janhoy commented on code in PR #96:
URL: https://github.com/apache/solr/pull/96#discussion_r1310122961


##########
solr/core/src/java/org/apache/solr/util/circuitbreaker/LoadAverageCircuitBreaker.java:
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@@ -0,0 +1,115 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.solr.util.circuitbreaker;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.management.ManagementFactory;
+import java.lang.management.OperatingSystemMXBean;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * <p>
+ * Tracks current system load average and triggers if the specified threshold 
is breached.
+ *
+ * This circuit breaker gets the load average (length of the run queue) over 
the last
+ * minute and uses that data to take a decision. We depend on 
OperatingSystemMXBean which does
+ * not allow a configurable interval of collection of data.
+ * //TODO: Use Codahale Meter to calculate the value locally.
+ * </p>
+ *
+ * <p>
+ * The configuration to define which mode to use and the trigger threshold are 
defined in
+ * solrconfig.xml
+ * </p>
+ */
+public class LoadAverageCircuitBreaker extends CircuitBreaker {
+  private static final Logger log = 
LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+  private static final OperatingSystemMXBean operatingSystemMXBean = 
ManagementFactory.getOperatingSystemMXBean();
+
+  private final boolean enabled;
+  private final double loadAverageThreshold;
+
+  // Assumption -- the value of these parameters will be set correctly before 
invoking getDebugInfo()
+  private static final ThreadLocal<Double> seenLoadAverage = 
ThreadLocal.withInitial(() -> 0.0);
+
+  private static final ThreadLocal<Double> allowedLoadAverage = 
ThreadLocal.withInitial(() -> 0.0);
+
+  public LoadAverageCircuitBreaker(CircuitBreakerConfig config) {
+    super(config);
+
+    this.enabled = config.getLoadAverageCBEnabled();
+    this.loadAverageThreshold = config.getLoadAverageCBThreshold();
+  }
+
+  @Override
+  public boolean isTripped() {
+    if (!isEnabled()) {
+      return false;
+    }
+
+    if (!enabled) {
+      return false;
+    }
+
+    double localAllowedLoadAverage = getLoadAverageThreshold();
+    double localSeenLoadAverage = calculateLiveLoadAverage();
+
+    if (localSeenLoadAverage < 0) {
+      if (log.isWarnEnabled()) {

Review Comment:
   After #1725 this is less of a problem. Users choose one or more CB plugins 
to activate, and each is named and documented precisely. The legacy 
`CircuitBreakerManager` is a deprecated plugin of its own, and its 
`cpuThreshold` has not changed semantics, still uses load avg.



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