Reduce registry calls in docker exporter checks

monitoring/docker-exporter/exporter.py

@@ -411,6 +411,9 @@ def check_containers():
     svc_map = parse_compose_services(compose_files, project_name, project_root)
 
     containers = client.containers.list()
+    pending_metrics = []
+    remote_targets = set()
+
     for container in containers:
         proj = container.labels.get("com.docker.compose.project")
         if not proj:
@@ -423,30 +426,47 @@ def check_containers():
         if svc in svc_map:
             compose_image = svc_map[svc]["image"]
 
-        update_flag = 0
-
         local_digest = get_local_digest(running)
         remote_target = compose_image or running
-        remote_digest = get_remote_digest(remote_target)
+
+        # If we cannot determine a local digest, we cannot compare and should
+        # avoid spending a registry lookup for this container.
+        if local_digest:
+            remote_targets.add(remote_target)
+
+        pending_metrics.append({
+            "container_name": container.name,
+            "service": svc,
+            "compose_image": compose_image,
+            "running_image": running,
+            "project_name": proj,
+            "remote_target": remote_target,
+            "local_digest": local_digest,
+        })
+
+    remote_digests = {target: get_remote_digest(target) for target in remote_targets}
+
+    for payload in pending_metrics:
+        local_digest = payload["local_digest"]
+        remote_target = payload["remote_target"]
+        remote_digest = remote_digests.get(remote_target)
+        update_flag = 1 if (local_digest and remote_digest and local_digest != remote_digest) else 0
 
         logger.info(
             "Digest comparison: container=%s service=%s running=%s target=%s local=%s remote=%s",
-            container.name,
-            svc,
-            running,
+            payload["container_name"],
+            payload["service"],
+            payload["running_image"],
             remote_target,
             local_digest,
             remote_digest,
         )
 
-        if local_digest and remote_digest and local_digest != remote_digest:
-            update_flag = 1
-
         set_container_update_metric(
-            container_name=container.name,
-            compose_image=compose_image,
-            running_image=running,
-            project_name=proj,
+            container_name=payload["container_name"],
+            compose_image=payload["compose_image"],
+            running_image=payload["running_image"],
+            project_name=payload["project_name"],
             update_flag=update_flag,
         )