Distributed Programming - Sockets

Transcription

1 Distributed Programming - Sockets Piet van Oostrum May 25, 2009

2 Concurrency In praktische situaties krijgen we concurrency door: Processen Threads Interrupts (alleen intern in het O.S.)

3 Processen Onafhankelijke executies van een programma Eigen adresruimte en andere resources Beschermd tegen andere processen (Processen komen uitgebreid aan de orde in opgave 2)

4 Starten van processen Op Unix: shell (script): progname param1 param2... Parallelle executie door starten in achtergrond met &. prog1 p1 p2 & prog2 p3 p4 & Op Windows: Door dubbelklikken op icon. Echter, resultaten (System.out) zijn vaak niet leesbaar. Daarom liever vanuit Command Prompt batch file aan te roepen: progname param1 param2... Starten in the achtergrond door start te gebruiken. start prog1 p1 p2 start prog2 p3 p4 Gebruik batch files is zelfs op korte duur al efficiënter bij testen.

5 Processen starten in Java Package java.lang.runtime Runtime r = Runtime.getRuntime(); Process p; p = r.exec("prog1 p1 p2"); Het Process object kan dan gebruikt worden om te wachten tot het process klaar is en/of informatie van/naar het process te verwerken.

6 Sockets Socket = mechanism that the Operating System offers to programs to make Internet connections. A process that wants to accept connections from other processes listens to a specific port. These are often servers E.g. a HTTP server listens on port 80. A process that wants to create a connection to another process creates a socket and connects the socket to the desired (IPaddress, port) combination This is often a client (e.g. Web browser)

7 Sockets again The server creates a socket on which it accepts connections from clients The server notices an incoming connection and accepts it (or could choose not to accept it). After accepting there is a new socket in the server. This is a new socket for each accepted connection. The client side also gets a port number from the operating system. Most of the time this is not interesting. The client and server can communicate over the socket in both directions.

8 Connection Server waits for connection Server s o ck e t connection request Client Server has accepted the connection Server socket s o ck e t data connection s o ck e t Client

9 Client-server structure With a client-server configuration the communication often has the following structure: Server listens on a specific port Client opens a connection to the server (that port) Client sends a request Server sends a response Request/response will continue a number of times Connection is closed

10 Sockets in Java Java has a number of classes for sockets (package java.net): Socket for normal (TCP) sockets DatagramSocket for datagrams (UDP) ServerSocket for the server-side (listening) This is a Java abstraction; for the OS it is just a socket but it will not be used for data

11 Sockets in Java Client side import java.net.*;... mysocket = new Socket(" 80); hostname, portnr "localhost" for the local machine or an InetAddress object A socket has an InputStream and an OutputStream. getinputstream() gives the InputStream getoutputstream() gives the OutputStream InputStream can be used to read bytes OutputStream can be used to write bytes

12 Different streams Reading and writing bytes is often not the best or easiest way to communicate. In some cases you must use it, e.g. when you have to read/write a GIF file In Java we can used more advanced Streams to use higher level communication: DataInputStream, DataOutputStream if you want to read/write binary data (ints, doubles, etc.) ObjectInputStream, ObjectOutputStream if you want to communicate with serialized objects. This is only useful if the other side is also written in Java. BufferedReader, PrintWriter if you want to communicate with text (like most Internet protocols).

13 Object Streams Output OutputStream ostream = mysocket.getoutputstream(); ObjectOutputStream os = new ObjectOutputStream(ostream); os.flush(); os.writeint(12345); os.writeobject("today"); os.writeobject(new Date()); os.flush(); ostream.close(); You must flush when the other side must do something, otherwise the data may be buffered in the sender. An ObjectOutputStream must also be flushed after opening, otherwise the other side may hang. Closing does an implicit flush

14 Object Streams Input InputStream istream = mysocket.getinputstream(); ObjectInputStream os = new ObjectInputStream(istream); int i = os.readint(); String today = (String)os.readObject(); Date date = (Date)os.readObject(); istream.close(); Of course you can also use objects from your own classes if they are serializable.

15 Text If we want to communicate with text then PrintWriter and BufferedReader should be used: PrintWriter out = new PrintWriter(mySocket.getOutputStream(), true); out.println("get /directory/file.html"); BufferedReader in = new BufferedReader(new InputStreamReader( mysocket.getinputstream())); instr = in.readline(); The second parameter of PrintWriter tells to flush when println is used.

16 Server side ServerSocket serversocket; try { serversocket = new ServerSocket(portnr); } catch (IOException e) {... } Socket clientsocket = null; try { clientsocket = serversocket.accept(); } catch (IOException e) {... } // Processing code here... clientsocket.close();

17 Multi-client server After the accept we usually want to: Communicate with the client Wait for new requests If the communication with the client is simple we can just have a loop with: accept and process Otherwise we must start a new Thread for the communication with the client

18 Example Socket clientsocket = null; while (notfinished) { try { clientsocket = serversocket.accept(); } catch (IOException e) {... } new ProcessingThread(clientSocket).start(); } serversocket.close(); The processing Threads will run parallel with the accept loop and with each other.

19 ProcessingThread public class ProcessingThread extends Thread { private Socket socket = null; public ProcessingThread(Socket socket) { this.socket = socket; } public void run() { try { PrintWriter out = new PrintWriter (socket.getoutputstream(), true); BufferedReader in = new BufferedReader( new InputStreamReader( socket.getinputstream()));... // Do the work... out.close(); in.close(); socket.close(); } catch (IOException e) {... } } }