January 12, 2009
The name GeoWebCache says it all - this piece of open
source software is about delivering geospatial data speedily from OGC Web Map Service
standard servers using a cache (a store of predigested tiles).
Directions Magazine interviewed Arne Kepp of OpenGeo to get a better handle on what
it is, how it works and how Web and mobile mappers might use it.
OpenGeo, if the name does not ring a bell, is the new name of the
geospatial division of The Open Planning Project, TOPP.
Directions Magazine (DM): GeoWebCache speeds up map rendering to Web
map clients. How does it do that?
Arne Kepp (AK): The key to GeoWebCache's speed improvements is to
divide the world up in to preset "tiles." This is how commercial
services like Google Maps and Virtual Earth achieve their speed, but
GeoWebCache is different as it operates against any compliant Web Map
Service (WMS) implementation. GeoWebCache acts as a proxy between the
client and the WMS server. When a client requests a tile, the program
will first check whether this tile has been requested before. If a tile
matching the parameters has been stored, then it will simply return
that. In this case, the tile cache will often be one hundred times
faster than a WMS server, because all it has to do is stream a file to
the network.
If it is a request for a new tile, it will create a request for a
larger tile and forward this to the WMS backend. Once the response is
received, the returned data are divided into tiles again, a process
called metatiling, and stored for later use. The particular tile
requested by the client is then returned. In this case the tile cache
is slightly slower than a direct request to the WMS server. But
GeoWebCache has built in tools to seed all the tiles in advance, so
that this never happens when the server is in production.
DM: Describe how it might be used to speed data being rendered from
GeoServer and Google Maps into a Google Maps client.
AK:
GeoWebCache includes an adapter for the
Google Maps client, so it
is very easy to add your layers to this service. The caching works
exactly the same way as it does for WMS; GeoWebCache will look for the
tile and return it, if possible, alternatively forwarding the request
to the backend. The only difference is that the Google Maps API
facility to request a remote tile layer uses requests with integer grid
coordinates instead of bounding boxes.
There is also an adapter for MS
Virtual Earth, which actually uses the
same grid, just a slightly different way of addressing the tiles. This
means that if you create a set of tiles for one of these services,
GeoWebCache will automatically let you use them in the other as well.
Both of these work great with GeoServer, but you can put GeoWebCache in
front of any WMS server and make your data available on both Google
Maps and MS Virtual Earth.
DM: What's the history of GeoWebCache? What was the impetus for
developing it? How did Google contribute?
AK:
The impetus to develop GeoWebCache came from a desire for a caching
solution that could be easily integrated with GeoServer. OpenGeo has
been focusing on an enterprise quality, fully supported open source Web
application stack. GeoWebCache plays a key role in the OpenGeo Stack,
sitting between OpenLayers, the JavaScript front end, and GeoServer,
which serves the data from a variety of data sources like PostGIS,
Oracle, ArcSDE and more. A tile cache was needed to ensure scalability
and improve the user experience. Since GeoServer was written in Java it
made sense to use the same language to make it easy to install on all
major platforms.
Through Google Summer of
Code 2007 (a Google program that offers
student developers stipends to write code for various open source
projects), student Chris Whitney was able to spend a summer creating
what became known as jTileCache. It had very basic functionality, but
also original ideas like using the Java Caching System to store image
objects and compress them on the fly. Over the next nine months
jTileCache was reworked into what is today known as GeoWebCache. In the
summer of 2008 the project benefited from another generous Summer
of Code grant, allowing a second student, Marius Suta, to contribute
code that enabled XML configuration using XStream and a RESTful
configuration interface. Google's Open Source Office has also funded
OpenGeo to add streaming Google Earth support to GeoServer, and
GeoWebCache benefited from this, gaining the ability to tile and cache
KML placemarks and vectors.
DM: GeoWebCache is licensed under LGPLv3, meaning it can be
included
in other open source and non-open source offerings. Has it been/will it
be included in commercial or open source offerings?
AK:
We chose LGPLv3 because it has always been a goal to use
GeoWebCache as a library. In fact, GeoWebCache is already integrated
into GeoServer
1.7.1. Currently this is not a very tight integration,
but it will become more so when we start expiring tiles automatically
based on transactions and similar events. There are also plans to
provide caching for WFS requests.
We are not currently aware of anyone using the code for commercial
products, but this is not surprising since we released the first stable
version this week. OpenGeo is fully committed to open source and will
not use it in closed products, but we encourage others to do so, and
hope they will join the community and improve it in collaboration with
all.
DM: What are the implications for GeoWebCache for mobile
applications?
AK:
The biggest problems on mobile devices are bandwidth and response
times. Tiling alleviates the former, partially because you do not have
to refresh the entire screen when you pan, only download tiles for the
missing areas. GeoWebCache is a natural fit anywhere you do tiling, and
the quick response ensures that no unnecessary delays are introduced.
Earlier in 2008 a user contributed a patch for GeoWebCache so that it
also has an adapter for Mobile GMaps. This is software that works
similarly to Google Maps, but on any Java J2ME-enabled phone. One idea
for future development is to export the tiles, so that you can put them
on flash memory in your device. This would make the map instantly
available at no charge.
DM: Besides speeding up delivery of maps to the client, are there
any
other big benefits of using GeoWebCache?
AK:
In addition to the adapters already mentioned, GeoWebCache also
supports Google Earth. With a regular WMS server this means you can
create image hierarchies like those provided by Google, where the
resolution increases as you zoom in. If you work against GeoServer it
is also possible to serve regionated KML. KML stands for Keyhole Markup
Lanuage and is an XML based vector format. Regionating in this context
means that you show a few features when you are zoomed far out, and
load more as you zoom in. This means you can serve a lot of data
without cluttering the display or overloading the client. Though it is
still evolving and more clients have to be developed, OpenGeo is
convinced that KML will play an important role for sharing information
in the years to come.
Finally, not only is GeoWebCache fast, but you can also serve hundreds
of simultaneous clients with a single instance of GeoWebCache. This can
translate into significant cost savings if you have a busy site.
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| Hi all. Just wanted to clarify that the software is called GeoWebCache, not OpenWebCache. Check it out at: http://geowebcache.org |
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| The typo has been corrected. I'm not sure why I wanted to rename the product! Adena Directions Media |
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