[OSM-talk] Long Ways and API 0.6

Dave Stubbs osm.list at randomjunk.co.uk
Sun Feb 8 19:53:27 GMT 2009


2009/2/8 Maarten Deen <mdeen at xs4all.nl>:
> Frederik Ramm wrote:
>> Ben Laenen wrote:
>>> OK, as long as there's no limit on the number of relation members, I'm
>>> happy :-)
>>
>> I'm pretty sure there will be, it is not implemented yet but I believe
>> we said it would be 1.000. Relations with more members become very hard
>> to work with.
>
> So, we're going to have to make relations to group relations? Isn't that a
> bit... daft?
> I thought relations were there to make the editors able to work on ways and not
> have to download all ways in the relation. How would a big relation then be hard
> to work with?


Because the API works on whole objects only. When you make a change to
an object you have to upload the entire new object. When you download
an object you have to download the whole object.

A relation with over 1000 members is a large object which will take
large amounts of bandwidth and database power to download/upload even
if you're not having to download every single member as well. Hence
the limit.

Also every editor needs to then cope with an object with that many
members -- they all do, it's just the performance and memory usage
that suffers.

There's very very few relation usages where we'll need groups of
relations. The large area multipolygons are about the only ones I can
think of.
Routes can happily be split -- the primary point of relations here is
to allow more than one route following the same way. It's not like
we're trying to build categories or similar -- we don't need to
because we can always match tags.

Dave




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