Software Concepts - Distributed Systems

Network Operating Systems (NOS)

• Simply loosely coupled OS for heterogeneous multicomputers (LAN, WAN)
• Goal is to offer local services to remote clients
• Each computer has its own NS and OS and works independently
• It fails to provide a single coherent view
• Degree of transparency is low
• Number of copies of OS is N
• Basis of communication is files
• Resource management is pernode
• Scalability possible
• Open

Distributed Operating System (DOS)

• Loosely coupled OS for multiprocessors and homogeneous multicomputers
• Multiprocessor uses different system services to manage resources connected in a system and use system calls to communicate with the processor
• Multi computer uses a separate uniprocessor OS on each computer for communicating between different computers
• Goal is to hide and manage service
• Services are transparent
• 
• Multiprocessors
  • Degree of transparency is very high
  • Same OS on all node
  • Number of copies of OS is 1
  • Basis of communication is shared memory
  • Resource management is Global central
• Multicomputers
  • Degree of transparency is high
  • Same OS on all node
  • Number of copies of OS is N
  • Basis of communication is messages
  • Resource management global distributed
  • Scalability moderate

Middleware Operating System (MOS)

• Additional layer on top of network OS implementing general purpose services
• Goal is to provide distribution transparency
• Services offered
  • Naming Service
  • Persistence Service
  • Message Service
  • Querying Service
  • Concurrency Service
  • Security Service
• Degree of transparency is high
• Number of copies of OS is N
• Basis of communication is model specific
• Resource management is pernode
• Scalability varies
• Open