System Models

1
System Models

• Bina Ramamurthy

2
Fundamental Issues

• There is no global time.
• All communications are by means of messages.
• Message communication may be affected by network
  delays and can suffer from a variety of failures
  and security attacks.
• How does one express a solution/process for
  handling an issue? One of the ways is to
  establish a model.

3
System Models

• Interaction model deals with performance and
  setting time limits in a distributed system, say,
  for message delivery.
• Failure model gives specification of faults and
  defines reliable communication and correct
  processes.
• Security model specifies possible threats and
  defines the concept of secure channels.
• Architectural model defines the way in which the
  components of the system interact with one
  another and the way in which they are mapped onto
  the underlying network of computers.

4
Architectural Model

• Abstracts the functions of the individual
  components.
• Defines patterns for distribution of data and
  workload.
• Defines patterns of communication among the
  components.
• Example Definition of server process, client
  process and peer process and protocols for
  communication among processes definition
  client/server model and its variations.

5
Software and hardware service layers in
distributed systems
6
Middleware

• Layer of software whose purpose is to mask the
  heterogeneity and to provide a convenient
  programming model for application programmers.
• Middleware supports such abstractions as remote
  method invocation, group communications, event
  notification, replication of shared data,
  real-time data streaming.
• Examples CORBA spec by OMG, Java RMI, MSs DCOM.

7
Clients invoke individual servers
EX 1. File server, 2. Web crawler
EX Web server
EX browser, web client
8
A service provided by multiple servers
EX akamai, altavista, Suns NIS (data
replication)
9
Web proxy server and caches
Proxy servers cache are used to provide
increased Availability and performance. They
also play a major role Firewall based security.
http//www.interhack.net/pubs/fwfaq/
10
A distributed application based on peer processes
Ex distributed Whiteboard Application EJB-base
d?
11
Web applets
EX Look at Object by value in CORBA
12
Java Object Serialization

• The capability to store and retrieve Java objects
  is essential to building persistence and
  streaming into application.
• We will discuss details about Serialization and
  saving and restoring objects in files/streams.

13
Object Serialization

• Key to storing and retrieving objects is
  representing the state of the objects in a
  serialized form sufficient to reconstruct the
  objects.
• Object ltgtbyte stream, other data to help in
  reconstruction
• Converting an object into an organized byte form
  for storage, streaming etc.

14
How to use Serialization?

• 1. import java.io.
• 2. Implement java.io.Serialization interface.
• 3. Use writeObject and readObject methods whose
  header are as given below
• void writeObject (Object obj) throws
  IOException
• Object readObject() thorws ClassNotFoundExcepti
  on, IOException

15
Writing to an Object Stream

• // serilaize various objects into a file
• FileOutputStream f new FileOutputStream(tmp)
• ObjectOutputStream s new ObjectOutputStream(f)
• s.writeObject(Today)
• s.writeObject(new Date())
• Theater t new Theater(4,6,10)
• //4 shows, 6 rows, 10 cols
• s.writeObject(t)

16
Reading from an Object Stream

• // Deserialize a objects from a file
• FileInputStream inf new FileInputStream(tmp)
• ObjectInputStream s1 new ObjectInputStream(inf)
  
• //read the Object and cast to retrieve the
• // actual object
• String (String)s1.readObject()
• Data date (Date)s1.readObject()

17
Exception Handling

• When a condition arises that the currently
  executing code cannot handle an exception occurs.
• Such conditions require special exception
  handling facilities.
• Traditionally these were handled by function
  return value.

18
Basics of Java Exception Handling

• Use try, throw, catch primitives.
• Enclose the code that you expect to result in an
  exception within a try block
• Attach a catch block that contains the code to
  handle the exception, following the try block.
  Use throw to transfer control to catch an
  exception (thrown).
• There can be many catch blocks following a single
  try to process the various types of exceptions.

19
Try blocks

• Syntax
• try
• normal statements
• statements that may result in exceptions
  //dynamic scope
• throw happens from here

20
Throwing an Exception

• throw is a keyword that can be used to announce
  that an exception has occurred.
• Throw normally specifies one operand.
• Thrown operand is an Object an object of
  Exception class is thrown.
• When an exception is thrown, control exits the
  try block and proceeds to the appropriate catch
  handler after the try block.

21
Catching an Exception

• Exception handlers are located within catch
  blocks.
• Each catch block starts with the keyword catch
  followed by parentheses containing a type of the
  exception this catch can handle.
• This is followed by the code for handling the
  exception enclosed within parentheses.

22
Exception handling examples

• For distributed computing Ex waiting for other
  host to respond.
• When dynamically allocating memory (large
  chunks).
• In large projects to handle error processing in a
  uniform manner project-wide.

23
Java Exception class

• java.lang.Exception class
• public class Exception extends Throwable
• public Exception() super()
• public Exception(String s)super(s)

24
User-defined Exception class

• For every project you implement you need to have
  a project dependent exception class so that
  objects of this type can be thrown.
• Java API also supports an extensive list of
  Exceptions.