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Announcements Reading for next week: 2 papers available on Blackboard Background Reading: 1.11, 12.1-12.8 in text About homework assignment 1 ... 1 Plan for Today Review of Database Design, Functional Dependency, and Normal Forms Choices for Application Design XML (briefly) Database Design What’s the problem here? Entity-Relationship Model (and Diagrams) Functional Dependencies Legal relations Decompositions Closures and Canonical Covers Dependency Preservation Normal Forms 1st Normal Form Boyce-Codd Normal Form (BCNF) 3rd Normal Form 3 Application Design Application Design What’s the Big Question/Problem in Application Design? In other words, why are we studying this? Client Side Scripting and Applets Browsers can fetch certain scripts (client-side scripts) or programs along with documents, and execute them in “safe mode” at the client site Javascript Macromedia Flash and Shockwave for animation/games VRML Applets Client-side scripts/programs allow documents to be active E.g., animation by executing programs at the local site E.g. ensure that values entered by users satisfy some correctness checks Permit flexible interaction with the user. Executing programs at the client site speeds up interaction by avoiding many round trips to server Client Side Scripting and Security Security mechanisms needed to ensure that malicious scripts do not cause damage to the client machine Easy for limited capability scripting languages, harder for general purpose programming languages like Java E.g. Java’s security system ensures that the Java applet code does not make any system calls directly Disallows dangerous actions such as file writes Notifies the user about potentially dangerous actions, and allows the option to abort the program or to continue execution. Web Servers A Web server can easily serve as a front end to a variety of information services. The document name in a URL may identify an executable program, that, when run, generates a HTML document. To install a new service on the Web, one simply needs to create and install an executable that provides that service. When a HTTP server receives a request for such a document, it executes the program, and sends back the HTML document that is generated. The Web client can pass extra arguments with the name of the document. The Web browser provides a graphical user interface to the information service. Common Gateway Interface (CGI): a standard interface between web and application server HTTP and Sessions The HTTP protocol is connectionless That is, once the server replies to a request, the server closes the connection with the client, and forgets all about the request In contrast, Unix logins, and JDBC/ODBC connections stay connected until the client disconnects Motivation: reduces load on server operating systems have tight limits on number of open connections on a machine Information services need session information retaining user authentication and other information E.g. user authentication should be done only once per session Solution: use a cookie Sessions and Cookies A cookie is a small piece of text containing identifying information Sent by server to browser on first interaction Sent by browser to the server that created the cookie on further interactions Server saves information about cookies it issued, and can use it when serving a request part of the HTTP protocol E.g., authentication information, and user preferences Cookies can be stored permanently or for a limited time Three-Tier Web Architecture Two-Tier Web Architecture Servlets Java Servlet specification defines an API for communication between the Web server and application program E.g. methods to get parameter values and to send HTML text back to client Application program (also called a servlet) is loaded into the Web server Two-tier model Each request spawns a new thread in the Web server thread is closed once the request is serviced Servlet API provides a getSession() method Sets a cookie on first interaction with browser, and uses it to identify session on further interactions Provides methods to store and look-up per-session information E.g. user name, preferences, .. Example Servlet Code public class BankQueryServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse result) throws ServletException, IOException { String type = request.getParameter(“type”); String number = request.getParameter(“number”); …code to find the loan amount/account balance … …using JDBC to communicate with the database.. …we assume the value is stored in the variable balance result.setContentType(“text/html”); PrintWriter out = result.getWriter( ); out.println(“<HEAD><TITLE>Query Result</TITLE></HEAD>”); out.println(“<BODY>”); out.println(“Balance on “ + type + number + “=“ + balance); out.println(“</BODY>”); out.close ( ); } } Server-Side Scripting Server-side scripting simplifies the task of connecting a database to the Web Define a HTML document with embedded executable code/SQL queries. Input values from HTML forms can be used directly in the embedded code/SQL queries. When the document is requested, the Web server executes the embedded code/SQL queries to generate the actual HTML document. Numerous server-side scripting languages JSP, Server-side Javascript, ColdFusion Markup Language (cfml), PHP, Jscript General purpose scripting languages: VBScript, Perl, Python Comparative Advantages JDBC and ODBC from Client Client-side Scripting and Applets Positive: Negative: Two-Tier Server Architecture Positive: Negative: Three-Tier Server Architecture Positive: Negative: Positive: Negative: Server-Side Scripting Positive: Negative: eXtensible Markup Language (XML) XML: Motivation Data interchange is critical in today’s networked world Examples: Banking: funds transfer Order processing (especially inter-company orders) Scientific data Chemistry: ChemML, … Genetics: BSML (Bio-Sequence Markup Language), … Paper flow of information between organizations is being replaced by electronic flow of information Each application area has its own set of standards for representing information XML has become the basis for all new generation data interchange formats XML Motivation (Cont.) Earlier generation formats were based on plain text with line headers indicating the meaning of fields Similar in concept to email headers Does not allow for nested structures, no standard “type” language Tied too closely to low level document structure (lines, spaces, etc) Each XML based standard defines what are valid elements, using XML type specification languages to specify the syntax Plus textual descriptions of the semantics XML allows new tags to be defined as required DTD (Document Type Descriptors) XML Schema However, this may be constrained by DTDs A wide variety of tools is available for parsing, browsing and querying XML documents/data Comparison with Relational Data Inefficient: tags, which in effect represent schema information, are repeated Better than relational tuples as a dataexchange format Unlike relational tuples, XML data is selfdocumenting due to presence of tags Non-rigid format: tags can be added Allows nested structures Wide acceptance, not only in database systems, but also in browsers, tools, and applications Structure of XML Data Tag: label for a section of data Element: section of data beginning with <tagname> and ending with matching </tagname> Elements must be properly nested Proper nesting Improper nesting <account> … <balance> …. </balance> </account> <account> … <balance> …. </account> </balance> Formally: every start tag must have a unique matching end tag, that is in the context of the same parent element. Every document must have a single top-level element XML Example <bank> <account> <account_number> A-101 </account_number> <branch_name> Downtown </branch_name> <balance> 500 </balance> </account> <depositor> <account_number> A-101 </account_number> <customer_name> Johnson </customer_name> </depositor> </bank> XML Document Schema Database schemas constrain what information can be stored, and the data types of stored values XML documents are not required to have an associated schema However, schemas are very important for XML data exchange Otherwise, a site cannot automatically interpret data received from another site Two mechanisms for specifying XML schema Document Type Definition (DTD) Widely used XML Schema Newer, increasing use Document Type Definition (DTD) The type of an XML document can be specified using a DTD DTD constraints structure of XML data DTD does not constrain data types What elements can occur What attributes can/must an element have What subelements can/must occur inside each element, and how many times. All values represented as strings in XML DTD syntax <!ELEMENT element (subelements-specification) > <!ATTLIST element (attributes) > Element Specification in DTD Subelements can be specified as names of elements, or #PCDATA (parsed character data), i.e., character strings EMPTY (no subelements) or ANY (anything can be a subelement) Example <! ELEMENT depositor (customer_name account_number)> <! ELEMENT customer_name (#PCDATA)> <! ELEMENT account_number (#PCDATA)> Subelement specification may have regular expressions <!ELEMENT bank ( ( account | customer | depositor)+)> Notation: “|” - alternatives “+” - 1 or more occurrences “*” - 0 or more occurrences Bank DTD <!DOCTYPE bank [ <!ELEMENT bank ( ( account | customer | depositor)+)> <!ELEMENT account (account_number branch_name balance)> <! ELEMENT customer(customer_name customer_street customer_city)> <! ELEMENT depositor (customer_name account_number)> <! ELEMENT account_number (#PCDATA)> <! ELEMENT branch_name (#PCDATA)> <! ELEMENT balance(#PCDATA)> <! ELEMENT customer_name(#PCDATA)> <! ELEMENT customer_street(#PCDATA)> <! ELEMENT customer_city(#PCDATA)> ]> Limitations of DTDs No typing of text elements and attributes All values are strings, no integers, reals, etc. Difficult to specify unordered sets of subelements Order is usually irrelevant in databases (unlike in the documentlayout environment from which XML evolved) (A | B)* allows specification of an unordered set, but Cannot ensure that each of A and B occurs only once IDs and IDREFs are untyped The owners attribute of an account may contain a reference to another account, which is meaningless owners attribute should ideally be constrained to refer to customer elements XML Schema XML Schema is a more sophisticated schema language which addresses the drawbacks of DTDs. Supports Typing of values User-defined, comlex types Many more features, including E.g. integer, string, etc Also, constraints on min/max values uniqueness and foreign key constraints, inheritance XML Schema is itself specified in XML syntax, unlike DTDs More-standard representation, but verbose XML Scheme is integrated with namespaces BUT: XML Schema is significantly more complicated than DTDs. XML Schema Version of Bank DTD <xs:schema xmlns:xs=http://www.w3.org/2001/XMLSchema> <xs:element name=“bank” type=“BankType”/> <xs:element name=“account”> <xs:complexType> <xs:sequence> <xs:element name=“account_number” type=“xs:string”/> <xs:element name=“branch_name” type=“xs:string”/> <xs:element name=“balance” type=“xs:decimal”/> </xs:squence> </xs:complexType> </xs:element> ….. definitions of customer and depositor …. <xs:complexType name=“BankType”> <xs:squence> <xs:element ref=“account” minOccurs=“0” maxOccurs=“unbounded”/> <xs:element ref=“customer” minOccurs=“0” maxOccurs=“unbounded”/> <xs:element ref=“depositor” minOccurs=“0” maxOccurs=“unbounded”/> </xs:sequence> </xs:complexType> </xs:schema> Where we are in the course … Fundamentals of Using a Database Implementing a Database Relational Model SQL Database Design Application Design System Architecture Storage Structure and Indexing Query Processing and Optimization Transactions Data Mining and Databases Pattern and Association Mining Information Retrieval