The Client-Server Paradigm 11/25/2018

Introduction The Client-Server paradigm is the most prevalent model for distributed computing protocols. It is the basis of all distributed computing paradigms at a higher level of abstraction. It is service-oriented, and employs a request-response protocol. 11/25/2018

The Client-Server Paradigm A server process, running on a server host, provides access to a service. A client process, running on a client host, accesses the service via the server process. The interaction of the process proceeds according to a protocol. 11/25/2018

Client-server applications and services An application based on the client-server paradigm is a client-server application. On the Internet, many services are Client-server applications. These services are often known by the protocol that the application implements. Well known Internet services include HTTP, FTP, DNS, finger, gopher, etc. User applications may also be built using the client-server paradigm. 11/25/2018

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Connectionless Server A connectionless server accepts one request at a time from any client, processes the request, and sends the response to the requestor. 11/25/2018

Connection-Oriented Client-Server applications A client-server application can be either connection-oriented or connectionless. In a connection-oriented client-server application: The server is passive: it listens and waits for connection requests from clients, and accepts one connection at a time. A client issues a connection request, and waits for its connection to be accepted. Once a server accepts a connection, it waits for a request from the client. When a client is connected to the server, it issues a request and waits for the response. When a server receives a request, it processes the request and sends a response, then wait for the next request, if any. The client receives the request and processes it. If there are further requests, the process repeats itself until the protocol is consumated. 11/25/2018

Connectionless Echo Server DatagramSocket ds = new DatagramSocket(port); while (true) { try { // create a new datagram packet byte[] buffer = new byte[MAXLEN]; DatagramPacket dp = new DatagramPacket(buffer, MAXLEN); ds.receive(dp); len = dp.getLength(); cAddr = dp.getAddress(); cPort = dp.getPort(); String s = new String(dp.getData(), 0, len); System.out.println(dp.getAddress() + " at port " + dp.getPort() + " says " + s); // create a datagram packet to send to client DatagramPacket theEcho = new DatagramPacket(buffer,len, cAddr, cPort); ds.send(theEcho); } // end try … } // end while 11/25/2018

The Basic Connection-Oriented Client-Server Model 11/25/2018

Concurrent, Connection-Oriented Server A connection-oriented server services one client at a time. If the duration of each client session is significant, then the latency or turnaround time of a client request becomes unacceptable if the number of concurrent client processes is large. To improve the latency, a server process spawns a child process or child thread to process the protocol for each client. Such a server is termed a concurrent server, compared to an iterative server. 11/25/2018

Concurrent, connection-oriented server - 2 A concurrent server uses its main thread to accept connections, and spawns a child thread to process the protocol for each client. Clients queue for connection, then are served concurrently. The concurrency reduces latency significantly. 11/25/2018

Connection-oriented server: latency analysis For a given server S, let Tc be the expected time that S takes to accept a connection, and Tp be the expected time S takes to process the protocol for a client. Further assume that the expected number of concurrent clients requiring the service of S is N. 11/25/2018

Connection-oriented Daytime Server … theServer = new ServerSocket(thePort); p = new PrintWriter(System.out); try { p.println("Echo Server now in business on port " + thePort ); p.flush(); theConnection = theServer.accept(); // read a line from the client theInputStream = new BufferedReader (new InputStreamReader (theConnection.getInputStream())); p = new PrintWriter(theConnection.getOutputStream()); while (!done){ theLine = theInputStream.readLine(); if (theLine == null) done = true; else{ p.println(theLine); } theConnection.close(); Connection acceptance Protocol processing 11/25/2018

Connection-oriented Concurrent DayTime Server ConcurrentDaytimeServer Connection-oriented Concurrent DayTime Server ConcurrentDaytimeServer.java DaytimeServerThread.java theServer = new ServerSocket(thePort); p = new PrintWriter(System.out); try { p.println("dayTime Server now in business on port " + thePort ); p.flush(); while (true) { theConnection = theServer.accept(); daytimeServerThread theThread = new daytimeServerThread(theConnection); theThread.start(); } public class daytimeServerThread extends Thread { Socket theConnection; public daytimeServerThread(Socket s) { theConnection = s; } public void run() { try { PrintWriter p; p = new PrintWriter(theConnection.getOutputStream()); p.println(new Date()); p.flush(); theConnection.close(); catch (IOException e) { System.err.println(e); } // end try } // end thread class 11/25/2018

Connection-oriented Echo Server public class echoServer { public static void main(String[] args) { ServerSocket theServer; int thePort; Socket theConnection; PrintWriter p; BufferedReader theInputStream; String theLine; boolean done = false; … theServer = new ServerSocket(thePort); p = new PrintWriter(System.out); try { theConnection = theServer.accept(); // read a line from the client theInputStream = new BufferedReader (new InputStreamReader (theConnection.getInputStream())); p = new PrintWriter(theConnection.getOutputStream()); while (!done){ theLine = theInputStream.readLine(); if (theLine == null) done = true; else { p.println(theLine); p.flush(); } // end if } //end while theConnection.close(); } // end try … 11/25/2018

Concurrent Echo Server See ConcurrentEchoServer.java See EchoServerThread.java 11/25/2018

Stateful server A stateful server maintain stateful information on each active client. Stateful information can reduce the data exchanged, and thereby the response time. 11/25/2018

Stateful vs. Stateless server Stateless server is straightforward to code. Stateful server is harder to code, but the state information maintained by the server can reduce the data exchanged, and allows enhancements to a basic service. Maintaining stateful information is difficult in the presence of failures. 11/25/2018

Stateful vs. stateless server In actual implementation, a server may be Stateless Stateful A hybrid, wherein the state data is distributed on both the server-side and the client-side. Which type of server is chosen is a design issue. 11/25/2018

A client can contact multiple servers A process may require the service of multiple servers. For example, it may obtain a timestamp from a daytime server, data from a database server, and a file from a file server. 11/25/2018

Middleware A process can serve as a intermediary, or middleware, between a client and a server. 11/25/2018