News Server Class Library

  The following sections show the implementation of the news server classes. These classes provide an interface to several kinds of news databases. The NServer class is an abstract class that acts only as common superclass for the news server classes. For facilitating the exploitation of polymorphism the methods of the NServer class are declared as abstract and virtual. Table 6.1 gives an explanation of these concepts.

 

 

Tabelle 6.1: Abstract and virtual methods in C++

Abstract Method

An abstract method only defines the interface for the method. The actual implementation for the method must be provided by classes that inherit the method from its parent class. Using abstract methods in a class ensures that all subclasses of the base class implement at least the abstract methods provided by the base class.

Virtual Method

Declaring methods of a class as virtual allows to exploit polymorphism across its subclasses. Consider a class GeometricShape and some subclasses (e.g., Rectangle , Circle , ...) that define the method draw . Whenever GeometricShape is used in the place of one of its subclasses, the correct method draw will be called.

The current version of the News Server Class Library already provides a class for the retrieval of news articles from a remote news server (RServer ), from a local news database (LServer ) and a class retrieving news articles from a remote news server with the ability to cache already requested data in a local news database (LServer ). The relationship between those classes has been explained in section 5.4 and is shown again in Figure 6.2. The classes of this library allow any program (e.g., news readers, proxy news servers, news caches, etc.) to access a news database in different ways.

  

Abbildung 6.2: Hierarchy of the News Server classes

News Server Class (NServer)

  The NServer class defines the interface for all methods needed to access a news server. These methods are declared abstract and virtual. The methods are declared abstract because the method has to be implemented by the subclass of this class. The methods are declared virtual to provide polymorphism across the news server classes.

An NVActiveDB class is used for the active database and an NVOverviewDB class for the overview database. Currently these two classes have to be used by all the NServer 's subclasses. However, this is likely to change in the future (See section 8.1.1) for a detailed explanation).

The following methods have been defined for the retrieval of newsgroups articles and the overview database:

active()

This method returns a pointer to the active database.

group(char *name)

This method returns a pointer to information about the newsgroup name. It contains the type of the newsgroup, the number of the first and last article and the total number of articles.

xoverfmt()

This method returns the order of the database entries stored in the overview database.

xoverdb(int fst, int lst)

This method returns a pointer to the overview database of the newsgroup. The overview database contains at least the overview records for the articles within the range fst-lst.

article(int nbr)

returns the article with the number nbr from the current newsgroup.

post(Article *article)

posts the article stored in article to the news database.

setserver(char *ns_name, char *ns_serv)

This method selects a new database. The parameters ns_name and ns_serv define the name of the database. For the RServer and CServer classes ns_name and ns_serv reflect the name and the service of the news server. This function has been added to allow multiplexing between different news databases. This interface may change in the future.

Remote Server Class (RServer)

The RServer class is a subclass of the NServer class and implements the news server interface as specified by theNServer class. The RServer class uses the news database provided by a news server. The data on the news server is accessed using NNRP.

The RServer class implements the following methods in addition to the methods specified in section 6.4.1:

connect()

This method connects to the news server specified by the setserver method.

is_connected()

This method returns, whether the RServer class currently is connected to the news server.

disconnect()

This method disconnects the RServer class from the news server.

Local Server Class (LServer)

The LServer class is a subclass of the NServer class and implements the news server interface as specified by the NServer class. The LServer class uses a local news database to provide newsgroups and news articles.

The LServer class needs not implement any method in addition to the ones defined in the NServer class.

Cache Server Class (CServer)

The CServer class is a subclass of the RServer and the LServer class and implements the news server interface as specified by theNServer class. The CServer class is used to retrieve newsgroups and articles from a news server using NNRP. Before these groups and articles are passed back to the caller, the data are cached in a local news database. The local news database is used to satisfy successive requests to the same data.

In addition to the methods defined in its superclasses, the CServer class implements the setttl(ttl_list, ttl_desc, ttl_group) method. This method is used to specify the timeouts used for the ActiveDB (ttl_list), the group description file (ttl_desc) and for the newsgroups (ttl_group). Whenever a client requests a database being expired, the databases is requested from the news server again.