Chapter 2-Architectures2.ppt
- 1. Chapter 2 - Architectures
- 2. 2 Introduction how to organize the collection of software components logical organization and physical organization i.e., software architectures: how they are organized and how they communicate we will discuss architectural styles system architectures: centralized vs decentralized ones
- 3. 3 2.1 Architectural Styles the logical organization of distributed systems into software components a component is a modular unit with well-defined required and provided interfaces that is replaceable within its environment a connector is a mechanism that mediates communication, coordination, or cooperation among components, e.g., facilities for RPC, message passing, or streaming data there are various architectural styles Layered architectures Object-based architectures Data-centered architectures Event-based architectures
- 4. 4 Layered architectures components are organized in a layered fashion where a component at layer Li is allowed to call components at the underlying layer Li-1, but not the other way around; e.g., network layers the layered architectural style
- 5. 5 Object-based architectures each object corresponds to a component and these components are connected through a remote procedure call mechanism (client-server paradigm) the object-based architectural style
- 6. 6 Data-centered architectures processes communicate through a common repository; e.g., a shared distributed file system Event-based architectures processes communicate through the propagation of events publish/subscribe systems processes publish events and the middleware ensures that only those processes that subscribed to those events will receive them the event-based architectural style
- 7. 7 shared data spaces event-based architectures combined with data-centered architectures processes are decoupled in time the shared data-space architectural style
- 8. 8 2.2 System Architectures the logical organization of distributed systems into software components or how are processes organized in a system 2.2.1 Centralized Architectures thinking in terms of clients requesting services from servers general interaction between a client and a server
- 9. 9 communication between client and server can be by a connectionless protocol if the underlying network is fairly reliable; efficient since there is no much overhead but assuring reliability is difficult when messages are lost or corrupted let the client send the request again; applicable only for idempotent operations an operation is idempotent if it can be repeated multiple times without harm; e.g., reading a record in a database see later in Chapter 8: Fault Tolerance by reliable connection-oriented protocol if the underlying network is unreliable establishing and terminating connections is expensive
- 10. 10 Application Layering no clear distinction between a client and a server; for instance a server for a distributed database may act as a client when it forwards requests to different file servers three levels exist the user-interface level: implemented by clients and contains all that is required by a client; usually through GUIs, but not necessarily the processing level: contains the applications the data level: contains the programs that maintain the actual data dealt with
- 11. 11 e.g., the general organization of an Internet search engine into three different layers Client-Server Architectures how to physically distribute a client-server application across several machines Multitiered Architectures
- 12. 12 Two-tiered architecture: alternative client-server organizations (a) put only terminal-dependent part of the user interface on the client machine and let the applications remotely control the presentation (b) put the entire user-interface software on the client side (c) move part of the application to the client, e.g. checking correctness in filling forms (d) and (e) are for powerful client machines (more popular)
- 13. 13 three tiered architecture: an example of a server acting as a client an example is the organization of Web sites
- 14. 14 2.2.2 Decentralized Architectures vertical distribution: the ones discussed so far where the different tiers correspond directly with the logical organization of applications; place logically different components on different machines horizontal distribution: physically split up the client or the server into logically equivalent parts an example is a peer-to-peer system where processes are equal and hence each process acts as a client and a server at the same time (servent) read about the different approaches of peer-to-peer architecture - pages 44 - 51 and about Architectures versus Middleware - pages 54 - 66
- 15. 15 another example is the horizontal distribution of a Web service