Web Development Lab. Bases de Dados e Aplicações Web MIEIC, FEUP 10/11 Sérgio Nunes 1
Summary • The Internet • The World Wide Web • Web Technologies 2
Introduction 3
Previous Experience? 4
Web and Internet • What is the Internet? • What is the Web? • Which technologies are used in WebDev? 5
Web Applications • What are Web Applications? • Advantages and disadvantages of WebApps? 6
Web Applications A Web Application is a software system based on Web standards and technologies, that is accessible through a Web browser. 7
Pros of Web Apps • Platform independence. • Easier updates & bug fixes. • Access from anywhere. • No piracy. • No installation hurdles. • Developers can measure user interaction. 8
Cons of Web Apps • Depends on network connectivity. • Less sophisticated UIs. • Limited hardware access. • No platform integration (e.g. drag&drop). • Need to address browser versions. • Harder to debug. • Security risks. • Infrastructure costs. 9
The Internet 10
The Internet’s origins • USA Department of Defense Advanced Research Projects Agency (ARPA) (1958). • Developed the idea of “networked computers” witch led to the creation of the ARPANET in 1967. • Other networks appeared worldwide. 11
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Packet-Switching • How to connect separate physical networks without dedicated links? • Point-to-point connections do not scale. Need to share the available resources. • Packet switching — divide data in small chunks and send them separately. • Multiple connections over the same medium. 14
Internetworking • Proliferation of different networking protocols became a problem when trying to connect all networks. • No network technology is ideal for all scenarios (e.g. Ethernet, Wireless, DSL, etc.). • To achieve a homogeneous service across heterogeneous networks we need both hardware and software. 15
Hardware • The router is the core hardware equipment used to connect different networks. • An internet is a set of networks connected by routers. 16
Software • To connect different networks we need communication protocols. • These protocols establish message formats and message exchanging rules. • The most important protocols for connecting different networks are called the Internet Protocols or TCP/IP Protocols. 17
net 2 net 1 net 4 net 3 user’s net 5 computers (a) net 2 net 1 net 4 net 3 net 5 router physical net (b) 18
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Internet Protocols BGP FTP HTTP SMTP TELNET SNMP TCP UDP ICMP IGMP OSPF RSVP IP BGP = Border Gateway Protocol OSPF = Open Shortest Path First FTP = File Transfer Protocol RSVP = Resource ReSerVation Protocol HTTP = HyperText Transfer Protocol SMTP = Simple Mail Transfer Protocol ICMP = Internet Control Message Protocol SNMP = Simple Network Management Protocol IGMP = Internet Group Management Protocol TCP = Transmission Control Protocol IP = Internet Protocol UDP = User Datagram Protocol 20
IP • The main function of the Internet Protocol is to offer a virtual network, hiding the underlying physical networks. • Offers two fundamental services: • Addressing system (IP addresses). • Datagram structure (packets). 21
TCP • The Transmission Control Protocol offers a reliable and ordered delivery of packets between applications in different computers. • Handles problems not addressed in the lower layers: packet duplication and loss, or communication delays. • Supports important applications such as the WWW, e-mail, FTP , etc. 22
Internet Services • DNS — IP addresses to symbolic names. • SMTP — electronic messages. • FTP — file transfer. • Gopher — document search and access. • WWW — hypertext information system. 23
DNS • The Domain Name System is an application layer service of the Internet. • Translates human-readable symbolic names to numeric addresses (IP). • Symbolic names are organized hierarchically. The right-most element is the top-level domain (TLD). 24
Gopher 25
The World Wide Web 26
WWW’s Origins • Invented in Europe at the European Council for Nuclear Research (CERN) in 1989. • Joint work by Tim Berners-Lee and Robert Cailliau to share and link information of various kinds, and where the user can browse at will. • Allow collaborators in remote sites to share their ideas relating to common projects. • Initial proposal “WorldWideWeb” (W3). 27
Initial Success • WorldWideWeb source code released into the public domain in 1993. • NCSA released Mosaic, a software program that was a combined web browser and Gopher client. • Mosaic’s popularity led to the growth of the World Wide Web. 28
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Mosaic • Marc Andreessen and Jim Clark left NCSA and founded Netscape Communications. • NCSA licensed Mosaic technology to Microsoft to form the basis of Internet Explorer. • The “Browser Wars” started for the dominance of the web browser market. 30
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Web Browser Features • A web browser is a client software application for retrieving, presenting and transversing information resources on the Web. • Web browsers communicate with web servers using the HTTP protocol. • Increasingly sophisticated. 33
Web Browsers • There are four major layout engines: • Trident (Microsoft) — Internet Explorer. • Gecko (Mozilla) — Netscape, Firefox. • WebKit (Apple, Google, Nokia et al.) — Safari, Chrome. • Presto (Opera) — Opera. 34
W3C • The World Wide Web Consortium was founded in 1994 by TBL. • International standards organization that develops technical specifications and guidelines for the Web. • Mission: “Led the Web to its full potential”. • The W3C does not enforce their recommendations. 35
W3C Standards • Accessibility • MathML • HTML, CSS • P3P • eGovernment • Web Services • Electronic Commerce • SVG, PNG • Geospatial • VoiceXML • Semantic Web • XML • Internationalization (i18n) • ... 36
Web Servers • A web server is a program whose primary function is to deliver resources on clients’ requests. Only acts when requests arrive. • The most common web servers are the Apache HTTP Server and Microsoft’s Internet Information Server (IIS). • Lightweight alternatives: nginx, lighttpd. • Typically different web servers coexist in a production environment. 37
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Web Architecture • The Web is supported by three core technologies: • Uniform Resource Locators (URL) • HyperText Transfer Protocol (HTTP) • HyperText Markup Language (HTML) 39
URL • Establishes a unique address for World Wide Web resources — e.g. pages, images, etc. • Used to locate Web resources. • Syntax: protocol://machine:port/directory/file.type • Protocol (http://, ftp://, file://) • Host (www.up.pt or 193.137.55.13) • Port (:80, the default) • Resource path (directory path to file) • Example: http://www.up.pt/sobre/index.html 40
HTTP • Defines how client machines communicate with web servers to obtain web resources. • Joint work of IETF and W3C. • Request-response protocol — client issues a request and waits for the server to respond. • Stateless protocol — each request is treated as an independent transaction. 41
Request Methods • GET — Request representation of the resource. • HEAD — Request the headers (without content). • POST — Submit data to be processed to the identified resource. • PUT — Upload data to the specified resource. • DELETE — Deletes the specified resource. 42
Status Codes • The first line of the HTTP response includes a numeric status code. Codes are organized in five classes of responses. • 1xx — Informational • 2xx — Success (e.g. 200 OK, 201 Created) • 3xx — Redirection (e.g. 301 Moved Permanently) • 4xx — Client Error (e.g. 404 Not Found, 403 Forbidden) • 5xx — Server Error (e.g. 500 Internal Server Error) 43
REST • The Web follows a RESTful design. • Representational State Transfer is an architectural style for distributed hypermedia systems. Key principles: • Identification of resources — all available resources are identified using a global naming scheme. • Uniform interfaces — servers adhere to a predefined set of generic methods. • Stateless interactions — the server keeps information about its resources, not about client interactions. 44
Typical HTTP Session 1. User types URL into browser. 2. The browser translates the symbolic name to an IP address using the DNS service. 3. The browser opens a TCP connection with the server, sends a HTTP GET request and waits for the response. 4. The servers responds with a set of headers and th contents of the requested resource. 5. The browser processes the response received from the remote server and proceeds with new requests if needed. 45
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