Prof. Leonardo Mostarda University of Camerino

1. Distributed Systems – Architectures Prof. Leonardo Mostarda University of Camerino Prof. Leonardo Mostarda-- Camerino,

2. Outline • Architectural styles • Layered architectures • Object-based architectures • Data-centered architectures • Event-based architectures • Shared-data space architectures • System architectures • Client server • Multitiered Architectures

3. Architectures • Distributed systems are very complex thus they need to be properly organised • Logical organisation • Physical organisation • Logical organisation is mostly about software architecture that is how software components are organised and how they interact. • The final instantiation of software components is referred to as system architecture

4. Architectural styles Example from the IEEE paper “Distributed Orchestration of Pervasive Services” by Leonardo Mostarda, Srdjan Marinovic, Naranker Dulay Required service Provided service • An architectural style defines: • Components • Connection • Data exchanged • Configuration • A component is a modular unit that provides and requires services • Components can be arranged in various configurations that have been classified into architectural styles.

5. Architectural Styles • Important styles of architecture for distributed systems: • Layered architectures • Object-based architectures • Data-centered architectures • Event-based architectures • Shared-data space architectures

6. Layered architectural style

7. Object-based architectural style Each object corresponds to a component Components are connected through a remote procedure call mechanism. This architecture is very general. For instance it matches the layered architecture.

8. Data-centered architectural style In a data-centered architecture processes communicate through a common repository For instance many networked applications use a shared distributed file system in which communication takes place through files. Web-based distributed systems use shared web-based data services.

9. Event-based architectural style • Components communicate by using events that can carry data • For instance pub/sub systems are event based systems • Components are loosely coupled

10. Shared-data space architectural style • Event-based architectures can be combined with data-centered architectures in order to obtain shared-data space architectural styles. • Components are decoupled in time • The shared repository can be accessed using a description rather than a reference

11. System architecture • Deciding software components, their interactions and their placement lead to a system architecture. • Centralised • Decentralised

12. Centralised Architectures A server implements a specific service. A client requests the service. The client-server interaction is also known as request replay behaviour. The protocol used for the implementation can be unreliable

13. Centralised Architectures • The communication is faster • What are the disadvantages of using a protocol that is not reliable? • If the reply is lost the client can send the request again which will lead to duplication of requests • For instance can you imagine duplicating a transfer of a £100000? • If the operation is “how much money I have left” than it is ok! • Operation idempotent can be repeated several times What are the advantages of using a protocol that is not reliable?

14. Centralised Architectures Type of application Connection set up required Should I use always TCP to implement the client-server request?

15. Application Layering • It is not easy to draw a clear distinction between a client and a server • For instance a server can act as a client • Many client-server applications support access to databases. In this case the following layered architectural style can be identified: • The user-interface level • The processing level • The data level

16. internet search engine example • The processing layer implements the core functionality.

17. Multitiered Architectures (1) • The three logical levels suggests a number of ways for physically distributing a client-server application. The simplest organisation have only two types of machines: • A client machine containing only the programs implementing (part of) the user-interface level • A server machine containing the rest, • the programs implementing the processing and data level

18. Multitiered Architectures (2) • We can distribute the three logical layers into two machines that are client and server • This is called physically two- tiered architecture. • (a) applications control the representation of data (thin clients) • (d) (e) workstations connected to a distributed file system (fat clients)

19. Multitiered Architectures (3) • Three-tiered architecture • programs that form part of the processing level reside on a separate server

20. Summary • Architectural styles • Layered architectures • Object-based architectures • Data-centered architectures • Event-based architectures • Shared-data space architectures • System architectures • Client server • Multitiered Architectures

21. 15 minutes pause Prof. Leonardo Mostarda-- Camerino,

22. Questions? Prof. Leonardo Mostarda-- Camerino,