university of pittsburgh computer science 1 week 5: introduction last week we discussedlast week we...

University of Pittsburgh Computer Science

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Week 5: IntroductionWeek 5: Introduction

• Last week we discussedLast week we discussed Difference between executing sequentially Difference between executing sequentially

and in paralleland in parallel

How multi-tasking allows computers to appear How multi-tasking allows computers to appear to do things in parallel, even with only one to do things in parallel, even with only one CPUCPU

How multi-tasking is done with Threads in JavaHow multi-tasking is done with Threads in Java Thread objects execute run() subprogramThread objects execute run() subprogram When run() finishes, thread is deadWhen run() finishes, thread is dead Moving.java and Moving1.java with one threadMoving.java and Moving1.java with one thread

Using more than one threadUsing more than one thread Independent threads – no data sharingIndependent threads – no data sharing


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Week 5: IntroductionWeek 5: Introduction

– Bounce.java demonstrationBounce.java demonstration

Threads share dataThreads share data– Data consistency issuesData consistency issues– Using monitors to restrict access to critical dataUsing monitors to restrict access to critical data

Other multi-thread issuesOther multi-thread issues– Deadlock – no thread can execute due to circular Deadlock – no thread can execute due to circular

dependency of resourcesdependency of resources– Starvation – some thread never gets to executeStarvation – some thread never gets to execute


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Week 5: Communicating with Other ComputersWeek 5: Communicating with Other Computers

Send and receive emailSend and receive email

• Computers need to “talk” to each Computers need to “talk” to each other for various reasonsother for various reasons

Upload and download filesUpload and download files

Allow multiple users to access information Allow multiple users to access information at the same timeat the same time

Ex. GamesEx. Games

• How can we do this?How can we do this?


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Week 5: Servers and ClientsWeek 5: Servers and Clients

• Typically, communicating Typically, communicating computers take on one of two roles:computers take on one of two roles:

Client: Client: computer makes requests to computer makes requests to server and utilizes the resultsserver and utilizes the results

Usually has an interface with userUsually has an interface with user

Server: Server: computer takes requests from computer takes requests from other computers and supplies them with other computers and supplies them with files, data, etcfiles, data, etc

Often does not deal directly with end Often does not deal directly with end usersusers


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Week 5: Types of CommunicationWeek 5: Types of Communication

• Computers can communicate in Computers can communicate in different waysdifferent ways Connection-oriented communicationConnection-oriented communication

Computers connect to each other and transmit Computers connect to each other and transmit data back and forthdata back and forth

Computers remain connected even if no data is Computers remain connected even if no data is sentsent

Similar in idea to a phone lineSimilar in idea to a phone line Connectionless communicationConnectionless communication Computers send Computers send data packetsdata packets to each other to each other No continuous connection between themNo continuous connection between them Similar in idea to U.S. MailSimilar in idea to U.S. Mail


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•Connection-oriented: all data follows the same path between the client and the server

•Connectionless: each data packet may follow a different route between client and server – no hard “connection” exists

CLIENT

SERVER

NETWORK


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• Advantages of connection-oriented:Advantages of connection-oriented: Data flow is consistent and reliableData flow is consistent and reliable

Order sent is always order receivedOrder sent is always order received

After set-up is complete, can be very fastAfter set-up is complete, can be very fast

• Disadvantages of connection-Disadvantages of connection-oriented:oriented: Each link in the connection is a vulnerable Each link in the connection is a vulnerable

pointpoint If a link goes down, connection must be If a link goes down, connection must be

reestablishedreestablished

Initial set-up can be time-consumingInitial set-up can be time-consuming For small amounts of data, overhead incurred For small amounts of data, overhead incurred

is too highis too high


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• Advantages of connectionless:Advantages of connectionless: No initial setup timeNo initial setup time

Short messages can be sent very quicklyShort messages can be sent very quickly

If a network link goes down, messages If a network link goes down, messages can easily be reroutedcan easily be rerouted

• Disadvantages of connectionless:Disadvantages of connectionless: Each packet must be routed individuallyEach packet must be routed individually

For long messages, this is time-consumingFor long messages, this is time-consuming

No guarantee that messages will arrive in No guarantee that messages will arrive in order, or even at allorder, or even at all Up to server-client to handle thisUp to server-client to handle this


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Week 5: Java SocketsWeek 5: Java Sockets

• We’ll look at connection-oriented We’ll look at connection-oriented communication in Javacommunication in Java

Utilizes Utilizes SOCKETSSOCKETS Think of these as a two-way connection Think of these as a two-way connection

between the client and the serverbetween the client and the server

Server starts up a Server starts up a ServerSocket This waits to accept a connection from a clientThis waits to accept a connection from a client

Client creates a new Client creates a new Socket by by connecting to Serverconnecting to Server Server creates a Socket on its end Server creates a Socket on its end

simultaneouslysimultaneously


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Socket

CLIENT

SERVER

Client Output

Server

Input

Server Output

Client Input


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• Let’s look at a simple exampleLet’s look at a simple example Server will wait for connection from clientsServer will wait for connection from clients

At each connection, client will send a At each connection, client will send a single message to the server, which the single message to the server, which the server will printserver will print

Server sends no data back to the clientServer sends no data back to the client Download SendToMe.java and ISend.java from Download SendToMe.java and ISend.java from

the Web sitethe Web site Start up two command promptsStart up two command prompts Compile and run SendToMe.java in the firstCompile and run SendToMe.java in the first Wait for instructions about ISend.javaWait for instructions about ISend.java


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• Computer addressesComputer addresses Computers connected directly to the internet Computers connected directly to the internet

have IP (Internet Protocol) Addresseshave IP (Internet Protocol) Addresses Technically a sequence of numbersTechnically a sequence of numbers

– Ex: 123.45.67Ex: 123.45.67

Usually accessed by nameUsually accessed by name– Ex: unixs1.cis.pitt.edu, nomad.cs.pitt.eduEx: unixs1.cis.pitt.edu, nomad.cs.pitt.edu

When connecting to a server, you must When connecting to a server, you must supply the supply the IP AddressIP Address and the and the port port numbernumber A single machine may be running many server A single machine may be running many server

programs, each on a different portprograms, each on a different port


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• Each computer in this lab has an IP addressEach computer in this lab has an IP address Names are written on the computer box and in the Names are written on the computer box and in the

CS department are CS department are hostname.cs.pitt.eduhostname.cs.pitt.edu

Look around and find out the addresses of some Look around and find out the addresses of some computers in the lab – just the first part up to the computers in the lab – just the first part up to the periodperiod

To send a message to someone else in the lab, type To send a message to someone else in the lab, type the following at the command prompt:the following at the command prompt:

java ISend hostname yourname “your message”java ISend hostname yourname “your message”

Try it a few times with various computersTry it a few times with various computers Try to guess who is sending you a messageTry to guess who is sending you a message But be nice!But be nice!


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Week 5: Java Sockets Week 5: Java Sockets

• Let’s look at the codeLet’s look at the code SendToMe.javaSendToMe.java

ServerSocket created, then goes into a loopServerSocket created, then goes into a loop Socket connection acceptedSocket connection accepted Reads a single line from client and prints it to Reads a single line from client and prints it to

consoleconsole

ISend.javaISend.java Gets host name, user name and message from Gets host name, user name and message from

command linecommand line Connects to correct host with a new socketConnects to correct host with a new socket Writes message to serverWrites message to server Closes connectionCloses connection


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Week 5: Two-Way Communication Week 5: Two-Way Communication

• In the first exampleIn the first example Client sends, server receivesClient sends, server receives

Server has simple loop, client just a few linesServer has simple loop, client just a few lines

• What if both client and server want What if both client and server want to send and receive data?to send and receive data? If sending and receiving are synchronized, If sending and receiving are synchronized,

we do not have to complicate things we do not have to complicate things muchmuch

What if server and client want to be able What if server and client want to be able to send to each other at any time?to send to each other at any time? We need to use ThreadsWe need to use Threads


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Week 5: Two-Way CommunicationWeek 5: Two-Way Communication

• Both sides of the communication must Both sides of the communication must be ready to send or receive a message be ready to send or receive a message at any timeat any time Without Threads, they could end up Without Threads, they could end up

deadlockeddeadlocked

CLIENT

SERVER

Client trying to send message to server

Server trying to send message to client


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• We need a way for each side to accept We need a way for each side to accept and receive messages at any timeand receive messages at any time One Thread for readingOne Thread for reading

One Thread for writingOne Thread for writing

SERVER

Client Thread waiting to accept messages Server Thread

sending messages

Server Thread waiting to accept messages

Client Thread sending messages

CLIENT


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• Look at next exampleLook at next example TwoWayServer.javaTwoWayServer.java

TwoWayClient.javaTwoWayClient.java

Compile TwoWayServer.javaCompile TwoWayServer.java Run with your name on command lineRun with your name on command line

– java TwoWayServer yournamejava TwoWayServer yourname– Now your computer will be a server that some Now your computer will be a server that some

other student can connect to in order to talkother student can connect to in order to talk

Compile TwoWayClient.javaCompile TwoWayClient.java Run with server name and your nameRun with server name and your name

– java TwoWayClient servername yournamejava TwoWayClient servername yourname– Where servername is the computer of another Where servername is the computer of another

student that you want to talk tostudent that you want to talk to


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• Let’s look at the codeLet’s look at the code Note that both programs look similarNote that both programs look similar

Threads are created for the input stream and Threads are created for the input stream and for the output streamfor the output stream

Each runs until the client/server has quitEach runs until the client/server has quit

The threads are NOT synchronized (no The threads are NOT synchronized (no monitors used)monitors used) Could cause a problem, but in this simple Could cause a problem, but in this simple

example it is not absolutely necessaryexample it is not absolutely necessary If we have more complicated communication, If we have more complicated communication,

monitors will be definitely requiredmonitors will be definitely required– Could lead to data consistency problems Could lead to data consistency problems

otherwise otherwise


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Week 5: More Complex CommunicationWeek 5: More Complex Communication

• Consider now a single server that Consider now a single server that serves many clientsserves many clients Each client connects to the serverEach client connects to the server

Clients interact with serverClients interact with server

Clients also interact with each other Clients also interact with each other through serverthrough server

Server needs to divide its time amongst Server needs to divide its time amongst clientsclients Server must coordinate interaction with Server must coordinate interaction with clients such that data is always consistentclients such that data is always consistent


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SERVER

CLIENT

CLIENT CLIENT

CLIENT

Additional links not shown


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• Server's job is quite complicatedServer's job is quite complicated Allows clients to log in and out and keeps Allows clients to log in and out and keeps

track of themtrack of them Takes messages from each client and Takes messages from each client and

passes them on to the other clients passes them on to the other clients Sees to it that clients' messages do not Sees to it that clients' messages do not

interfere with one anotherinterfere with one another "If you all talk at once, no one will understand "If you all talk at once, no one will understand

what you are saying!"what you are saying!" Server has to regulate how messages are passedServer has to regulate how messages are passed


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SERVER

CLIENT

CLIENT CLIENT

CLIENT

Hey, listen to me!

One at a time and you'll all get your turn!

No, listen to me!

No, listen to me!

No, listen to me!


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• How can we do this in Java?How can we do this in Java? The server needs to have a thread for The server needs to have a thread for

each clienteach client Each thread will in a sense serve the needs of Each thread will in a sense serve the needs of

one clientone client

The threads MUST be synchronizedThe threads MUST be synchronized There is a lot of critical data in the server that There is a lot of critical data in the server that

must only be accessed by one thread at a timemust only be accessed by one thread at a time– Updating the number of clients onlineUpdating the number of clients online– Sending a message to the clients onlineSending a message to the clients online

Let’s look at an example – a Chat ServerLet’s look at an example – a Chat Server


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• General idea:General idea: Client logs into server and is added to list of Client logs into server and is added to list of

clientsclients Client is assigned a thread in the server programClient is assigned a thread in the server program Server synchronizes client threads for data Server synchronizes client threads for data

consistencyconsistency

Client also has a thread for receiving Client also has a thread for receiving messagesmessages However, no thread used to send messages – a However, no thread used to send messages – a

Button and ActionEvent are used to process themButton and ActionEvent are used to process them They are then sent to the serverThey are then sent to the server

Client also has a GUI so that users will be Client also has a GUI so that users will be comfortablecomfortable


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• Let’s try the programLet’s try the program Download JRChatServer.javaDownload JRChatServer.java

Download JRChatClient.javaDownload JRChatClient.java

Compile and run JRChatClient.javaCompile and run JRChatClient.java

Use it for a while … log in and out … send Use it for a while … log in and out … send messages … see what is going onmessages … see what is going on

Now look at the codeNow look at the code DiscussDiscuss


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Week 5: SummaryWeek 5: Summary

• This week we discussedThis week we discussed Idea of servers and clientsIdea of servers and clients

Communication between server and Communication between server and clientsclients Connection-oriented communication with Connection-oriented communication with

socketssockets Connectionless communication with packetsConnectionless communication with packets If two-way, need for threadsIf two-way, need for threads

Using a server with multiple clientsUsing a server with multiple clients How to coordinate the clients with threads and How to coordinate the clients with threads and

monitorsmonitors

university of pittsburgh computer science 1 week 5: introduction last week we discussedlast week we...

Documents

data sharing slide

data data consistency

data packets

critical data

data flow

discussedlast week

hard connection

continuous connection