refactor(trainer): address PR review feedback for pod selection logic

- Give Running and Succeeded pods equal priority (both are healthy states)
- Update docstring to clearly explain equal priority and timestamp tiebreaker
- Remove JOB_INDEX_LABEL .get() default, use direct access
- Move _select_best_pod_for_role method after public methods per convention

Addresses review comments from @kramaranya on PR #160

Signed-off-by: HKanoje <hrithik.kanoje@gmail.com>

Author: HKanoje

kubeflow/trainer/backends/kubernetes/backend.py

@@ -58,58 +58,6 @@ def __init__(self, cfg: KubernetesBackendConfig):
 
         self.namespace = cfg.namespace
 
-    def _select_best_pod_for_role(
-        self, pods: list[models.IoK8sApiCoreV1Pod]
-    ) -> Optional[models.IoK8sApiCoreV1Pod]:
-        """
-        Select the best Pod for a role based on status priority and creation timestamp.
-
-        Priority order:
-        1. Running or Succeeded Pods (prefer most recent)
-        2. Failed Pods (prefer most recent)
-        3. Pending Pods (prefer most recent)
-        4. Unknown Pods (prefer most recent)
-        """
-        if not pods:
-            return None
-
-        # Pod status priority (higher number = higher priority)
-        status_priority = {
-            constants.POD_RUNNING: 4,  # Highest priority
-            constants.POD_SUCCEEDED: 3,  # Second highest
-            constants.POD_FAILED: 2,  # Third priority
-            constants.POD_PENDING: 1,  # Low priority
-            constants.POD_UNKNOWN: 0,  # Lowest priority
-        }
-
-        # Group Pods by status priority
-        pods_by_status = {}
-        for pod in pods:
-            status = pod.status.phase if pod.status else constants.POD_UNKNOWN
-            priority = status_priority.get(status, 0)
-
-            if priority not in pods_by_status:
-                pods_by_status[priority] = []
-            pods_by_status[priority].append(pod)
-
-        # Find the highest priority status that has Pods
-        highest_priority = max(pods_by_status.keys()) if pods_by_status else 0
-        candidate_pods = pods_by_status[highest_priority]
-
-        # Among Pods with the same status, select the most recent one
-        if len(candidate_pods) == 1:
-            return candidate_pods[0]
-
-        # Sort by creation timestamp (most recent first)
-        candidate_pods.sort(
-            key=lambda p: (
-                p.metadata.creation_timestamp or datetime.datetime.min.replace(tzinfo=timezone.utc)
-            ),
-            reverse=True,
-        )
-
-        return candidate_pods[0]
-
     def list_runtimes(self) -> list[types.Runtime]:
         result = []
         try:
@@ -498,6 +446,62 @@ def __get_runtime_from_cr(
             ),
         )
 
+    def _select_best_pod_for_role(
+        self, pods: list[models.IoK8sApiCoreV1Pod]
+    ) -> Optional[models.IoK8sApiCoreV1Pod]:
+        """
+        Select the best Pod for a role based on status priority and creation timestamp.
+
+        Priority order (higher priority = preferred):
+        1. Running or Succeeded Pods (equal priority, prefer most recent)
+        2. Failed Pods (prefer most recent)
+        3. Pending Pods (prefer most recent)
+        4. Unknown Pods (prefer most recent)
+
+        Both Running and Succeeded are considered healthy states with equal priority.
+        When multiple pods share the same priority, the most recently created pod is selected.
+        """
+        if not pods:
+            return None
+
+        # Pod status priority (higher number = higher priority)
+        # Running and Succeeded have equal priority as both are healthy states
+        status_priority = {
+            constants.POD_RUNNING: 4,  # Highest priority (healthy)
+            constants.POD_SUCCEEDED: 4,  # Highest priority (healthy)
+            constants.POD_FAILED: 2,  # Lower priority
+            constants.POD_PENDING: 1,  # Low priority
+            constants.POD_UNKNOWN: 0,  # Lowest priority
+        }
+
+        # Group Pods by status priority
+        pods_by_status = {}
+        for pod in pods:
+            status = pod.status.phase if pod.status else constants.POD_UNKNOWN
+            priority = status_priority.get(status, 0)
+
+            if priority not in pods_by_status:
+                pods_by_status[priority] = []
+            pods_by_status[priority].append(pod)
+
+        # Find the highest priority status that has Pods
+        highest_priority = max(pods_by_status.keys()) if pods_by_status else 0
+        candidate_pods = pods_by_status[highest_priority]
+
+        # Among Pods with the same priority, select the most recent one
+        if len(candidate_pods) == 1:
+            return candidate_pods[0]
+
+        # Sort by creation timestamp (most recent first)
+        candidate_pods.sort(
+            key=lambda p: (
+                p.metadata.creation_timestamp or datetime.datetime.min.replace(tzinfo=timezone.utc)
+            ),
+            reverse=True,
+        )
+
+        return candidate_pods[0]
+
     def _read_pod_logs(self, pod_name: str, container_name: str, follow: bool) -> Iterator[str]:
         """Read logs from a pod container."""
         try:
@@ -585,7 +589,7 @@ def __get_trainjob_from_cr(
                 # - For training nodes: use role + job index (node-0, node-1, launcher-0, etc.)
                 role = pod.metadata.labels[constants.JOBSET_RJOB_NAME_LABEL]
                 if role in {constants.LAUNCHER, constants.NODE}:
-                    job_index = pod.metadata.labels.get(constants.JOB_INDEX_LABEL, "0")
+                    job_index = pod.metadata.labels[constants.JOB_INDEX_LABEL]
                     key = f"{role}-{job_index}"
                 else:
                     key = role
@@ -627,7 +631,7 @@ def __get_trainjob_from_cr(
                             pod.status,
                             trainjob.runtime,
                             pod.metadata.labels[constants.JOBSET_RJOB_NAME_LABEL],
-                            int(pod.metadata.labels.get(constants.JOB_INDEX_LABEL, "0")),
+                            int(pod.metadata.labels[constants.JOB_INDEX_LABEL]),
                         )
                     )
         except multiprocessing.TimeoutError as e: