Design goals: maintainability, flexibility, portability

Architectural Style vs. Architecture

• Subsystem decomposition: identification of subsystems, services, and their relationships to each other
  • Architectural Style: a pattern for a subsystem decomposition
    • Software Architecture: instance of an architectural style

Client-Server Architecture

Often used in the design of database systems

• Client: user application
• Server: database access and manipulation
• Client requests a service from the server

Portability High performance Reliability Scalability Flexibility

Functions performed by the client

• Input by the user (customized user interface)
• Sanity checks of input data

Functions performed by the server

• Centralized data management
• Provision of data integrity and database consistency
• Provision of database security

• One or more servers provide services to clients
• Each client calls a service offered by the server
  • Server performs service and returns result to client
  • Client knows interface of the server
  • Server does not know the interface of the client
• Response is typically immediate (i.e. less than a few seconds)
• End users interact only with the client

Problems

• Client server systems use a request-response protocol
• Peer to peer communication is often needed

Example: a database must process queries from application 1 and should be able to send notifications to application 2 when data in the database has changed

Peer to peer Architectural Style

• Generalization of the client server architectural style
• Clients can be servers and servers can be clients

Layered Architectural Style

• A layer is a subsystem that provides a service to another subsystem with the following restrictions
  • A layer only depends on services from lower layers
  • A layer has no knowledge of higher layers

Closed architecture (opaque layering)

A layered architecture is closed, if each layer can only call operations from the layer directly below (also called “direct addressing”)

Design goals: maintainability, flexibility, portability

more portable → low coupling , but potentially a bottleneck

Open architecture (transparent layering)

A layered architecture is open, if a layer can call operations from any layer below (also called “indirect addressing”)

Design goals: high performance, real-time operations support

more efficient → high coupling :(

3 layered architectural style

Often used for the development of web applications

Example

1. The web browser implements the user interface
2. The web server serves requests from the web browser
3. The database manages and provides access to the persistent data

4 layered architectural style

Hierarchically ordered layers

Example

1. A web browser provides the user interface
2. A web server serves static HTML requests
3. An application server provides session management (for example the contents of an electronic shopping cart) and processes dynamic HTML requests
4. A database manages and provides access to the persistent data

→ If these layers reside on different hardware nodes, then it is a 4 tier architecture

Layer vs. tier

• 3 layered architectural style: an architectural style where an application consists of 3 hierarchically ordered layers
• 3 tier architecture: a software architecture where the 3 layers are allocated on 3 separate hardware nodes
• Note: Layer is a type (e.g. class, subsystem) and tier is an instance (e.g. object, hardware node)