A distributed system is a collection of independent computers that appears to its users as a single coherent (connected) system.
 | Hardware: Easy to expand and scale (independant computers), |
| Software: Users and applications interact with a distributed system in a uniform way. |
Distributed systems uses middleware between the different operating systems and the distributed applications.
Goals:
| Easy access for users to remote resources |
| Transparency (able to present itself as if it is a single computer) |
| Openness (interoperability, portability ) |
| Scalability (size, geographically and administratively)(distributing, replication/caching and hiding communication latencies) |
Hardware concepts:
| Multiprocessors (all CPUs have direct access to the shared memory) |
| Homo/Hetero Multicomputer (each CPU has direct connection with its own local memory) |
Software concepts:
| 1: Distributed operating systems (uniprocessor, multiprocessor and multicomputer). Same OS distributed on the different machines. Communicating with messages. CLOSED. |
| 2: Network Operating systems (collection of uniprocessor systems, each with its own OS). rlogin. communicating with shared files. OPEN |
NEITHER OF THESE TWO QUALIFIES AS DITRIBUTED SYSTEMS. WE NEED AN ADITIONAL LAYER OF SOFTWARE!!
Middleware:
Sits between the application and the operating software.
| Middleware models: 1: All IO as keyboard are treated like files (UNIX), 2: RPC(remote procedure call), 3: Distributed objects. |
| Middleware services: naming, persistence (storage), distributed transactions (multiple read and write automatically), its OPEN(interfaces are complete). |
The client-server model:
| Client: A process that requests a service from a server by sending and then waiting. |
| Server: A process implementing a specific service like a file system service or a database service. |
Application layering:
| User interface level |
| Processing level (core functionality of an application) |
| Data level (database) |
Cient-server architectures:
| Client machine only containing the programs implementing the user-interface level |
| A server machine with the programs implementing the processing and datalevel |
| Multitired Architectures (GUI, Application and database divided not stricktly in 3). Vertical. |
| Horizontal distribution (webservers replicated across several machines) |