Transcript Lec 2

Revision
Week 13 – Lecture 2
The exam
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5 questions
Multiple parts
Read the question carefully
Look at the marks as an indication of how
much thought and writing required
• Requires more understanding of the principles
than detailed technical knowledge
Course structure
• Client Server model – message oriented/protocol
driven
• Networks – passing the message
• Distributed components – understanding the
message
• Other key issues
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Security
Platforms – Hardware/OS/DBMS/Middleware
Reliability
Performance
Internationalisation
Protocols
Really understand the definition
A protocol defines the format and order of
messages exchanged between two
communicating entities, and the actions
taken on receipt or transmission of a
message.
Host 1
Client
process
Operating system
Proc
A
Proc
B
Proc
C
Server
process
Proc. A asks Proc. X to
Perform a service.
Proc. Z asks Proc. C to
Perform a service
Host 1 does not ask
Host 2 to perform a
Service
Requests are made by
passing messages
Operating system
Server
process
Proc
X
Proc
Y
Host 2
Proc
Z
Messages are sent in
accordance with a protocol
Client
process
Client Server model
• Distinguish between servers as processes as and hosts
• Understand why we want to distribute our system over
multiple hosts
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Scalability
Heterogeneity
Openness and
Fault tolerance
• Because there is a significant overhead
• Understand the advantages and disadvantages of using
a browser as the workstation client
• Understand the concept of transparency
Networks
• Know the layers of both the OSI and the Internet
models – look for the logic in the layering
• Understand the comms. concepts we discussed in
week 2 so that you are able to understand the
functions at each layer of the network
• Remember the function of the network – to pass
messages reliably from a process in one host to a
process in another host.
• Make sure you know the functions of TCP and IP as
the two main protocols in the Internet architecture.
Know the limitations of each and be able to describe
the approaches to overcoming them.
Distributed components
• Protocol - format and order – action to be taken
• Service request has three parts
– The name of the component
– The service to be performed
– The list of parameters
• Requirements for success of a service request
– Locate the host & process
– Pass the request (& the results) reliably and quickly
– Understand the message – format & meaning
• Know how far XML helps with understanding the
message
• Be able to describe how RosettaNet extends that
understanding within a specific domain
The role of
Middleware
Client
process
Host A
Middleware
Request
Network
Reply
Middleware
Host B
Server
process
What do want Middleware to do?
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Locate the host and process
Provide total transparency
Provide load balancing and failover
Resolve data heterogeneity – provide a common IDL (XML)
Synchronise client and server (parallelism)
Use the Network to transfer messages
Start the process if it is not active (and deactivate it afterwards)
Handle errors
Handle two phase commits where there are 2+ DBMS
Manage message queues for asynchronous connectivity
Help manage data conversion
Not all Middleware products do all of these – you have to choose
one that suits your needs.
Security
• Risk analysis
• Policy is the guiding legislation
• Within secure communications, understand
– Secrecy
– Authentication
– Message integrity
• And the methods for achieving these
Performance
• Understand the concepts of response time and
throughput
• Why do we have commit to performance targets in
a Service Level Agreement
• At what stage in the project, and how, do we
establish these targets
• Be able to articulate the process of achieving them
• Be able to describe who is responsible for
achieving them