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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B-YF. Istanbul 2004
same time as updating spatial entities in a layer in spatial
database. Offline refreshment means updating invalid spatial
data of a layer in cache if their versions are invalid when
accessing to them.
Information in spatial cache includes /ayerVersion, slotVersion,
entityVersion and the corresponding relations between all
entities and slots of a layer. When accessing to entities in a
layer, we can determine whether spatial data in cache is valid or
not through comparing version information in cache and spatial
database.
(1) If /ayerVersion of a layer in cache is smaller than that in
spatial database, it means that all entities of the layer are
changed, and the whole layer should be refreshed in cache.
Otherwise,
(2) If slotVersion of a layer in cache is smaller than that in
spatial database, it means that all slots of the layer are
rearranged, and the corresponding relations between all entities
and slots of the layer should be refreshed in cache. Otherwise,
(3) If entityVersion of a layer in cache is smaller than that in
spatial database, it means that some entities of the layer are
changed, and those entities should be refreshed in cache.
(4) If above three conditions are all equal to those in spatial
database, it means spatial data of a layer in cache is same as
that in spatial database, and no refreshment is required.
When accessing to spatial data, if we can pre-load some spatial
data into cache, it will make spatial cache more effective. But
how to predict what kind of spatial data will be accessed to?
There are two rules of accessing to memory: there is every
probability of accessing to just being accessed memory, and
there is every probability of accessing to neighbors of just being
accessed memory. There are same rules in accessing to spatial
data: there is every probability of accessing to just being
accessed spatial data, and there is every probability of accessing
to neighbors of just being accessed spatial entities. According to
those rules, we can arrange slots by spatial scope, and a slot is
an accessing unit of spatial data. When the network is idle, we
can pre-load some neighbors of spatial data in cache from
spatial database.
3. CONCLUSIONS
Geo-Union has finished a preliminary component-based model
for distributed WebGIS, and has got into use in many fields
with sound effects. Although spatial cache and other techniques
are adopted in Geo-Union to improve its performance, a lot of
works still wait us ahead to make Geo-Union more practicable
and effective:
(1) Dynamic load balancing policy. In Geo-Union, most works
are completed at client side, and server is only responsible for
data access and simple data query. Therefore it is not well
balanced between client and server. Especially there will exists
a lot of transmission for massive data between client and server.
Although, spatial index and spatial cache techniques can
improve the performance to a certain extent, we still want to
take full advantage of the computing capability of GIS server,
so as to lighten load at client side and decrease the transmission
203
of redundant data in network. This needs a more reasonable
component design for the system.
(2) System concurrency. WebGIS is open to millions of users.
How to ensure the correctness, validity, stability and scalability
of Geo-Union to satisfy users’ requests is another key problem
for practicable WebGIS.
4. ACKNOWLEDGEMENT
Supported by the National Grand Fundamental Research 973
Program of China under Grant No.2002CB312000; the National
Research Foundation for the Doctoral Program of Higher
Education of China under Grant No. 20020001015; the National
Science Foundation of China under Grant No.60203002; the
National High Technology Development 863 Program under
Grant No. 2002AA 135330 and No. 2002AA 134030: the Beijing
Science Foundation under Grant No.4012007.
5. REFERENCES
Li Bin, 2001. A Component Perspective on Geographic
Information Services. — Cartography and Geographic
Information Science, 27(1), pp.75-86.
Dept. of Computer Science and Technology, Peking University,
2002. Operation and Component Guide for Geo-Union
Enterprise (in Chinese). Technology Material,
http://gis.pku.edu.cn. (accessed 20 Oct. 2002).