feat: Warn on unsafe serve_features_while_materializing configs

Emit advisory (non-fatal) warnings when the flag is combined with a non-sql registry (no-op) or cache_mode='thread' (state transitions may be observed stale). Signed-off-by: Alan Gauthier <alan.gauthier@jobteaser.com>
feast-dev · ntkathole · Sep 29, 2026 · Aug 27, 2026 · Sep 2, 2026 · Sep 10, 2026
commit c473fa3a2acfae750430495ea7d1c2aa86c5a5fb
@@ -247,6 +247,30 @@ def validate_path(cls, path: str, values: ValidationInfo) -> str:
             return cls._normalize_postgres_scheme(path, "path")
         return path
 
+    @model_validator(mode="after")
+    def _warn_on_unsafe_serve_while_materializing(self) -> "RegistryConfig":
+        """Warn when ``serve_features_while_materializing`` is combined with a
+        registry configuration where it is a no-op or where its guarantees may
+        not hold. These are advisory warnings only, never hard failures.
+        """
+        if self.serve_features_while_materializing:
+            if self.registry_type != "sql":
+                _logger.warning(
+                    "`serve_features_while_materializing` is enabled but "
+                    f"`registry_type` is '{self.registry_type}'. The flag targets "
+                    "the serving interruption caused by concurrent servers sharing "
+                    "a `sql` registry that materialization flips to `MATERIALIZING`; "
+                    "on other registry types it typically has no effect."
+                )
+            if self.cache_mode == "thread":
+                _logger.warning(
+                    "`serve_features_while_materializing` is enabled together with "
+                    "`cache_mode='thread'`. In thread mode the registry cache may lag "
+                    "by up to `cache_ttl_seconds`, so feature-view state transitions "
+                    "(including the exit from `MATERIALIZING`) may be observed stale."
+                )
+        return self
+
 
 class MaterializationConfig(BaseModel):
     """Configuration options for feature materialization behavior."""

@@ -135,3 +135,47 @@ def test_pathlib_does_not_treat_s3_as_absolute(self):
 
         joined = Path("/app").joinpath(s3_path)
         assert str(joined).startswith("/app/s3:")
+
+
+class TestServeWhileMaterializingWarnings:
+    """`serve_features_while_materializing` emits advisory warnings when combined
+    with a registry configuration where the flag is a no-op or where its
+    guarantees may not hold. These are warnings only, never hard failures."""
+
+    def test_no_warning_when_flag_disabled(self, caplog):
+        with caplog.at_level("WARNING", logger="feast.repo_config"):
+            RegistryConfig(registry_type="sql")
+        assert "serve_features_while_materializing" not in caplog.text
+
+    def test_no_warning_for_safe_sql_sync_config(self, caplog):
+        with caplog.at_level("WARNING", logger="feast.repo_config"):
+            RegistryConfig(registry_type="sql", serve_features_while_materializing=True)
+        assert "serve_features_while_materializing" not in caplog.text
+
+    def test_warns_on_non_sql_registry(self, caplog):
+        with caplog.at_level("WARNING", logger="feast.repo_config"):
+            RegistryConfig(
+                registry_type="file", serve_features_while_materializing=True
+            )
+        assert "registry_type" in caplog.text
+        assert "cache_mode" not in caplog.text
+
+    def test_warns_on_thread_cache_mode(self, caplog):
+        with caplog.at_level("WARNING", logger="feast.repo_config"):
+            RegistryConfig(
+                registry_type="sql",
+                cache_mode="thread",
+                serve_features_while_materializing=True,
+            )
+        assert "cache_mode='thread'" in caplog.text
+        assert "registry_type" not in caplog.text
+
+    def test_warns_on_both_when_both_incompatible(self, caplog):
+        with caplog.at_level("WARNING", logger="feast.repo_config"):
+            RegistryConfig(
+                registry_type="file",
+                cache_mode="thread",
+                serve_features_while_materializing=True,
+            )
+        assert "registry_type" in caplog.text
+        assert "cache_mode='thread'" in caplog.text