IS5413 Chap.11 Application Architectures and Cloud Delivery
Application Architectures and Cloud Delivery
Three kinds of logic, and one of them keeps moving
Every application built on a database places three kinds of logic somewhere. Presentation logic handles input and output, through a keyboard, a mouse, a screen or a printer. Processing logic handles input and output processing, business rules and data management, expressed as procedures, functions and programs.
Storage logic handles the retrieval and writing of data and is the work of the management system itself.
The history the chapter tells is almost entirely the history of where the middle one lives, moving from mainframe systems, through client and server systems, to cloud delivered systems.
Two tier, three tier, and what each buys
In a two tier arrangement the workstation carries presentation, data processing and business rules while the server performs storage, access and processing of the data, which is why the client is described as thick or fat.
That works well for a small user population and fails on two counts: it does not scale, and a change to a business rule has to reach every workstation. A three tier arrangement separates presentation, application and data tiers, leaving a thin client used mainly for the interface.
The published benefits are scalability, technological flexibility, long term cost reduction, systems that fit business need more closely, better service to customers, a competitive edge, and lower risk.
The published challenges are equally specific: high short term costs, specialised tools and training, a need for experienced architects, incompatible standards between products, and gaps in client side tooling.
Renting the stack: three service models and two deployment models
Cloud computing is described as an arrangement that makes networked services available widely, easily and on request, characterised by on demand self service, broad network access, resource pooling, rapid elasticity and measured service.
Infrastructure as a service rents computing power for a limited period, supplying machines, storage, firewalls, load balancers and networks so that existing applications can run on somebody else's hardware. Platform as a service supplies an operating system, a runtime, a database and a web server behind an interface, so that developers build their own service on top.
Software as a service supplies a finished application reachable from anywhere with no additional hardware or software.
Public and private clouds are a separate axis, distinguished by who owns the pool rather than by what is rented, so a private platform service is perfectly coherent.
Why markup exchange belongs in a database course
Extensible markup language matters here for one reason: it is how structured data crosses an organisational boundary.
A schema definition for such a document is a record definition and therefore metadata, which is the same role the repository played in the opening chapter.
Storing such documents as ordinary files reintroduces every file processing problem the course began with, which is why four options exist: shredding the document into relations, storing it whole in a large object column, using dedicated markup columns, or using a native document store.
The service protocols built on top, for describing, discovering and exchanging services, are what allow loosely coupled components to be assembled into a service oriented architecture, and the microservice pattern is presented as the direction that idea has taken since.
What this chapter covers
- 01
Presentation, processing and storage logic
- 02
Thick and thin clients
- 03
Two tier against three tier benefits and challenges
- 04
Infrastructure, platform and software service models
- 05
Public against private deployment
- 06
Four ways to store a marked up document
Worked example · free
Place the logic and choose the service model
- 3Name the current arrangement and the evidence for it.
- 3Name the target arrangement and the specific problem it solves.
- 3Choose the service model and tie it to the stated constraint.
Key terms
- Presentation Logic
- The part of an application handling input and output through the interface, as distinct from the processing and storage of the data behind it.
- Thin Client
- A workstation used mainly for the user interface, with little or no application processing and limited or no local data storage.
- Application Server
- The middle tier of a three tier arrangement, holding the business rules so that they can be changed in one place rather than on every client.
- Stored Procedure
- Logic embedded in the management system and called by name, improving performance and reducing network traffic at the cost of being product specific.
- Service Oriented Architecture
- A collection of loosely coupled, highly interoperable services that talk to each other by handing over data or by jointly driving one business activity.
Application Architectures and Cloud Delivery FAQ
What actually distinguishes two tier from three tier?
Where the processing logic lives. In two tier it rides on the client alongside presentation, which is what makes the client thick and what makes a business rule change expensive. In three tier it becomes a tier of its own that can be replaced or scaled without touching the client or the database. Counting servers is not the test; locating the business rules is.
How do the three cloud service models differ?
By how much of the stack you rent. Infrastructure as a service rents machines, storage and networking and expects you to run what sits on them. Platform as a service adds an operating system, runtime, database and web server behind an interface, so that you write the application. Software as a service supplies the finished application itself.
Public and private are a separate question about who owns the pool, so the two axes combine freely.
Exam move
Sketch the three logic types as three bands and then redraw them for a two tier and a three tier arrangement, saying in one sentence which band moved and what that purchased. Then take three systems you personally use and place each in a service model, justifying the placement by what you would have to administer yourself.
Finish by listing the five published challenges of three tier, since questions that ask only for benefits are rarer than students expect.
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