Q: Are the hashes indexed spatially so that the client can synchronize very
fast a small area of interest? I saw "p: position" in the PDF but I am not sure
if it means the position in the hast tree or in the world or if they are
related.
A: The hashes are organized into a Merkle Tree (
http://en.wikipedia.org/wiki/Merkle_tree ) and are not indexed spatially.
p:position is the position in the tree, since we try to just send the parts of
the tree that are different. However, the use case of synchronizing a small
area of interest is supported by combining the contributed Sha1 filter along
with an area filter against the features. Since the features are indexed
spatially, it should be very fast. The spatially filtered features skip SHA-1
computation. We've used other types of feature filters to split up the features
based on feature attributes and then synchronized each set separately. For
example, suppose you have a Vehicle feature with a type attribute that can be
'Plane', 'Train', or 'Automobile'. You could synchronize the planes more
frequently by having two sets, one with a filter for 'Plane' and the other that
has a filter for not 'Plane'.
Q: Can Geoserver act also as a client? Then there could be network of
Geoservers acting as regional mirror servers which could be synchronized from
the master service. The master could be reserved for maintaining data and
synchoronizing and clients could take the burden of WMS, WFS etc services which
could be flexible, scalable and cost effective.
A: Some more code is needed to make Geoserver act as a client. The client
library has a listener interface for what it detects as the required changes to
get in sync with the server:
public interface FeaturesChangedListener {
void featuresCreate(Feature[] feature);
void featuresUpdate(Feature[] feature);
void featuresDelete(Feature[] feature);
}
The code that is missing is to implement this interface and apply the changes
to Geoserver. I guess the mirrow Geoserver would need some way to configure
where the master is and what feature sets (or subsets) to sync. The current
client library is combined with some other FSI code that updates World Wind.
The model implemented is master-mirror. We are doing some more research into
master-master synchronization where either side can make a change and then
exchange messages to get back into synch. I'd have to get government permission
to release this new work to opensource if folks are interested.
Finally, I just turned off digests to help me manage the mailings from the list
better. If I've missed responding to anyone, poke me again please.
Robert Hastings, Lab-SDL: 619-553-7006, Cell: 858-395-4436, Moebius:
858-455-5431x210, Lab-250: 619-553-2263
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