1. Ch 12 Distributed Systems Architectures
Types of systems:
designed to run on a personal computer or workstation (MS Word)
embedded systems that run on a single processor or on an integrated group of processors (helicopter display)
distributed systems where the software runs on a loosely integrated group of co-operating processors linked by a network (corporate accounting system)
Virtually all large systems are distributed
Distributed software engineering is increasingly important

2. Distributed systems architectures
Client-server
distributed services which are called on by clients
servers that provide services differ from clients that use services
Distributed object
no distinction between clients and servers
any object may provide and use services from other objects
often called peer-to-peer
Combinations of the two

3. 12.1 Multiprocessor architectures
Simplest distributed model, used for many large real-time systems
Multiple processes which may (or not) run on different processors
A traffic control system:

4. 12.2 Client-server architectures
Services are provided by servers
Clients use them
Clients know servers
Servers need not know clients

5. Layered application architecture
Client/Server often organized in layers
Presentation layer
presents the results of a computation to system users and with collecting user inputs
Application processing layer
application specific functionality
e.g., in a banking system, functions such as open account, close account...
Data management layer
manages the system databases
CS 330 examples?

6. Thin and fat clients * **
* All application processing and data management are on the server
The client is responsible only for the presentation software
** The server is only responsible for data management
The client implements application logic and interacts with the user

7. Thin versus fat client model
Thin client
used when legacy systems are migrated to client server architectures. The legacy system acts as server with a gui implemented on a client
heavy processing load on the server and the network
Fat client
application processing is locally executed
most suitable when the client system capabilities are known in advance
more complex to manage (new versions of the application have to be installed on all clients)

8. Three-tier architectures
Each layer may execute on a separate processor
Better performance than a thin-client
Simpler to manage than a fat-client
More scalable - as demands increase, add extra servers

9. 12.3 Distributed object architecture
No distinction between clients and servers
Each entity is an object that provides services to other objects and receives services from other objects
Object communication is through a middleware system called an object request broker (software bus)
More complex to design
than client/server
systems

10. Another example: data mining
A way to integrate data in different formats
Each integrator is looking for specific information
The number of databases can be increased without disrupting the system
New types of relationships can be mined by adding new integrator objects

11. CORBA
An international standard for Object Request Broker (ORB) - middleware to manage distributed object communications
A CORBA object encapsulates a state with an interface defined in an IDL (interface definition language)
The object may be written in any language
The ORB knows of all objects in the system and their interfaces
Example services:
trading services allow objects to use other objects on the net
notification services notify other objects that an event has occurred
transaction services support atomic transactions and rollback on failure
Alternative approach by Microsoft: .NET, DLLs

12. CORBA application structure** *
***
* Objects defined for a specific domain, e.g. banking
** Objects used by many domains, e.g. user interface
*** General services, e.g. directories, security management

13. 12.4 Interorganizational distributed computing
Old: intraorganizational distributed computing
org. resources shared behind a security wall
corporate look and feel
Newer models share resources among organizations
Peer to peer: decentralized architectures with no distinction between clients and servers
big application: file sharing
Napster, Kazaa
outlawed by Spain in 2006
Service-oriented systems
created by linking software services provided by different suppliers
examples?

14. Services standards Services based on communication via XML
can be provided on any platform
in any programming language
Key standards
SOAP - Simple Object Access Protocol (example)
WSDL - Web Services Description Language
describes a web service
location
methods it exposed
UDDI - Universal Description, Discovery and Integration
allows businesses to list themselves and make services available
programs wanting a web service (e.g. credit card authentication) go through a service broker
often located inside a company

15. Not all SOA*s are web-based
* service oriented architecture