Chapter 4 Interprocess Communication. Copyright 2015 Prof. Amr El-Kadi

Transcription

1 Chapter 4 Interprocess Communication

2 Outline Introduction The API for the Internet Protocols External Data Representation and Marshaling Client-Server Communication Case Study: interprocess communication in UNIX

3 Introduction Applications, services RMI and RPC This chapter request-reply protocol marshalling and external data representation Middleware layers UDP and TCP

4 The API for the Internet Protocols Synchronous and asynchronous communication. Message destinations. Reliability. Ordering.

5 Sockets socket any port agreed port socket client Internet address = message other ports server Internet address =

6 The same socket may be used both for sending and receiving of messages. Each socket is associated with exactly one protocol (i.e., UDP or TCP).

7 External Data Representation and Marshaling Flattening of data. What is marshaling and unmarshaling? CORBA s Common Data Representation (CDR) and Sun XDR, for use by a variety of programming languages. Java s Object Serialization, for use only by Java. Both marshal data in binary form (as opposed to the ASCII-based approach used by HTTP).

8 CORBA s CDR Typ e seq ue nce stri ng Re pr esenta tio n le ngth (unsi gned lo ng ) fo llow ed by el e me nts in order le ngth (unsi gned lo ng ) fo llow ed by ch arac ters in o rder (ca n al so ca n hav e w ide ch arac ters) a rra y array e le ments i n orde r (no l en g th s pe ci fie d b eca us e it is fixe d) stru ct in t he orde r o f de c laration o f the co mp onents enum erated unsig ne d long (the v alue s a re s pe c ifie d by t he orde r dec lare d) unio n ty pe tag f ollo we d b y the s el e cte d m emb er

9 Java Object Serialization In Java RMI, both objects and primitive data values may be exchanged. Java supports reflection and it uses it in serialization and deserialization.

10 Case Study: interprocess communication in UNIX Those are based on the socket abstraction. The socket system call creates a socket for a process. It returns a socket descriptor. The socket lasts until it is closed or until every process having its descriptor exists. The recipient process must bind its socket descriptor to a socket address (same is true for a sender if it expects replies). The bind system call is used for this purpose. Note that the address of a socket cannot change once its bound.

11 Datagram communication Sending a message Receiving a message s = socket(af_inet, SOCK_DGRAM, 0) s = socket(af_inet, SOCK_DGRAM, 0) bind(s, ClientAddress) sendto(s, "message", ServerAddress) bind(s, ServerAddress) amount = recvfrom(s, buffer, from) ServerAddress and ClientAddress are socket addresses

12 Stream communication There is a bound on the size of the recipient queue. The sender blocks if the queue is full. Normally, the server would listen and upon accepting a connection, it would fork a new process to communicate with the client leaving the parent listening to other clients. When a connection is accepted, UNIX automatically creates a new socket and pairs it with the client s socket so that the server would continue listening for other clients using the original socket.

13 The listen operation is used to listen and specifies the size of the queue. The client process uses connect system call to request a connection via the socket address and no prior binding is needed as it automatically binds the client s socket name to its socket.

14 Sockets used for streams Requesting a connection Listening and accepting a connection s = socket(af_inet, SOCK_STREAM,0) connect(s, ServerAddress) write(s, "message", length) s = socket(af_inet, SOCK_STREAM,0) bind(s, ServerAddress); listen(s,5); snew = accept(s, ClientAddress); n = read(snew, buffer, amount) ServerAddress and ClientAddress are socket addresses