【正文】 stance of each item is being sold234。 Highest bidder for an item wins234。 More plicated with multiple copies, and buyers bid for specific number of copiesn Exchange: multiple buyers, multiple sellers234。 ., stock exchange234。 Buyers specify maximum price, sellers specify minimum price 234。 exchange matches buy and sell bids, deciding on price for the trade 216。 . average of buy/sell bids61169。Silberschatz, Korth and System ConceptsOrder Settlementn Order settlement: payment for goods and deliveryn Insecure means for electronic payment: send credit card number234。 Buyers may present some one else’s credit card numbers234。 Seller has to be trusted to bill only for agreedon item234。 Seller has to be trusted not to pass on the credit card number to unauthorized peoplen Need secure payment systems234。 Avoid abovementioned problems234。 Provide greater degree of privacy216。 . not reveal buyers identity to seller234。 Ensure that anyone monitoring the electronic transmissions cannot access critical information62169。Silberschatz, Korth and System ConceptsSecure Payment Systemsn All information must be encrypted to prevent eavesdropping234。 Public/private key encryption widely usedn Must prevent personinthemiddle attacks 234。 . someone impersonates seller or bank/credit card pany and fools buyer into revealing information 216。 Encrypting messages alone doesn’t solve this problem216。 More on this in next sliden Threeway munication between seller, buyer and creditcard pany to make payment234。 Credit card pany credits amount to seller234。 Credit card pany consolidates all payments from a buyer and collects them together216。 . via buyer’s bank through physical/electronic check payment63169。Silberschatz, Korth and System ConceptsSecure Payment Systems (Cont.)n Digital certificates are used to prevent impersonation/maninthe middle attack234。 Certification agency creates digital certificate by encrypting, ., seller’s public key using its own private key216。 Verifies sellers identity by external means first!234。 Seller sends certificate to buyer234。 Customer uses public key of certification agency to decrypt certificate and find sellers public key 216。 Maninthemiddle cannot send fake public key234。 Sellers public key used for setting up secure municationn Several secure payment protocols234。 . Secure Electronic Transaction (SET)64169。Silberschatz, Korth and System ConceptsDigital Cashn Creditcard payment does not provide anonymity234。 The SET protocol hides buyers identity from seller234。 But even with SET, buyer can be traced with help of credit card panyn Digital cash systems provide anonymity similar to that provided by physical cash234。 . DigiCash234。 Based on encryption techniques that make it impossible to find out who purchased digital cash from the bank234。 Digital cash can be spent by purchaser in parts216。 much like writing a check on an account whose owner is anonymous65169。Silberschatz, Korth and SudarshanDatabase System Concepts遺留系統(tǒng)169。Silberschatz, Korth and System Concepts遺留系統(tǒng)n 遺留系統(tǒng)是與當(dāng)代標(biāo)準(zhǔn)和系統(tǒng)不兼容但是仍處于生產(chǎn)應(yīng)用的老一代系統(tǒng)234。 例如運(yùn)行于主機(jī)之上的用 Cobol寫(xiě)的應(yīng)用216。 今天的熱門新系統(tǒng)是明天的遺留系統(tǒng) !n 將遺留系統(tǒng)應(yīng)用移到更現(xiàn)代的環(huán)境中是有問(wèn)題的234。 非常昂貴 , 因?yàn)檫z留系統(tǒng)可能涉及多年來(lái)形成的幾百萬(wàn)行代碼216。 原來(lái)的程序員通常不再找得到234。 從老系統(tǒng)轉(zhuǎn)換到新系統(tǒng)也是問(wèn)題216。 稍后詳述n 一種方法 : 在遺留應(yīng)用之上構(gòu)造 包裝器 層以允許新系統(tǒng)與遺留應(yīng)用之間的互操作234。 例如使用 ODBC 或 OLEDB 作為包裝器67169。Silberschatz, Korth and System Concepts遺留系統(tǒng) (續(xù) )n 重寫(xiě)遺留應(yīng)用首先需要弄懂其功能234。 遺留代碼經(jīng)常沒(méi)有文檔或只有過(guò)時(shí)的文檔234。 反向工程 : 檢查遺留代碼的過(guò)程 , 以便216。 提供用 ER或面向?qū)ο竽Ｐ兔枋龅哪Ｊ皆O(shè)計(jì)216。 找出實(shí)現(xiàn)了哪些過(guò)程和進(jìn)程 , 以得到對(duì)系統(tǒng)的一個(gè)高層視圖n 再工程 : 反向工程后進(jìn)行新系統(tǒng)的設(shè)計(jì)234。 在此過(guò)程中對(duì)現(xiàn)有系統(tǒng)設(shè)計(jì)進(jìn)行了改進(jìn)68169。Silberschatz, Korth and System Concepts遺留系統(tǒng) (續(xù) )n 從老系統(tǒng)到新系統(tǒng)的切換是個(gè)主要問(wèn)題234。 生產(chǎn)系統(tǒng)是每天要用的 , 生成新數(shù)據(jù)234。 暫停系統(tǒng)可能導(dǎo)致整個(gè)公司的活動(dòng)停頓 , 帶來(lái)巨大損失n 大爆炸方法 : H 實(shí)現(xiàn)完整的新系統(tǒng)H 用來(lái)自老系統(tǒng)的數(shù)據(jù)加載到新系統(tǒng)4 執(zhí)行這一步時(shí)不能有事務(wù)運(yùn)行4 創(chuàng)建腳本以快速做此工作H 關(guān)閉老系統(tǒng)并開(kāi)始使用新系統(tǒng)234。 這個(gè)方法的危險(xiǎn)性 : 新代碼有錯(cuò)誤或性能問(wèn)題或錯(cuò)失特性怎么辦216。 公司可能停頓69169。Silberschatz, Korth and System Concepts遺留系統(tǒng) (續(xù) )n Chickenlittle 方法 : 234。 一次只替換遺留系統(tǒng)的一部分234。 利用包裝器在遺留代碼和新代碼之間互操作216。 例如先替換前端 , 遺留系統(tǒng)后端使用包裝器– 在此階段如果新前端出現(xiàn)問(wèn)題 , 老前端可以繼續(xù)工作216。 替換后端 , 一次替換一個(gè)功能單元– 共享數(shù)據(jù)庫(kù)的所有部分也許必須一道替換 , 否則在數(shù)據(jù)庫(kù)上也需要包裝器234。 缺點(diǎn) : 需要大量額外開(kāi)發(fā)來(lái)創(chuàng)建包裝器并確保平滑互操作216。 如果公司命脈依賴于系統(tǒng)，還是值得的70169。Silberschatz, Korth and SudarshanDatabase System ConceptsEnd of Chapter