concepts of ds

• A distributed system is one where several computations coordinate across machine boundaries
  • To coordinate, they must communicate, and must know of each other’s existence
  • To coordinate, they must have common things (state) to “talk” about

1. Global Resource Access
2. Concurrency
3. High Availability / Fault Tolerance (Inevitable)
4. Transparency
5. Management and Maintenance
6. Security
7. List item
8. Scalability
9. Timeliness (Intolerant for real-time system)
10. Openness (Publishing key programming interfaces)

Note: simple Web-based DS work

Client-server

• Clients: Generalized, large number, consumer of service
• Protocol: File retrievals, SQL, HTTP over TCP/IP
• Server: specialized, a provider of services (file services, db services, groupware services, web services)
• Naming, authentication, authorization, auditing and logging, billing
• Load balancing

Difference between consistency and consensus

Consistency means the content and time ordering should be consistent in different nodes, consensus means that different nodes share an agreement on certain protocol or value.