1Distributed systems programming Distributed systems programming is needed in many of today’s applications Web servers, web caches, client-server applications, network devices, … From low level to services Sockets (low-level API) TODAY! RPC, RMI (higher-level API discussed earlier) CORBA (also adds services, component model) LATER! EJB, J2EE, (Java version); .net (Microsoft) Jini (Java’s answer to networked services and devices)Sockets Low-level communication interface (API) Network Programming Defined 1981Bolt, Beranek and Newman (BBN) Originally developed for the C language TCP/IP version with Unix called BSD sockets Similar API was developed for System V a.k.a TLI To some extent similar to file IO Sockets What is a socket? It is a peer-to-peer communication “endpoint” abstraction Hides details of network for programming communication situations Interfaces to some common network protocols (e.g. TCP, UDP, XNS, NetBIOS, IP, etc.) Many high level APIs are based on sockets 1985 Sun introduced RPC and NFS (Network File System) over sockets.Socket types Three types: Stream (e.g. interface to TCP),  session based reliable service that takes care of connection maintenance, packet reassembly, etc. Mimics UNIX file system semantics Datagram (e.g. interface to UDP, NetBIOS),  handles independent packets.  Unreliable; 5% of datagrams don’t make it.  Needs higher level acknowledgement service. Raw (e.g. interface to IP, ICMP, etc.),  Programming interface to lower-level protocolsSocket Ports On TCP/IP networks: Socket = Internet Address(IP) + Port Address IP is a 32 bit number using dotted string notation, e.g., 206.10.47.09. Port is an “entry point”to an application that resides on a host. Commonly used and defined for server applicationsSocket Ports cont. Typically a communication is assigned to a port, a process can request/assign to access a port-number. TCP and UDP protocols (most common TCP/IP transport protocols) use port numbers 1-1023 that are reserved. Standard Internet apps (FTP, TELNET, TFTP, SMTP, SNMP,..) use ports between 1-255. > 1023 can be used for new user-level servers2Sockets To initiate a connection one must fix the roles: client or server A network connection can be connection-oriented or connectionless With a connectionless protocol there is nothing like an “Open” communication; every network IO operation can be with different process/host In a connection based operation a peer-to-peer channel is openedConnection-oriented use Connection is first established between client and server System Calls at server side socket() // creates endpoint, specify protocol (TCP, UDP, XNS, etc.), specify type (i.e. stream, datagram, etc.) Returns a small integer similar to file descriptor. bind() // register and bound network address to socket listen() // signal willing to receive connections accept() // accept a connection read(), write() // transfer dataConnection-oriented protocol Client side socket() // same meaning as in server connect() //establish contact between local-remote site read() write() //transfer dataExample, connection-oriented client-server sequencesocket()bind()listen()accept()read()write()socket()connect()write()read()SERVERCLIENTConnection establishmentData(request)Data (reply)BlocksUntilconnectSystem calls connectionless case sendto() // send datagrams (data packets) recvfrom() // receive datagrams Compared to the connection-oriented read()and write() these calls have arguments for remote protocol-specific addresses.Example 2, connectionless client-server sequence of callssocket()bind()recvfrom()sendto()socket()bind()sendto()recvfrom()SERVERCLIENTData(request)Data (reply)BlocksUntil datareceived from client3Differences connectionless vs. connection-oriented Connection establishing not required Blocks until data is received as opposed until connection is established Note: Client could also use connect() and read() write() in the connectionless case, but not “real connect” as it returns immediately, i.e. does not block as there is no connection established. Use of the sendto() and recvfrom() as opposed to the read() and write() system calls  The client registers an address for itself with the bind() to be able to receive messages back (this address is sent with data automatically)More on the differences Connectionless is often useful Lower overhead, no session creation required Discovery type of situations: broadcasting queries to the network and learning who is out there… Quick messages (e.g. heartbeat, “I am alive”) Can send to 500 nodes, would need 500 sessions with the connection-oriented model If one of the messages are not received no problem… Sockets and Java Before RMI and CORBA support has been designed for Java this was the only way to do network programming Java.net package contains Set of classes that let you download and manipulate URL objects A native Java implementation of Berkeley (BSD) socketsJava sockets Semantics the same as we discussed earlier Java.net contains most of the classes needed Java.io contains socket related stream classes A socket may be associated with a “stream” in Java (provided by the Socket class implementation) Streams are fun as they hide devices, its like a data pipe You can use OutputStream, InputStream, and other stream classes derived from these with sockets. Somewhat similar to what you have seen with file access.Summary Sockets Abstraction for network programming. Java provides a nice implementation. Many known high-level protocols use sockets as underlying communication mechanisms. Disadvantages:  lots of details to address, many parameters to control. Requires understanding of system issues and network protocols  Advantage:  Best performance in distributed applications, fine-tuning opportunities. CORBA Programming distributed systems and services Objective of this class is to give you an idea of CORBA capabilities and programming approach4What is CORBA? CORBA = Common Object Request Broker Architecture What is CORBA: A “middleware” or intermediate software layers that allow heterogeneous client and server applications to communicate in a distributed system. CORBA works with distributed systems written in : C,C++,COBOL, Java, or ADA...