International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
C ied
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Interface
Mobile Client 4*--——-—-— - Synchronization Server «—» Database Server
Fig.3 The three-tier architecture of wireless GIS database
As Fig.3 and Fig.4 shows, WGIS mobile database is utilized a
synchronization server to promote the efficiency of spatial data
synchronization. [t provides a replication model for database
server (DBSVR) and WGIS. Besides, it ensures serializability
and consistency of the mobile database system through the
reconciliation of transaction-level conflict.
The function of Sync Server which is connected wireless
communication interface is making distributed processing,
controlling spatial data exchange with WGIS and database
server, and sustaining a wireless cell. Mobile client conserves a
database duplicated copy, and through the GIS manages local
geographic database. With the ODBC interface, sync server
could exchange spatial data with database server.
DBSVR
ogao
| 2800
]
Rep
Fig. 4 The three-tier WGIS replication model
4.3 Replication dynamic
Replication is a general technique to increase the data
availability. However, the generally available replication
technologies assume the deployment on a fixed distributed
environment.
According to the behaviour of replicas in replication schemes,
we can categorize the schemes into two replications, static
replication and dynamic replication. In this paper, dynamic
replication for wireless GIS database is considered.
In static replication schemes, the location and the number of
replicas are chosen prior to the deployment. Traditional
replication schemes, for example, fall into this category.
Manual recalculation of the access cost and redistribution of
replicas are necessary to reflect newly accessing patterns. In a
mobile environment, however, static replication schemes may
not perform well since the assumptions about fixed locations
and static accessing patterns are no longer held.
On the other hand, in dynamic replication schemes, the location
and/ or the number of replicas are changed to follow the
accessing patterns to data being replicated. Dynamic replication
schemes try to overcome the problems mentioned above by
continuously maintaining statistics about accessing patterns
and/or system workload so as to dynamically recalculate access
cost and reconfigure the replication structure to adapt to the
changes in accessing patterns. In general, this is desirable for a
mobile environment.
In a mobile environment, however, mobile hosts are dynamic.
They could move to anywhere and for unpredictable length of
time. Furthermore, the users of replicas may need to work at
several *well-known' sites. In such cases, it may be more
advantageous to deploy multi-replication, i.e., placing a replica
on each “activity centre’ of its users.
308
In a mobile computing environment, however, mobile hosts can
move anywhere and anytime, resulting in a highly dynamic
system. Accordingly, the centre of activity of replica readers is
not static in general. Not only that, the ability of mobile users to
move can also make it more costly to find current location of
mobile users and of course the replicas if they are dynamic. In
this sense, it is necessary to take a balance between the cost of
finding location and replica maintenance.
5. COMPARISON OF VARIOUS DATA STORAGE
STRATEGIES IN WGIS
In this section, the comparison of the dynamic multi-tier
replication and dynamic-single replication strategy strategies
for wireless databases, which combine the two aspects of
replication policies mentioned above, is performed. We intend
to show how some important parameters related to the
characteristics of data access in wireless environment affect the
performance of these strategies.
As the performance measure, the average access cost of data is
used. In a network environment, cost is mostly associated with
the number of network packets transferred to do an activity
which is observed until it is completed. In a mobile computing
environment, generally, network packets can be divided into
two classifications, i.e., the data packet and the signal packet.
The data packet consists of user data transferred from the server
to the client and vice versa. On the other hand, the signal packet
consists of data such as routing information, location lookup,
and location update used by the system,. However, for the
reasons mentioned below, we simply ignore the signal packet
from our model.
Generally, the size of the signal packet is much smaller than
that of the data packet. Thus, when considering both of them,
ignoring the signal packet will improve the clarity of analysis
and the resulting observation on the characteristic of each
strategy. In a mobile environment, signal packets are exchanged
using a separated channel from data packets. Therefore,
separating the analysis of these two kinds of packets is more
logical and makes our model closer to the real situation in
mobile computing environments.
Without losing the generality, 50 mobile users are assumed to
be sharing their databases with each other. The accesses include
both ‘read’ and ‘update’. Access requests arrive according to
the Poisson distribution and the access configuration, i.e., the
portions of ‘read’ and ‘update’, are determined by the ‘write’
ratio. We assume that an update operation will be preceded by a
‘read’ operation.
As for dynamic replication strategies, the implementations are
as follows. As indicated by the name, the dynamic-single
replication strategy is implemented by making each master
database to only have single replica. In this strategy, each
replica is initially allocated on the home location. As the
simulation runs, mobile users start to move and access the
shared data. The replication manager in the registration area
where the user resides currently records every ‘read’ request
from a mobile user. Periodically, the access statistics from all
registration areas are collected and compared. Based on the
comparison result, the system makes a replica relocated to the
registration area where the ‘read’ accesses to it are requested
mostly, and the statistics is reinitialized. In this way, the
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