The top most layer of an application. This is the layer we see when we use the software. By using this layer we can access the web pages. The main function of this layer is to communicate with the Application layer. This layer passes the information which is given by the user in terms of keyboard actions, mouse clicks to the Application Layer. For example, the login page of Gmail where an end-user could see text boxes and buttons to enter user id, password, and to click on sign-in.
It is also known as Business Logic Layer which is also known as the logical layer. As per the Gmail login page example, once the user clicks on the login button, the Application layer interacts with the Database layer and sends required information to the Presentation layer. This layer acts as a mediator between the Presentation and the Database layer. Complete business logic will be written in this layer. The data is stored in this layer.
The application layer communicates with the Database layer to retrieve the data. It contains methods that connect the database and performs required action e. One-tier architecture has all the layers such as Presentation, Business, Data Access layers in a single software package. Applications that handle all the three tiers such as MP3 player, MS Office come under the one-tier application. The data is stored in the local system or a shared drive. The client system handles both Presentation and Application layers and the Server system handles the Database layer.
It is also known as a client-server application. So why not allow the presentation layer direct access to either the persistence layer or database layer?
After 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. 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. 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 , 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. 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. 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 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.
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. Plus, if you need to alter contextual behavior pass in an entirely different object , that is a snap with DI. My experience is that it's been cumbersome without it. There are full stack frameworks that require you to fire up the framework itself just to get at your services.
If you're not there, and it sounds like you're not, it's all gravy! Thanks Ben. Your architecture is very similar to what I ended up with. I ended up building my own framework in order to learn about these issues.
However, since a form requires a single "object" while a grid is an array of objects and a tree is nested data the construction of the say json will differ depending upon what component is asking for the data. Now, given that a server side "entity" may potentially have it's data represented on the client side as a form, grid or tree, but it should not know that, my ClientFormatting layer takes the CF data structure returned from the Application layer and rearranges it to produce the kind of json layout and metadata that the specific component needs.
The client web request contains a parameter like forForm, forGrid, forTree and the ClientFormatting layer is the only layer that uses those parameters. I set it up so that the ClientFormatting can also handle other server side languages as "plugins".
So, the ClientFormatting layer is all that needs to change if I start using a new client side library, or if I want the same server app to provide data as a published "API" with different data types json, xml, etc.
So, to sum up, the server can then "speak" multiple languages and "dialects" without touching the Controller or Application layers. The REST request determines what action to take and what data is requested via the Controller , while the ClientFormatting layer controls the format of the data being passed back to the Controller depending upon "who" the client side library is.
Formatting the response for the client is definitely something that I am "hacking" right now. We jump through a number of hoops to strip out unsafe data ex. All to say, that's one of the weakest parts of my app. But, part of that is based on the shoe-horning of a single model into both the Command AND Query aspects of the application as you know from our other discussion. Often my controller layer manages transaction boundaries, so I think transaction management is the responsibility of both the controller and application layers.
I tend to keep my domain layer fairly light. My domain classes have methods relevant to those, but functionality that requires coordinating multiple entities usually ends up in my application layer. Many of my choices are influenced by how course-grained or fine-grained I want my application layer API to be.
I often avoid adding course-grained methods to the application layer by allowing the controller layer to manage transactions across multiple calls to the application layer. To be completely philosophical for a second, one thing I don't like about putting my transaction in the Controller layer is that it requires the controller to know about the type of storage that is being used. If you keep the transaction control in the Application layer, then all the Controller has to know about is the data that can go into and come out of the application - it doesn't know anything else.
That said, to get off my philosophical soap box : in 12 years of web development, I've only ever changed my data storage layer twice. I tried to continue this thought experiment with a specific use-case in mind. I've found myself getting a bit lost and concerned with "necessary" duplication of effort. Light Dark. Right now, I am thinking in terms of building software with the following layers: Controller Layer Application Layer Domain Layer Infrastructure Layer Before I get into the details of each layer, let me make some broad sweeping philosophical statements: Each of the above layers depends exclusively on the layer immediately below it.
Outside of entities ex. Linked Lists , circular references are to be avoided. The more I think about it, the more I tend to feel that circular references are a sign that I am missing orchestration at a higher level of the application.
Controller Layer It is the public face of the application. It routes incoming requests and returns responses. Both incoming and outgoing data is restricted to simple data types ex. It performs high-level security and authentication. Since different requests may be using different forms of identification ex. The Controller layer may invoke the application layer to help with authentication. However, if requirements are simple enough ex.
Basic Authentication for 3rd-party Web Hooks , the controller layer may perform its own authentication. It is the only part of the entire application that knows about session management. It is very thin, using the Application layer to perform most of the work. Application Layer It is the programmatic boundary of the application.
It accepts and returns simple data structures. Rather, it deals with non-serialized data structures and relies on the Controller to serialize and deserialize data representations. It performs low-level security and checks access permissions. In the case of a request made by a user, the application would be responsible for checking whether or not the requesting user has the right to perform the given action ex.
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