Software architecture patterns OReilly Media.Key Concepts.Notice in Figure 1 2 that each of the layers in the architecture is marked as being closed.This is a very important concept in the layered architecture pattern.A closed layer means that as a request moves from layer to layer, it must go through the layer right below it to get to the next layer below that one.For example, a request originating from the presentation layer must first go through the business layer and then to the persistence layer before finally hitting the database layer.Figure 1 2.Closed layers and request access.So why not allow the presentation layer direct access to either the persistence layer or database layerAfter all, direct database access from the presentation layer is much faster than going through a bunch of unnecessary layers just to retrieve or save database information.The answer to this question lies in a key concept known as layers of isolation.The layers of isolation concept means that changes made in one layer of the architecture generally dont impact or affect components in other layers the change is isolated to the components within that layer, and possibly another associated layer such as a persistence layer containing SQL.An Introduction To Software Architecture Shaw' title='An Introduction To Software Architecture Shaw' />If you allow the presentation layer direct access to the persistence layer, then changes made to SQL within the persistence layer would impact both the business layer and the presentation layer, thereby producing a very tightly coupled application with lots of interdependencies between components.This type of architecture then becomes very hard and expensive to change.The layers of isolation concept also means that each layer is independent of the other layers, thereby having little or no knowledge of the inner workings of other layers in the architecture.To understand the power and importance of this concept, consider a large refactoring effort to convert the presentation framework from JSP Java Server Pages to JSF Java Server Faces.Assuming that the contracts e.While closed layers facilitate layers of isolation and therefore help isolate change within the architecture, there are times when it makes sense for certain layers to be open.For example, suppose you want to add a shared services layer to an architecture containing common service components accessed by components within the business layer e.Creating a services layer is usually a good idea in this case because architecturally it restricts access to the shared services to the business layer and not the presentation layer.Without a separate layer, there is nothing architecturally that restricts the presentation layer from accessing these common services, making it difficult to govern this access restriction.In this example, the new services layer would likely reside below the business layer to indicate that components in this services layer are not accessible from the presentation layer.However, this presents a problem in that the business layer is now required to go through the services layer to get to the persistence layer, which makes no sense at all.This is an age old problem with the layered architecture, and is solved by creating open layers within the architecture.As illustrated in Figure 1 3, the services layer in this case is marked as open, meaning requests are allowed to bypass this open layer and go directly to the layer below it.In the following example, since the services layer is open, the business layer is now allowed to bypass it and go directly to the persistence layer, which makes perfect sense.Figure 1 3.Open layers and request flow.Leveraging the concept of open and closed layers helps define the relationship between architecture layers and request flows and also provides designers and developers with the necessary information to understand the various layer access restrictions within the architecture.Failure to document or properly communicate which layers in the architecture are open and closed and why usually results in tightly coupled and brittle architectures that are very difficult to test, maintain, and deploy.Pattern Example.To illustrate how the layered architecture works, consider a request from a business user to retrieve customer information for a particular individual as illustrated in Figure 1 4.The black arrows show the request flowing down to the database to retrieve the customer data, and the red arrows show the response flowing back up to the screen to display the data.In this example, the customer information consists of both customer data and order data orders placed by the customer.The customer screen is responsible for accepting the request and displaying the customer information.It does not know where the data is, how it is retrieved, or how many database tables must be queries to get the data.Once the customer screen receives a request to get customer information for a particular individual, it then forwards that request onto the customer delegate module.This module is responsible for knowing which modules in the business layer can process that request and also how to get to that module and what data it needs the contract.See the latest news and architecture related to 2017chicagoarchitecturebiennial, only on ArchDaily.The customer object in the business layer is responsible for aggregating all of the information needed by the business request in this case to get customer information.This module calls out to the customer dao data access object module in the persistence layer to get customer data, and also the order dao module to get order information.These modules in turn execute SQL statements to retrieve the corresponding data and pass it back up to the customer object in the business layer.Once the customer object receives the data, it aggregates the data and passes that information back up to the customer delegate, which then passes that data to the customer screen to be presented to the user.Figure 1 4. Billing Management Software Open Source . Layered architecture example.From a technology perspective, there are literally dozens of ways these modules can be implemented.For example, in the Java platform, the customer screen can be a JSF Java Server Faces screen coupled with the customer delegate as the managed bean component.The customer object in the business layer can be a local Spring bean or a remote EJB3 bean.The data access objects illustrated in the previous example can be implemented as simple POJOs Plain Old Java Objects, My.Batis XML Mapper files, or even objects encapsulating raw JDBC calls or Hibernate queries.From a Microsoft platform perspective, the customer screen can be an ASP active server pages module using the.NET framework to access C modules in the business layer, with the customer and order data access modules implemented as ADO Active.Architecture logicielle Wikipdia.Larchitecture logicielle dcrit dune manire symbolique et schmatique les diffrents lments dun ou de plusieurs systmes informatiques, leurs interrelations et leurs interactions.Contrairement aux spcifications produites par lanalyse fonctionnelle, le modle darchitecture, produit lors de la phase de conception, ne dcrit pas ce que doit raliser un systme informatique mais plutt comment il doit tre conu de manire rpondre aux spcifications.Lanalyse dcrit le quoi faire alors que larchitecture dcrit le comment le faire .La phase de conception logicielle est lquivalent, en informatique, la phase de conception en ingnierie traditionnelle mcanique, civile ou lectrique cette phase consiste raliser entirement le produit sous une forme abstraite avant la production effective.Par contre, la nature immatrielle du logiciel model dans linformation et non dans la matire, rend la frontire entre larchitecture et le produit beaucoup plus floue que dans lingnierie traditionnelle.Lutilisation doutils CASE Computer aided software engineering ou bien la production de larchitecture partir du code lui mme et de la documentation systme permettent de mettre en vidence le lien troit entre larchitecture et le produit.Larchitecture logicielle constitue le plus gros livrable dun processus logiciel aprs le produit le logiciel lui mme.En effet, la phase de conception devrait consommer autour de 4.Leffort dpend grandement du type de logiciel dvelopp, de lexpertise de lquipe de dveloppement, du taux de rutilisation et du processus logiciel.Les deux objectifs principaux de toute architecture logicielle sont la rduction des cots et laugmentation de la qualit du logiciel la rduction des cots est principalement ralise par la rutilisation de composants logiciels et par la diminution du temps de maintenance correction derreurs et adaptation du logiciel.La qualit, par contre, se trouve distribue travers plusieurs critres la norme ISO 9.Linteroprabilit extrinsque exprime la capacit du logiciel communiquer et utiliser les ressources dautres logiciels comme les documents crs par une certaine application.Linteroprabilit intrinsque exprime le degr de cohrence entre le fonctionnement des commandes et des modules lintrieur dun systme ou dun logiciel.La portabilit exprime la possibilit de compiler le code source etou dexcuter le logiciel sur des plates formes machines, systmes dexploitation, environnements diffrentes.La compatibilit exprime la possibilit, pour un logiciel, de fonctionner correctement dans un environnement ancien compatibilit descendante ou plus rcent compatibilit ascendante.La validit exprime la conformit des fonctionnalits du logiciel avec celles dcrites dans le cahier des charges.La vrifiabilit exprime la simplicit de vrification de la validit.Lintgrit exprime la facult du logiciel protger ses fonctions et ses donnes daccs non autoriss.La fiabilit exprime la facult du logiciel grer correctement ses propres erreurs de fonctionnement en cours dexcution.La maintenabilit exprime la simplicit de correction et de modification du logiciel, et mme, parfois, la possibilit de modification du logiciel en cours dexcution.La rutilisabilit exprime la capacit de concevoir le logiciel avec des composants dj conus tout en permettant la rutilisation simple de ses propres composants pour le dveloppement dautres logiciels.Lextensibilit exprime la possibilit dtendre simplement les fonctionnalits dun logiciel sans compromettre son intgrit et sa fiabilit.Lefficacit exprime la capacit du logiciel exploiter au mieux les ressources offertes par la ou les machines o le logiciel sera implant.Lautonomie exprime la capacit de contrle de son excution, de ses donnes et de ses communications.La transparence exprime la capacit pour un logiciel de masquer lutilisateur humain ou machine les dtails inutiles lutilisation de ses fonctionnalits.La composabilit exprime la capacit pour un logiciel de combiner des informations provenant de sources diffrentes.La convivialit dcrit la facilit dapprentissage et dutilisation du logiciel par les usagers.Graphique de dgradation du logiciel en fonction de larchitecture.Inspir de Pressman R.S., Software Engineering A Practitioners Approach, Fifth Edition.Mc.Graw Hill. Chapitre 1, p.Une architecture faible ou absente peut entraner de graves problmes lors de la maintenance du logiciel.En effet, toute modification dun logiciel mal architectur peut dstabiliser la structure de celui ci et entraner, la longue, une dgradation principe dentropie du logiciel.Larchitecte informatique devrait donc concevoir, systmatiquement, une architecture maintenable et extensible.Dans les processus itratifs comme UP Unified Process, la gestion des changements est primordiale.En effet, il est implicitement considr que les besoins des utilisateurs du systme peuvent changer et que lenvironnement du systme peut changer.Larchitecte informatique a donc la responsabilit de prvoir le pire et de concevoir larchitecture en consquence la plus maintenable possible et la plus extensible possible.Bien des logiciels ont t crs sans architecture par plusieurs gnrations de dveloppeurs, ayant chacun us dune imagination dbordante pour russir maintenir lintgrit du systme.Une telle absence darchitecture peut tre qualifie darchitecture organiqueNote 1.En effet, un dveloppeur confront une telle architecture a plus limpression de travailler avec un organisme vivant quavec un produit industriel.Il en rsulte que la complexit du logiciel fait en sorte que celui ci est extrmement difficile comprendre et modifier.La rutilisation de composants logiciels est lactivit permettant de raliser les conomies les plus substantielles2, encore faut il possder des composants rutiliser.De plus, la rutilisation de composants ncessite de crer une architecture logicielle permettant une intgration harmonieuse de ces composants3.Le dveloppement par la rutilisation logicielle impose donc un cycle de production rutilisation perptuel et une architecture logicielle normalise.Une rutilisation bien orchestre ncessite la cration et le maintien dune bibliothque logicielle et un changement de focus crer une application revient crer les composants de bibliothque ncessaires puis construire lapplication laide de ces composants.Une telle bibliothque, facilitant le dveloppement dapplication est un framework cadriciel dentreprise et son architecture, ainsi que sa documentation sont les pierres angulaires de la rutilisation logicielle en entreprise.Le rle de larchitecte informatique se dplace donc vers celui de bibliothcaire.Larchitecte informatique doit explorer la bibliothque pour trouver les composants logiciels appropris puis crer les composants manquants, les documenter et les intgrer la bibliothque.Paragon Hard Drive Manager 2009 Full Size '>Paragon Hard Drive Manager 2009 Full Size .
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