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− Computers were large and expensive.
− No way to connect them.
− All systems were Centralized Systems.
− Powerful microprocessors.
− High Speed Computer Networks (LANs , WANs).
Then came the
What are Distributed Systems ? ?
A distributed system is a piece of software that
a collection of independent computers appears
to its users as a single coherent system.
(1) independent computers and (2) single
system => middleware.
World Wide Web (WWW) is the biggest
example of distributed system.
An intranet which is a portion of the internet
managed by an organization
SYSTEMS ? ?
availability of powerful yet cheap
microprocessors (PCs, workstations),
continuing advances in communication
ADVANTAGES OF D.S. OVER
A collection of microprocessors offer a better
price/performance than mainframes. Low
price/performance ratio: cost effective way to
increase computing power.
• If one machine crashes, the system as a whole
can still survive. Higher availability and
Speed: a distributed system may have more
total computing power than a mainframe.
Ex.: 10,000 CPU chips, each running at 50
MIPS. Not possible to build 500,000 MIPS
Enhanced performance through load
Incremental growth: Computing power
can be added in small increments. This
leads to Modular expandability
ADVANTAGES OF D.S. OVER
Data sharing: allow many users to access
to a common data base.
Resource Sharing: expensive peripherals
like color printers.
Communication: enhance human-to-
human communication. E.g.: email, chat.
Flexibility: spread the workload over the
ORGANIZATION OF D.S.:
A distributed system organized as middleware.
−The middleware layer extends over multiple machines, and offers each
application the same interface.
With Distributed Systems, it is easier for users to
access remote resources and to share resources
with other users.
− Examples: printers, files, Web pages, etc
A distributed system should also make it easier for
users to exchange information.
Easier resource and data exchange could cause
security problems – a distributed system should
deal with this problem.
The openness of DS is determined primarily
by the degree to which new resource-
sharing services can be added and be made
available for use by a variety of client
It hides the fact that the processes and
resources are physically distributed across
Transparency is of various forms as
A system is described as scalable if it
remains effective when there is a significant
increase in the number of resources and the
number of users.
Controlling the cost of resources or money.
Controlling the performance loss.
There is a possibility that several clients
will attempt to access a shared resource at
the same time.
Any object that represents a shared resource
in a distributed system must be responsible
for ensuring that operates correctly in a
Goal: High performance computing tasks.
Cluster Computing Systems:
− A “supercomputer” built from “off the
shelf” computer in a high-speed network
(usually a LAN)
− Most common use: a single program is run
in parallel on multiple machines
Grid Computing Systems:
− Contrary to clusters, grids are usually
composed of different types of computers
(hardware, OS, network, security, etc.)
− Resources from different organizations are
brought together to allow collaboration
− Examples: SETI@home, WWW…
Goal: Distribute information across several
− Remote processes called Clients access the
servers to manipulate the information
− Different communication models are used.
The most usual are RPC (Remote Procedure
Calls) and the object oriented RMI (Remote
− Often associated with Transaction systems
− These are the distributed systems involving mobile
and embedded computer devices like Small,
wireless, battery-powered devices (PDA’s, smart
phones, sensors, wireless surveillance cams,
portable ECG monitors, etc.)
− These systems characterized by their “instability”
when compared to more “traditional” distributed
− Pervasive Systems are all around us, and ideally
should be able to adapt to the lack of human
Automatically connect to a different network;
Discover services and react accordingly;
Automatic self configuration (E.g.: UPnP –
Universal Plug and Play)…
− Examples: Home Systems, Electronic Health Care
Systems, Sensor Networks, etc.
Distributed systems are everywhere
Internet, intranet, wireless networks.
Resource sharing is the main motivating
factor for constructing distributed systems.
The construction of distributed systems
produces many challenges like Secure
communication over public networks.