Chapter 5: Web Applications and Distributed Systems

Question 1

What model do web applications adopt?

Answer 1

Client/server architecture

Question 2

What are some of the performance concerns of web application?

Answer 2

• Forecasting volume of activity
• Mechanisms for accessing database
• Location of database

Question 3

What are performance problems with good security?

Answer 3

SSL, server needs to create a key for each request (computationally expensive)

Question 4

What is Distributed Object Technology (DOT)?

Answer 4

DOT is merging of object orientated technology with distributed systems technology

• The object is the unit of computation and distribution in a heterogenous multi-platform computing environment

Question 5

What is middleware?

Answer 5

A region in between the client and server. Contains it’s own APIs

Question 6

What is latency?

Answer 6

Latency is the difference in response time between a local and remote operation invocation

Question 7

What are sources of latency?

Answer 7

• Network speed
• Middleware overhead
• Communication overhead due to objects in different address space

Question 8

What are the differences between local vs remote object access?

Answer 8

• Programmer must be aware of ultimate location of object on server
• Execution environment must provide a uniform mechanism for accessing objects that hides their location

Question 9

What is partial failure?

Answer 9

Failure occurs in a component or network while others continue to operate normally

Question 10

What are two alternatives for coping with failures?

Answer 10

• Treat all objects as if they were local (partial failures might be issue)
• Treat all objects as if they were remote (adds extra latency)

Question 11

What are approaches to treating all objects uniformly?

Answer 11

• Ignore the concurrency
• Make all objects single threaded
• Include concurrency semantics in all objects

Question 12

How to have effective development with DOT?

Answer 12

Object locations and concurrency should be dealt with early as possible

Question 13

What is the essence of developing distributed systems?

Answer 13

• Prioritize placement of objects and the communication and synchronization between objects.
• Adapt keep-it-simple approach, represent synchronization as a delay between client and server
• Use software model approximation techniques to solve software execution models

Question 14

Types of System Interactions in Middleware

Answer 14

• Synchronous
• Asynchronous
• Deferred Synchronous
• Asynchronous Callback communication