From fb81270c2c26198310b09fdc8322644f46e0b4fc Mon Sep 17 00:00:00 2001
From: water <672684719@qq.com>
Date: Tue, 25 Aug 2026 05:08:37 +0800
Subject: [PATCH] fix: Make HttpLoadQueuePeon degrade gracefully instead of
 throwing on fetch failure
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When a historical server returns a non-200/404 response or is unreachable,
fetchSegmentLoadingCapabilities() throws an RE, which propagates through
LoadQueueTaskMaster.resetPeonsForNewServers() into PrepareBalancerAndLoadQueues.
The top-level DruidCoordinator.DutiesRunnable.run() catches and logs it, but
segment management stops entirely for ALL servers — not just the unhealthy one.

Fix: degrade gracefully by returning default SegmentLoadingCapabilities derived
from the configured batch size, with a warning log. The peon is still created,
the server is still managed with conservative defaults, and the rest of the
duty group proceeds normally.
---
 .../coordinator/loading/HttpLoadQueuePeon.java       | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/server/src/main/java/org/apache/druid/server/coordinator/loading/HttpLoadQueuePeon.java b/server/src/main/java/org/apache/druid/server/coordinator/loading/HttpLoadQueuePeon.java
index 37ebaa8d9a07..b402c729dc8e 100644
--- a/server/src/main/java/org/apache/druid/server/coordinator/loading/HttpLoadQueuePeon.java
+++ b/server/src/main/java/org/apache/druid/server/coordinator/loading/HttpLoadQueuePeon.java
@@ -178,7 +178,13 @@ private SegmentLoadingCapabilities fetchSegmentLoadingCapabilities()
         return defaultCapabilities;
       } else if (HttpServletResponse.SC_OK != responseHandler.getStatus()) {
         log.makeAlert("Received status[%s] when fetching loading capabilities from server[%s]", responseHandler.getStatus(), serverId);
-        throw new RE("Received status[%s] when fetching loading capabilities from server[%s]", responseHandler.getStatus(), serverId);
+        int batchSize = config.getBatchSize() == null ? 1 : config.getBatchSize();
+        SegmentLoadingCapabilities defaultCapabilities = new SegmentLoadingCapabilities(batchSize, batchSize);
+        log.warn(
+            "Failed to fetch loading capabilities from server[%s]. Received status[%s]. Using default capabilities[%s].",
+            serverId, responseHandler.getStatus(), defaultCapabilities
+        );
+        return defaultCapabilities;
       }
 
       return jsonMapper.readValue(
@@ -187,7 +193,9 @@ private SegmentLoadingCapabilities fetchSegmentLoadingCapabilities()
       );
     }
     catch (Throwable th) {
-      throw new RE(th, "Received error while fetching historical capabilities from Server[%s].", serverId);
+      log.warn(th, "Failed to fetch loading capabilities from server[%s]. Using default capabilities.", serverId);
+      int batchSize = config.getBatchSize() == null ? 1 : config.getBatchSize();
+      return new SegmentLoadingCapabilities(batchSize, batchSize);
     }
   }