1. 1 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)

2. Distributed Computing Class: BSIT-8 Instructor: Raihan Ur Rasool Chapter 04: Inter-process Communication

3. 3  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 ?

4. 4  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]

5. 5  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 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  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

6. Reading Assignment ???  CORBA’s Common Data Representation (CDR)  Java Object Serialization  Extensible Markup language (XML)  Next Time:  Recap  Client-server communication 6 

7. 7  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 Constructed types  Types that composed by several primitive types  A message example 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)

8. 8 CORBA CDR for constructed types

9. 9 CORBA CDR message The flattened form represents aPerson struct with value: {‘Smith’, ‘London’, 1934} 0–30–3 4–74–7 8–11 12–15 16–19 20-23 24–27 5 "Smit" "h___" 6 "Lond" "on__" 1934 index in sequence of bytes4 bytes notes on representation length of string ‘Smith’ length of string ‘London’ unsigned long Struct Person { string name; string place; long year; };

10. External data representation-The SUN XDR standard 10  The entire XDR message consists of a sequence of 4-byte objects:  example: ‘Hi’, ‘There’, 2001 2 “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

11. 11  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 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

12. 12 Indication of Java serialized form The true serialized form contains additional type markers; h0 and h1 are handles Serialized values Person 3 1934 8-byte version number int year 5 Smith java.lang.String name: 6 London h0 java.lang.String place: 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);

13. 13 Representation of a remote object reference Internet addressport numbertimeobject number interface of remote object 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

14. Objectives of this lecture 14  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).

15. 15  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 ?

16. 16  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

17. 17 The request-reply protocol Request ServerClient doOperation (wait) (continuation) Reply message getRequest execute method message select 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.

18. 18 Request-reply message structure messageType requestId objectReference methodId arguments int (0=Request, 1= Reply) int RemoteObjectRef int or Method array of bytes Internet addressport numbertimeobject number interface of remote object 32 bits information to be transmitted in a request or reply message Contains message identifier Representation of Remote Object Reference

19. 19 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

20. 20  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

21. 21  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

22. 22  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

23. HTTP - methods 23  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.

24. 24 HTTP request / reply messages GET//www.dcs.qmw.ac.uk/index.htmlHTTP/ 1.1 URL or pathnamemethodHTTP versionheadersmessage body HTTP/1.1200OK resource data HTTP versionstatus codereasonheadersmessage 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

25. Assignment  Install LAM-MPI on Linux  Home Assignment 2 [30/12/08] 25

26. Objectives of this lecture 26  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.

27. 27  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 ?

28. 28  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]

29. 29  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

30. 30  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

31. 31 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

32. 32 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();} }

33. 33  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

34. 34  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 ?

35. 35  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

36. Home Assignment 2 36 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

37. Reading Material Home Assignment 2: Learn MPI Programming 37 Discussion date: 1 st January, 2009 http://www.netlib.org/utk/papers/mpi-book/mpi-book.html

38. Extra Slides for Reference 38

39. 39  Datagram communication Datagram communication  Datagram Socket  Bind  Sendto  recvfrom  Stream communication Stream communication  stream socket, bind  Accept  Connect  Write and read UNIX socket

40. 40 Sockets used for datagrams ServerAddress and ClientAddress are socket addresses Sending a messageReceiving a message bind(s, ClientAddress) sendto(s, "message", ServerAddress) bind(s, ServerAddress) amount = recvfrom(s, buffer, from) s = socket(AF_INET, SOCK_DGRAM, 0)

41. 41 Sockets used for streams Requesting a connectionListening 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