Distributed Systems 13 Java Sockets Simon Razniewski Faculty

Distributed Systems 13. Java Sockets Simon Razniewski Faculty of Computer Science Free University of Bozen-Bolzano A. Y. 2015/2016

Inter-Process Communication • Methods for making remote processes or threads exchange data • Typically associated to the transport layer • Two main modes – Message passing Datagrams, UDP – Bidirectional channel Byte stream, TCP

Socket • Endpoint of inter-process communication flow over a network • Communication is bidirectional – Provides “local” APIs to send/receive messages over a network • Send: typically asynchronous – The thread copies the message to a local buffer and then continues – The Transport Layer takes care of sending the message • Receive: typically synchronous (with timeout) – Asynchronous forms (polling/interrupt) are more difficult and can be anyway realized thanks to multithreading

Sockets in JAVA • Sockets in JAVA: java. net package • Connectionless socket UDP – “symmetric”: Datagram. Socket class • Connection-Oriented socket TCP – “asymmetric”: Socket class for client, Server. Socket for server – The connection is a full-duplex byte stream, but the two interacting processes are distinguished (client is proactive, server waits)

Internet Sockets and Identifiers • Network protocol: IP • Given a Transport protocol (UDP, TCP) – Each socket endpoint is identified by (IP address, port number) – Hence, “globally”, each socket endpoint is univocally identified by the triple (IP address, port number, protocol type) – Yes, it is possible to use at the same time an IP address + port number with TCP and UDP • Message communication: between two T-entities using a protocol (source IP addr. , source port no. , target IP addr. , target port no. , protocol)

Naming and Addressing • java. net. Inet. Address represents an IP address • Provides conversions to/from logical names (DNS facilities) • Principal (static) method: Inet. Address. get. By. Name(<name>) – Returns an Inet. Address starting from a string – <name>: IPv 4, IPv 6, logical name – Relies on the two subclasses Inet 4 Address and Inet 6 Address for managing IPv 4 and IPv 6 addresses – Throws Unknown. Host. Exception if no IP address matching <name> can be found • Inet. Address. get. All. By. Name(<logical_name>) returns a list of addresses associated to the same logical name

Loopback Address and Testing • To test applications locally, one can use the “loopback address”: address of a virtual network interface representing the local host • Traffic sent to the loopback address is simply moved back to the source without any change • Loopback address – IP: 127. 0. 0. 1 – Logical: localhost • Automatically used when we call Inet. Address. get. By. Name(null) or Inet. Address. get. Local. Host()

Datagram Socket • Supports connectionless interaction among (remote) threads • The thread creates a local socket used to send/receive messages: local address + port number • The pair (local IP address, port number) is a Inet. Socket. Address • Datagram. Socket() creates a socket bound to a wildcard socket address(“any” local IP address, “any” available port) • dsocket. bind(<socket address>) binds the socket to the specified socket address • dsocket. close() “close” the socket (cannot be used to send/receive messages anymore)

Datagram Socket • Typical constructor Datagram. Socket(int local. Port, Inet. Address local. Address) throws Socket. Exception – Creates a new datagram socket that binds to the specified (address, port) socket address – Throws Socket. Exception if • The IP Address cannot be assigned (not local) • Port is reserved (<1024) • Another socket is already bound to the socket address (address, port) – Throws Security. Exception if there is a security manager forbidding the use of (address, port)

Information Exchange • The unit of information to be sent/received is encapsulated into a Datagram. Packet • The datagram packet contains – Payload information: the content • byte[] buffer for the content • int length how long is the content • int offset starting point inside the buffer – Control information: info about receiver/sender • Socket. Address or Inet. Address + int (port)

Sending a Datagram. Packet • • New Datagram. Packet created Socket address is set to the destination endpoint Information serialized as a buffer of bits Message composed by extracting a portion of the buffer offset + length • Sequence of multiple sending by shifting the offset buffer split into datagrams • Sending: socket. send(<packet>)

Receiving a Datagram. Packet • New Datagram. Packet created • Buffer of bits specified to state where to put the incoming information, with offset and length buffer offset + length • Sequence of multiple receives by shifting the offset possible • Receiving: socket. receive(<packet>) • When the receive operation is completed, the buffer is filled with the content, and the socket. Address of <packet> contains the sender endpoint infos

Receiving a Datagram. Packet • The receive operation is blocking thread waits forever • Possibility of associating a timeout to the socket. set. So. Timeout(<millisecon ds>) • When the timeout expires, a java. net. Socket. Timeout. Exception is triggered

Datagram. Socket and Connection • socket. connect(<remote socket address>) • What does it mean to “connect” a datagram socket? It is connectionless! • Connection provides some useful facilities (syntactic sugar) – For sending no need of specifying the destination information into the datagram packet (it is taken from the connection) – For receiving selective receive: only packets coming from the address specified in the connection are accepted • socket. disconnect() to reset this behavior

Closing a Datagram. Socket • Sockets in JAVA do not only have an impact at the application level, but they also use resources • It is therefore recommended to close them when they are not useful anymore – Resources are freed • Programming pattern Datagram. Socket ds = null; try { <code, including the creation of ds> } finally { if (ds != null) ds. close(); }

Conversions from/to bytes String s = “hello”; byte [] m = s. get. Bytes(); String s = new String(m, “UTF 8”);

Conversion (2) public class Conversions { public static byte[] from. Int(int n) throws IOException { Byte. Array. Output. Stream array. Output. Stream = new Byte. Array. Output. Stream(); Data. Output. Stream data. Output. Stream = new Data. Output. Stream(array. Output. Stream); data. Output. Stream. write. Int(n); data. Output. Stream. flush(); return array. Output. Stream. to. Byte. Array(); } public static int to. Int(byte[] buf) throws IOException { Byte. Array. Input. Stream array. Input. Stream = new Byte. Array. Input. Stream(buf); Data. Input. Stream data. Input. Stream = new Data. Input. Stream(array. Input. Stream); return data. Input. Stream. read. Int(); } } http: //stackoverflow. com/questions/6374915/java-convert-int-to-byte-array-of-4 -bytes

TCP Sockets

Java Stream Sockets • Connection-oriented reliable byte streams • Two kinds of stream sockets – java. net. Socket used by Client and Server – java. net. Server. Socket used only by the Server to accept connections from the clients • Guiding principle: classes tend to hide the protocol-related information just passed to the constructor • Remember: each connection is denoted by protocol (TCP) + socket addresses of the endpoints

Client Socket • Needs – Socket address of the server – Port number of the client (if = 0 chosen by the system) • When created, atomically connects to the server – Throws IOException in case of problems or when the server cannot be contacted

Client Socket – Connection Creation • public Socket(Inet. Address remote. Host, int remote. Port) throws IOException – Creates client socket connecting it to the provided remote socket address • public Socket (String remote. Host, int remote. Port) throws IOException – Creates client socket connecting it to the remote socket address obtained after having resolved the logical name of the remote host • public Socket(Inet. Address remote. Host, int remote. Port, Inet. Address local. Host, int local. Port)throws IOException – Creates client socket connecting it to the two end-points – If local. Port = 0, it is automatically assigned by the system • Socket creation automatically handles also the connection to the server (CONNECT primitive) – Transparent three-way handshake protocol

Client Socket - Usage • Socket is bidirectional: provides two streams – One for receiving information (input stream) socket. get. Input. Stream(); – One for sending information (output stream) socket. get. Output. Stream(); • Streams are simply byte streams the abstract classes java. io. Input. Stream and java. io. Output. Stream are used

Data Streams • Support the marshaling/unmarshaling of JAVA primitive types • Can be connected to the byte streams associated to the socket – Realization of interaction protocols by exchanging typed information Data. Input. Stream in = new Data. Input. Stream(client. Socket. get. Input. Stream()); Data. Output. Stream out = new Data. Output. Stream(client. Socket. get. Output. Stream()); Data. Output. Stream String void write. UTF(String str) char void write. Char(int v) int void write. Int(int v) float void write. Float(float v) Data. Input. Stream String read. UTF() char read. Char() int read. Int() float read. Float() UTF = Unified Transformation Format Primitive data types and their length: http: //docs. oracle. com/javase/tutorial/java/nutsandbolts/datatypes. html

Writing & Reading • Other useful patterns are… • out = new Print. Writer (socket. get. Output. Stream(), true); – Print. Writer is a Java Writer managing the printing of primitive types – “true” indicates autoflushing, applied when one of the println() methods is invoked – Otherwise, data may be buffered • in = new Buffered. Reader( new Input. Stream. Reader(socket. get. Intput. Stream()); – Buffered. Reader supports reading text line per line

Server Socket – Connection Creation • Server socket is used by the server process to wait for connection requests from the clients • public Server. Socket(int local. Port) throws IOException; – Create a server socket binding it to specified local port – Bind. Exception if the local port is busy • public Server. Socket (int local. Port, int backlog, Inet. Address bind. Addr) – Explicit local IP address – Backlog is the size of the queue for incoming connection indications (connections requested but not yet accepted)

Server Socket – Waiting… • public Socket accept() throws IOException; • States that the server is willing to accept incoming connection • If the accept queue is not empty, a request is accepted, otherwise the server waits for new connection requests (blocking) – Timeout can be set • When the request is processed, a dedicated Socket is created • Such a socket represents the real server’s endpoint for the connection with the client

Sequential Server <server. Socket created> while(true) { Socket conn. Socket = server. Socket. accept(); <execute service using conn. Socket> socket. close(); }

Parallel Server <server. Socket created> while(true) { Socket conn. Socket = server. Socket. accept(); Service. Thread t = new Service. Thread(conn. Socket); t. start(); //or put start in the //constructor of Service. Thread } N. B. : conn. Socket closed by the service thread

Graceful Shutdown • socket. close() closes both the input and output stream of the socket • Possibility of closing only one side (typically the output one) • Typically, a process is responsible for the output direction – socket. shutdown. Output() disables the output stream for the socket – An IOException is thrown if the process tries to write – All data that were inserted before are still delivered to the other endpoint, then the TCP termination protocol is executed – It is still possible to receive data using the input stream of the socket