![](bg4.png)
completion. In case of failure at any particular node,
the Hub issues messages, using Web services, to
cause a rollback at every node involved. If every
node completes its task successfully, the Hub issues
messages to cause a commit at every node. The suc-
cess in the implementation of this procedure requires
the implementation of a Web services-based distrib-
uted commit protocol.
The RDM implements such a protocol to pro-
mote synchronization of data that exist in different
nodes and are possibly replicated. RDM uses a
publish/subscribe method. The nodes that subscribe
to a topic receive change notifications. If a node
becomes offline, the previously notified changes
must be processed at the moment it becomes online
again, before it can accept any further commands.
The DRS is a mechanism that is responsible for
finding out whether or when a node becomes inac-
cessible, and for determining which redundant copy
(if any) can replace it during the period of unavail-
ability. This mechanism is integrated to the RDM
since, if the distributed database manages redundant
data, every modification is propagated using the
publish/subscribe notification system, in order to en-
sure the synchronization. Therefore, available nodes
receive changes immediately, and unavailable nodes
receive updates later, in a queue.
4 SODDA PROTOTYPE
To validate proposed architecture, we have
implemented a SODDA prototype as a Microsoft
.NET data provider. However, we cannot provide
many implementation details due to space
limitations. Results obtained so far demonstrate the
feasibility of the ideas and proposals presented here,
along with a large potential to support future work,
both in research and in commercial applications.
In order to test our prototype, we developed three
Data Source Wrappers, respectively for SQL Server,
SQL Server CE and Access. Support for data
sources such as MySQL, Oracle, XML files, and
CSV files is planned for future releases. The
prototype currently only implements data retrieval
features. Data manipulation is to be included in the
next releases of the SODDA data provider.
5 CONCLUSIONS
SODDA intends to use some of the most interesting
features of SOA to implement distributed databases.
Expected benefits include easier implementation,
lower communications costs, and greater access ca-
pillarity. The proposed statistics service facilitates
query optimization, since it unifies the treatment of
performance-oriented metadata and allows
implementation of automatic statistics updating.
An extensive list of possibilities for future work
presents itself at this stage. We observe that the
distributed database nodes do not, necessarily, need
to be managed by a full DBMS. Since they are
accessed only through Web services, it would be
possible to have other data sources, such as
spreadsheets, Web pages and others. Data Source
Wrappers can be written to use these sources as
nodes on a SODDA-based environment. Another
possibility involves the implementation of “hot-
swapping” of nodes, making it possible to achieve
full availability for, say, equipment maintenance,
through the simple modification of catalog entries.
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