Distributed Systems

Definition

Systems where software and hardware components, placed in a network, communicate and coordinate their actions by passing messages.

DS Coulouris

A set of independent computers that are presented to the user as a single integrated system. DS Tanembaum

Characteristics

Collection of autonomous computing elements.

• Global clocks.

• Membership management.

  • Open.

  • Close.

Single coerent system.

Middleware

Available services:

• Communication (eg. RPC).

• Transactions.

• Service composition.

• Reliability.

Objective

• Support of resource sharing.

• Transparent distribution (application and user)

• Openness.

• Scalability.

Transparent Distribution

Transparancy	Description
Access	Hides the differences in data representation and in the method of access.
Localization	Hides where an object is located
Relocation	Hides the fact that an object can be moved to another location while in use.
Migration	Hides the fact that an object can be moved to another location.
Replication	Hides the fact that an object can be replicated
Concurrency	Hides the fact that an object can be shared between concurrent users.
Flaws	Hides the flaws in recoveries of an object.

Openness

"Being open".

Using components that can be used, or integrated, into other systems.

Interoperability.

Composition.

Extensibility.

Scalability

Dimension

• Size.

• Geographic.

• Administrative.

Vertical vs. Horizontal

Pitfalls

The Network is reliable

The Network is secure

The Network is homogenous

The topology is inmutable

Zero latency

Infinit bandwith

No cost of transport

There is a single administrator