Talk:Featured tile layers/Updates
I only get DNS errors for tah.openstreetmap.org. I can't seem to request tiles or view request status at all. :( Help! Miqrogroove 19:55, 16 July 2012 (BST)
- After much surfing, I have found a solution. I will replace the Z12 Update instructions, which were obsolete. Miqrogroove 20:43, 16 July 2012 (BST)
- Lower zoom levels do not allow /dirty requests if there's too much data (too many nodes) in it, for performance reasons because the renderer needs to perform large database requests, and draw many more objects, so the "metatile" requires longer time and much more computing ressources on the renderer.
- Note: A "metatile" is a square of NxN tiles rendered and cached at the same time on the server; the tiles you request are smaller. For each zoom level, the boundaries of metatiles are at "x" and "y" positions multiple of 8, so that it's not necessary to send multiple /dirty request for tiles at ".../z/(8m)/(8n)/dirty" and ".../z/(8m+7)/(8n+7)/dirty".
- These metatiles will be just marked for later refreshing, but will be scheduled in a job queue by dedicated rendering server resources, or on a secondary renderer (also used for /dirty requests).
- Only high levels allow /dirty requests (they are also queued but only on a secondary renderer, but they usually render in about 1 minute or 2; this renderer is also used when you browse the map on these metatiles and there's no prerendered metatile in the cache; the last ones are not using /dirty requests but standard requests and visibly have priority to /dirty requests for tiles that are already in the cache).
- In the past it was already not possible to make /dirty requests on lower zoom levels, but the minimum level at which they are allowed is apparently increased.
- The exact parameters are tuned but not documented. They may change without notice if the admins see that there's too much work in the job queue, notably for tiles on large areas that are updated much more frequently (and so are almost always dirty).
- Also if a metatile is already in the job queue, repeating the /dirty request will not accelerate the rendering: that metatile will remain in its current position in the queue, and the extra /dirty request will be just ignored.
- Finally a /dirty request may be ignored if the metatile is already in the cache and its age is too young (was already regenerated in a near past). This avoids service attacks against the renderers by filling its queue. There may also be other limitations such as limiting the number of queued /dirty requests demanded by the same user (IP).
- Technically, the "/dirty" is just an optional (unofficially supported) facility, but it has a price as it competes with the normal scheduling of renderers, that have limited computing resources. A way to get faster refreshes is to donate to the Foundation so that they can invest in more hardware for the renderers, or for the database servers. There's no support of "/dirty" request on all rendering services (e.g. Mapquest doesn't support them and manages the scheduling of rendering completely itself, based on their own usage statistics and priorities).
- — Verdy_p (talk) 13:55, 30 May 2016 (UTC)
Thanks for the details - I'll not repeat /dirty anymore, hoping for a faster rendering ;-) I was using /dirty (Z12) hoping a feature I modified two months ago shows up properly since the tiles at lower zoom levels (<12) were up-to-date. Zooming 10-14 over the following link shows a trunk road (ref=112) except at Z12 where it still shows a primary road http://www.openstreetmap.org/#map=13/46.0495/-71.3708 --jfd553 (talk) 15:42, 30 May 2016 (UTC)