distributed architectures introduction to internet · distributed architectures http and other...

Introduction

DistributedArchitectures

HTTP and OtherProtocols

User InterfaceDesign

Introduction to InternetApplications

Internet Applications, ID1354

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Contents

Distributed Architectures

HTTP and Other Protocols

User Interface Design

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Section

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Local Application

I We are familiar with anarchitecture where the entireapplication resides on the samecomputer.

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Introducing a Server

I Now, the application willbe split on two tiers(computers).

I A client that has theview and a server thathas controller andmodel.

I The view is displayed ina web browser.

This architecture is not good, we also needlayers for communication

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II Now, the application willbe split on two tiers(computers).

This architecture is not good, we also needlayers for communication

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This architecture is not good, we also needlayers for communication.

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This architecture is not good, we also needlayers for communication.

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Server-Side CommunicationII First, we add a server layer,

normally called view (a bitconfusing).

I However, the server sideview layer performs taskstypical of a view:

I Creates views (HTML),which are sent to the client.

I Interprets user gestures, aclick in a web page creates arequest to the server.

It might seem that we need yet a layer, fornetwork handling. There is such a layer, but it isin the web server. We don’t write it ourselves.

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Server-Side CommunicationI First, we add a server layer,

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Client-Side Communication

I Next, we add a clientlayer for communication,the net layer.

I Actually, the browserhandles most of thecommunication.

I The small network codewritten by us is normallyconsidered part of theclient-side view, the netlayer is omitted.

I This is a traditional web application.

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The MVVM PatternI The trend is that data is

stored also on the client,therefore we get aclient-side model.

I This reduces the networkcommunication, since wedo not need to resendthe entire view each timethe user does something.

I Thereby, the applicationbecomes faster.

I This is referred to as the MVVM,model-view-viewmodel pattern.

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Programming Languages

I This is the architecturewe will normally useduring the course.

I The view is programmedin HTML and CSS, clientside behavior isprogrammed inJavaScript and the entireserver side code iswritten in PHP.

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Programming Languages

I This is the architecturewe will normally useduring the course.

I The view is programmedin HTML and CSS, clientside behavior isprogrammed inJavaScript and the entireserver side code iswritten in PHP.

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Three-Tier Architecture

I Of course, we also needto store data. That isdone in the data layer,which is often adatabase.

I We also introduce theintegration layer, tohandle the databasecalls.

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Three-Tier Architecture

I Of course, we also needto store data. That isdone in the data layer,which is often adatabase.

I We also introduce theintegration layer, tohandle the databasecalls.

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Three-Tier Architecture (Cont’d)

I In a bigger application, we would most likelyplace the database in a separate node.

I This is called three-tier architecture and is,since long time, the dominating architecturefor web applications.

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Three-Tier Architecture (Cont’d)

I In a bigger application, we would most likelyplace the database in a separate node.

I This is called three-tier architecture and is,since long time, the dominating architecturefor web applications.

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Event-Driven Architecture

I In the latest year, there is a growing tendency tomove business logic to the client, perhapscompletely remove the server-side model.

I This is made possible with web sockets, whichenable full duplex browser-server communication.

I The motive is to reduce communication latency. Thebrowser informs the server about user actions, butdoes not wait for response before updating the view.

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Section

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The IP ProtocolI All Internet communication is based on the

Internet Protocol (IP).I IP provides basic functionality for sending

and receiving data.

I Data is sent in chunks, called packages. Apackage is like an envelope for a letter. Ithas sender and a receiver addresses and acontent, which is the data beingtransmitted.

I A node (computer) receiving a packet canaccept it, ignore it or retransmit it.

I A node dedicated to retransmitting packetsacross subnet borders is called a router.

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and receiving data.I Data is sent in chunks, called packages. A

package is like an envelope for a letter. Ithas sender and a receiver addresses and acontent, which is the data beingtransmitted.

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IP AddressI An internet (version 4) address has 32 bits

divided into four bytes,[0-255].[0-255].[0-255].[0-255]. Each nodeconnected to the internet has one or moreaddresses.

I Normally, an IP address must be unique,assigned only to one node.

I Some addresses, like 192.168.X.X arededicated to private networks and can beused freely. Such an address is nottransmitted on the public internet. Instead, itis translated to a public address by a router.

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The TCP Protocol

I TCP, Transmission Control Protocol, is usedon top of the IP protocol.

I TCP adds transport guarantees, forexample the following.

I Packets are delivered to the receiver in thesame order they are sent by the sender.

I Delivered packets have the same content assent packets.

I There are no lost packets.

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The TCP Protocol

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The TCP Protocol

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The TCP Protocol

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The TCP Protocol

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The TCP Protocol (Cont’d)

I TCP is connection-oriented, think of atelephone line as opposed to sending aletter. To establish a TCP connection is aslow operation.

I TCP handles ports, which makes it possibleto have multiple connections with the sameIP address open simultaneously. A port isidentified by a number. An endpoint of aTCP connection has an IP address and aport number.

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The TCP Protocol (Cont’d)

I TCP is connection-oriented, think of atelephone line as opposed to sending aletter. To establish a TCP connection is aslow operation.

I TCP handles ports, which makes it possibleto have multiple connections with the sameIP address open simultaneously. A port isidentified by a number. An endpoint of aTCP connection has an IP address and aport number.

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I IP addresses are normally translated tonames (instead of numbers). Such a nameis called domain name.

I Domain names are divided intosubdomains, divided by dots (.)

I The address www.ict.kth.se consists ofthe subdomain www, which is part of thesubdomain ict, which is part of kth, whichis part of se, which is part of the root, .

I The translation between numbers andnames is managed by DNS, Domain NameSystem.

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I A Uniform Resource Locator, URL definesa resource’s location on the internet.

I A URL consists of four parts.1. A protocol, e.g., http

2. A host (IP address or name),http://www.kth.se

3. A port number (optional). The default HTTPport number is 80.http://www.kth.se:8080

4. A path, which identifies the resource’s locationon the server.http://www.kth.se:8080/abc/index.html

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I A URL consists of four parts.1. A protocol, e.g., http2. A host (IP address or name),

http://www.kth.se

3. A port number (optional). The default HTTPport number is 80.http://www.kth.se:8080

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http://www.kth.se3. A port number (optional). The default HTTP

port number is 80.http://www.kth.se:8080

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URN and URI

I A Uniform Resource Name, URN is aresource identifier without host name andport number. A typical example is a isbnidentifying a book.

I A Uniform Resource Identifier, URI is eithera URL or URN.

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URN and URI

I A Uniform Resource Name, URN is aresource identifier without host name andport number. A typical example is a isbnidentifying a book.

I A Uniform Resource Identifier, URI is eithera URL or URN.

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I HyperText Transfer Protocol, HTTP is usedfor communication between web browsersand web servers.

I HTTP is based on TCP, which means aTCP connection is established for eachbrowser-server communication.

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I HyperText Transfer Protocol, HTTP is usedfor communication between web browsersand web servers.

I HTTP is based on TCP, which means aTCP connection is established for eachbrowser-server communication.

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The Request-Response Cycle

A HTTP communication typically proceeds as follows.

1. The client opens a TCP connection to the server.

2. The client sends a request for a resource on theserver. The request consists of a HTTP header, anddata if the user submitted data to the server.

3. The server sends a response to the client. Also theresponse consists of HTTP headers, and data if theresponse required data.

4. The server closes the TCP connection.

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4. The server closes the TCP connection.

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4. The server closes the TCP connection.22 / 52

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4. The server closes the TCP connection.22 / 52

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The Request-Response Cycle(Cont’d)

I HTTP is stateless. Neither server norbrowser remembers anything aboutprevious request-response cycles. Sessionhandling must be added in server-sidecode.

I To establish a TCP connection isexpensive. Therefore, TCP connectionsmight be kept alive and reused for multiplerequest-response cycles. This is specifiedwith the keep-alive HTTP header, sebelow.

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The Request-Response Cycle(Cont’d)

I HTTP is stateless. Neither server norbrowser remembers anything aboutprevious request-response cycles. Sessionhandling must be added in server-sidecode.

I To establish a TCP connection isexpensive. Therefore, TCP connectionsmight be kept alive and reused for multiplerequest-response cycles. This is specifiedwith the keep-alive HTTP header, sebelow.

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Cookies

I A cookie is a piece of data that is stored onthe client.

I The cookie is tagged with the server’sdomain name and included in every requestto that server.

I This enables the server to associate datawith a specific client.

I Cookies can be used to store the user’ssettings, for example display language.

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Cookies

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Cookies

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Cookies

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HTTP SessionsI As mentioned above, HTTP is stateless.I Still, the server must be able to recognize

which calls originate from the same client.Otherwise for example log in is impossible.

I One commonly used method to solve thisproblem is to use cookies.

I If a request has a cookie with a sessionidentifier, it identifies the user. If there is nosuch cookie, the user does not have arunning session.

I On the server, the session id can beassociated with any amount of data relatedto the user with that session. This is calledconversational state.

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I On the server, the session id can beassociated with any amount of data relatedto the user with that session. This is calledconversational state. 25 / 52

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I On the server, the session id can beassociated with any amount of data relatedto the user with that session. This is calledconversational state. 25 / 52

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HTTP Message Format

I A HTTP message has start-line,headers and body.

I The request start-line consists ofHTTP method (se left), URL pathand HTTP version, e.g., GET/page1.html HTTP/1.1

I The response start-line consists ofHTTP version, status code andreason, e.g.,HTTP/1.1 200 OK

I Sample request (top) andresponse (bottom) messages aredepicted to the left.

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HTTP Message Format

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HTTP Message Format

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HTTP Message Format

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Status Codes

I A HTTP response contains a status code toindicate the outcome of the request. Thereare five different categories of status codes.

1xx Reply contains information, forexample 101, Switch Protocol.

2xx Success, for example 200, OK.

3xx Redirection, for example 301, MovedPermanently.

4xx Client error, for example 404, NotFound.

5xx Server error, for example 500, InternalServer Error

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Status Codes

2xx Success, for example 200, OK.3xx Redirection, for example 301, Moved

Permanently.

4xx Client error, for example 404, NotFound.

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Status Codes

Permanently.4xx Client error, for example 404, Not

Found.

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Status Codes

Found.5xx Server error, for example 500, Internal

Server Error

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Status Codes

Found.5xx Server error, for example 500, Internal

Server Error

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HTTP MethodsI HTTP 1.1 has eight different methods that

requires the following server actions.GET Deliver resource identified by the

specified URL.POST Accept message body and deliver it to

the resource at the specified URL.

PUT Accept message body and store it as aresource with the specified URL.

DELETE Delete the resource at the given URL.HEAD Like GET, but only deliver headers.

TRACE Return the request message.OPTIONS Tell which HTTP methods can be used

with the specified URL.CONNECT Connect to another host.

I GET and POST are the most common methods andthe only ones we will use in this course.

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the resource at the specified URL.PUT Accept message body and store it as a

resource with the specified URL.

DELETE Delete the resource at the given URL.HEAD Like GET, but only deliver headers.

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resource with the specified URL.DELETE Delete the resource at the given URL.

HEAD Like GET, but only deliver headers.TRACE Return the request message.

OPTIONS Tell which HTTP methods can be usedwith the specified URL.

CONNECT Connect to another host.I GET and POST are the most common methods and

the only ones we will use in this course.

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HEAD Like GET, but only deliver headers.

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CONNECT Connect to another host.

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Safe and Idempotent MethodsI GET and HEAD are safe methods, which

means they should not take any actionother than to retrieve the specifiedresource.

I GET, HEAD, PUT, DELETE, OPTIONSand TRACE are idempotent methods,which means the same request can be sentmultiple times without any side-effects onthe server.

I POST is not idempotent. If you submit thesame purchase order multiple times in aweb shop you will probably by multipleitems. The purchase is typically a POSTrequest.

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I POST is not idempotent. If you submit thesame purchase order multiple times in aweb shop you will probably by multipleitems. The purchase is typically a POSTrequest. 29 / 52

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I POST is not idempotent. If you submit thesame purchase order multiple times in aweb shop you will probably by multipleitems. The purchase is typically a POSTrequest. 29 / 52

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When to Use GET

I Use GET whenI The only desired action is to retrieve the

specified resource.I If it shall be possible to bookmark the link.

I The URL is shorter than 255 bytes. Note thata GET URL is longer than a POST URL sincedata is included in the URL which GET, but isin the message body with POST (see below).

I You want to be able to write the entire request,including data, in the browser. This is usefulwhen debugging.

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When to Use GET

specified resource.I If it shall be possible to bookmark the link.I The URL is shorter than 255 bytes. Note that

a GET URL is longer than a POST URL sincedata is included in the URL which GET, but isin the message body with POST (see below).

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When to Use GET

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When to Use GET

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When to Use POST

I Use POST whenI The required action updates server state, for

example saves something in a database.I The data does not fit within the 255 byte limit

for URLs.

I The data shall not appear in the URL. Notethat this is not a matter of security, data is sentin clear text also when using POST.

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When to Use POST

for URLs.I The data shall not appear in the URL. Note

that this is not a matter of security, data is sentin clear text also when using POST.

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When to Use POST

for URLs.I The data shall not appear in the URL. Note

that this is not a matter of security, data is sentin clear text also when using POST.

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HTTP Parameters

I HTTP parameters are data included in arequest to a web server.

I A typical example is when the user hasentered data in a HTML form.

I When using the GET method, parametersare appended to the URL as a query string,http://some.domain/some/path?city=stockholm&country=sweden

I When using the POST method, parametersare included in the message body.

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HTTP ParametersI HTTP parameters are data included in a

request to a web server.I A typical example is when the user has

entered data in a HTML form.

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HTTP ParametersI HTTP parameters are data included in a

request to a web server.I A typical example is when the user has

entered data in a HTML form.

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HTTP Parameters

I HTTP parameters are data included in arequest to a web server.

I A typical example is when the user hasentered data in a HTML form.

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HTTP HeadersI HTTP headers have the syntaxname: value

I There are several predefined headers, andit is also allowed to add new headers.

I Sample request headers are:

Host The receiver address or domain name.User-Agent Identifies the sender browser and

operating system.Content-Length Message body length in bytes.Connection Keep connection open future requests.

I Sample response headers are:

Content-Length Message body length in bytes.Content-Type Media Type (see below) of response.

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I Sample request headers are:

Host The receiver address or domain name.User-Agent Identifies the sender browser and

operating system.Content-Length Message body length in bytes.Connection Keep connection open future requests.

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I Sample request headers are:Host The receiver address or domain name.

User-Agent Identifies the sender browser andoperating system.

Content-Length Message body length in bytes.Connection Keep connection open future requests.

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Content-Length Message body length in bytes.

Connection Keep connection open future requests.I Sample response headers are:

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I Sample response headers are:Content-Length Message body length in bytes.

Content-Type Media Type (see below) of response.

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I Sample response headers are:Content-Length Message body length in bytes.Content-Type Media Type (see below) of response.

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I Sample response headers are:Content-Length Message body length in bytes.Content-Type Media Type (see below) of response.

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Media Type

I Media Type (or MIME Type) definesmessage content. This tells the receiverhow to interpret the data.

I Some media types are:text/html HTML markuptext/plain Plain text

image/png A png imagevideo/ogg A ogg video.

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Media Type

I Media Type (or MIME Type) definesmessage content. This tells the receiverhow to interpret the data.

I Some media types are:text/html HTML markuptext/plain Plain text

image/png A png imagevideo/ogg A ogg video.

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Web Browsers

I It is important to test the web applicationwith all different browsers that shall be ableto display it.

I Browsers behave differently, and youshould expect that some breakspecifications.

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Web Browsers

I It is important to test the web applicationwith all different browsers that shall be ableto display it.

I Browsers behave differently, and youshould expect that some breakspecifications.

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Web Servers

I The web server can deliverstatic content and also callserver-side programs, likePHP, Java or .NET programs.

I The most commonly usedweb server is apache,https://httpd.apache.org/

I Other common web serversare nginx,http://wiki.nginx.org/Main

and Microsoft IIS.

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Web Servers

and Microsoft IIS.

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Web Servers

and Microsoft IIS.

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Web Servers (Cont’d)

I You need to install a web server on yourlaptop. All labs will be reported on your ownlaptop, there is no web server in ICT schoolwhere you can run all the labs.

I It might take time to get the web serverrunning. You are advised to start installingthe web server now.

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Web Servers (Cont’d)

I You need to install a web server on yourlaptop. All labs will be reported on your ownlaptop, there is no web server in ICT schoolwhere you can run all the labs.

I It might take time to get the web serverrunning. You are advised to start installingthe web server now.

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Section

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Web Development Tools

I There are many tools that facilitatesdeveloping web applications.

I Browser support varies between tools, mostexamples will be using Firefox.

I You are strongly advised to start usingsome of the following tools, they will helpyou a lot.

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Browser Web ConsoleI Most browsers have a

built-in console.

I The console logsinformation associatedwith the web page, forexample errors andwarnings related toJavaScript, CSS andnetwork requests.

I It enables you to runJavaScript expressionsin the web page.

I It also lets you choose elements from the web pageand have their HTML and CSS displayed.

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built-in console.

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built-in console.

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built-in console.

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Browser Web Console (Cont’d)

I The console is opened withCtrl-Shift-K in Firefox andCtrl-Shift-J in Chrome.

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Firebug

I Firebug is a powerful plug-in to Firefox.I In addition to console features, you can for

example debug JavaScript, mark HTMLelements, edit CSS and log network traffic.

I There are also many plug-ins to Firebug.I There is a cross-browser version of

Firebug, written in JavaScript, that offers asubset of the functionality for most otherbrowsers.

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Firebug

I There are also many plug-ins to Firebug.

I There is a cross-browser version ofFirebug, written in JavaScript, that offers asubset of the functionality for most otherbrowsers.

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Firebug

Firebug, written in JavaScript, that offers asubset of the functionality for most otherbrowsers. 42 / 52

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Firebug

Firebug, written in JavaScript, that offers asubset of the functionality for most otherbrowsers. 42 / 52

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Web DeveloperI Web Developer is a powerful

plug-in to Firefox, whichallows you to:

I edit HTML and CSS.

I See the area coveredby a chosen element.

I See the page indifferent screenresolutions.

I Edit cookies.I Validate HTML and

I Web Developer has beenported to Chrome.

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I edit HTML and CSS.I See the area covered

by a chosen element.

I See the page indifferent screenresolutions.

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by a chosen element.I See the page in

different screenresolutions.

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I Edit cookies.

I Validate HTML andCSS.

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Validators

I There are online validators for both HTMLand CSS. Links can be found on the courseweb site.

I Remember to always validate your HTMLand CSS code.

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Validators

I There are online validators for both HTMLand CSS. Links can be found on the courseweb site.

I Remember to always validate your HTMLand CSS code.

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NetBeans

I There are many different IDEs for webdevelopment, all have their pros and cons.

I NetBeans will be used for examples duringthe course. Make sure to download the Allversion, see image above.

I Most important is that you actually use anIDE, do not program in a text editor unlessyou are really sure it is what you prefer.

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NetBeans

I Most important is that you actually use anIDE, do not program in a text editor unlessyou are really sure it is what you prefer. 45 / 52

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NetBeans

I Most important is that you actually use anIDE, do not program in a text editor unlessyou are really sure it is what you prefer. 45 / 52

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JSFiddle and JSLint

I JSFiddle is an online editor where you cantest HTML, CSS and JavaScript.

I JSLint is an online tool for testingJavaScript code quality.

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JSFiddle and JSLint

I JSFiddle is an online editor where you cantest HTML, CSS and JavaScript.

I JSLint is an online tool for testingJavaScript code quality.

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W3Schools Try It Yourself

I w3schools.com has excellent tutorialsfor all languages covered in the course.

I All examples are presented with an onlineeditor where you can experiment with yourcode.

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W3Schools Try It Yourself

I w3schools.com has excellent tutorialsfor all languages covered in the course.

I All examples are presented with an onlineeditor where you can experiment with yourcode.

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Section

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Use UI Guidelines!I This is not a course in human-computer

interaction. Still, it is mandatory to considerbasic heuristics for user interface design.

I There are some short introductory texts onuser interface design available at NielsenNorman Group, such as:

I 10 Usability Heuristics for User InterfaceDesign,http://www.nngroup.com/articles/ten-usability-heuristics/

I Top 10 Mistakes in Web Design,http://www.nngroup.com/articles/top-10-mistakes-web-design/

I Other lists linked from the latter.I Here follows the 10 usability heuristics

mentioned above.

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mentioned above.

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mentioned above.

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I Other lists linked from the latter.

I Here follows the 10 usability heuristicsmentioned above.

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mentioned above. 49 / 52

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mentioned above. 49 / 52

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J. Nielsen’s UI Design Principles

1. The system should always keep usersinformed about what is going on.

2. Use words, phrases and concepts familiarto the user, rather than system-orientedterms.

3. Implement undo and redo.4. Follow platform conventions, users should

not have to wonder whether different words,situations, or actions mean the same thing.

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3. Implement undo and redo.

4. Follow platform conventions, users shouldnot have to wonder whether different words,situations, or actions mean the same thing.

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5. Eliminate error-prone conditions or checkfor them and ask users to confirm beforethey commit to the action.

6. Minimize the user’s memory load by makingobjects, options, etc visible. The usershould not have to remember information.

7. Use accelerators to speed up interaction forexpert users.

8. Remove irrelevant information.

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9. Error messages should be expressed inplain language, precisely indicate theproblem, and suggest a solution.

10. If necessary, provide help anddocumentation. The help should be easy tosearch, focused on the user’s task, and listconcrete steps to be carried out.

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9. Error messages should be expressed inplain language, precisely indicate theproblem, and suggest a solution.

10. If necessary, provide help anddocumentation. The help should be easy tosearch, focused on the user’s task, and listconcrete steps to be carried out.

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