[OSM-talk] Tile refresh on openstreetmap.org

[Christoph Hormann]

On Sunday 02 March 2014, Tom Hughes wrote:
> >
> > Well - in principle i think re-rendering of very old tiles (like >2
> > weeks age) at the low zooms should take precedence over style
> > induced updates at the high zooms.  Not sure how feasible it is to
> > implement this.
>
> They do. The algorithm goes something like this:
>
> * Update stylesheet, remembering time
> * Render z0 to z10 in the background
> * Mark planet as dirty, using saved time
> * Start rendering z11 to z12 in the background
>
> [...]
>
> Note that nothing in z0 to z12 is marked dirty as a result of changes
> to the data, so they only re-render when the style changes, or once a
> month as a background job.

That pretty much explains my observations.  By having one or several 
stylesheet updates within the monthly update cycle the actual update of 
some tiles is stretched from a month to the ~7 weeks i observed.  The 
solution i could envision would be to have a single render queue for 
the pre-rendered tiles that is prioritized based on age of the existing 
tile, possibly modulated with the zoom level (i.e. the age of the 
lowest zooms could be considered more severe than z11 and z12).  This 
means the queue always renders the tile which has the highest need for 
update at the moment and you do not have the problem that several style 
sheet updates induce rapid re-renders of some tiles but at the same 
time delay updates of other tiles.  In fact such a system would be 
fully agnostic to style sheet changes but still make sure they show up 
in the map as fast as possible.

Not sure if this is possible with the current render stack of course.

-- 
Christoph Hormann
http://www.imagico.de/