Continuous integration: Difference between revisions

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{{short description|Software development practice of building and testing frequentingfrequently}}

{{more footnotes|date=July 2016}}

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[[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.

'''Continuous integration''' ('''CI''') is the practice of frequently building and testing a [[software system]] during its development. It is intended to ensure that [[source code|code]] written by [[programmers]] is always buildable, runnable and passes [[automated testing]]. Developers [[Merge (version control)|merge]] to an [[Branching (revision control)|integration branch]] and an automated system [[Software build|builds]] and [[software testing|tests]].<ref name="martinfowler" />

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.

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{{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".

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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 CICD system. While his post was originally met with scepticismskepticism, 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 [[Leanlean software development]] methodology, also based on IMVU.

== Goal ==

A stated goal of CI is to run the automated process frequently enough that no intervening window remains between [[Commit (version control)|commit]] and [[Software build|build]], and such that no errors can arise without developers noticing them and correcting them immediately.<ref name="martinfowler" /> Generally, this means triggering builds on each commit to a repository. Due to processing limitations, sometimes multiple changes are committed between automation runs.

== 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.

A [[Server (computing)|server]] builds from the integration branch frequently. Usually this is after each commit or periodically like once a day.

Once built, all tests should run.<ref>{{Cite web |title=Continuous integration |url=https://www.atlassian.com/agile/continuous-integration |last=Radigan |first=Dan |website=Atlassian Agile Coach}}</ref>

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.

The server may also run other [[quality control]] and [[software quality]] processes such as static analysis, measuring performance, extracting documentation from the source code, and facilitating manual [[Quality assurance|QA]] processes.

== Related practices ==

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=== Build automation ===

{{Main|Build automation}}

[[Build automation]] is a best practice.<ref name="Brauneis, [OSLC] Possible new Working Group - Automation">{{Cite mailing list |urllast=http://open-services.net/pipermail/community_open-services.net/2010-January/000214.htmlBrauneis |first=David |title=[OSLC] Possible new Working Group – Automation |mailing-list=open-services.net Community |date=1 January 2010 |last=Brauneis |first=David |mailing-listurl=http://open-services.net Community/pipermail/community_open-services.net/2010-January/000214.html |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 |last=Taylor |first=Bradley |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 |website=Rails machine |type=[[World Wide Web|blog]] |ref=Taylor, Rails Deployment and Automation with ShadowPuppet and Capistrano}}</ref> [[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).

=== Atomic commits ===

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=== Continuous delivery and continuous deployment ===

{{See also|CI/CD}}

[[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 ===

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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>

=== 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

=== Automate the build ===

|title=Continuous Integration: Improving Software Quality and Reducing Risk

{{Main|Build automation}}

|author1=Paul M. Duvall

|author2=Steve Matyas

|author3=[[Andrew Glover]]

|isbn=978-0-321-33638-5

|publisher=[[Addison-Wesley Professional]]

|date=2007

}}</ref>

=== Automate theDaily build ===

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

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

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).

=== Everyone commits to the baseline every day ===

By committing regularly, every committer can reduce the number of conflicting changes. Checking in a week's worth of work runs the risk of conflicting with other features and can be very difficult to resolve. Early, small conflicts in an area of the system cause team members to communicate about the change they are making.<ref name=":0">{{Cite web |title=Continuous Integration |url=https://www.thoughtworks.com/continuous-integration |website=Thoughtworks}}</ref> Committing all changes at least once a day (once per feature built) is generally considered part of the definition of Continuous Integration. In addition, performing a [[nightly build]] is generally recommended.{{Citation needed|date =April 2012}} These are lower bounds; the typical frequency is expected to be much higher.

=== Every commit should be built ===

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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>

== Costs and benefitsBenefits ==

{{POV section|date=May 2016}}

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

CI benefits include:

Continuous integration is intended to produce benefits such as:

* 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

* ConstantFrequent availability of a "current"known-good build for testing, demo, orand release purposes

* Frequent code check-incommit pushes developers to createencourages 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>

* ImmediateQuick feedback on the system-wide impact of localcode changes

* Supports collection of [[software metrics]] such as [[code coverage]], [[code complexity]]

== GoalRisks ==

Risks of CI include:

* There is some work involved to set up a [[buildBuild system]], and it can become complex, making it difficult tosetup modifyrequires flexibly.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 and how testable the code really is.<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 rigorous documentation and in-process review thatwhich aremay be difficult to achieve using continuous integration. This type of life cycle often requires additional steps to be completed prior to product release when regulatory approval of the product is required.

== Best practices for cloud systems ==

* [[Software testing#Integration testing|Integration bugs]] are detected early and are easy to track down due to small changesets. This saves both time and money over the lifespan of a project.

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>

* Avoids last-minute chaos at release dates, when everyone tries to check in their slightly incompatible versions

* When unit tests fail or a [[Software bug|bug]] emerges, if developers need to revert the codebase to a bug-free state without [[debugging]], only a small number of changes are lost (because integration happens frequently)

* Constant availability of a "current" build for testing, demo, or release purposes

* Frequent code check-in pushes developers to create 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>

* '''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.

With continuous automated testing, benefits can include:

* '''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.

* Enforces discipline of frequent automated testing

* Immediate feedback on the system-wide impact of local changes

* [[Software metrics]] generated from automated testing and CI (such as metrics for [[code coverage]], [[code complexity]], and [[feature complete]]ness) focus developers on developing functional, quality code, and help develop momentum in a team{{Citation needed|date=November 2009}}

Some downsides of continuous integration can include:

* Constructing an automated test suite requires a considerable amount of work, including ongoing effort to cover new features and follow intentional code modifications.

** [[Software testing|Testing]] is considered a [[best coding practices|best practice for software development]] in its own right, regardless of whether or not continuous integration is employed, and automation is an integral part of project methodologies like [[test-driven development]].

** Continuous integration can be performed without any test suite, but the cost of [[quality assurance]] to produce a releasable product can be high if it must be done manually and frequently.

* There is some work involved to set up a [[build system]], and it can become complex, making it difficult to modify flexibly.<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>

** However, there are a number of [[Comparison of continuous integration software|continuous integration software projects]], both proprietary and open-source, which can be used.

* Continuous integration is not necessarily valuable if the scope of the project is small or contains untestable legacy code.

* Value added depends on the quality of tests and how testable the code really is.<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>

* Larger teams mean that new code is constantly added to the integration queue, so tracking deliveries (while preserving quality) is difficult and builds queueing up can slow down everyone.<ref name=":1" />

* With multiple commits and merges a day, partial code for a feature could easily be pushed and therefore integration tests will fail until the feature is complete.<ref name=":1" />

* Safety and mission-critical development assurance (e.g., [[DO-178C]], [[ISO 26262]]) require rigorous documentation and in-process review that are difficult to achieve using continuous integration. This type of life cycle often requires additional steps to be completed prior to product release when regulatory approval of the product is required.

== 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 ==