Distributed SystemsConcepts and Design

Chapter 4

4.1. Introduction –p. 132-133

4.1. Introduction –p. 132-133

4.1. Introduction –p. 132-133

This Chapter Will Cover

Characteristics of interprocess communication UDP and TCP from Programmers point of view Objects and data structures translated Design of suitable protocols

4.2. The API for the Internet protocols –p. 133-144

Characteristics of interprocess communication

Connect Send Receive Disconnect

4.2. The API for the Internet protocols –p. 133-144

Characteristics of interprocess communication

Synchronous and Asynchronous blocking send blocking receive non-blocking receive synchronous asynchronous

Message Destination IP address & port Location transparency Send directly to processes Multicase to a group of process

Reliability

Ordering

4.2. The API for the Internet protocols –p. 133-144

Characteristics of interprocess communication

Sockets – Both UDP and TCP use the socket abstraction, with provides an endpoint for communication between processes.

4.2. The API for the Internet protocols –p. 133-144

UDP datagram communication

•Message size (up to 216 bytes)

•Blocking: non-blocking send, blocking receive

•Timeouts

•Receive from any

•Failure Model (Omission, Ordering)

•Use of UDP (DNS, Less overhead)

4.2. The API for the Internet protocols –p. 133-144

Java API for UDP datagramUDP client sends a message to the server and gets a reply

import java.net.*; import java.io.* public class UDPClient{ public static void main(String args[]){ //args give message contents and server hostname try{ DatagramSocket aSocket = new DatagramSocket(); byte[] m = args[0].getBytes(); InetAddress aHost = InetAddress.getByName(args[1]); int serverPort = 6789; DatagramPacket request =     new DatagramPacket(m,args[0].length(), aHost,serverPort); aSocket.send(request); byte[] buffer = new byte[1000]; DatagramPacket reply =     new DatagramPacket(buffer,buffer.length); aSocket.receive(reply); System.out.println("Reply:"+new String(reply.getData())); aSocket.close(); }catch(SocketException e)     {System.out.println("Socket:"+e.getMessage()); }catch(IOException e){System.out.println("IO:"+e.getMessage();} } }

4.2. The API for the Internet protocols –p. 133-144

Java API for UDP datagramUDP server repeatedly receives a request and sends it back to the client

import java.net.* import java.io.* public class UDPServer{

public static void main(String args[]){ try{ DatagramSocket aSocket = new DatagramSocket(6789); byte[] buffer = new byte[1000]; while(true){ DatagramPacket request = new datagramPacket(buffer, buffer.length); aSocket.receive(request); DatagramPacket reply = new DatagramPacket(request.getData(), request.getLength(), request.getAddress(), request.getPort()); aSocket.send(reply); } }catch(SocketException e) {System.out.println("Socket:" + e.getMessage()); }catch(IOException e) {System.out.println("IO:" + e.getMessage());} } }

4.2. The API for the Internet protocols –p. 133-144

TCP stream communication

Characteristics•Message size: Is Unlimited•Lost Messages •Flow Control•Message duplication and order•Message destination

Outstanding Issues•Matching of data items•Blocking•Treads •Failure model•Use of TCP: http, ftp, telnet, smtp

4.3. External data representation –p. 144-155

•Different ways to represent int, float char...

•byte ordering for integer

•standard external data representation

•send in sender's format and indicates what format, receivers translate if necessary

•External data representation

Three Approaches to External Data Representation•CORBA•Java’s object serialization•XML

4.3. External data representation –p. 144-155

CORBA CDR message

•Primitive types•Construction types•CORBA IDL complier generates marshalling and unmarshalling routines•Structure with string, unsigned long

4.3. External data representation and marshalling –p. 144-155

Java object serialization

•serialization and de-serialization•flattened to be transmitted or stored on the disk

4.3. External data representation and marshalling –p. 144-155

Extensible markup language (XML)

•User-defined tags

•Different Apps agree on different set of tags.

•e.g. SOAP for web serves, tags are published

•Tags are in plain text

Illustration of the use of a namespace in the person structure

4.4. Client-Server communication –p. 155ff

Client-server communication

•Synchronous (client waits for a reply)•Asynchronous (client doesn't wait)

Request/Reply Protocol p.157

Request/Reply Protocol pp.158-160

UDP – Failure Handling Timeout Discard of duplicates Lost replies – idempotent operations History

R, RR, RRA protocolsRequest - Request/Reply –

Request/Reply/Acknowledge Reply

Request/Reply Protocols pp. 160-163

TCP implementation of Request/Reply Protocols HTTP example – allows persistent connection HTTP methods – GET, HEAD, POST, PUT,

DELETE, OPTIONS, TRACE HTTP Message contentsRequest:Method/URL/HTTP Version/Header/msgReply:HTTP Version/status code/reason/header/msg

Group Communication –p. 164

Multicast Operation a single message sent from one process to all

members of a group High fault tolerance Can locate servers Better performance thru replication Propagation of event notifications

Group Communication p. 165

IP Multicast

Multicast Group Multicast Routers Multicast Address Allocation

Group Communications pp 166-168

IP Multicast – Failure Models Same as UDP – no guarantee of delivery Effects:

Replicated services – all or none msg receipt Discovery servers – repeat requests Replicated Data –broadcast of data, not methods Event Notifications – app determines qualities

Unix Inter-process Communication pp. 168-169 IPC in Unix

Layered on TCP and UDP protocol Socket System call – binding to an address Message destinations = socket address

Msg queues at sending socket Networking protocol transmits msg Msg queues at receiving socket Receiving process makes system call to receive

msg.

Unix Inter-process Communication pp 169-170 Datagram Communication (UDP)

Sockets identified in each communication Socket call Bind call Send to call Receive from call

Unix Inter-process Communication pp. 170-171 Stream Communication (TCP)

One server is ‘listening’ for requests Socket call for stream socket + bind + listen Accept call, create new socket Client process issues socket, connect Both use write/read Both close when communication is finished

Inter-process Communication

Summary UDP vs. TCP Marshalling data – CORBA, Java, XML Request/Reply Protocols Multicast Messages