1 tuesday, december 23, 2008 if one ox could not do the job they did not try to grow a bigger ox,...
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Tuesday, December 23, 2008
If one ox could not do the job they did not try to grow a bigger
ox, but used two oxen.
- Grace Murray Hopper(1906-1992)
Distributed ComputingClass: BSIT-8
Instructor: Raihan Ur Rasool
Chapter 04: Chapter 04:
Inter-process CommunicationInter-process Communication
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Introduction The API for the Internet protocols External data representation and
marshalling Client-Server communication Group communication Case study: interprocess communication in
UNIX Summary
Where are we ?
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Why does the communication data need external data representation and marshalling? [data structures must be flattened]
Different data format on different computers, e.g., big-endian/little-endian integer order, ASCII (Unix) / Unicode character coding
How to enable any two computers to exchange data values?1. The values be converted to an agreed external format
before transmission and converted to the local form on receipt
2. The values are transmitted in the sender’s format, together with an indication of the format used, and the receipt converts the value if necessary
External data representation and marshalling introduction [data structures and a sequence of bytes]
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Why does the communication data need external data representation and marshalling?
Different data format on different computers, e.g., big-endian/little-endian integer order, ASCII (Unix) / Unicode character coding
How to enable any two computers to exchange data values?1. The values be converted to an agreed external format before
transmission and converted to the local form on receipt2. The values are transmitted in the sender’s format, together with an
indication of the format used, and the receipt converts the value if necessary
External data representation An agreed standard for the representation of data structures
and primitive values Marshalling
The process of taking a collection of data items and assembling them into a form suitable for transmission in a message
Usage: for data transmission or storing in files Three alternative approaches to EDR
CORBA’s common data representation / Java’s object serialization & XML
External data representation and marshalling introduction
Reading Assignment ??? CORBA’s Common Data Representation (CDR) Java Object Serialization Extensible Markup language (XML)
Next Time: Recap Client-server communication
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Represent all of the data types that can be used as arguments and return values in remote invocations in CORBA
15 primitive types Short (16bit), long(32bit), unsigned short, unsigned long, float, char,
… Constructed types
Types that composed by several primitive types A message example The type of a data item is not given with the data
representation in message It is assumed that the sender and recipient have common
knowledge of the order and types of the data items in a message.
RMI and RPC are on the contrary
CORBA’s Common Data Representation (CDR)
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CORBA CDR for constructed types
Type Representation
sequence length (unsigned long) followed by elements in order
string length (unsigned long) followed by characters in order (can also
can have wide characters)
array array elements in order (no length specified because it is fixed)
struct in the order of declaration of the components
enumerated unsigned long (the values are specified by the order declared)
union type tag followed by the selected member
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CORBA CDR message
The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1934}
0–34–78–1112–1516–1920-2324–27
5"Smit""h___" 6"Lond""on__"1934
index in sequence of bytes 4 bytes
notes on representation
length of string
‘Smith’
length of string‘London’
unsigned long
Struct Person { string name; string place; long year;};
External data representation-The SUN XDR standard
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The entire XDR message consists of a sequence of 4-byte objects:
example: ‘Hi’, ‘There’, 2001
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“Hi--”5
“Ther”“e---”2001
sequence length‘Hi’
‘There’
Integer 2001
Must also define which end of each object is the most significant
The marshalled message of the example: 2 Hi 5 There 2001
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Serialization (deserialization) The activity of flattening an object or a connected set of
objects into a serial form that is suitable for storing on the disk or transmitting in a message
Include information about the class of each object Handles: references to other objects are serialized as
handles Each object is written once only
Example Make use of Java serialization
ObjectOutputStream.writeObject, ObjectInputStream.readObject
The use of reflection Reflection : The ability to enquire about the properties of a
class, such as the names and types of its instance variables and methods
Reflection makes it possible to do serialization (deserialization) in a completely generic manner
Java object serialization
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Indication of Java serialized form
The true serialized form contains additional type markers; h0 and h1 are handles
Serialized valuesPerson
31934
8-byte version number
int year
5 Smith
java.lang.Stringname:
6 London
h0
java.lang.Stringplace:h1
Explanation
class name, version number
number, type and name of instance variables
values of instance variables
Public class Person implements Serializable {private String name;private String place;private int year;public Person (String aName, String aPlace, int aYear){
name = aName;place = aPlace;year = aYear;
}// followed by methods for accessing the instance variables
}
Person p = new Person(“Smith”, “London”, 1934);
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Representation of a remote object reference
Internet address port number time object number interface of remote object
32 bits 32 bits 32 bits 32 bits
•An identifier for a remote object that is valid throughout a distributed system
•Must ensure uniqueness over space and time
•Existence of remote object references
•Life of the remote object references
•Remote object references may not be used for relocated objects
Objectives of this lecture
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To study the general characteristics of interprocess communication and the particular characteristics of both datagram and stream communication in the Internet.
To be able to write Java applications that use the Internet protocols and Java serialization.
The design issues for Request-Reply protocols and how collections of data objects may be represented in messages (RMI and language integration are left until Chapter 5).
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Introduction The API for the Internet protocols External data representation and
marshalling Client-Server communication Group communication Case study: interprocess communication in
UNIX Summary
Where are we ?
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The request-reply protocol Normally synchronous [client blocks until the reply arrives] May be reliable
Reply is ACK Asynchronous may be alternative
Client may retrieve the reply later. Mostly implemented over UDP but not TCP
Acknowledgements are redundant since requests are followed by replies
Establishing a connection involves two extra pairs of messages in addition to the pair required for a request and a reply
Flow control is redundant for the majority of invocations, which pass only small arguments and results
The request-reply protocol
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The request-reply protocol
Request
ServerClient
doOperation
(wait)
(continuation)
Replymessage
getRequest
executemethod
messageselect object
sendReply
public byte[] doOperation (RemoteObjectRef o, int methodId, byte[] arguments)sends a request message to the remote object and receives the reply. The arguments specify the remote object, the method to be invoked and the arguments of that method.
public byte[] getRequest ();acquires a client request via the server port.
public void sendReply (byte[] reply, InetAddress clientHost, int clientPort); sends the reply message reply to the client at its Internet address and port.
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Request-reply message structure
messageType
requestId
objectReference
methodId
arguments
int (0=Request, 1= Reply)
int
RemoteObjectRef
int or Method
array of bytes
Internet address port number time object number interface of remote object
32 bits 32 bits 32 bits 32 bits
information to be transmitted in a request or reply message
Contains message identifier
Representation of Remote Object Reference
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Request-reply message structure
Fields: 1. Message Type
Request or reply 2. A doOperation generates requestId for each message A server copies in the reply message 3 Remote object reference
marshalled 4. methodId
Message Identifier: requestId increasing sequence of U_integers (reset to zero) Identfier for the sender process, for example its port and internet
address
4,294,967,295
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Failure model Problems:
Omission failures. (and failure of processes—crash failure) No guarantee of delivery in order.
Timeout doOperation return exception when repeatedly issued
requests are all timeout Duplicate request messages:
filter out duplicates by requestID if the server has not yet sent the reply, transmit the reply
after finishing operation execution Lost Reply Messages
If the server has already sent the reply, execute the operation again to obtain the result. Note idempotent operation, e.g., add an element to a set, and a contrary example, append an item to a sequence
History History: server contains a record of reply messages that
have been transmitted to avoid re-execution of operations –memory cost
The request-reply protocol
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HTTP used by web browsers to make requests to web servers and to receive replies from them.
Client requests specify a URL that includes the DNS hostname of a web server, an optional port number and an identification of a resource.
HTTP specifies: Messages. Methods. Arguments. Results. Rules for representing data in messages.
Fixed set of methods, unlike other RMI Content negotiation and password style
authentication
HTTP: an example of a request – reply protocol
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If over TCP Each client-server interaction consists of the following
steps The client requests and the server accepts a connection at the
default server port or at a port specified in the URL The client sends a request message to the server The server sends a reply message to the client The connection is closed
Persistent connection (http1.1, RFC 2616) Connections that remain open over a series of request-reply
exchanges between client and server Closing
Marshalling Request and replies are marshalled into messages as ASCII
text string Resources are represented as byte sequences and may be
compressed MIME (Multipurpose Internet Mail Extensions) to send text,
images etc, in emails HTTP Methods
GET, HEAD, POST, PUT, DELETE, OPTIONS, TRACE
HTTP: an example of a request – reply protocol
HTTP - methods
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GET: Requests the resource whose URL is given as argument. HEAD: Similar to GET, but no data returned. POST: Can deal with data supplied with the request. PUT: data supplied in the request is stored with the given
URL as its identifier either as modification of the an existing resource or as a new resource.
DELETE: Server deletes resource identified by the URL. OPTIONS: Server supplies client with list of methods that it
allows. TRACE: Server sends the request back. Each client request specifies the name of a method to be
applied to a resource at the server and the URL of that resource.
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HTTP request / reply messages
GET //www.dcs.qmw.ac.uk/index.html HTTP/ 1.1
URL or pathnamemethod HTTP version headers message body
HTTP/1.1 200 OK resource data
HTTP version status code reason headers message body
Request/Reply: Header: for example acceptable content type The message body contains data associated with the
URL specified in the request
Contents of HTTP request message
Contents of HTTP reply message
/303
Objectives of this lecture
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To study the general characteristics of interprocess communication and the particular characteristics of both datagram and stream communication in the Internet.
To be able to write Java applications that use the Internet protocols and Java serialization.
To be aware of the design issues for Request-Reply protocols and how collections of data objects may be represented in messages (RMI and language integration are left until Chapter 5).
To be able to use the Java API to IP multicast and to consider the main options for reliability and ordering in group communication.
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Introduction The API for the Internet protocols External data representation and
marshalling Client-Server communication Group communication [multicast
operation] Case study: interprocess communication in
UNIX Summary
Where are we ?
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Fault tolerance based on replicated services Client request are multicast to all the members of the
group, each of which performs an identical operation Finding the discovery servers in spontaneous
networking Multicast message can be used by servers and clients to
locate available discovery services to register their interfaces or to look up the interfaces of other services
The characteristics of Multicast
Service Interface provides an entry point that consumers use to access the functionality exposed by the application [network addressable]
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Fault tolerance based on replicated services Client request are multicast to all the members of the
group, each of which performs an identical operation Finding the discovery servers in spontaneous
networking Multicast message can be used by servers and clients to
locate available discovery services to register their interfaces or to look up the interfaces of other services
Better performance through replicated data Data are replicated to increase the performance of a
service, e.g., Web Cache. Each time the data changes, the new value is multicast to the processes managing the replicas [bandwidth]
Propagation of event notification Multicast to a group may be used to notify processes
when something happens, e.g., the Jini system uses multicast to inform interested clients when new lookup services advertise their existence. [newsgroup]
The characteristics of Multicast
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A multicast group is specified by a class D Internet address Built on top of IP Sender is unaware of the identities of the individual recipients Available only via UDP
The membership of a group is dynamic It is possible to send datagram to a multicast group without being a
member and join or leave the group at any time. IPv4
Multicast routers [Internet] use the broadcast capability of the local network [Ethernet] MTTL (time to live) - specify the number of routers a multicast message
is allowed to pass. [to limit the distance of propagation] Multicast address allocation
Permanent group – exist even without members 224.0.0.1 to 224.0.0.255
Temporary group – the other addresses, set TTL to a small value Temporary groups, free multicast address, avoid accidents, X IP
multicast Failure model: due to UDP, [no guarantees, omission failure –
unreliable multicast] Java API to IP multicast
IP Multicast Protocol– group communication
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Multicast peer joins a group and sends and receives datagrams
import java.net.*;import java.io.*;public class MulticastPeer{
public static void main(String args[]){ // args give message contents & destination multicast group (e.g. "228.5.6.7")
MulticastSocket s =null; try {
InetAddress group = InetAddress.getByName(args[1]); s = new MulticastSocket(6789); s.joinGroup(group);
byte [] m = args[0].getBytes(); DatagramPacket messageOut =
new DatagramPacket(m, m.length, group, 6789); s.send(messageOut);
// this figure continued on the next slide
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Multicast peers example… continued
// get messages from others in group byte[] buffer = new byte[1000];
for(int i=0; i< 3; i++) { DatagramPacket messageIn =
new DatagramPacket(buffer, buffer.length); s.receive(messageIn); System.out.println("Received:" + new String(messageIn.getData())); }
s.leaveGroup(group); }catch (SocketException e){System.out.println("Socket: " + e.getMessage());
}catch (IOException e){System.out.println("IO: " + e.getMessage());}}finally {if(s != null) s.close();}
} }
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Failures Router failure prevent all recipients beyond it receiving
the message Members of a group receive the same array of messages
in different orders Some examples of the effects of reliability and ordering
Fault tolerance based on replicated services if one of them misses a request, it will become inconsistent
with the others Finding the discovery servers in spontaneous networking
an occasional lost request is not an issue when locating a discovery server
Replicated Data [lost messages, ordering]—replication technique
Reliable multicast or unreliable multicast? According to application’s requirement
Reliability and ordering of multicast
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Introduction The API for the Internet protocols External data representation and
marshalling Client-Server communication Group communication Case study: interprocess communication in
UNIX – FOR SELF STUDY Summary
Where are we ?
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Two alternative building blocks Datagram Socket: based on UDP, efficient but
suffer from failures Stream Socket: based on TCP, reliable but
expensive Marshalling
CORBA’s CDR and Java serialization Request-Reply protocol
Base on UDP or TCP Multicast
IP multicast is a simple multicast protocol
Summary
Home Assignment 2
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Q.1 From the book, answer the following questions
a) 4.1
b) 4.2
c) 4.5
d) 4.6
e) 4.7
f) 4.25
g) 4.27 Quiz 02
Reading Material
Home Assignment 2: Learn MPI Programming
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Discussion date: 1st January, 2009
http://www.netlib.org/utk/papers/mpi-book/mpi-book.html
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Datagram communication Datagram Socket Bind Sendto recvfrom
Stream communication stream socket , bind Accept Connect Write and read
UNIX socket
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Sockets used for datagrams
ServerAddress and ClientAddress are socket addresses
Sending a message Receiving a message
bind(s, ClientAddress)
sendto(s, "message", ServerAddress)
bind(s, ServerAddress)
amount = recvfrom(s, buffer, from)
s = socket(AF_INET, SOCK_DGRAM, 0)s = socket(AF_INET, SOCK_DGRAM, 0)
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Sockets used for streams
Requesting a connection Listening and accepting a connection
bind(s, ServerAddress);listen(s,5);
sNew = accept(s, ClientAddress);
n = read(sNew, buffer, amount)
s = socket(AF_INET, SOCK_STREAM,0)
connect(s, ServerAddress)
write(s, "message", length)
s = socket(AF_INET, SOCK_STREAM,0)
ServerAddress and ClientAddress are socket addresses