Better error when collections across separate backends #153

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is roughly based on [Keep a Changelog](https://keepachangelog.com/en/
 
 <!-- start changelog -->
 
+## 0.37.1
+
+- Better error message when requested collections are spread across separate backends ([#153](https://github.com/Open-EO/openeo-aggregator/issues/153))
+
 ## 0.37.0
 
 - move example configs inside package source tree ([#117](https://github.com/Open-EO/openeo-aggregator/issues/117))

src/openeo_aggregator/about.py

@@ -2,7 +2,7 @@
 import sys
 from typing import Optional
 
-__version__ = "0.37.0a1"
+__version__ = "0.37.1a1"
 
 
 def log_version_info(logger: Optional[logging.Logger] = None):

src/openeo_aggregator/backend.py

@@ -321,9 +321,16 @@ def get_backend_candidates_for_collections(self, collections: Iterable[str]) ->
             if intersection:
                 backend_candidates = list(intersection)
             else:
+                # No single backend that provides each of requested collections
+                # Report collections that are only available on certain backends
                 union = functools.reduce(lambda a, b: set(a).union(b), backend_combos)
+                only_on = [
+                    f"{cid!r} only on {list(backends)}"
+                    for cid, backends in collection_backends_map.items()
+                    if union.difference(backends)
+                ]
                 raise BackendLookupFailureException(
-                    message=f"Collections across multiple backends ({union}): {collections}."
+                    message=f"Requested collections are not available on a single backend, but spread across separate ones: {', '.join(only_on)}."
                 )
 
         _log.info(f"Backend candidates {backend_candidates} for collections {collections}")

tests/test_backend.py

@@ -1077,7 +1077,10 @@ def test_get_best_backend_for_collections_basic(self, catalog, backend1, backend
         assert catalog.get_backend_candidates_for_collections(["S3", "S4"]) == ["b1"]
         assert catalog.get_backend_candidates_for_collections(["S4", "S5"]) == ["b2"]
 
-        with pytest.raises(OpenEOApiException, match="Collections across multiple backends"):
+        with pytest.raises(
+            OpenEOApiException,
+            match=r"Requested collections are not available on a single backend, but spread across separate ones: 'S3' only on \['b1'\], 'S5' only on \['b2'\]\.",
+        ):
             catalog.get_backend_candidates_for_collections(["S3", "S4", "S5"])
 
     def test_get_collection_metadata_basic(self, catalog, backend1, backend2, requests_mock):