Ser321 Principles of Distributed Software Systems 3...

cnSockets.fm Ser321 Class Notes

e Systems

Principles of Distributed Software Systems © T. Lindquist 2015 August 2015

Ser321 Principles of Distributed Softwar

3. Programming with Sockets


ple Index

l servers/clients and ocols and how are ns?

he internet, naming,

olved with sockets.TCP/IP sockets to

mmunicate and the sed TCP/IP sockets.


3.a Motivation, References and Exam3.a.1 Motivation

• How do client-server applications such as ftp, telnet, maiWeb browsers and servers communicate? What are protthey used to govern communication for these applicatio

• Outcomes- To understand the basics of client-server computing; t

internet resources and socket connections.- To be able to use the Java API classes and methods inv- To be able to design and implement systems that use

realize distributed processing.- To understand how socket-based clients and servers co

importance of a communication protocol to stream-ba


ormation

ing

putingText:ssing, User ol


3.a.2 References, Readings and Sources of Inf• References

- The Java Tutorial, linked below, see Custom Network- http://java.sun.com/docs/books/tutorial/

• In Java Network Programming and Distributed Com- Chapters 1,3,5,6: Networking Theory, Internet Addre

Datagram Protocol, and Transmission Control Protoc• In Java Network Programming 4th Edition Text:

- Chapter 1: Basic Network Concepts

http://www.davidreilly.com/jnpbook/

http://shop.oreilly.com/product/0636920028420.do

http://java.sun.com/docs/books/tutorial/


kets (in java.net)ckets in Java


3.a.3 Index of Examples in this Section• Java simple: Section 3.c.3 Java Classes Related to Soc• Java threaded: Section 3.c.4 Example of Threaded So


f Sockets

onents operating on

er)

a communications rranged protocol.t) via a browser,

r. These objects can server.ed objects, objects RMI and EJB.


3.b Network Addressing and Types o3.b.1 Basics of Distributed Programming

• The task of developing a software system that has compmore than one computer.

• Multiple users (clients) sharing the same resource (serv• Many Paradigms for distributed systems

- Application software resides on client and server, datover the internet through streams according to a pre-a

- Client may download application specific code (applewhich embodies a majority of the client’s logic.

- Client may download application objects from a servereceive messages and have state as established on the

- Client may send messages to server objects. Distributcreated in frameworks such as DCOM, CORBA, Java


)*

some differentiate).ee:

chapter1

h?query#fragment


3.b.2 Network Addressing• Internet (Network) Address

- IP = 4 byte numeric address (example, 129.219.38.79- Domain Address = machineName { . domainName }- Example domain address: roo.eas.asu.edu

• Universal Resource Locator (URL) as opposed to URI (- URL specifies what and where: for a full definition s

http://www.w3.org/Addressing/- example,

http://quay.poly.asu.edu:8072/Ser321/index.html#- protocol, as http:// (http, ftp, mailto, file, orb)- internet address, as quay.poly.asu.edu- port number, as 8072- file, /Ser321/index.html- anchor #chapter1- The general form is: protocol://user@host:port/pat

http://www.w3.org/Addressing/

http://www.w3.org/Addressing/


ide or sent to the

r the system

a client and server. th directions.municate between


3.b.3 Network Addressing• Port - a place where things are brought in from the outs

outside- 16 bit integer- port numbers less than 1024 are typically reserved fo

• Socket - A plug. Sockets have two components- Internet address- port (roo.east.asu.edu:8080)

• Connection - establishing a linkage via a socket betweenConnections may last for several communications in bo

• Protocol - an agreement of what, when and how to comthe client and server, examples include:- http for web connections- smtp for transmitting mail messages- ftp file transfer protocol- telnet establishing remote logins to a unix system


process

m:

libraries java.net)


3.b.4 Sockets Context and History• First appeared in Berkeley Unix in 1981 (BSD 4.2 inter

communications - IPC)• Sockets are supported on virtually every operating syste

- Windows socket API is WinSock- BSD Unix systems have sockets as part of the kernel- MS-DOS, Windows, Mac-OS and OS/2 have socket - Java provides sockets as part of core classes (package


and received) in

rotocol

yer)ls


3.b.5 Three flavors of Sockets• Datagram sockets

- not session oriented - independent transmissions, - no sequencing assured or checksums provided,- no acknowledgement provided,- limit data transfer to 32K Bytes.- fastest form of communication

• Stream Sockets (also called TCP/IP)- Session (or connection) based service- guarantees that packets are sent without errors, sent (

sequence and without duplication,- unlimited data size in packets- TCP/IP = Transmission Control Protocol / Internet P

• Raw Sockets- interface to low level protocols (such as IP network la- no peer-to-peer services, but used to develop protoco


berct queueg its socket. In Java, on is established

establish a session

et object.nts are waiting, the ts. The main thread e socket.s for passing ut streams perform

allow resources to


3.b.6 Scenario for Stream Sockets• Sockets get created by both client and server,

- Server creates a ServerSocket specifying a port num- Server may also specify the length of the client conne- Client specifies the internet address and port in creatin

when the client creates its Socket instance, a connectiwith the SocketServer

• Server listens for arriving requests (accept method) to (connection) with a client.- Client requests a connection by creating a new Sock- If no connections are pending, the server blocks. If clie

server creates new threads (Stay tuned) to service clienre-issues the accept to service other clients on the sam

• The client and server identify input and output streaminformation according to a protocol. The input and outpthe work of the application.

• The client and server must both close the connection tobe used in another connection.


a

InputStream bytes) to the output ernal buffer, which stream buffering.

treamta types (int, ble (machine-

tStreame types to/from a le objects) are

that support the t streams.


3.c Socket Connections in Jav3.c.1 Sockets have Input and Output Streams

• Buffered Streamjava.io.BufferedOutputStream, and java.io.Buffered- Buffered writing and reading single bytes (or arrays of

stream or from the input stream. Data goes into an intcan be marked or reset. The implementation handles

• Data Streamjava.io.DataOutputStream, and java.io.DataOutputS- Lets an application write and read primitive Java da

boolean, double, char, ...) to/from a stream in a portaindependent) way.

• Object Streamjava.io.ObjectOutputStream, and java.io.ObjectInpu- Serializes and deserializes Java Objects and primitiv

stream. Object graphs (supports nesting of serializabplaced onto or obtained from a Stream. Only objects java.io.Serializable interface can be written to objec


ationd reading to/from

lls its accept ket object.e host and port in on, the constructor

eams from the

eam, and the client

n read the data from ta streams, or object match the other


3.c.2 Java Sockets Use Steams for Communic• TCP/IP Clients and servers communicate by writing an

streams.

• Java TCP/IP (stream-based) socketsjava.net.ServerSocket, and java.net.Socket- Server creates a java.net.ServerSocket object and ca

method. When a client connects, accept returns a Soc- Client creates a java.net.Socket object, specifying th

the constructor. When the server accepts the connecticompletes.

- Server and client can obtain both input and output strconnected Socket objects.

- The server has both an input stream and an output stralso has both an input stream and an output stream.

• When the server writes to its output stream, the client caits input stream, and vice versa. So, buffered streams, dastreams can be used in communication. But, they shouldside.


.net)leSock.jar domain addresses.

.poly.asu.edu.219.40.47

provides the ability

p://www.google.com

a connectionless ramSend.java

. Execute below rver.java


3.c.3 Java Classes Related to Sockets (in java• For examples corresponding to this slide see: javaSimp• InetAddress - class encapsulates relation between IP and

See: GetIP.java and GetNameForIP.java- Run as: java -cp classes ser321.sockets.GetIP quay- java -cp classes ser321.sockets.GetNameForIP 129

• URL - the URL class encapsulates web resources. URLto get the content of a URL. See: SimpleBrowser.java- Run as: java -cp classes ser321.sockets.SimpleBrowser htt

• Datagrams - Datagram packets are used to implement packet delivery service: DatagramReceive.java, Datag- java -cp classes ser321.sockets.DatagramReceive- java -cp classes ser321.sockets.DatagramSend

• Socket and ServerSocket - Support for Stream Socketsfrom different shells. See: SockClient.java and SockSe- java -cp classes ser321.sockets.SockServer- java -cp classes ser321.sockets.SockClient

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/javaSimpleSock.jar


ad the server. Here a client where bi-t Streams.

anguage ject) streams can be uage environments, to read from a Java . But, at the level of ty between


3.c.4 Example of Threaded Sockets in Java• The first step toward handling multiple clients is to thre

is an example of a threaded socket server together with directional communication is accomplished using Objec- See: javaThreadSock.jar

• Sockets are platform independent and to a large extent lindependent. We can’t expect that language-specific (Obused to communicate over sockets to/from different langsuch as Java to/from C++. For example, we can’t expectObjectInputStream, something that was written in C++communicating with Byte arrays, there is interoperabililanguages.- See: simpleInterop.jar

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/javaThreadSock.jar

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/simpleInterop.jar


ET

and received) in

gh streamseam.r serialized objects.ockets.UDPClient


3.d Socket Connections in C# .N3.d.1 Stream Sockets in C# .NET

• Stream Sockets (TCP)- Session (or connection) based service- guarantees that packets are sent without errors, sent (

sequence and without duplication,- unlimited data size in packets

• Communication between server and client occurs throu- The base class for this communication is NetworkStr- Streams can be used for binary, text, and string data o

• Datagram sockets are supported by class System.Net.S


ueue, etc)g its socket. In C#,

nection is

cceptTcpClient t. or more clients are

s a new thread (stay ues the accept to

s for passing he input and output

allow resources to


3.d.2 Scenario for Stream Sockets• Sockets are created by both client and server,

- Server specifies a port number- Server may customize aspects of connections (wait q- Client specifies the internet address and port in creatin

when the client creates its TCPClient instance, a conestablished with the TCPListener

• Server listens for arriving requests (AcceptSocket or Amethod) to establish a session (connection) with a clien- If no connections are pending, the server blocks. If one

waiting, they are queued and in turn, the server createtuned) to service each client. The parent thread re-issservice another client on the same socket.

• The client and server identify input and output streaminformation according to a protocol they both agree to. Tstreams perform the work of the application.

• The client and server must both close the connection tobe used in another connection.


er

");

ver.exe portNum

ing.ASCII);

ocketClient.exeebRequest and

a web page.hen prompted.


3.d.3 C# .NET Sample Socket Client and Serv• See the example: csSockets.jar In the server:

TcpListener tcpl = new TcpListener(9090);tcpl.Start();Console.Write("TCPSocketServer up and waiting for connections on 9090TcpClient sock = tcpl.AcceptTcpClient();...tcpl.Stop();sock.Close();

- Server (TCPSocketServer.cs): ./bin/TCPSocketSer

• In the client:TcpClient tcpc = new TcpClient("localhost", 9090);Stream tcpStream = tcpc.GetStream();StreamReader reader = new StreamReader(tcpStream, System.Text.Encod...reader.Close();tcpc.Close();

- Run the client (TCPSocketClient.cs) as: ./bin/TCPS• Among other supporting classes, .NET includes HttpW

WebResponse classes to support http requests.- The example: WebRequest.cs downloads and prints - Run as: ./bin/WebRequest.exe and provide a URL w

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/csSockets.jar


d Servern C#.

d build the project


3.d.4 .NET Sample Threaded Socket Client an• Here is an example of a threaded socket server written i

- See: csThreadSock.jar- Extract the jar file, change to the project directory an

with: ant build

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/csThreadSock.jar


nge the same object.

hronization.is necessary.nges the object?

state, for example:

ack pointer.tep 2, then another = stack.top()).e

(item)(item) completes


3.d.5 Sharing Objects: Synchronization• What is a Shared Object?

- Multiple threads of single program may access or cha• When should access to shared objects be controlled?

- When multiple threads only read the object - no sync- If any thread modifies the object - synchronization

• Why must access be controlled when some thread cha- consider: stack.push(item), for example- multiple actions are necessary to achieve consistent- (1) increment stack pointer;- (2) place the new item into the collection using the st- If one thread is interrupted after step 1, but before s

thread may see an inconsistent stack state (visualize a• Monitor - a lock used to protect a critical section of cod

- Each thread calls Enter() to get the lock before push- Each thread calls Exit() to release the lock after push


Span) lock the r’s before first Exit)

to unlock the object

struct:

d Exit methods

ush or pop


3.d.6 C# .NET Monitors• A Monitor protects access to the Object

- See System.Threading.Monitor class

- All Monitor methods are static

• Basic Monitor methods to lock and unlock an object

- void Enter(object) and bool TryEnter(object, Timeobject. A thread may be recursive (call multiple Ente

- void Exit(object) indicate the calling thread is ready

- Alternatively, a Thread may use the C# language conlock (object){ ... }

• Recall that Java uses monitors without explicit Enter an

- public synchronized void push(String item)

- The stack object’s monitor (lock) is acquired before p

- See example: csmonitor.jar

http://pooh.poly.asu.edu/Ser321/Examples/Sockets/csmonitor.jar


ck an object based

tack isn’t full.ck has storage.

ass


3.d.7 Conditional locking• Some shared objects need the ability to conditionally lo

on the state of the shared structure.• Consider a Stack that is implemented with finite storage

- Want to accept (and lock) for a push only when the s- Want to accept (and lock) for a pop only when the sta

• C# handles this with Wait(), Pulse() and PulseAll()- Static methods in the System.Threading.Monitor cl


after an Enter)

d the thread blocks a Pulse (or time es.

Pulse(this); }

Item; Pulse(this); }


3.e These are only used only within a lock (or• Wait, Pulse and PulseAll

- When a thread executes a Wait, the lock is released an(optionally, for up to a timespan). When awakened byexpiration), the thread reacquires the lock and continu

• In Push:- lock { if (stack_is_full) {Wait(this);} addTheItem;

• Correspondingly in Pop- lock { if (stack_is_empty) {Wait(this);} removeThe

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