Chapter 8: Application Design and Development
اسلاید 1: Chapter 8: Application Design and Development
اسلاید 2: Chapter 8: Application Design and Development User Interfaces and ToolsWeb Interfaces to DatabasesWeb FundamentalsServlets and JSPBuilding Large Web ApplicationsTriggersAuthorization in SQLApplication Security
اسلاید 3: User Interfaces and ToolsMost database users do not use a query language like SQL.FormsGraphical user interfacesReport generatorsData analysis tools (see Chapter 18)Many interfaces are Web-basedBack-end (Web server) uses such technologies asJava servletsJava Server Pages (JSP)Active Server Pages (ASP)
اسلاید 4: The World Wide WebThe Web is a distributed information system based on hypertext.Most Web documents are hypertext documents formatted via the HyperText Markup Language (HTML)HTML documents containtext along with font specifications, and other formatting instructionshypertext links to other documents, which can be associated with regions of the text.forms, enabling users to enter data which can then be sent back to the Web server
اسلاید 5: A formatted report
اسلاید 6: Web Interfaces to DatabasesWhy interface databases to the Web?Web browsers have become the de-facto standard user interface to databasesEnable large numbers of users to access databases from anywhereAvoid the need for downloading/installing specialized code, while providing a good graphical user interfaceExamples: banks, airline and rental car reservations, university course registration and grading, an so on.
اسلاید 7: Web Interfaces to Database (Cont.)Dynamic generation of documentsLimitations of static HTML documentsCannot customize fixed Web documents for individual users.Problematic to update Web documents, especially if multiple Web documents replicate data.Solution: Generate Web documents dynamically from data stored in a database. Can tailor the display based on user information stored in the database.E.g. tailored ads, tailored weather and local news, …Displayed information is up-to-date, unlike the static Web pagesE.g. stock market information, ..
اسلاید 8: Uniform Resources LocatorsIn the Web, functionality of pointers is provided by Uniform Resource Locators (URLs).URL example: http://www.bell-labs.com/topics/book/db-book The first part indicates how the document is to be accessed “http” indicates that the document is to be accessed using the Hyper Text Transfer Protocol.The second part gives the unique name of a machine on the Internet.The rest of the URL identifies the document within the machine.The local identification can be:The path name of a file on the machine, orAn identifier (path name) of a program, plus arguments to be passed to the programE.g. http://www.google.com/search?q=silberschatz
اسلاید 9: HTML and HTTPHTML provides formatting, hypertext link, and image display features.HTML also provides input featuresSelect from a set of optionsPop-up menus, radio buttons, check listsEnter valuesText boxesFilled in input sent back to the server, to be acted upon by an executable at the serverHyperText Transfer Protocol (HTTP) used for communication with the Web server
اسلاید 10: Sample HTML Source Text <html> <body> <table border cols = 3> <tr> <td> A-101 </td> <td> Downtown </td> <td> 500 </td> </tr> … </table> <center> The <i>account</i> relation </center> <form action=“BankQuery” method=get> Select account/loan and enter number <br> <select name=“type”> <option value=“account” selected> Account <option> value=“Loan”> Loan </select> <input type=text size=5 name=“number”> <input type=submit value=“submit”> </form> </body> </html>
اسلاید 11: Display of Sample HTML Source
اسلاید 12: Client Side Scripting and AppletsBrowsers can fetch certain scripts (client-side scripts) or programs along with documents, and execute them in “safe mode” at the client siteJavascriptMacromedia Flash and Shockwave for animation/gamesVRMLAppletsClient-side scripts/programs allow documents to be activeE.g., animation by executing programs at the local siteE.g. ensure that values entered by users satisfy some correctness checksPermit flexible interaction with the user.Executing programs at the client site speeds up interaction by avoiding many round trips to server
اسلاید 13: Client Side Scripting and SecuritySecurity mechanisms needed to ensure that malicious scripts do not cause damage to the client machineEasy for limited capability scripting languages, harder for general purpose programming languages like JavaE.g. Java’s security system ensures that the Java applet code does not make any system calls directlyDisallows dangerous actions such as file writesNotifies the user about potentially dangerous actions, and allows the option to abort the program or to continue execution.
اسلاید 14: Web ServersA 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.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.To install a new service on the Web, one simply needs to create and install an executable that provides that service.The Web browser provides a graphical user interface to the information service.Common Gateway Interface (CGI): a standard interface between web and application server
اسلاید 15: Three-Tier Web Architecture
اسلاید 16: Two-Tier Web ArchitectureMultiple levels of indirection have overheadsAlternative: two-tier architecture
اسلاید 17: HTTP and SessionsThe HTTP protocol is connectionlessThat is, once the server replies to a request, the server closes the connection with the client, and forgets all about the requestIn contrast, Unix logins, and JDBC/ODBC connections stay connected until the client disconnects retaining user authentication and other informationMotivation: reduces load on server operating systems have tight limits on number of open connections on a machineInformation services need session informationE.g. user authentication should be done only once per sessionSolution: use a cookie
اسلاید 18: Sessions and CookiesA cookie is a small piece of text containing identifying informationSent by server to browser on first interactionSent by browser to the server that created the cookie on further interactionspart of the HTTP protocolServer saves information about cookies it issued, and can use it when serving a requestE.g., authentication information, and user preferencesCookies can be stored permanently or for a limited time
اسلاید 19: ServletsJava Servlet specification defines an API for communication between the Web server and application programE.g. methods to get parameter values and to send HTML text back to clientApplication program (also called a servlet) is loaded into the Web serverTwo-tier modelEach request spawns a new thread in the Web server thread is closed once the request is servicedServlet API provides a getSession() method Sets a cookie on first interaction with browser, and uses it to identify session on further interactionsProvides methods to store and look-up per-session informationE.g. user name, preferences, ..
اسلاید 20: Example Servlet CodePublic class BankQuery(Servlet 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 balanceresult.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 ( ); }}
اسلاید 21: Server-Side ScriptingServer-side scripting simplifies the task of connecting a database to the WebDefine 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 languagesJSP, Server-side Javascript, ColdFusion Markup Language (cfml), PHP, JscriptGeneral purpose scripting languages: VBScript, Perl, Python
اسلاید 22: Improving Web Server PerformancePerformance is an issue for popular Web sites May be accessed by millions of users every day, thousands of requests per second at peak timeCaching techniques used to reduce cost of serving pages by exploiting commonalities between requestsAt the server site:Caching of JDBC connections between servlet requestsCaching results of database queriesCached results must be updated if underlying database changesCaching of generated HTMLAt the client’s networkCaching of pages by Web proxy
اسلاید 23: TriggersA trigger is a statement that is executed automatically by the system as a side effect of a modification to the database.To design a trigger mechanism, we must:Specify the conditions under which the trigger is to be executed.Specify the actions to be taken when the trigger executes.Triggers introduced to SQL standard in SQL:1999, but supported even earlier using non-standard syntax by most databases.
اسلاید 24: Trigger Example Suppose that instead of allowing negative account balances, the bank deals with overdrafts by setting the account balance to zerocreating a loan in the amount of the overdraftgiving this loan a loan number identical to the account number of the overdrawn accountThe condition for executing the trigger is an update to the account relation that results in a negative balance value.
اسلاید 25: Trigger Example in SQL:1999create trigger overdraft-trigger after update on account referencing new row as nrow for each row when nrow.balance < 0 begin atomic insert into borrower (select customer-name, account-number from depositor where nrow.account-number = depositor.account-number); insert into loan values (n.row.account-number, nrow.branch-name, – nrow.balance); update account set balance = 0 where account.account-number = nrow.account-number end
اسلاید 26: Triggering Events and Actions in SQLTriggering event can be insert, delete or updateTriggers on update can be restricted to specific attributesE.g. create trigger overdraft-trigger after update of balance on accountValues of attributes before and after an update can be referencedreferencing old row as : for deletes and updatesreferencing new row as : for inserts and updatesTriggers can be activated before an event, which can serve as extra constraints. E.g. convert blanks to null.create trigger setnull-trigger before update on r referencing new row as nrow for each row when nrow.phone-number = ‘ ‘ set nrow.phone-number = null
اسلاید 27: Statement Level TriggersInstead of executing a separate action for each affected row, a single action can be executed for all rows affected by a transactionUse for each statement instead of for each rowUse referencing old table or referencing new table to refer to temporary tables (called transition tables) containing the affected rowsCan be more efficient when dealing with SQL statements that update a large number of rows
اسلاید 28: External World ActionsWe sometimes require external world actions to be triggered on a database updateE.g. re-ordering an item whose quantity in a warehouse has become small, or turning on an alarm light, Triggers cannot be used to directly implement external-world actions, BUTTriggers can be used to record actions-to-be-taken in a separate tableHave an external process that repeatedly scans the table, carries out external-world actions and deletes action from tableE.g. Suppose a warehouse has the following tablesinventory (item, level ): How much of each item is in the warehouseminlevel (item, level ) : What is the minimum desired level of each itemreorder (item, amount ): What quantity should we re-order at a timeorders (item, amount ) : Orders to be placed (read by external process)
اسلاید 29: External World Actions (Cont.)create trigger reorder-trigger after update of amount on inventoryreferencing old row as orow, new row as nrowfor each row when nrow.level < = (select level from minlevel where minlevel.item = orow.item) and orow.level > (select level from minlevel where minlevel.item = orow.item) begininsert into orders (select item, amount from reorder where reorder.item = orow.item) end
اسلاید 30: Triggers in MS-SQLServer Syntax create trigger overdraft-trigger on account for update as if inserted.balance < 0 begin insert into borrower (select customer-name,account-number from depositor, inserted where inserted.account-number = depositor.account-number) insert into loan values (inserted.account-number, inserted.branch-name, – inserted.balance) update account set balance = 0 from account, inserted where account.account-number = inserted.account-number end
اسلاید 31: When Not To Use TriggersTriggers were used earlier for tasks such as maintaining summary data (e.g. total salary of each department)Replicating databases by recording changes to special relations (called change or delta relations) and having a separate process that applies the changes over to a replica There are better ways of doing these now:Databases today provide built in materialized view facilities to maintain summary dataDatabases provide built-in support for replicationEncapsulation facilities can be used instead of triggers in many casesDefine methods to update fieldsCarry out actions as part of the update methods instead of through a trigger
اسلاید 32: Authorization in SQL (see also Section 4.3)Forms of authorization on parts of the database:Read authorization - allows reading, but not modification of data.Insert authorization - allows insertion of new data, but not modification of existing data.Update authorization - allows modification, but not deletion of data.Delete authorization - allows deletion of data
اسلاید 33: Authorization (Cont.)Forms of authorization to modify the database schema:Index authorization - allows creation and deletion of indices.Resources authorization - allows creation of new relations.Alteration authorization - allows addition or deletion of attributes in a relation.Drop authorization - allows deletion of relations.
اسلاید 34: Authorization and ViewsUsers can be given authorization on views, without being given any authorization on the relations used in the view definitionAbility of views to hide data serves both to simplify usage of the system and to enhance security by allowing users access only to data they need for their jobA combination or relational-level security and view-level security can be used to limit a user’s access to precisely the data that user needs.
اسلاید 35: View ExampleSuppose a bank clerk needs to know the names of the customers of each branch, but is not authorized to see specific loan information.Approach: Deny direct access to the loan relation, but grant access to the view cust-loan, which consists only of the names of customers and the branches at which they have a loan.The cust-loan view is defined in SQL as follows:create view cust-loan as select branchname, customer-name from borrower, loan where borrower.loan-number = loan.loan-number
اسلاید 36: View Example (Cont.)The clerk is authorized to see the result of the query: select * from cust-loanWhen the query processor translates the result into a query on the actual relations in the database, we obtain a query on borrower and loan.Authorization must be checked on the clerk’s query before query processing replaces a view by the definition of the view.
اسلاید 37: Authorization on ViewsCreation of view does not require resources authorization since no real relation is being createdThe creator of a view gets only those privileges that provide no additional authorization beyond that he already had.E.g. if creator of view cust-loan had only read authorization on borrower and loan, he gets only read authorization on cust-loan
اسلاید 38: Granting of PrivilegesThe passage of authorization from one user to another may be represented by an authorization graph.The nodes of this graph are the users.The root of the graph is the database administrator.Consider graph for update authorization on loan.An edge Ui Uj indicates that user Ui has granted update authorization on loan to Uj.U1U4U2U5U3DBA
اسلاید 39: Authorization Grant GraphRequirement: All edges in an authorization graph must be part of some path originating with the database administratorIf DBA revokes grant from U1:Grant must be revoked from U4 since U1 no longer has authorizationGrant must not be revoked from U5 since U5 has another authorization path from DBA through U2Must prevent cycles of grants with no path from the root:DBA grants authorization to U7U7 grants authorization to U8U8 grants authorization to U7DBA revokes authorization from U7Must revoke grant U7 to U8 and from U8 to U7 since there is no path from DBA to U7 or to U8 anymore.
اسلاید 40: Security Specification in SQLThe grant statement is used to confer authorizationgrant <privilege list>on <relation name or view name> to <user list><user list> is:a user-idpublic, which allows all valid users the privilege grantedA role (more on this later)Granting a privilege on a view does not imply granting any privileges on the underlying relations.The grantor of the privilege must already hold the privilege on the specified item (or be the database administrator).
اسلاید 41: Privileges in SQLselect: allows read access to relation,or the ability to query using the viewExample: grant users U1, U2, and U3 select authorization on the branch relation:grant select on branch to U1, U2, U3insert: the ability to insert tuplesupdate: the ability to update using the SQL update statementdelete: the ability to delete tuples.references: ability to declare foreign keys when creating relations.usage: In SQL-92; authorizes a user to use a specified domainall privileges: used as a short form for all the allowable privileges
اسلاید 42: Privilege To Grant Privilegeswith grant option: allows a user who is granted a privilege to pass the privilege on to other users. Example:grant select on branch to U1 with grant optiongives U1 the select privileges on branch and allows U1 to grant thisprivilege to others
اسلاید 43: RolesRoles permit common privileges for a class of users can be specified just once by creating a corresponding “role”Privileges can be granted to or revoked from roles, just like userRoles can be assigned to users, and even to other rolesSQL:1999 supports roles create role teller create role manager grant select on branch to teller grant update (balance) on account to teller grant all privileges on account to manager grant teller to manager grant teller to alice, bob grant manager to avi
اسلاید 44: Revoking Authorization in SQLThe revoke statement is used to revoke authorization.revoke<privilege list>on <relation name or view name> from <user list> [restrict|cascade]Example:revoke select on branch from U1, U2, U3 cascadeRevocation of a privilege from a user may cause other users also to lose that privilege; referred to as cascading of the revoke.We can prevent cascading by specifying restrict:revoke select on branch from U1, U2, U3 restrictWith restrict, the revoke command fails if cascading revokes are required.
اسلاید 45: Revoking Authorization in SQL (Cont.)<privilege-list> may be all to revoke all privileges the revokee may hold.If <revokee-list> includes public all users lose the privilege except those granted it explicitly.If the same privilege was granted twice to the same user by different grantees, the user may retain the privilege after the revocation.All privileges that depend on the privilege being revoked are also revoked.
اسلاید 46: Limitations of SQL AuthorizationSQL does not support authorization at a tuple levelE.g. we cannot restrict students to see only (the tuples storing) their own gradesWith the growth in Web access to databases, database accesses come primarily from application servers. End users dont have database user ids, they are all mapped to the same database user idAll end-users of an application (such as a web application) may be mapped to a single database userThe task of authorization in above cases falls on the application program, with no support from SQLBenefit: fine grained authorizations, such as to individual tuples, can be implemented by the application.Drawback: Authorization must be done in application code, and may be dispersed all over an applicationChecking for absence of authorization loopholes becomes very difficult since it requires reading large amounts of application code
اسلاید 47: Audit TrailsAn audit trail is a log of all changes (inserts/deletes/updates) to the database along with information such as which user performed the change, and when the change was performed.Used to track erroneous/fraudulent updates.Can be implemented using triggers, but many database systems provide direct support.
اسلاید 48: Application SecurityData may be encrypted when database authorization provisions do not offer sufficient protection.Properties of good encryption technique:Relatively simple for authorized users to encrypt and decrypt data.Encryption scheme depends not on the secrecy of the algorithm but on the secrecy of a parameter of the algorithm called the encryption key.Extremely difficult for an intruder to determine the encryption key.
اسلاید 49: Encryption (Cont.) Data Encryption Standard (DES) substitutes characters and rearranges their order on the basis of an encryption key which is provided to authorized users via a secure mechanism. Scheme is no more secure than the key transmission mechanism since the key has to be shared.Advanced Encryption Standard (AES) is a new standard replacing DES, and is based on the Rijndael algorithm, but is also dependent on shared secret keys Public-key encryption is based on each user having two keys:public key – publicly published key used to encrypt data, but cannot be used to decrypt data private key -- key known only to individual user, and used to decrypt data. Need not be transmitted to the site doing encryption. Encryption scheme is such that it is impossible or extremely hard to decrypt data given only the public key.The RSA public-key encryption scheme is based on the hardness of factoring a very large number (100s of digits) into its prime components.
اسلاید 50: AuthenticationPassword based authentication is widely used, but is susceptible to sniffing on a networkChallenge-response systems avoid transmission of passwordsDB sends a (randomly generated) challenge string to userUser encrypts string and returns result. DB verifies identity by decrypting resultCan use public-key encryption system by DB sending a message encrypted using user’s public key, and user decrypting and sending the message backDigital signatures are used to verify authenticity of dataE.g. use private key (in reverse) to encrypt data, and anyone can verify authenticity by using public key (in reverse) to decrypt data. Only holder of private key could have created the encrypted data.Digital signatures also help ensure nonrepudiation: sender cannot later claim to have not created the data
اسلاید 51: Digital CertificatesDigital certificates are used to verify authenticity of public keys. Problem: when you communicate with a web site, how do you know if you are talking with the genuine web site or an imposter?Solution: use the public key of the web siteProblem: how to verify if the public key itself is genuine?Solution:Every client (e.g. browser) has public keys of a few root-level certification authoritiesA site can get its name/URL and public key signed by a certification authority: signed document is called a certificateClient can use public key of certification authority to verify certificateMultiple levels of certification authorities can exist. Each certification authority presents its own public-key certificate signed by a higher level authority, and Uses its private key to sign the certificate of other web sites/authorities
اسلاید 52: End of Chapter
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