Interprocess CommunicationPC

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Interprocess CommunicationPC

• DCS

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Interprocess Communication (IPC)

• Communication and synchronization between
  co-operating processes.
• Building Blocks
• Client-Server Communication
• Client-server communication using sockets in Java
• Group Communication
• Sockets in Java

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Building blocks

• Data representation and messages
• External data representation
• Mapping data structures and data items to
  messages
• Marshaling and unmarshalling
• Flattening data structures before transmission
  and rebuilding them on arrival
• Send and Receive operations
• Synchronous and asynchronous communication
• Message destinations

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External data representation

• Two Methods
• Convert data to an agreed (standard) external
  format and convert to local form on receipt.
• Transmit data values in their native form
  together with an architecture identifier- so the
  recipients convert the data if necessary.

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External data representation-The SUN XDR standard

• The entire XDR message consists of a sequence of
  4-byte objects
• example Hi, There, 2001

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sequence length Hi There Integer 2001
Hi--
Must also define which end of each object is the
most significant
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Ther
e---
2001
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Marshaling

• Take a collection of data items and assemble them
  into a suitable form for transmission.
• Marshaling flattens structured data items into a
  sequence of data items and translates them to an
  external data representation
• Unmarshaling translates from external data
  representation to local one and unflattens the
  data items

The marshalled message of last example 2 Hi 5
There 2001
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Send and Receive Operations

• Message passing is supported by send and receive
  operations

B
A
Receive (Port-id, message)
Send (Port-id, message)
Process A blocks until the message has been sent
Process B blocks until message arrives
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Blocking Send or Receive

• The sending process blocks while the message is
  being sent. Similarly a receive primitive blocks
  the caller until the message has been received.

Client Blocked
Client Running
Client Running
Return from Kernel, Process Released
Trap to kernel process blocked
Message being sent
TIME
A blocking send
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Non-blocking Send or Receive

• With non-blocking send (or receive) the control
  is returned to the caller immediately after the
  message is sent (or received)
• i.e continue computing in parallel with message
  transmission

Client Running
Client Blocked
Client Running
Return
Trap
message copied to kernel buffer
Message being sent
TIME
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A non-blocking send
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Timeouts in blocking send and receive

• A blocking send or receive could wait forever!
• A timeout can be associated with a port so that
  the caller can terminate with an error status.

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Synchronous and Asynchronous communication

• Synchronous
• The sending process and the receiving process
  both synchronize at every message.
• e.g. occam
• Asynchronous
• Send operation is non-blocking but the receive
  operation can be blocking
• e.g. BSD 4.x UNIX, Mach, Chorus

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Buffered primitives

• A queue (like a mail box) is associated with each
  message destination.
• Sending processes add messages to the queue.
• Receiving processes remove messages from the
  queue.

Queue(Mail Box)
Send
Receive
(block) if Queue is empty!
block if Queue is full!
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Message destinations

• Depending on operating systems and distributed
  systems environment

Message Destinations OS Distributed
Location
system environment independent
Process V yes Ports Mach, Chorus
Amoeba Yes Sockets BSD 4.x UNIX No Group of
processes Chorous Yes Objects Clouds,
Emerald Yes
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Client-Server Communication

• The OSI model generates a substantial overhead in
  LAN-based distributed systems.

N1
N1
N1 Data
N1 Data
N
N N1 Data
N
N N1 Data
N-1
N-1 N N1 Data
N-1
N-1 N N1 Data
Communication Path
Communication Path
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Transmitting Site (Adds Headers)
Receiving Site (Strips Headers)
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Something more is needed....

• Use the client-server model which is based on
  request-reply protocol.
• The client-server model structures the operating
  system as a group of co-operating processes
  called servers.
• The servers offer services to clients.
• The client-server machines run all on the same
  microkernel as user processes.

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The request-reply protocol

• The client sends a message to the server asking
  for some service.
• The server does the work and returns the
  data/error code back to client.

Server
DoOperation .......... (Wait) .......... (Continu
ation)
GetRequest Execute request Send Reply
Client
Request Message
Reply Message
Kernel
Kernel
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Advantages of request-reply protocol

• No routing is required
• Simplicity and efficiency
• layer 5 defines a set of legal requests and reply
  primitives. No routing or session management.
• The stack is shorter and works efficiently

7 6 5 4 3 2 1
Request/reply

• DoOperation()
• GetRequest()
• SendReply()

Data Link Physical
request-reply primitives
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Request-reply primitive DoOperation

• Client uses it to invoke remote operation

PROCEDURE DoOperation (serverportId,
requestMessage, replyMessage )
send request to

• Implementation uses
• A Send operation
• followed by a Receive operation

and receive reply
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Request-reply primitives GetRequest and SendReply

• Server uses GetRequest to acquire service
  requests and
• uses SendReply to send the reply to the client.
• When the reply message is received by the client
  the original DoOperation is unblocked.

PROCEDURE GetRequest (serverportId,
requestMessage) PROCEDURE SendReply
(serverportId, replyMessage )
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Request-Reply message structure

• The information to be transmitted in a request or
  reply message looks like

Message Type (Request, Reply)
Generated by DoOperation for each request. The
server copies it into the corresponding reply
message
RequestId ProcedureId (a server
procedure) Arguments (flattened list)
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Group Communication

• Communication between one process and a group of
  processes is not best done in a pair-wise
  exchange
• Instead a multicast operation is more appropriate
  where group membership is transparent to the
  sender
• A very useful infrastructure for distributed
  systems offering
• Fault tolerance based on replicated objects
• Easy to locate Discovery Services for spontaneous
  networking (Mobile other devices)
• Propagation of event notification
• Example Jini system (see next slide) uses
  multicast to inform interested clients when a new
  lookup service advertises its existence

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Jini System

• A service-oriented architecture (SOA)
• SOA takes the existing software components on the
  network and lets them to be published, invoked
  and discovered by each other.
• Web services will be regarded as an SOA
  implementations

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Jini Architecture

• Service provider
• Service registry
• Service requestor

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Socket class in Java

• A socket is a network access point that can be
  used by an application to send or receive data
• TCP connections are created using the Socket
  class.
• When you create the client socket you must supply
  a remote address and port number

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The Socket class in Java

• The socket on creation in Java connects to a
  remote machine on a given port to a given host.
• The local address and port numbers are assigned
  automatically
• Once a connection has been created we can
  transfer streams between remote applications.

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More on Sockets

• Each socket connection can be defined by a
  protocol, port number and a host address
• So
• TCP,80,128.1.20.10
• is different to
• UDP,80,128,1,20,10

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TCP Client Sockets in Java

• Can be used to send a data stream of bytes over
  the network
• snew Socket(stmail.staffs.ac.uk,110)
• This creates a socket connecting to the server
  stmail.staffs.ac.uk
• On port 110 (POP3, Post Office Protocol)
• This socket could be then used to download from
  the mail server stmail.staffs.ac.uk

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Receiving data from a socket

• To receive data from a socket, you can receive it
  by creating a BufferedReader from the sockets
  input stream
• input new BufferedReader
• (new inputStreamReader(my_socket.getInputStream(
  )))
• To read a string from the socket
• my_stringinput.readline()

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Writing Data to a Socket

• Easiest way to send text data to a socket is to
  send it a PrintWriter create the PrintWriter
  like this
• outnew PrintWriter(smtp_socket.getOutputStream(),
  true)
• Now send data to the socket like this
• out.println(USER mail01)

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Send and Receive

• To send/receive objects create your IO stream as
• ObjectOutputStream
• ObjectInputStream
• To send/receive arrays of bytes create your IO
  stream as
• DataOutputStream
• DataInputStream
• Closing (important conserves resources !!)
• my_sock.close()

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Example Getting Web Pages with a client
socket(source code handout Grab-page)

• Using a socket to download pages from the Web

Web Server
host
GrabPage
Get /file.html HTTP/1.0
HTTP
http//host/file.html
Internet
lthtmlgtltheadgtlttitlegt....
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Exploring the code

• The program sits in a loop waiting for the user
  to type in URLs
• For each URL it creates a new GrabPage object.
• This object attempts to connect to the Web
  server, and download the specified page
• If successful, it displays the page contents on
  the screen

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Exploring the code .... The class GrabPage

• downloads and displays the Web-page
• It first parses the URL (dissect())
• then connects to the server(connect())
• Issues an HTTP request and displays the response
  (grab())