format=wiki |type=a collegial discussion |publisher=C2}}
* {{Cite web |title=Continuous Integration: The Cornerstone of a Great Shop |url=http://www.methodsandtools.com/archive/archive.php?id=42 |last=Richardson |first=Jared |type=introduction}}
* {{Cite web |title=A Recipe for Build Maintainability and Reusability |url=http://jayflowers.com/joomla/index.php?option=com_content&task=view&id=26 |last=Flowers |first=Jay |access-date=28 May 2006 |archive-date=25 June 2020 |archive-url=https://web.archive.org/web/20200625060704/https://jayflowers.com/joomla/index.php?option=com_content&task=view&id=26 |url-status=dead }}
* {{Cite web |title=Developer works |url=http://www.ibm.com/developerworks/java/library/j-ap11297/ |last=Duvall |first=Paul |website=[[IBM]] |date=4 December 2007}}
* {{Cite web |title=Version lifecycle |date=June 2024 |url=http://www.mediawiki.org/wiki/Version_lifecycle |publisher=MediaWiki}}

[[Category:Continuous integration| ]]
[[Category:Agile software development]]
[[Category:Extreme programming]]
[[Category:Software development process]]