> From: Stephan Knauss [mailto:[email protected]] > Sent: Tuesday, July 23, 2013 5:30 AM > Cc: [email protected] > Subject: Re: [OSM-talk] Is there some lag in the backend data? > > For me this sounds more like a replication problem. Data is available > immediately. It's not related to closing a changeset. > > Is the API delivering /map call responses from the main database or from > a read-only replication?
The main API is either from the main database or from a database set up with postgresql replication where the latency is measured in milliseconds. > If its the later than JOSM might need to be extended to evaluate a > timestamp to detect outdated responses. Although this talk@ message isn't relevant for JOSM accessing the main API as it currently exists, it made me think. My GSOC project is a service a mirror that uses replication *and* you can use as a replacement for the main API in your client. E.g. with JOSM you can change the API URL in it and have everything work seamlessly. What this talk@ message got me wondering is what if anything needs changing in an editor to account for replication lag. Due to the nature of replication there can be a 0-5 minute delay in information getting into the replicated database. A good server can average about 45 seconds and streaming replication has the potential to bring this down to under 15 seconds. This does not make conflicts with other users worse. The time required for download, editing and upload is long enough in comparison to replication lag that you won't have a significant increase in conflicts. Where it might run into problems is if there is a scenario where a user uploads changes and then immediately requests the latest version from the API. In this scenario my service would of transparently proxied the main API for the upload then answered from its own data to the latest version request. The reply for the upload and the reply to the latest version request would then be inconsistent. On the other hand, I can't see when a client would actually do this. Is anyone aware of any clients where this scenario can come up, or of other scenarios where different API calls having different delays could matter? _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
