Write a Simple Proxy Program

Code for program can use Java, C#, or C++

Implement the Database class as described in lesson 5 to just read databases from files. Heres the C++ version of the interface it should implement:

class IDatabase {

public:

virtual string getID()= 0;

virtual bool exists(const string& key) = 0;

virtual string get(const string& key) = 0;

};

This time, however, your concrete database class will actually retrieve values from a file. The format will be as before:

on each line. Keys do not contain spaces but values can. Whenever a user calls get(key), the value for that key is returned (throw an exception if its not in the file). The database id is the file name. Use it to open the file, and keep the file open for the life of the object.

Each time you try to access the data via get() or exists(), go to the file (dont read in all the data at first and store it in the object).

Since retrieving values is now expensive, write a proxy named CacheDB that wraps an IDatabase* and keeps a most-recently-used list of up to SIZE elements (use 5). When users call get( ) or exists() on such a database, retrieve from the cache if its previously been fetched and is still in the cache. Otherwise retrieve it from the file and add it to the cache. Dont let the cache exceed SIZE entries.

Note that when you retrieve anything from the cache, you must update the most-recently-used order.

Write a method of CacheDB called inspect(), which returns the keys in the cache in order, so we can see what is in the cache at any time. Call it after using the cache. Note that this is not part of a typical proxy, but is only there to show that you implemented the cache correctly. The inspect() method will not show up in the IDatabase interface or any other proxy (see below.)

Finally, write a SecureDB proxy that automatically requires users to login before allowing them to call the IDatabase methods. Have a special Database of users and passwords that authenticates on the first access attempt. Throw an exception if authentication fails.

Note that both get() and exists() are accesses and must be secured.

You should be able to mix and match the proxies at will.

Here is a sample driver in C++:

void test(IDatabase* db) {

try {

cout << db->get("one") << endl;

cout << db->get("two") << endl;

cout << db->exists("two") << endl; // Call to exists

cout << db->get("three") << endl;

cout << db->get("four") << endl;

cout << db->get("four") << endl;

cout << db->get("five") << endl;

cout << db->get("six") << endl;

cout << db->get("one") << endl;

cout << db->get("seven") << endl;

}

catch (runtime_error& x) {

cout << x.what() << endl;

}

cout << endl;

}

int main() {

Database db("db.dat"); // Hard code this exact name

test(&db);

Database userdb("userdb.dat"); // Hard code this exact name

SecureDB sdb(&db, &userdb);

test(&sdb);

CacheDB cdb(&sdb);

test(&cdb);

cout << "Cache contents: " << cdb.inspect() << endl;

try {

Database db2("noname.dat");

}

catch (runtime_error& x) {

cout << x.what() << endl;

}

}

Heres the input file, db.dat:

one the first value

two the second value

three the third value

four the fourth value

five the fifth value

six the sixth value

Heres the expected output:

the first value

the second value

true // or 1 (depending how true is printed)

the third value

the fourth value

the fourth value

the fifth value

the sixth value

the first value

No such record: seven

Enter username: chuck

Enter password: foo

the first value

the second value

true

the third value

the fourth value

the fourth value

the fifth value

the sixth value

the first value

No such record: seven

the first value

the second value

found key two in cache

true

the third value

the fourth value

found key four in cache

the fourth value

the fifth value

the sixth value

the first value

No such record: seven

Cache contents: [one], [six], [five], [four], [three],

noname.dat does not exist

Note the trace statement when accessing the cache, and the exceptions for non-existent records and files.

Note: this program is quite easy, but there are several important details that people miss if they are sloppy. Read the program specification carefully to make sure you get the details right.

Implementation notes:

Hard-code the following input files: db.dat for the database contents, and userdb.dat for the database of usernames and passwords. Do NOT use other names.

The SecureDB uses lazy authentication: It does NOT authenticate until the first request to get() or exists().

In CacheDB, note the way exists() works:

First, it checks the cache, (and reorders the cache if it is found).

If it isnt found in the cache, then go to the database itself. However, since a call to the database exists() function doesnt return the value, you will not be updating the cache.

Note that if you call get() and the key isnt in the database, do NOT return a value like NULL or even .