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What are the components interacting with each other?
What type of components? dll, exe

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State ONE special feature that makes you think this pattern is client server?

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State ONE special feature that makes you think this pattern is Model View Controller?

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State ONE special feature that makes you think this pattern is pipe and filter?

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State ONE special feature that makes you think this pattern is repository?

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State ONE special feature that makes you think this pattern is layered?

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Describes the organization and wiring of the physical components in a system.

Component :-

1. Component

b) Interface

c) Dependencies

d) Port

Example :

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Client-Server Architecture consists of two types of components: clients and servers.

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This pattern consists of 2 parities which is  client and server. This server components will provide services to multiple client components.  In many cases, the server is a database with application logic represented as stored procedures. Example is online application which is banking, email and document sharing.

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MVC separates domain, application and business logic from the rest of the user interface. It does this by separating the application into three parts which is the model, the view, and the controller.

Advantages of using MVC
- Reusability
Use the same model for different
views
- Encourages well-defined
interfaces

- Simplicity
Each part is simpler and easier
to understand, implement and
test.

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In the diagram, Bank transaction authorizer, ATM monitoring server and ATM reconfiguration server provide client request from ATM main process, Reconfigure and update process and monitoring station program.

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Layered pattern can be used to structure programs that can be decomposed into groups of subtasks, each of which is at a particular level of abstraction. Each layer provides services to the next higher layer.
Layered Architecture Pattern

4 layers of a general information system are as follows:

--> Presentation layer (also known as UI layer)

The presentation layer is responsible for the following:

Data encryption/decryption

Character/string conversion

Data compression

Graphic handling

-->Application layer (also known as service layer)

The application layer provides many services, including:

Simple Mail Transfer Protocol

File transfer

Web surfing

Web chat

Email clients

Network data sharing

Virtual terminals

Various file and data operations

--> Business logic layer (also known as domain layer)
Business logic is the programming that manages communication between an end user interface and a database.
Business rule describes :

specific procedure

a workflow consists of the tasks

procedural steps

required input and output information

tools needed for each step of that procedure.

-->Data access layer (also known as persistence layer)
Data access layer.in computer software, is a layer of a computer program which provides simplified access to data stored in persistent storage of some kind, such as an entity-relational database.

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1. Model: The HTML is the "skeleton" of bedrock content. Text that communicates information to the reader.

2. View: The CSS adds visual style to the content. It is the "skin" that we use to flesh out our skeleton and give it a particular look. We can swap in different skins via CSS without altering the original content in any way. They are relatively, but not completely, independent.

3. Controller: The browser is responsible for combining and rendering the CSS and HTML into a set of final, manipulatible pixels on the screen. It gathers input from the user and marshals it to any JavaScript code necessary for the page to function. But here, too, we have flexibility: we can plug in a different brower and get comparable results. Some browsers might render it faster, or with more fidelity, or with more bells and whistles.

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Pipes and Filters consist of a chain of processes or other data processing entities, arranged so that the output of each element of the chain is the input of the next.

implemented in a multitasking operating system, by launching all processes at the same time, and automatically servicing the data read requests by each process with the data written by the upstream process .

Filter
•Independent entities
•Does not share state with other filters.
•No do not know the identity to upstream and downstream filters.

Pipes
•Stateless data stream
•Source end feeds filter input and sink receives output.

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Repository means the central location of data, hence the special feature is that all of the information comes from a central database.

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Used for developing user interface that divides an application into three interconnected parts.

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This architectural pattern is divided into two applications, where the client make the request to the server. The server is actually a database with application logic represented as stored procedures. A server too can be specialized in server applications, files, terminal or email.

ONE of the most special features of client server architectural pattern is mix and match platforms. Client and server can be mixed and matched for different platforms. One of the example is the hardware and operating system can be well intermixed. The figure below shows how client and server architectural pattern works.

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Used to structure programs that can be decomposed into groups of subtasks, each of which is at a particular level of abstraction. Each layer provides services to the next higher layer.

The most commonly found 4 layers of a general information system are as follows.

Presentation layer (also known as UI layer)

Application layer (also known as service layer)

Business logic layer (also known as domain layer)

Data access layer (also known as persistence layer)

Usage

General desktop applications.

E commerce web applications.

One special feature that makes us think this pattern is layered is when there is a grouping of related functionality into distinct layers that are stacked on top of each other. And it is an  inverted pyramid of reuse where each layer aggregates the responsibilities and abstractions of the layer directly beneath it .

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This architectural pattern consist of Client and Server. Diagram below shows the client-server for application.

A computer network diagram of clients communicating with a server via the Internet.

The client–server model is a distributed application structure that partitions tasks between servers and clients.

Clients and servers communicate over a computer network on separate hardware.

A server host share their resources with clients.

A client does not share any of its resources, but requests a server's content or service function.

Clients initiate communication sessions with servers which await incoming requests.

Examples of computer applications that use the client–server model are Email, network printing, and the World Wide Web.

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The Pipe and Filter is an architectural design pattern that allows for stream/asynchronous processing. In this pattern, there are many components, which are referred to as filters, and connectors between the filters that are called pipes. Each filter is responsible for applying a function to the given data; this is known as filtering. Filters can work asynchronously. The final output is given to the consumer, known as a sink

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A repository architecture is a system that will allow several interfacing components to share the same data. Each component interfaces the same dataset that is utilized system wide. Data manipulation taking place in one component will reflect an identical representation of data in another component.

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It is important to think of the repository pattern so that if the Model layers needs some breaking changes, then it should have minimum or no maintenance impact on the controller layer.

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is a pattern used to isolate business logic from the user interface.

Model  => the information (the data) of the application and the business rules used to manipulate the data

View => corresponds to elements of the user interface such as text, checkbox items, and so forth, and the Controller manages details involving the communication between the model and view.

controller => handles user actions such as keystrokes and mouse movements and pipes them into the model or view as required

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Layered architectures consist of four standard layers: presentation, business, persistence, and database

- presentation layer : Presentation of the web pages, UI forms and end user interracting API’s
-  business layer : executing specific business rules associated with the request
-Persistence layer : contains all the classes responsible for doing the technical stuff ->persisting the data in the database, like DAOs

Feature :  Partitions the concerns of the application into stacked groups

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The most common architecture pattern.  Components within the layered architecture pattern are organized into horizontal layers, each layer performing a specific role within the application.

Most layered architectures consist of four standard layers:
1) Presentation
2) Business
3) Persistence
4) Database

Each layer of the layered architecture pattern has a specific role and responsibility within the application.

Layered architecture is one of more  powerful features of the layered architecture pattern which is a way to provide for separation of concerns, encapsulation and decoupling, by grouping code units by their functional role within the application.

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Same data model can be display in multiple view using a controller.

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Layered architecture is an architecture in which data moves from one defined level of processing to another.  Components within the layered architecture pattern are organized into horizontal layers, each layer performing a specific role within the application. It have 4 types of layer which are presentation, business, presistence and database.

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This post is part of The Software Architecture Chronicles, a series of posts about Software Architecture. In them, I write about what I've learned on Software Architecture, how I think of it, and how I use that knowledge. The contents of this post might make more sense if you read the previous posts in this series.
Layering is a common practice to separate and organise code units by their role/responsibilities in the system.

In an object-oriented program, UI, database, and other support code often gets written directly into the business objects. Additional business logic is embedded in the behaviour of UI widgets and database scripts. This happens because it is the easiest way to make things work, in the short run.

When the domain-related code is diffused through such a large amount of other code, it becomes extremely difficult to see and to reason about. Superficial changes to the UI can actually change business logic. To change a business rule may require meticulous tracing of UI code, database code, or other program elements. Implementing coherent, model-driven objects becomes impractical. Automated testing is awkward. With all the technologies and logic involved in each activity, a program must be kept very simple or it becomes impossible to understand.

Eric Evans 2014, Domain-Driven Design Reference

What it means to be layered
In a layered system, each layer:

Depends on the layers beneath it;

Is independent of the layers on top of it, having no knowledge of the layers using it.

In a layered architecture, the layers can be used in a strict way, where a layer only knows the layer directly beneath it, or in a more flexible approach where a layer can access any layer beneath it. Both Martin Fowler and my own experience tells me that the second case seems to work better in practice as it avoids the creation of proxy methods (or even complete proxy classes) in the intermediary layers, and can degrade into the anti-pattern of the Lasagna Architecture (more about it below).

Sometimes the layers are arranged so that the domain layer completely hides the data source from the presentation. More often, however, the presentation accesses the data store directly. While this is less pure, it tends to work better in practice.

Fowler 2002, Patterns of Enterprise Application Architecture

The advantages are:

We only need to understand the layers beneath the one we are working on;

Each layer is replaceable by an equivalent implementation, with no impact on the other layers;

Layers are optimal candidates for standardisation;

A layer can be used by several different higher-level layers.

The disadvantages are:

Layers can not encapsulate everything (a field that is added to the UI, most likely also needs to be added to the DB);

Extra layers can harm performance, especially if in different tiers.

The 60s and 70s

Although software development started during the 50s, it was during the 60s and 70s that it was effectively born as we know it today, as the activity of building applications that can be delivered, deployed and used by others that are not the developers themselves.

At this point, however, applications were very different than today. There was no GUI (which only came into existence in the early 90s, maybe late 80s), all applications were usable only through a CLI, displayed in a dumb terminal who would just transmit whatever the user typed to the application which was, most likely, being used from the same computer.

Applications were quite simple so weren't built with layering in mind and they were deployed and used on one computer making it effectively a one-tier application, although at some point the dumb client might even have been remote. While these applications were very simple, they were not scalable, for example, if we needed to update the software to a new version, we would have to do it on every computer that would have the application installed.

Layering during the 80s and 90s

During the 1980s, enterprise applications come to life and we start having several users in a company using desktop computers who access the application through the network.

At this time, there were mostly three layers:

User Interface (Presentation): The user interface, be it a web page, a CLI or a native desktop application;
ie: A native Windows application as the client (rich client), which the common user would use on his desktop computer, that would communicate with the server in order to actually make things happen. The client would be in charge of the application flow and user input validation;

Business logic (Domain): The logic that is the reason why the application exists;
ie: An application server, which would contain the business logic and would receive requests from the native client, act on them and persist the data to the data storage;

Data source: The data persistence mechanism (DB), or communication with other applications.
ie: A database server, which would be used by the application server for the persistence of data.

With this shift in usability context, layering started to be a practise, although it only started to be a common widespread practice during the 1990s (Fowler 2002) with the rise of client/server systems. This was effectively a two-tier application, where the client would be a rich client application used as the application interface, and the server would have the business logic and the data source.

This architecture pattern solves the scalability problem, as several users could use the application independently, we would just need another desktop computer, install the client application in it and that was it. However, if we would have a few hundred, or even just a few tenth of clients, and we would want to update the application it would be a highly complex operation as we would have to update the clients one by one.

Layering after the mid 90s

Roughly between 1995 and 2005, with the generalised shift to a cloud context, the increase in application users, application complexity and infrastructure complexity we end up seeing an evolution of the layering scheme, where a typical implementation of this layering could be:

A native browser application, rendering and running the user interface, sending requests to the server application;

An application server, containing the presentation layer, the application layer, the domain layer, and the persistence layer;

A database server, which would be used by the application server for the persistence of data.

This is a three-tier architecture pattern, also known as n-tier. It is a scalable solution and solves the problem of updating the clients as the user interface lives and is compiled on the server, although it is rendered and ran on the client browser.

Layering after the early 2000s

In 2003, Eric Evans published his emblematic book Domain-Driven Design: Tackling Complexity in the Heart of Software. Among the many key concepts published in that book, there was also a vision for the layering of a software system:

User Interface

Responsible for drawing the screens the users use to interact with the application and translating the user's inputs into application commands. It is important to note that the "users" can be human but can also be other applications, which corresponds entirely to the Boundary objects in the EBI Architecture by Ivar Jacobson (more on this in a later post);

Application Layer

Orchestrates Domain objects to perform tasks required by the users. It does not contain business logic. This relates to the Interactors in the EBIArchitecture by Ivar Jacobson, except that Jacobson's interactors were any object that was not related to the UI nor an Entity;

Domain Layer

This is the layer that contains all business logic, the Entities, Events and any other object type that contains Business Logic. It obviously relates to the Entity object type of EBI. This is the heart of the system;

Infrastructure

The technical capabilities that support the layers above, ie. persistence or messaging.

Anti-pattern: Lasagna Architecture

Lasagna Architecture is the name commonly used to refer to the anti-pattern for Layered Architecture. It happens when:

We decide to use a strict layering approach, where a layer only knows about the layer immediately below it. In such case, we will end up creating proxy methods, and even proxy classes, just so we go through the intermediate layers instead of directly using the layer we need;

We lead the project into over-abstraction in an urge to create the perfect system;

Small updates reverberate through all areas of an application, for example, tidying up a single layer can be a large undertaking with huge risks and small payoff.

We end up with too many layers, which increases the complexity of the overall system;

We end up with too many tiers, which both increases the complexity and damages the performance of the overall system;

We explicitly organise our monolith according to its layers (ie. UI, Domain, DB), instead of organising it by its sub-domains/components (ie. Product, Payment, Checkout), destroying modularity and encapsulation of the domain concepts.

Conclusion

Layered Architecture is one more way to provide for separation of concerns, encapsulation and decoupling, by grouping code units by their functional role within the application.

However, as most things in life, too much is counter productive! So the rule-of-thumb is: Use just the layers we need, the tiers we need, and nothing more! We must not get carried away chasing an architectural holy grail, which does not exist. What does exist is a need, and the best possible fit for that need. Which is part of Lean, btw.

Furthermore, it's important to note that this top/down approach to layering is outdated. It is no longer what we should do in modern software development, there are new and better ways of thinking of an application layer. I will talk about then in following posts.

Sources

2002 - Martin Fowler - Patterns of Enterprise Application Architecture

2003 - Eric Evans - Domain-Driven Design: Tackling Complexity in the Heart of Software

2011 - Chris Ostrowski - Understanding Oracle SOA - Part 1 - Architecture

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In a layered architecture, the layers can be used in a strict way, where a layer only knows the layer directly beneath it, or in a more flexible approach where a layer can access any layer beneath it, though the second case seems to work better in practice.

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i) The repository mediates between the data source layer and the business layers of the application.
ii) Repository queries the data source for the data, maps the data from the data sources to a business entity.
iii) Repository persists changes in the business entity to the data source
iv) A repository separates the business logic from the interactions with the underlying data source or Web service

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Layered architecture is an architecture where a data moves from level of processing to another level of processing
A layer provides services to the layer above it so the lowest-level layers represent core services that are likely to be used throughout the system.

Advantage:

Allows replacement of entire layers so long as the interface is maintained. Redundant facilities (e.g., authentication) can be provided in each layer to increase the dependability of the system.
Disadvantage:
Providing a clean separation between layers is often difficult and a high-level layer may have to interact directly with lower-level layers rather than through the layer immediately below it. Performance can be a problem because of multiple levels of interpretation of a service request as it is processed at each layer.

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Used when there are multiple ways to view and interact with data. Also used when the future requirements for interaction and presentation of data are unknown.
Model–view–controller (MVC) is used for developing user interfaces.
It divides an application into three interconnected parts.

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In image processing, rendering is a jargon word that has come to mean "the collection of operations necessary to project a view of an object or a scene onto a view surface". The best way to break down the overall rendering process is to consider each object as a different coordinate space, in which independent operations can be carried out simultaneously [Watt93]. The input to a polygonal renderer is a list of polygons and the output is a colour for each pixel on the screen. To build up, for instance, a 3D scene, four general tasks are to be performed per object (Figure).

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Some of the components included in the diagram are documents, database table, files, executables and all physical elements with a location.

One example would be a component diagram for an ATM system; the components shown below are mostly physical elements, interacting with each other or a database table.

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The component diagram's main purpose is to show the structural relationships between the components of a system.  It have 4 types which are component, interface, dependencies and port.

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Pipe and filter is a very simple and powerful architecture that consist of several filters that filter data before it passes it via pipe to other components. All the filters work simultaneously.

Advantages of Pipe & Filter
- Reusability
Filters can be recombined
- Extensibility
It is possible to add new filters.
It is also possible to substitute
old filters with new one.
- Independence
Independent verification of each
filter
- Performance
Enables concurrent execution
of filters.

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Pipe and Filter consists of any number of components (filters) that transform or filter data, before passing it on via connectors (pipes) to other components. The filters are all working at the same time.

One special feature that makes us think this pattern is Pipe and Filter because it has message queue as pipe and it also has filters.

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Pipe and Filter is a very simple, yet powerful architecture, that is also very robust. It consists of any number of components (filters) that transform or filter data, before passing it on via connectors (pipes) to other components.

Use the Pipes and Filters architectural style to divide a larger processing task into a sequence of smaller, independent processing steps (Filters) that are connected by channels (Pipes).

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A repository architecture is a system that would provide both a console and graphical user interface to update both the structure and data set of any particular database and allow several interfacing components to share the same data.

Shared data is held in a central database or repository and may be accessed by all sub-systems

Each sub-system maintains its own database and passes data explicitly to other sub-systems.

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Consist of client and server.
Client request while server provide services.

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There are 5 layers in the system which are presentation layer, business layer, services layer, persistence layer and database layer. Each layer is related to each other

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Categorized into specific layers. Each layer are related functionality associated with each layer.

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Components

Server

listens for requests from client components.

When a request is received, the server processes the request, and then sends a response back to the client.

Servers may be further classified as stateless or stateful.

managing disk drives (file servers), printers (print servers), or network traffic (network servers).

Clients

PCs or workstations on which users run applications.

make composite requests that consist of multiple atomic requests.

This enables a more conversational or transactional interactions between client and server.

server keeps a record of the requests from each current client called a session.

Sequence Diagram

PROS

CONS

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The repository pattern is an abstraction. It’s purpose is to reduce complexity and make the rest of the code persistent ignorant.

One special feature that makes us think this pattern is repository because it has the Repository which is responsible to provide and implement these query methods.

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The list repository contains a query object and a data mapper object that are specific to SharePoint. These are the ListItemFieldMapper and CAMLQueryBuilder classes. Both classes act as repository

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As what the name stated, Model View Controller (MVC) consists of Model(M), View(V) and Controller(C).
Model- contain the brain of the application. (Software logic and application data)
View- contain the interface to be display. (colour, layout, etc.)
Controller- handle communication between user and model.

Basically, user interact with controller (using interface produce by View), interaction is send to model, model process and return data back to controller, data then display via View.

Main objective: To create seperation of concern (To seperate user interface with software logic.)

Image below shows the process flows in MVC.

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Component diagrams are different in terms of nature and behavior. Component diagrams are used to model the physical aspects of a system.

Component diagrams are used to visualize the organization and relationships among components in a system. These diagrams are also used to make executable systems.

The purpose of the component diagram can be summarized as −

Visualize the components of a system.

Construct executables by using forward and reverse engineering.

Describe the organization and relationships of the components.

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Centralized data access. Because data is stored only on the server, access and updates to the data are far easier to administer than in other architectural styles.

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All data can be managed consistently. All data in a system is managed in a central repository that is accessible to system components.

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It is a  MVC based user interface component diagram of  Algorithmic Trading System

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-The pattern is envisaged as a stream of data f;owing througn a set of processing stages
-Data can be processed sequentially or in parallel, based on the context
-pipes and Filters can be combined in different ways to create a family of systems.

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MODEL-VIEW-CONTROLLER(MVC)

Separates presentation and interaction from the system data.

Structured to three logical components :

Models for handling data and business logic and it’s only represent the data
and does nothing else. Does NOT depend on the controller or view

Controllers for handling the user interface and application

Views for handling graphical user interface objects and presentation

Figure 1: MVC class structure

By separating the software applications to three components is a good idea where it helps to:

1. Improved scalability (the ability for an application to grow)
For example, if your application begins experiencing performance issues because database access is slow, you can upgrade the hardware running the database without other components being affected.

2. Ease of maintenance, as the components have a low dependency on each other, making changes to one (to fix bugs or change functionality) does not affect another.

3. Reusability where a model may be reused by multiple views.

Disadvantages of MVC
1. The complexity is high to develop the applications using this pattern.
2. Not right suitable for small applications which has adverse effect in the application’s performance and design.
3. In terms of servlet and JSP, both often contains business logic and presentation tier.
4. The isolated development process by UI authors, business logic authors and controller authors may lead to delay in their respective modules development.

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Component diagrams are used to:-
- Visualize the organization and
relationships among
components in a system.

- Make executable systems.

Component diagrams can be used to −

Model the components of a system.

Model the database schema.

Model the executables of an application.

Model the system's source code.

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Consists of any number of components (filters) that transform or filter data, before passing it on via connectors (pipes) to other components.

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UML 1.0, a component is modeled as a rectangular block with two smaller rectangles protruding from the side. In UML 2.0, a component is modeled as a rectangular block with a small image of the old component diagram shape.

Represents hardware or software objects, which are of a higher level than components.

Shows input or materials that a component either receives or provides.

Specifies a separate interaction point between the component and the environment.

Groups together multiple elements of the system and is represented by file folders in Lucidchart

Allows developers to affix a meta-analysis to the component diagram.

Shows that one part of your system depends on another.

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The components that included in the diagram are Web Store, Accounting and Warehouses.
WebStore subsystem contains three components related to online shopping - Search Engine, Shopping Cart, and Authentication.
Accounting subsystem provides two interfaces - Manage Orders and Manage Customers.
Warehouses subsystem provides two interfaces Search Inventory and Manage Inventory