[mapguide-internals] Avoiding unnecessary Tile Cache invalidation (Trac Ticket 1332)

[Jackie Ng]

Hi All,

At Zac's suggestion, I'm spawning this thread off of discussions on Trac
#1332 (http://trac.osgeo.org/mapguide/ticket/1332) to stimulate some more
discussion on this list.

Sometime last week, I heard a tale of total frustration about how a Tile
Cache of 1.5 million tiles that was built over 1.5-2 days was instantly
trashed the moment a dynamic untiled layer was added to the map definition
and saved back into the repository.

That got me thinking about whether something as innocuous as changing the
description or background color of a map definition w/ tiled layers would
invalidate the tile cache so I tested this out and just as I feared, it
indeed wiped out the tile cache.

My initial thoughts were that tiling's only concerned with the base layer
settings in the Map Definition. So theoretically speaking, doing
modifications that don't affect the base layers should not affect the
integrity of the original tiles. 

Now according to Tom, there is a whole stack of dependencies that would have
to be checked such as Feature Source changes, Symbol changes, Layer
Definition changes in order to determine whether a Map Definition's Tile
Cache needs to be rebuilt. I wonder what overhead would be incurred by
performing these checks (if it hasn't been done already).

There's also renaming the tile cache directory before saving (also suggested
by Tom) and then renaming back. I wonder what other things can be done so
that the Tile Cache is cleared *only* if it has to be.

- Jackie
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