Add graph version information to response

[gardster]

I encountered such a problem while using OSRM: I have an OSRM cluster behind a load balancer that processes a continuous stream of requests. To update the road graph, I'm restarting servers one by one. At some point, several servers running with the old graph and some servers with the new one. As far as I understand, there is no way to find out on which version of the graph it was created from the OSRM answer. I suggest to add such field into the response. This can be a data timestamp from the pbf file, or the value of some parameter passed on the stage of osrm-extract.

[TheMarex]

Can you say more about what the use-case there is? Why does the client need to know about the data version?

[gardster]

Sure.
For example I have a "toll roads expenses" service. I have a matched tracks from the OSRM and some server with information about toll pricing. This server looks for toll roads in OSRM answer and returns prices if it can find toll boothes. So both services have geography information from the OSM (geometry, osm_ids and so on) and ideally need to deploy simultaneously. When someone make a vast graph modification and OSRMs answer has a different geometry than pricing server supposed it can lead to mistakes when the old pricing server tries to find objects on track. So we need additional information to be sure that all services works on same version of geography information.

[TheMarex]

Ah interesting. So we did have something similar not too long ago but removed it because we did not have a use-case for it. 😅

To add this feature I would recommend the following steps:

• Get the timestamp from here
• Add a new functions to writeTimestamp/readTimestamp to https://github.com/Project-OSRM/osrm-backend/blob/master/include/extractor/files.hpp and pass the timestamp there
• Add an entry .osrm.timestamp here
• Create a create_timestmap_view function here that uses index.GetBlockPtr<T>(name) to get a shared memory region for the timestamp
• Call create_timestamp_view and readTimestamp from somewhere here
• Extend the interface BaseDatafacade for a new method GetTimestamp
• Extend the RouteApi to add the timestamp to the JSON response here by calling facade.GetTimestamp()

[gardster]

Thank you for the detailed description of necessary steps.