Engineering

• Basic principles of Engineering:
  • Reproducibility (do the same thing more than once),
  • Measurability (define what is same),
  • Portability (as a result of the above),
  • Standardization (formal or industrial),
  • Documentation (what else is needed for reproducibility),
  • Organization (no randomness).
• Apply these principles in Software Engineering.

Application

• Application is a computer program created for users.
• Application can be logically divided in followint parts:
  • Presentation layer
    • input, display results, user interface, logic of user interface
  • Application layer
    • data processing, calculations, business logic
  • Data layer
    • storage and retrieval of data, storage logic

Birth of application

• Includes:
  • analysis of requirements
  • architecture design
  • choice of technologies
  • programming
  • testing
• Choice of technologies and architectural desing are usually caried out together
• Computer is really needed only for programming and testing
• Application also has to be deployed and maintained
  • Includes end of life procedures

Step 1 - analysis of requirements

• Most important part of whole development process
  • Depends on experience, size of team/company (yours and customer’s), budget, …
• Requires analys of customer’s environment
  • Different approach for different types of customers
• Significant impact on the outcome
• Project cartoon

Step 1 - analysis of requirements

• General (non-functional)
  • Web/desktop/console application
  • Operating system and technologies
• What the appliction should do (functional)
  • List of use-cases + scenarios
• Who will use the application
  • User roles and other actors (time, other apps)
• How the user interface should work
  • Graphical design / wireframes / no UI at all
• What kinds information will be stored (if any)
• Interfaces for other applications (if any)

Step 2 - architecture

• Structure and distribution of application’s services
  • presentation layer
    • presentation services (display UI elements)
    • presentation logic (UI layout and function)
  • application layer
    • logic of application (application algorithms)
    • logic of data storage (data algorithms)
  • data layer
    • data control (storage access control)
• Generally a 3 layer architecture is most suitable
• Yet a 2 layer application is used sometimes
  • Multiple variants

Two layer - option 1

• Client-server with remote data storage (file server)
• Heavy load on client
• Heavy load on transmission channel
• Small load on server
• Examples:
  • SVN, DFS, NFS

Two layer - option 2

• Client-server with remote presentation
• Small load on client and transmission channel
• Heavy load on server
• It is impossible to distinguish from three-layer architecture from client’s perspective
• Many web applications

Two layer - option 3

• Client-server with separated logic
• Good load balance
• Diffucult to expand and/or port application logic
• Some very complex web applications

Two layer model

• Data are stored in format given by needs of application - diffuclt to transfer between applications
• Data store change requires all clients to reflect that change
• Reasons to use:
  • quick first desing and implementation
  • unavailable or non-developed universal solutions
    • database servers
    • application servers
    • components

Three layer model

• Good load balance
• Small load on transmission channel
• Good expandability
  • When using standardized interfaces
• Flexible architecture
• Example of many web applications

Three layer advantages

• Presentation and data layer are separated by application layer
  • client presentation can be changed when using appropriate interfaces
    • web app/mobile app with single backend
  • data storage can be changed when using appropriate interfaces
    • ORM
  • change in presentation layer cannot influence data storage
    • flexible, safe, reliable
• Interface is something between two things
  • well documented communication protocol

Example

• Application rendered in web browser does not depend on a database server
  • Is is not important where data was gathered
  • We can change data store completely and presentation will remain same
  • Application can be viewed in different browsers and data store is not aware of this
• Application layer is an interface between data store and presentation layer

Distributed systems support with three layer application

SOA, API

• An application with good API can be used from another applications
  • Presentation layer can be bypassed and work directly with application from another app code
• A good interface uses well known standard format (SOAP, XML-RPC - XML, REST - JSON, XML) and is documented
• More such apps is used to create new application by combining existing abilities
• Cloud services

Step 3 - Technologies

• By requested application properties:
  • network / local
  • client-server / stand-alone
  • compiled / interpreted
  • native / runtime
• Database application
• Web application
• Language and development environment
  • not just IDE
• Available libraries and their distribution system

Different approaches

Web applications

• Web page only delivers content (read only)
• Web application interacts with user
• Advantages
  • Rapid development (RAD)
  • Portability
  • Low operational maintenance costs
  • A lot of prepared and well documented services (HTML, CSS, Web browser)
• Disadvantages
  • A programmer cannot influence many things in presentation (given by browser)

An impossible chart

Checkpoint

• Why a user should not communicate directly with data layer?
• Does every web application have to have an API?
• Which layer is best to solve a request that “searching for products should not be influenced by diacritics”?
• Is it better to have more or less application layers?
• Does web/network application have to have client-server architecture?
• Is is better to choose architecture or technologies first?
• What is important to choose a technology?