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
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
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