I would like to use osm2po as a routing engine/library. My problem is that in geojson response, there is no osm_id value returned, only id field which, if I assume correctly is not a fixed value. I mean if I made new graph file from newer osm data, id of the same feature/edge could be different. Is there a way to get osm_id from service response?
No. (Not yet). But if you can give me a good use case, I will think about it. Alternatively use the returned SegmentId for Database-lookups. The value corresponds to the ID-Field in the 2po-table where all corresponding osm-IDs are in.
Nevertheless, osm2po-IDs change because OSM-IDs do as well.