Only use multiprocess mode is env var is set.

tiled/config.py

@@ -172,24 +172,7 @@ def construct_build_app_kwargs(
             "response_bytesize_limit"
         )
         server_settings["database"] = config.get("database", {})
-        metrics = config.get("metrics", {})
-        if metrics.get("prometheus", False):
-            prometheus_multiproc_dir = os.getenv("PROMETHEUS_MULTIPROC_DIR", None)
-            if not prometheus_multiproc_dir:
-                raise ValueError(
-                    "prometheus enabled but PROMETHEUS_MULTIPROC_DIR env variable not set"
-                )
-            elif not Path(prometheus_multiproc_dir).is_dir():
-                raise ValueError(
-                    "prometheus enabled but PROMETHEUS_MULTIPROC_DIR "
-                    f"({prometheus_multiproc_dir}) is not a directory"
-                )
-            elif not os.access(prometheus_multiproc_dir, os.W_OK):
-                raise ValueError(
-                    "prometheus enabled but PROMETHEUS_MULTIPROC_DIR "
-                    f"({prometheus_multiproc_dir}) is not writable"
-                )
-        server_settings["metrics"] = metrics
+        server_settings["metrics"] = config.get("metrics", {})
         for structure_family, values in config.get("media_types", {}).items():
             for media_type, import_path in values.items():
                 serializer = import_object(import_path, accept_live_object=True)

tiled/server/app.py

@@ -605,6 +605,24 @@ def override_get_validation_registry():
 
     metrics_config = server_settings.get("metrics", {})
     if metrics_config.get("prometheus", True):
+
+        # PROMETHEUS_MULTIRPOC_DIR puts prometheus_client in multiprocess mode
+        # (for e.g. gunicorn) which uses a directory of memory-mapped files.
+        # If that environment variable is set, check that the directory exists
+        # and is writable.
+        prometheus_multiproc_dir = os.getenv("PROMETHEUS_MULTIPROC_DIR", None)
+        if prometheus_multiproc_dir:
+            if not Path(prometheus_multiproc_dir).is_dir():
+                raise ValueError(
+                    "prometheus enabled and PROMETHEUS_MULTIPROC_DIR is set but "
+                    f"({prometheus_multiproc_dir}) is not a directory"
+                )
+            if not os.access(prometheus_multiproc_dir, os.W_OK):
+                raise ValueError(
+                    "prometheus enabled and PROMETHEUS_MULTIPROC_DIR is set but "
+                    f"({prometheus_multiproc_dir}) is not writable"
+                )
+
         from . import metrics
 
         app.include_router(metrics.router, prefix="/api/v1")

tiled/server/metrics.py

@@ -4,6 +4,7 @@
 `tiled_<noun>_<verb>_<type_suffix>`.
 """
 
+import os
 from functools import lru_cache
 
 from fastapi import APIRouter, Request, Response, Security
@@ -140,13 +141,17 @@ def prometheus_registry():
 
     This is run the first time the /metrics endpoint is used.
     """
-    # The multiprocess configuration makes it compatible with gunicorn.
-    # https://github.com/prometheus/client_python/#multiprocess-mode-eg-gunicorn
-    from prometheus_client import CollectorRegistry
-    from prometheus_client.multiprocess import MultiProcessCollector
+    if os.environ.get("PROMETHEUS_MULTIPROC_DIR"):
+        # The multiprocess configuration makes it compatible with gunicorn.
+        # https://github.com/prometheus/client_python/#multiprocess-mode-eg-gunicorn
+        from prometheus_client.multiprocess import MultiProcessCollector
+        from prometheus_client import CollectorRegistry
+
+        registry = CollectorRegistry()
+        MultiProcessCollector(registry)  # This has a side effect.
+    else:
+        from prometheus_client import REGISTRY as registry
 
-    registry = CollectorRegistry()
-    MultiProcessCollector(registry)  # This has a side effect, apparently.
     return registry