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<!doctype html><html lang=en class=no-js><head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1,shrink-to-fit=no"><meta name=generator content="Hugo 0.107.0"><meta name=robots content="index, follow"><link rel="shortcut icon" href=/favicons/favicon.ico><link rel=apple-touch-icon href=/favicons/apple-touch-icon-180x180.png sizes=180x180><link rel=icon type=image/png href=/favicons/favicon-16x16.png sizes=16x16><link rel=icon type=image/png href=/favicons/favicon-32x32.png sizes=32x32><link rel=icon type=image/png href=/favicons/android-36x36.png sizes=36x36><link rel=icon type=image/png href=/favicons/android-48x48.png sizes=48x48><link rel=icon type=image/png href=/favicons/android-72x72.png sizes=72x72><link rel=icon type=image/png href=/favicons/android-96x96.png sizes=96x96><link rel=icon type=image/png href=/favicons/android-144x144.png sizes=144x144><link rel=icon type=image/png href=/favicons/android-192x192.png sizes=192x192><title>Façade | alguidelines.dev - Business Central Design Patterns</title><meta name=description content="Created by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (Gang of Four), Described by Jesper Schulz-Wedde (Microsoft)
<!doctype html><html lang=en class=no-js><head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1,shrink-to-fit=no"><meta name=generator content="Hugo 0.110.0"><meta name=robots content="index, follow"><link rel="shortcut icon" href=/favicons/favicon.ico><link rel=apple-touch-icon href=/favicons/apple-touch-icon-180x180.png sizes=180x180><link rel=icon type=image/png href=/favicons/favicon-16x16.png sizes=16x16><link rel=icon type=image/png href=/favicons/favicon-32x32.png sizes=32x32><link rel=icon type=image/png href=/favicons/android-36x36.png sizes=36x36><link rel=icon type=image/png href=/favicons/android-48x48.png sizes=48x48><link rel=icon type=image/png href=/favicons/android-72x72.png sizes=72x72><link rel=icon type=image/png href=/favicons/android-96x96.png sizes=96x96><link rel=icon type=image/png href=/favicons/android-144x144.png sizes=144x144><link rel=icon type=image/png href=/favicons/android-192x192.png sizes=192x192><title>Façade | alguidelines.dev - Business Central Design Patterns</title><meta name=description content="Created by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (Gang of Four), Described by Jesper Schulz-Wedde (Microsoft)
Abstract
The intent …"><meta property="og:title" content="Façade"><meta property="og:description" content="Created by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (Gang of Four), Described by Jesper Schulz-Wedde (Microsoft)
Abstract The intent of this pattern is to provide a unified API to a single or a collection of potentially complex subsystems. If you apply this pattern as a general pattern, you will ensure improved:
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</span></span></span></code></pre></div><p>There are no rules for the subsystem, except that access needs to be <strong>internal</strong>. How you implement, how much you document, how you test, is entirely up to you and not the business of the outside caller. Of course, you should apply all of the best practices and patterns anyway, as you and possibly other developers will have to understand, extend and maintain the subsystem too. But from the view of this pattern, the complexity of the subsystem is irrelevant - just as long as it&rsquo;s not accessible.</p><h2 id=usage>Usage</h2><p>The facade pattern is one of the most prominent patterns in the <a href=https://github.com/microsoft/ALAppExtensions/tree/main/Modules/System>system application</a>. You will find plenty of examples here.</p><h2 id=benefits>Benefits</h2><p>The benefits of this rather simple pattern should be abundantly clear by now. But let&rsquo;s go over them once more, structured by the advantages this patterns brings:</p><h3 id=decoupling>Decoupling</h3><p>As the entire subsystem is inaccessible to outside systems, no dependencies can be taken. Hence this patterns strongly promotes the decoupling of objects.</p><h3 id=encapsulation>Encapsulation</h3><p>The entire purpose of this very pattern is to encapsulate complexity; you hide away the implementation details behind an easy to understand facade.</p><h3 id=readability>Readability</h3><p>If done right, the developer doesn&rsquo;t need to be able to understand the details of the subsystem. Everything relevant to using the subsystem is described in the facade.</p><h3 id=testability>Testability</h3><p>Ensuring the correct behaviour of the subsystem can be done by testing the facade. The facade defines the contract of your subsystem - what does it expose and how should it behave. That contract should be covered with adequate tests, which will ensure that it is upheld, even if you decide to change the implementation of the subsystem.</p><h3 id=maintainability>Maintainability</h3><p>The one thing you may not change freely, is the facade and the test of the facade. It can be extended, but you should not break any existing APIs. But that leaves the entire subsystem to be completely rewritten, if you desire to do so. As no external dependencies can exist, there is no risk of introducing any syntactical breaking changes to the outside world. And as the tests of the public facade remain, there is no risk of introducing semantical breaking changes either - the contract is upheld, as long as your tests pass.</p><h2 id=when-not-to-use>When not to use</h2><p>This is a very applicable pattern, which can be used in most cases. While it may appear to be overkill at the beginning of the design of your subsystem, chances are your subsystem will evolve in complexity over time. The pattern may complicate implementation of extensibility, as you hide away all implementation details and hence also take away the opportunity to extend those, but that does not mean that extensibility cannot be achieved - but it requires more careful designing for extensibility. Extensibility with facades could be a topic of its own, which for now isn&rsquo;t covered by this pattern description. As always, use your common sense when to use and when not to use this pattern.</p><h2 id=snippets>Snippets</h2><p>Given the simplicity of this pattern, there are no snippets available at the moment.</p><h2 id=list-of-references>List of references</h2><p>This is one of the most commonly used and discussed, initially described here:</p><p><a href=https://archive.org/details/designpatternsel00gamm/page/185>Design Patterns: Elements of Reusable Object-Oriented Software</a>. Addison Wesley. pp. 185ff. ISBN 0-201-63361-2.</p><p>It is also a key pattern in the design of our system application modules, which is described here:</p><p><a href=https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-blueprint>Module Architecture</a></p><div class="text-muted mt-5 pt-3 border-top">Last modified February 24, 2022: <a href=https://github.com/microsoft/alguidelines/commit/fb17dc9640ceef53d1f3b8bbf9e9791ef92fabbc>Removed Discussion Links. Using new Theme solution. (fb17dc96)</a>
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