Databases Reference
In-Depth Information
Some proposals work with the notion of weak transactions, transactions
that read or write local and probably inconsistent data [23]. Other proposals
present mechanisms so that applications have views of the DBs consistent
with their own actions, mechanisms known as session guarantees (see [24]).
Others present notions of transactions based on escrow methods, which
are especially interesting in sales transactions. The total number of available
items is distributed in the different sites, and local transactions can commit if
the demand does not exceed the quantity in the local site (see [25]). In [26]
isolation-only transactions are proposed, but the rest of the ACID properties
are not. In [27] a technique is explained whereby the broadcast channel is
used so that the mobile clients know if they have to abort the transactions
that are running. To do that, certification reports sent through the broadcast
channel contain items over which commits are going to be made. Another
related idea, called transaction proxies and presented in [28], consists of defin-
ing dual transactions (one for each transaction performed in a mobile host)
that will be executed in an FH that acts as the host support of the mobile
one. The dual transactions contain only the updates made by the mobile
transaction in case recovery is needed.
10.6.2
Data Dissemination by Broadcasting
The feature of asymmetry in mobile communications along with the power
limitation of the mobile computing framework make the model of broad-
casting data to clients an interesting alternative. Broadcasting is the delivery
of data from a server to a large set of clients (sometimes it is also referred to as
being push-based ). By pushing data, the server avoids interruptions caused
by requests of clients and thus optimizes the use of the bandwidth in the
upstream direction.
The main aspects that a broadcasting system must take into considera-
tion are the clients needs and whether to send the data periodically or aperi-
odically. Periodic push has the advantage of allowing clients to disconnect
for certain periods and still not miss items. In [29, 30] there appears the
use of a periodic dissemination architecture in the context of mobile systems.
Aperiodic dissemination, on the other hand, is a more effective way of using
the bandwidth available. In [31, 32] those authors work with the concept of
indexing on air, that is, transmitting an index along with the data, so cli-
ents can tune in only during the times they need to. One issue that arises in
the former approach is how the index is multiplexed with the data to make
the latency and tuning time minimal.
