[OSM-dev] Two technology related questions

[Sandor Seres]

I am not sure am I in the right forum to ask the two tiling related
questions here. If not, I do hope someone could redirect me to the right
place/forum. 

As I understand the OSM raster-tiles serving system is one of the
fundamental OSM services. Even the front page map (the SlippyMap and its
layers) is based on this service as well as many private mapping systems.
Further I understand that these tiles are pre-generated, edge aligned colour
raster images in PNG format of a uniform dimension and created per zoom
levels. At the same time, the OSM source data is (implicitly and apart from
the attributes) in a vector format. So, at some stage the mentioned service
must perform a vector-to-raster transformation and here is my first
dilemma/question:

1. Are you rasterizing geometry/vector objects first for a larger area and
then tiling this raster image, or the contrary? First tiling the
geometry/vector objects and so rasterizing these vector tiles?

Next, I think I understand how the maintenance of this raster tiles database
happens. When a mapper uploads a change all related tiles are marked for
re-rendering. So, at an appropriate time the marked tiles are re-rendered
and the users have an impression (more or less) that the updates happen
on-the-fly. I could not find some more details about this marking process.
So, my second dilemma/question is:

2. When an object (given by a relation) has changed, do you mark all the
related/coverage tiles for re-rendering or only those affected by the
change?

Just to be a bit more precise regarding the second question. Assume we have
an area object, for example a larger lake with islands, given by a relation.
Assume, a local editor will refine the object by changing just a short
section of the outer polygon, or/and just to insert a new small missing
island. The whole object is covered by hundreds of tiles while the actual
change is covered by only a few.

Thanks for the help/answers, Sandor.

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