Geography Reference
In-Depth Information
5.2. Tile pre-fetching
For a given tile request, tile pre-fetching methods try to anticipate which tiles will be
requested immediately afterwards. There are several works in the literature that address
object prefetching in Web GIS: [9, 10] approximate which tiles will be used in advance based
on the global tile access pattern of all users and the semantics of query; [11, 12] use an heuristic
method that considers the former actions of a given user.
Buffer=1
3x3 metatile
requested to
the WMS server
Tile requested
by the client
Figure 8.
Metatile 3x3 centered in the requested tile.
We propose another pre-fetching strategy, known as metatiling, that works as follows [13]:
when the proxy receives a tile request from a client and a cache miss is produced, it requests a
larger image tile (called metatile) to the remote backend. This metatile includes the requested
tile and also the surrounding ones contained in a specified buffer, as shown in Figure 8.
Then, the proxy cuts the metatile into individual tiles, returns the requested tile to the client,
and stores all these fragments into the cache, as shown in Figure 9. The main advantage of
metatiling is that it can reduce the bottleneck between the proxy cache and the remote Web
Map Server.
Proxy Cache
getMap(tile)
getMap(metatile)
no (cache miss)
1
2
3
remote
WMS
Is in cache?
4
6
5
cache
requested tile
cut & insert tiles
Client
metatile
requested tile
Figure 9.
Tile request flow with metatiling.
Moreover metatiling reduces the problem of duplicating the labeling of features that span
more than one tile. This problem is illustrated in Figure 10. Depending on the WMS server's
configuration, this feature can be labeled once on each tile (Figure 10(a)). By increasing
the geographic bounding box of tile requests, the WMS server avoids label duplicates
(Figure 10(b)).
