Re: Application design question....

"Frederic Lachasse" <>
23 Jul 2006 08:33:11 -0400
"MF" <> wrote in message


I have the following scenario for making a DataSourceHandler

- The DataSourceHandler use different adapters in order to get the data
from several sources.

- Each adapter fetches data ONLY from one data source implementing the
corresponding API for the data source.

- It shall be possible for DataSourceHandler to change the adapter, if
one data source becomes unavailable. For example, if DataSourceHandler
uses AdapterA for fetching the data, and suddenly the DataSourceA for
AdapterA becomes unavailable, it shall be possible to change to
AdapterB, which use DataSourceB to fetch data.

- The fetched data shall be converted to a common data structure (say
CommonDataStruct), which then is the used by DataSourceHandler to send
these data to some clients.

Now, how this DataSourceHandler and Adapter application shall be
designed and structured?
How can Factory or Adatper patterns and inheritance be used to design
this DataSourceHandler?
Any suggestions for class definitions or class diagrams?

I would do the following:

A data source interface that defines the functionality used by the
application to fetch data:

class DataSourceInterface
    virtual ErrorType fetchData(CommonDataStruct& data) = 0;
    virtual ~DataSourceInterface() {};

The ErrorType will return success or failure information about the fetch
(could be a simple boolean type).

In your terminology, an Adapter is an implementation of a

class XyzAdapter : public DataSourceInterface
    // runtime data used for connecting to the data source
    ZyzAdapter(ConfigData& config)
        // initialize the data source adapter
    virtual ErrorType fetchData(CommonDataStruct& data)
        // code to fetch an convert the data
        return errorCode;

Still following your terminology, a DataSource is an instance of an adapter
created with the necessary configuration data. They will be created from
factories using configuration data. Simple algorithm: getDataSourceType,
findFactory, call the factory with the data source configuration data. The
factory itself can be a pointer to a function or an object with a virtual

The DataSourceHandler is itself an implementation of DataSourceInterface.
Its fetchData() method just iterates on all the configured DataSource
objects, and call its fetchData() method until one succeed. If all failed,
then it returns an error.

The application should work with the master DataSourceHandler the same way
as with any other DataSource.

Fr?d?ric Lachasse - ECP86

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