Advantages and disadvantages of DS

Definition

A distributed system is a collection of independent computers, connected by a network and running distribution middleware, that appears to its users as a single coherent system. Its components communicate and coordinate their actions only by passing messages (Tanenbaum; Coulouris).

Advantages

• Economics: many cheap computers give a better price/performance ratio than one mainframe.
• Performance and speed: work is shared among processors (parallel processing, load balancing).
• Inherent distribution: applications such as banking, e-mail and the web are naturally spread over many places.
• Reliability and availability: if one machine fails the others continue, particularly with replication.
• Resource sharing: hardware, software and data are shared by remote users.
• Incremental growth (scalability): capacity is added by adding machines.
• Flexibility and openness: parts from different vendors can be combined.

Disadvantages

• Complexity: design, development and debugging are harder than for a single machine.
• Software: little general-purpose distributed software exists; middleware adds overhead.
• Network problems: the network may be slow, saturated or fail, and lost messages hurt the system.
• Security: open networks and shared resources raise the risk of attacks and data leakage.
• Partial failure: parts fail independently, so the state is hard to know and keep consistent.
• No global clock or shared memory: synchronisation and agreement are difficult.
• Higher management cost: many machines need to be administered, upgraded and kept consistent.