On Wed, Jun 19, 2013 at 11:13 AM, Andy Allan <[email protected]> wrote:
> On 19 June 2013 16:25, Ian Dees <[email protected]> wrote: > > > The last bit of work before I can call it complete is to correctly expire > > tiles based on the minutely diffs. The naive and incorrect approach > would be > > to expire based on node changes. This ignores changes to way and relation > > tags, for example. > > > Any thoughts on how to do this correctly and efficiently? > > I think the correct way is, for each tile, to store the list of nodes, > ways and relations returned within that tile. For example, the map > call walks up the tree from nodes -> ways, but then also back down the > three from ways -> nodes in order to complete the way. So if a > minutely diff marks an entity as expired, I can't see any other way to > determine which tiles that request is mentioned in, beyond re-creating > every tile from scratch, or doing some complex graph walking. Even for > a simple node tag change, that node could appear in any (or all) > tiles, and doing an exhaustive search of all the ways and relations > that mention it, and relation members in turn, to find every possible > bounding box seems the wrong way. > > So basically, when generating a tile, store an entity -> tile lookup > table too. When regenerating a tile, flush the entity -> tile store > for that tile and repopulate. So we're talking potentially several billion (entity type)+(entity id) -> [(tile x)+(tile y), ...] rows in a database of some sort along with the forward and reverse indexes.
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