tailieunhanh - DISTRIBUTED SYSTEMS principles and paradigms Second Edition phần 9

Thay vào đó, nó là an toàn hơn nhiều để sử dụng các phím persession, bởi vì nếu như một phím được thỏa hiệp, lúc tồi tệ nhất, chỉ có một phiên duy nhất bị ảnh hưởng. Các tin nhắn được gửi trong các buổi khác giữ bí mật. Liên quan đến điểm cuối cùng này là Alice có thể muốn trao đổi một số dữ liệu bí mật với Bob, nhưng cô không tin tưởng anh ta rất nhiều mà cô sẽ | 550 DISTRIBUTED WEB-BASED SYSTEMS CHAP. 12 program that is to be executed at the server side along with parameter values that are filled in by the user. Once the form has been completed the program s name and collected parameter values are sent to the server as shown in Fig. 12-3. 3. Start process to fetch document Web server CGI process Database server Figure 12-3. The principle of using server-side CGI programs. When the server sees the request it starts the program named in the request and passes it the parameter values. At that point the program simply does its work and generally returns the results in the form of a document that is sent back to the user s browser to be displayed. CGI programs can be as sophisticated as a developer wants. For example as shown in Fig. 12-3 many programs operate on a database local to the Web server. After processing the data the program generates an HT1 1Ldocument and returns that document to the server. The server will then pass the document to the client. An interesting observation is that to the server it appears as if it is asking the CGI program to fetch a document. In other words the server does nothing but delegate the fetching of a document to an external program. The main task of a server used to be handling client requests by simply fetching documents. With CGI programs fetching a document could be delegated in such a way that the server would remain unaware of whether a document had been generated on the fly or actually read from the local file system. Note that we have just described a two-tiered organization of server-side software. However servers nowadays do much more than just fetching documents. One of the most important enhancements is that servers can also process a document - before passing it to the client. In particular a document may contain a server-side script which is executed by the server when the document has been fetched locally. The result of executing a script is sent along with the rest of the .

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