On Mon, Aug 23, 2010 at 8:39 AM, Frederik Ramm <[email protected]> wrote: > Hi, > > Scott Crosby wrote: >> >> My idea is that we assume that a given snapshot of the database at a >> given changeset number is unique. > > I don't know if that breaks your whole edifice but the above assumption is > false; changesets are not generally applied in a transaction so at any one > time, hundreds of changesets may be open and may, during the course of their > lifetime, modify the same objects.
Non-atomic changesets are both a fatal problem in designing and constructing a history and at the same time, irrelevant. Creating a true history is impossible. The information about the concurrent updates across different non-atomic changesets is presumably lost. This is not actually a problem. To present a history, all that is needed is to define *an* ordering that is usefully consistent with the OSM database history. Pretending changsets are atomic lets me define/construct one such ordering, using changeset numbers or ending timestamps. Peter's design seems to make a similar assumption about atomicity of changesets. Other orderings, such ignoring changeset numbers entirely and using timestamps of entity changes would have their own tradeoffs. Scott _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
