CS 4504 Lecture Notes - Lecture 7: Directory Service, Mobile Agent, Mutual Exclusion

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28 Sep 2016
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Chapter 3
Distributed Computing Paradigms
Instructor Dr. P. Bobbie
Lecture Notes #6
Chapter 3
Remote Procedure Call
The message-passing model works well for basic network protocols. However, as the programs
grew in size and complexity another abstraction method was required to run on a single
processor.
The remote procedure call (RPC) provides a model for a communication between two
processes in a similar manner to local procedure call.
RPC involves two independent processes. They could be on separate machines or on the
same.
Process A issues a procedure call to B. A passes a list of argument values.
RPC triggers a predefined action in a procedure provided by process B.
At the completion process B returns a value to process A.
Note: Message-passing model is data orientated with the actions triggered by the message
exchange while RPC model is action-orientated with the data passed as arguments.
Distributed Objects Paradigms
Remote Method Invocation (RMI) – object-orientated version of RPC.
RMI – a process invokes the methods in an object, which may reside on a remote host. Value
may be returned when the method call is completed.
Implementation example:
oProgram provides a remote method for each participant to register itself and
another method for a participant.
oEach participant program provides following methods:
Allow main process to announce an event.
Main process updates
Main process informs about task completion and disconnects.
The Object request broker Paradigm (ORB)
ORB issues a request to an appropriate object that provides the desired service. ORB acts as a
middleware allowing an application to access multiple remote (or local) objects.
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Example:
Each participant issues a request to the main process to register for the session and make calls.
Through ORB main process invokes the methods of each participant and sends a confirmation,
updates status, and finish and then disconnect.
The Object Space
Object space – logical entities.
The participants of an application converge in a common object space. A provider places
objects as entries into an object space and requestor who subscribe to the space may access
the entries.
Object space provides virtual space/meeting room among providers and requestors of network
resources. Mutual exclusion is inherent in the paradigm.
The Mobile Agent Paradigm
Mobile agent – transportable program or object.
The agent is launched ffrom an originating host. The agent then travels autonomously from host
to host according to an itinerary that it carries.
The paradigm offers abstraction for a transportable program or object. Data is carried by the
program/object as the program is itself transported among participants.
The Network Services Paradigm
NSP is an extension of Remote Method Invocation paradigm. The only difference is that service
objects are registered with a global directory service allowing them to be looked up and
accessed by service requestors on a federated network.
The Collaborative Application (Groupware ) paradigm
Processes participate in a collaborative session as a group. Each participating process may
contribute input to part or all of the group. Processes may do so using multicasting to send data
to all or part of the group or they can use sketchpads to allow each participant to read and write
data to a shared display.
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