Revision Week 13 – Lecture 2

The exam 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 –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.

Operating system Proc A Proc B Proc C Operating system Proc X Proc Y Proc Z Client process Client process Server process Server process Host 1 Host 2 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 Messages are sent in accordance with a protocol

Client Server model Distinguish between servers as processes as and hosts Understand why we want to distribute our system over multiple hosts –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

Client process Server process Middleware Network Request Reply Host A Host B The role of Middleware

What do want Middleware to do? 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