SOLR-8906: Make transient core cache pluggable

solr/CHANGES.txt

@@ -196,6 +196,8 @@ Other Changes
 
 * SOLR-9601: Redone DataImportHandler 'tika' example, removing all unused and irrelevant definitions (Alexandre Rafalovitch)
 
+* SOLR-8906: Make transient core cache pluggable (Erick Erickson)
+
 ==================  6.5.1 ==================
 
 Bug Fixes

solr/core/src/java/org/apache/solr/core/CoreContainer.java

@@ -131,6 +131,7 @@ public class CoreContainer {
 
   protected CoreAdminHandler coreAdminHandler = null;
   protected CollectionsHandler collectionsHandler = null;
+  protected TransientSolrCoreCache transientSolrCoreCache = null;
   private InfoHandler infoHandler;
   protected ConfigSetsHandler configSetsHandler = null;
 
@@ -145,6 +146,8 @@ public class CoreContainer {
 
   private UpdateShardHandler updateShardHandler;
 
+  private TransientSolrCoreCacheFactory transientCoreCache;
+  
   private ExecutorService coreContainerWorkExecutor = ExecutorUtil.newMDCAwareCachedThreadPool(
       new DefaultSolrThreadFactory("coreContainerWorkExecutor") );
 
@@ -495,7 +498,7 @@ public class CoreContainer {
     updateShardHandler = new UpdateShardHandler(cfg.getUpdateShardHandlerConfig());
     updateShardHandler.initializeMetrics(metricManager, SolrInfoMBean.Group.node.toString(), "updateShardHandler");
 
-    solrCores.allocateLazyCores(cfg.getTransientCacheSize(), loader);
+    transientCoreCache = TransientSolrCoreCacheFactory.newInstance(loader, this);
 
     logging = LogWatcher.newRegisteredLogWatcher(cfg.getLogWatcherConfig(), loader);
 
@@ -541,9 +544,9 @@ public class CoreContainer {
     String registryName = SolrMetricManager.getRegistryName(SolrInfoMBean.Group.node);
     metricManager.registerGauge(registryName, () -> solrCores.getCores().size(),
         true, "loaded", SolrInfoMBean.Category.CONTAINER.toString(), "cores");
-    metricManager.registerGauge(registryName, () -> solrCores.getCoreNames().size() - solrCores.getCores().size(),
+    metricManager.registerGauge(registryName, () -> solrCores.getLoadedCoreNames().size() - solrCores.getCores().size(),
         true, "lazy",SolrInfoMBean.Category.CONTAINER.toString(), "cores");
-    metricManager.registerGauge(registryName, () -> solrCores.getAllCoreNames().size() - solrCores.getCoreNames().size(),
+    metricManager.registerGauge(registryName, () -> solrCores.getAllCoreNames().size() - solrCores.getLoadedCoreNames().size(),
         true, "unloaded",SolrInfoMBean.Category.CONTAINER.toString(), "cores");
     metricManager.registerGauge(registryName, () -> cfg.getCoreRootDirectory().toFile().getTotalSpace(),
         true, "totalSpace", SolrInfoMBean.Category.CONTAINER.toString(), "fs");
@@ -635,6 +638,16 @@ public class CoreContainer {
     }
   }
 
+  public TransientSolrCoreCache getTransientCacheHandler() {
+
+    if (transientCoreCache == null) {
+      log.error("No transient handler has been defined. Check solr.xml to see if an attempt to provide a custom " +
+          "TransientSolrCoreCacheFactory was done incorrectly since the default should have been used otherwise.");
+      return null;
+    }
+    return transientCoreCache.getTransientSolrCoreCache();
+  }
+  
   public void securityNodeChanged() {
     log.info("Security node changed, reloading security.json");
     reloadSecurityProperties();
@@ -1082,10 +1095,10 @@ public class CoreContainer {
   }
 
   /**
-   * @return a Collection of the names that cores are mapped to
+   * @return a Collection of the names that loaded cores are mapped to
    */
   public Collection<String> getCoreNames() {
-    return solrCores.getCoreNames();
+    return solrCores.getLoadedCoreNames();
   }
 
   /** This method is currently experimental.
@@ -1098,6 +1111,8 @@ public class CoreContainer {
   /**
    * get a list of all the cores that are currently loaded
    * @return a list of al lthe available core names in either permanent or transient core lists.
+   * 
+   * Note: this implies that the core is loaded
    */
   public Collection<String> getAllCoreNames() {
     return solrCores.getAllCoreNames();

solr/core/src/java/org/apache/solr/core/NodeConfig.java

@@ -52,6 +52,8 @@ public class NodeConfig {
 
   private final Integer coreLoadThreads;
 
+  @Deprecated
+  // This should be part of the transientCacheConfig, remove in 7.0
   private final int transientCacheSize;
 
   private final boolean useSchemaCache;
@@ -62,6 +64,8 @@ public class NodeConfig {
 
   private final PluginInfo[] metricReporterPlugins;
 
+  private final PluginInfo transientCacheConfig;
+
   private NodeConfig(String nodeName, Path coreRootDirectory, Path configSetBaseDirectory, String sharedLibDirectory,
                      PluginInfo shardHandlerFactoryConfig, UpdateShardHandlerConfig updateShardHandlerConfig,
                      String coreAdminHandlerClass, String collectionsAdminHandlerClass,
@@ -69,7 +73,7 @@ public class NodeConfig {
                      LogWatcherConfig logWatcherConfig, CloudConfig cloudConfig, Integer coreLoadThreads,
                      int transientCacheSize, boolean useSchemaCache, String managementPath, SolrResourceLoader loader,
                      Properties solrProperties, PluginInfo[] backupRepositoryPlugins,
-                     PluginInfo[] metricReporterPlugins) {
+                     PluginInfo[] metricReporterPlugins, PluginInfo transientCacheConfig) {
     this.nodeName = nodeName;
     this.coreRootDirectory = coreRootDirectory;
     this.configSetBaseDirectory = configSetBaseDirectory;
@@ -90,6 +94,7 @@ public class NodeConfig {
     this.solrProperties = solrProperties;
     this.backupRepositoryPlugins = backupRepositoryPlugins;
     this.metricReporterPlugins = metricReporterPlugins;
+    this.transientCacheConfig = transientCacheConfig;
 
     if (this.cloudConfig != null && this.getCoreLoadThreadCount(false) < 2) {
       throw new SolrException(SolrException.ErrorCode.SERVER_ERROR,
@@ -182,6 +187,8 @@ public class NodeConfig {
     return metricReporterPlugins;
   }
 
+  public PluginInfo getTransientCachePluginInfo() { return transientCacheConfig; }
+
   public static class NodeConfigBuilder {
 
     private Path coreRootDirectory;
@@ -195,13 +202,16 @@ public class NodeConfig {
     private String configSetsHandlerClass = DEFAULT_CONFIGSETSHANDLERCLASS;
     private LogWatcherConfig logWatcherConfig = new LogWatcherConfig(true, null, null, 50);
     private CloudConfig cloudConfig;
-    private Integer coreLoadThreads;
+    private int coreLoadThreads = DEFAULT_CORE_LOAD_THREADS;
+    @Deprecated
+    //Remove in 7.0 and put it all in the transientCache element in solrconfig.xml
     private int transientCacheSize = DEFAULT_TRANSIENT_CACHE_SIZE;
     private boolean useSchemaCache = false;
     private String managementPath;
     private Properties solrProperties = new Properties();
     private PluginInfo[] backupRepositoryPlugins;
     private PluginInfo[] metricReporterPlugins;
+    private PluginInfo transientCacheConfig;
 
     private final SolrResourceLoader loader;
     private final String nodeName;
@@ -210,7 +220,7 @@ public class NodeConfig {
     //No:of core load threads in cloud mode is set to a default of 8
     public static final int DEFAULT_CORE_LOAD_THREADS_IN_CLOUD = 8;
 
-    private static final int DEFAULT_TRANSIENT_CACHE_SIZE = Integer.MAX_VALUE;
+    public static final int DEFAULT_TRANSIENT_CACHE_SIZE = Integer.MAX_VALUE;
 
     private static final String DEFAULT_ADMINHANDLERCLASS = "org.apache.solr.handler.admin.CoreAdminHandler";
     private static final String DEFAULT_INFOHANDLERCLASS = "org.apache.solr.handler.admin.InfoHandler";
@@ -284,6 +294,8 @@ public class NodeConfig {
       return this;
     }
 
+    // Remove in Solr 7.0
+    @Deprecated
     public NodeConfigBuilder setTransientCacheSize(int transientCacheSize) {
       this.transientCacheSize = transientCacheSize;
       return this;
@@ -314,11 +326,16 @@ public class NodeConfig {
       return this;
     }
     
+    public NodeConfigBuilder setSolrCoreCacheFactoryConfig(PluginInfo transientCacheConfig) {
+      this.transientCacheConfig = transientCacheConfig;
+      return this;
+    }
+
     public NodeConfig build() {
       return new NodeConfig(nodeName, coreRootDirectory, configSetBaseDirectory, sharedLibDirectory, shardHandlerFactoryConfig,
                             updateShardHandlerConfig, coreAdminHandlerClass, collectionsAdminHandlerClass, infoHandlerClass, configSetsHandlerClass,
                             logWatcherConfig, cloudConfig, coreLoadThreads, transientCacheSize, useSchemaCache, managementPath, loader, solrProperties,
-                            backupRepositoryPlugins, metricReporterPlugins);
+                            backupRepositoryPlugins, metricReporterPlugins, transientCacheConfig);
     }
   }
 }

solr/core/src/java/org/apache/solr/core/SolrCores.java

@@ -17,6 +17,7 @@
 package org.apache.solr.core;
 
 import com.google.common.collect.Lists;
+import org.apache.http.annotation.Experimental;
 import org.apache.solr.common.SolrException;
 import org.apache.solr.common.util.ExecutorUtil;
 import org.apache.solr.logging.MDCLoggingContext;
@@ -32,6 +33,8 @@ import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Observable;
+import java.util.Observer;
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.concurrent.ConcurrentHashMap;
@@ -39,15 +42,12 @@ import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
 
 
-class SolrCores {
+class SolrCores implements Observer {
 
   private static Object modifyLock = new Object(); // for locking around manipulating any of the core maps.
   private final Map<String, SolrCore> cores = new LinkedHashMap<>(); // For "permanent" cores
 
-  //WARNING! The _only_ place you put anything into the list of transient cores is with the putTransientCore method!
+  private final Map<String, CoreDescriptor> lazyDescriptors = new LinkedHashMap<>();
-  private Map<String, SolrCore> transientCores = new LinkedHashMap<>(); // For "lazily loaded" cores
-
-  private final Map<String, CoreDescriptor> dynamicDescriptors = new LinkedHashMap<>();
 
   private final CoreContainer container;
   
@@ -67,32 +67,18 @@ class SolrCores {
     this.container = container;
   }
   
-  // Trivial helper method for load, note it implements LRU on transient cores. Also note, if
+  protected void putDynamicDescriptor(String rawName, CoreDescriptor cd) {
-  // there is no setting for max size, nothing is done and all cores go in the regular "cores" list
-  protected void allocateLazyCores(final int cacheSize, final SolrResourceLoader loader) {
-    if (cacheSize != Integer.MAX_VALUE) {
-      log.info("Allocating transient cache for {} transient cores", cacheSize);
-      transientCores = new LinkedHashMap<String, SolrCore>(cacheSize, 0.75f, true) {
-        @Override
-        protected boolean removeEldestEntry(Map.Entry<String, SolrCore> eldest) {
-          if (size() > cacheSize) {
     synchronized (modifyLock) {
-              SolrCore coreToClose = eldest.getValue();
+      if (cd.isTransient()) {
-              log.info("Closing transient core [{}]", coreToClose.getName());
+        if (container.getTransientCacheHandler() != null) {
-              pendingCloses.add(coreToClose); // Essentially just queue this core up for closing.
+          container.getTransientCacheHandler().addTransientDescriptor(rawName, cd);
-              modifyLock.notifyAll(); // Wakes up closer thread too
+        } else {
+          log.error("Tried to add transient core to transient handler, but no transient core handler has been found. "
+              + " Descriptor: " + cd.toString());
         }
-            return true;
+      } else {
+        lazyDescriptors.put(rawName, cd);
       }
-          return false;
-        }
-      };
-    }
-  }
-
-  protected void putDynamicDescriptor(String rawName, CoreDescriptor p) {
-    synchronized (modifyLock) {
-      dynamicDescriptors.put(rawName, p);
     }
   }
 
@@ -101,19 +87,25 @@ class SolrCores {
   protected void close() {
     Collection<SolrCore> coreList = new ArrayList<>();
 
+    
+    TransientSolrCoreCache transientSolrCoreCache = container.getTransientCacheHandler();
+    // Release observer
+    if (transientSolrCoreCache != null) {
+      transientSolrCoreCache.close();
+    }
+
     // It might be possible for one of the cores to move from one list to another while we're closing them. So
     // loop through the lists until they're all empty. In particular, the core could have moved from the transient
     // list to the pendingCloses list.
-
     do {
       coreList.clear();
       synchronized (modifyLock) {
         // make a copy of the cores then clear the map so the core isn't handed out to a request again
         coreList.addAll(cores.values());
         cores.clear();
-
+        if (transientSolrCoreCache != null) {
-        coreList.addAll(transientCores.values());
+          coreList.addAll(transientSolrCoreCache.prepareForShutdown());
-        transientCores.clear();
+        }
 
         coreList.addAll(pendingCloses);
         pendingCloses.clear();
@@ -147,10 +139,12 @@ class SolrCores {
 
   //WARNING! This should be the _only_ place you put anything into the list of transient cores!
   protected SolrCore putTransientCore(NodeConfig cfg, String name, SolrCore core, SolrResourceLoader loader) {
-    SolrCore retCore;
+    SolrCore retCore = null;
     log.info("Opening transient core {}", name);
     synchronized (modifyLock) {
-      retCore = transientCores.put(name, core);
+      if (container.getTransientCacheHandler() != null) {
+        retCore = container.getTransientCacheHandler().addCore(name, core);
+      }
     }
     return retCore;
   }
@@ -161,6 +155,17 @@ class SolrCores {
     }
   }
 
+  /**
+   *
+   * @return A list of "permanent" cores, i.e. cores that  may not be swapped out and are currently loaded.
+   * 
+   * A core may be non-transient but still lazily loaded. If it is "permanent" and lazy-load _and_
+   * not yet loaded it will _not_ be returned by this call.
+   * 
+   * Note: This is one of the places where SolrCloud is incompatible with Transient Cores. This call is used in 
+   * cancelRecoveries, transient cores don't participate.
+   */
+
   List<SolrCore> getCores() {
     List<SolrCore> lst = new ArrayList<>();
 
@@ -170,16 +175,34 @@ class SolrCores {
     }
   }
 
-  Set<String> getCoreNames() {
+  /**
+   * Gets the cores that are currently loaded, i.e. cores that have
+   * 1> loadOnStartup=true and are either not-transient or, if transient, have been loaded and have not been swapped out
+   * 2> loadOnStartup=false and have been loaded but either non-transient or have not been swapped out.
+   * 
+   * Put another way, this will not return any names of cores that are lazily loaded but have not been called for yet
+   * or are transient and either not loaded or have been swapped out.
+   * 
+   * @return List of currently loaded cores.
+   */
+  Set<String> getLoadedCoreNames() {
     Set<String> set = new TreeSet<>();
 
     synchronized (modifyLock) {
       set.addAll(cores.keySet());
-      set.addAll(transientCores.keySet());
+      if (container.getTransientCacheHandler() != null) {
+        set.addAll(container.getTransientCacheHandler().getLoadedCoreNames());
+      }
     }
     return set;
   }
 
+  /** This method is currently experimental.
+   * @return a Collection of the names that a specific core is mapped to.
+   * 
+   * Note: this implies that the core is loaded
+   */
+  @Experimental
   List<String> getCoreNames(SolrCore core) {
     List<String> lst = new ArrayList<>();
 
@@ -189,26 +212,26 @@ class SolrCores {
           lst.add(entry.getKey());
         }
       }
-      for (Map.Entry<String, SolrCore> entry : transientCores.entrySet()) {
+      if (container.getTransientCacheHandler() != null) {
-        if (core == entry.getValue()) {
+        lst.addAll(container.getTransientCacheHandler().getNamesForCore(core));
-          lst.add(entry.getKey());
-        }
       }
     }
     return lst;
   }
 
   /**
-   * Gets a list of all cores, loaded and unloaded (dynamic)
+   * Gets a list of all cores, loaded and unloaded 
    *
-   * @return all cores names, whether loaded or unloaded.
+   * @return all cores names, whether loaded or unloaded, transient or permenent.
    */
   public Collection<String> getAllCoreNames() {
     Set<String> set = new TreeSet<>();
     synchronized (modifyLock) {
       set.addAll(cores.keySet());
-      set.addAll(transientCores.keySet());
+      if (container.getTransientCacheHandler() != null) {
-      set.addAll(dynamicDescriptors.keySet());
+        set.addAll(container.getTransientCacheHandler().getAllCoreNames());
+      }
+      set.addAll(lazyDescriptors.keySet());
     }
     return set;
   }
@@ -251,14 +274,15 @@ class SolrCores {
   protected SolrCore remove(String name) {
 
     synchronized (modifyLock) {
-      SolrCore tmp = cores.remove(name);
+      SolrCore ret = cores.remove(name);
-      SolrCore ret = null;
-      ret = (ret == null) ? tmp : ret;
       // It could have been a newly-created core. It could have been a transient core. The newly-created cores
       // in particular should be checked. It could have been a dynamic core.
-      tmp = transientCores.remove(name);
+      TransientSolrCoreCache transientHandler = container.getTransientCacheHandler(); 
-      ret = (ret == null) ? tmp : ret;
+      if (ret == null && transientHandler != null) {
-      dynamicDescriptors.remove(name);
+        ret = transientHandler.removeCore(name);
+        transientHandler.removeTransientDescriptor(name);
+      }
+      lazyDescriptors.remove(name);
       return ret;
     }
   }
@@ -268,8 +292,8 @@ class SolrCores {
     synchronized (modifyLock) {
       SolrCore core = cores.get(name);
 
-      if (core == null) {
+      if (core == null && container.getTransientCacheHandler() != null) {
-        core = transientCores.get(name);
+        core = container.getTransientCacheHandler().getCore(name);
       }
 
       if (core != null && incRefCount) {
@@ -282,7 +306,9 @@ class SolrCores {
 
   protected CoreDescriptor getDynamicDescriptor(String name) {
     synchronized (modifyLock) {
-      return dynamicDescriptors.get(name);
+      CoreDescriptor cd = lazyDescriptors.get(name);
+      if (cd != null || container.getTransientCacheHandler() == null) return cd;
+      return container.getTransientCacheHandler().getTransientDescriptor(name);
     }
   }
 
@@ -295,7 +321,7 @@ class SolrCores {
       if (cores.containsKey(name)) {
         return true;
       }
-      if (transientCores.containsKey(name)) {
+      if (container.getTransientCacheHandler() != null && container.getTransientCacheHandler().containsCore(name)) {
         // Check pending
         for (SolrCore core : pendingCloses) {
           if (core.getName().equals(name)) {
@@ -314,7 +340,7 @@ class SolrCores {
       if (cores.containsKey(name)) {
         return true;
       }
-      if (transientCores.containsKey(name)) {
+      if (container.getTransientCacheHandler() != null && container.getTransientCacheHandler().containsCore(name)) {
         return true;
       }
     }
@@ -324,13 +350,16 @@ class SolrCores {
 
   protected CoreDescriptor getUnloadedCoreDescriptor(String cname) {
     synchronized (modifyLock) {
-      CoreDescriptor desc = dynamicDescriptors.get(cname);
+      CoreDescriptor desc = lazyDescriptors.get(cname);
+      if (desc == null) {
+        if (container.getTransientCacheHandler() == null) return null;
+        desc = container.getTransientCacheHandler().getTransientDescriptor(cname);
         if (desc == null) {
           return null;
         }
+      }
       return new CoreDescriptor(cname, desc);
     }
-
   }
 
   // Wait here until any pending operations (load, unload or reload) are completed on this core.
@@ -412,9 +441,9 @@ class SolrCores {
     synchronized (modifyLock) {
       if (cores.containsKey(coreName))
         return cores.get(coreName).getCoreDescriptor();
-      if (dynamicDescriptors.containsKey(coreName))
+      if (lazyDescriptors.containsKey(coreName) || container.getTransientCacheHandler() == null)
-        return dynamicDescriptors.get(coreName);
+        return lazyDescriptors.get(coreName);
-      return null;
+      return container.getTransientCacheHandler().getTransientDescriptor(coreName);
     }
   }
 
@@ -494,4 +523,13 @@ class SolrCores {
     }
     return false;
   }
+
+  // Let transient cache implementation tell us when it ages out a corel
+  @Override
+  public void update(Observable o, Object arg) {
+    synchronized (modifyLock) {
+      pendingCloses.add((SolrCore) arg); // Essentially just queue this core up for closing.
+      modifyLock.notifyAll(); // Wakes up closer thread too
+    }
+  }
 }

solr/core/src/java/org/apache/solr/core/SolrXmlConfig.java

@@ -91,6 +91,7 @@ public class SolrXmlConfig {
     NodeConfig.NodeConfigBuilder configBuilder = new NodeConfig.NodeConfigBuilder(nodeName, config.getResourceLoader());
     configBuilder.setUpdateShardHandlerConfig(updateConfig);
     configBuilder.setShardHandlerFactoryConfig(getShardHandlerFactoryPluginInfo(config));
+    configBuilder.setSolrCoreCacheFactoryConfig(getTransientCoreCacheFactoryPluginInfo(config));
     configBuilder.setLogWatcherConfig(loadLogWatcherConfig(config, "solr/logging/*[@name]", "solr/logging/watcher/*[@name]"));
     configBuilder.setSolrProperties(loadProperties(config));
     if (cloudConfig != null)
@@ -456,5 +457,9 @@ public class SolrXmlConfig {
     }
     return configs;
   }
+  private static PluginInfo getTransientCoreCacheFactoryPluginInfo(Config config) {
+    Node node = config.getNode("solr/transientCoreCacheFactory", false);
+    return (node == null) ? null : new PluginInfo(node, "transientCoreCacheFactory", false, true);
+  }
 }
 

solr/core/src/java/org/apache/solr/core/TransientSolrCoreCache.java

@@ -0,0 +1,127 @@
+/*
+ * 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.core;
+
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Observable;
+import java.util.Set;
+
+import org.apache.http.annotation.Experimental;
+
+/**
+ * The base class for custom transient core maintenance. Any custom plugin that want's to take control of transient
+ * caches (i.e. any core defined with transient=true) should override this class.
+ *
+ * Register your plugin in solr.xml similarly to:
+ *
+ *   <transientCoreCacheFactory name="transientCoreCacheFactory" class="TransientSolrCoreCacheFactoryDefault">
+ *        <int name="transientCacheSize">4</int>
+ *   </transientCoreCacheFactory>
+ *
+ *
+ * WARNING: There is quite a bit of higher-level locking done by the CoreContainer to avoid various race conditions
+ *          etc. You should _only_ manipulate them within the method calls designed to change them. E.g.
+ *          only add to the transient core descriptors in addTransientDescriptor etc.
+ *          
+ *          Trust the higher-level code (mainly SolrCores and CoreContainer) to call the appropriate operations when
+ *          necessary and to coordinate shutting down cores, manipulating the internal structures and the like..
+ *          
+ *          The only real action you should _initiate_ is to close a core for whatever reason, and do that by 
+ *          calling notifyObservers(coreToClose); The observer will call back to removeCore(name) at the appropriate 
+ *          time. There is no need to directly remove the core _at that time_ from the transientCores list, a call
+ *          will come back to this class when CoreContainer is closing this core.
+ *          
+ *          CoreDescriptors are read-once. During "core discovery" all valid descriptors are enumerated and added to
+ *          the appropriate list. Thereafter, they are NOT re-read from disk. In those situations where you want
+ *          to re-define the coreDescriptor, maintain a "side list" of changed core descriptors. Then override
+ *          getTransientDescriptor to return your new core descriptor. NOTE: assuming you've already closed the
+ *          core, the _next_ time that core is required getTransientDescriptor will be called and if you return the
+ *          new core descriptor your re-definition should be honored. You'll have to maintain this list for the
+ *          duration of this Solr instance running. If you persist the coreDescriptor, then next time Solr starts
+ *          up the new definition will be read.
+ *          
+ *
+ *  If you need to manipulate the return, for instance block a core from being loaded for some period of time, override
+ *  say getTransientDescriptor and return null.
+ *  
+ *  In particular, DO NOT reach into the transientCores structure from a method called to manipulate core descriptors
+ *  or vice-versa.
+ */
+public abstract class TransientSolrCoreCache extends Observable {
+
+  // Gets the core container that encloses this cache.
+  public abstract CoreContainer getContainer();
+
+  // Add the newly-opened core to the list of open cores.
+  public abstract SolrCore addCore(String name, SolrCore core);
+
+  // Return the names of all possible cores, whether they are currently loaded or not.
+  public abstract Set<String> getAllCoreNames();
+  
+  // Return the names of all currently loaded cores
+  public abstract Set<String> getLoadedCoreNames();
+
+  // Remove a core from the internal structures, presumably it 
+  // being closed. If the core is re-opened, it will be readded by CoreContainer.
+  public abstract SolrCore removeCore(String name);
+
+  // Get the core associated with the name. Return null if you don't want this core to be used.
+  public abstract SolrCore getCore(String name);
+
+  // reutrn true if the cache contains the named core.
+  public abstract boolean containsCore(String name);
+  
+  // This method will be called when the container is to be shut down. It should return all
+  // transient solr cores and clear any internal structures that hold them.
+  public abstract Collection<SolrCore> prepareForShutdown();
+
+  // These methods allow the implementation to maintain control over the core descriptors.
+  
+  // This method will only be called during core discovery at startup.
+  public abstract void addTransientDescriptor(String rawName, CoreDescriptor cd);
+  
+  // This method is used when opening cores and the like. If you want to change a core's descriptor, override this
+  // method and return the current core descriptor.
+  public abstract CoreDescriptor getTransientDescriptor(String name);
+
+
+  // Remove the core descriptor from your list of transient descriptors.
+  public abstract CoreDescriptor removeTransientDescriptor(String name);
+
+  // Find all the names a specific core is mapped to. Should not return null, return empty set instead.
+  @Experimental
+  public List<String> getNamesForCore(SolrCore core) {
+    return Collections.emptyList();
+  }
+  
+  /**
+   * Must be called in order to free resources!
+   */
+  public abstract void close();
+
+
+  // These two methods allow custom implementations to communicate arbitrary information as necessary.
+  public abstract int getStatus(String coreName);
+  public abstract void setStatus(String coreName, int status);
+}
+
+
+  

solr/core/src/java/org/apache/solr/core/TransientSolrCoreCacheDefault.java

@@ -0,0 +1,198 @@
+/*
+ * 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.core;
+
+import java.lang.invoke.MethodHandles;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Observer;
+import java.util.Set;
+
+import org.apache.solr.common.util.NamedList;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class TransientSolrCoreCacheDefault extends TransientSolrCoreCache {
+
+  private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
+  private int cacheSize = NodeConfig.NodeConfigBuilder.DEFAULT_TRANSIENT_CACHE_SIZE;
+
+  protected Observer observer;
+  protected CoreContainer coreContainer;
+
+  protected final Map<String, CoreDescriptor> transientDescriptors = new LinkedHashMap<>();
+
+  //WARNING! The _only_ place you put anything into the list of transient cores is with the putTransientCore method!
+  protected Map<String, SolrCore> transientCores = new LinkedHashMap<>(); // For "lazily loaded" cores
+
+  /**
+   * @param container The enclosing CoreContainer. It allows us to access everything we need.
+   */
+  public TransientSolrCoreCacheDefault(final CoreContainer container) {
+    this.coreContainer = container;
+    this.observer= coreContainer.solrCores;
+    
+    NodeConfig cfg = container.getNodeConfig();
+    if (cfg.getTransientCachePluginInfo() == null) {
+      // Still handle just having transientCacheSize defined in the body of solr.xml  not in a transient handler clause.
+      // deprecate this for 7.0?
+      this.cacheSize = cfg.getTransientCacheSize();
+    } else {
+      NamedList args = cfg.getTransientCachePluginInfo().initArgs;
+      Object obj = args.get("transientCacheSize");
+      if (obj != null) {
+        this.cacheSize = (int) obj;
+      }
+    }
+    doInit();
+  }
+  // This just moves the 
+  private void doInit() {
+    NodeConfig cfg = coreContainer.getNodeConfig();
+    if (cfg.getTransientCachePluginInfo() == null) {
+      // Still handle just having transientCacheSize defined in the body of solr.xml not in a transient handler clause.
+      this.cacheSize = cfg.getTransientCacheSize();
+    } else {
+      NamedList args = cfg.getTransientCachePluginInfo().initArgs;
+      Object obj = args.get("transientCacheSize");
+      if (obj != null) {
+        this.cacheSize = (int) obj;
+      }
+    }
+
+    log.info("Allocating transient cache for {} transient cores", cacheSize);
+    addObserver(this.observer);
+    // it's possible for cache
+    if (cacheSize < 0) { // Trap old flag
+      cacheSize = Integer.MAX_VALUE;
+    }
+    // Now don't allow ridiculous allocations here, if the size is > 1,000, we'll just deal with
+    // adding cores as they're opened. This blows up with the marker value of -1.
+    transientCores = new LinkedHashMap<String, SolrCore>(Math.min(cacheSize, 1000), 0.75f, true) {
+      @Override
+      protected boolean removeEldestEntry(Map.Entry<String, SolrCore> eldest) {
+        if (size() > cacheSize) {
+          SolrCore coreToClose = eldest.getValue();
+          setChanged();
+          notifyObservers(coreToClose);
+          log.info("Closing transient core [{}]", coreToClose.getName());
+          return true;
+        }
+        return false;
+      }
+    };
+  }
+
+  
+  @Override
+  public Collection<SolrCore> prepareForShutdown() {
+    // Returna copy of the values
+    List<SolrCore> ret = new ArrayList(transientCores.values());
+    transientCores.clear();
+    return ret;
+  }
+
+  @Override
+  public CoreContainer getContainer() { return this.coreContainer; }
+
+  @Override
+  public SolrCore addCore(String name, SolrCore core) {
+    return transientCores.put(name, core);
+  }
+
+  @Override
+  public Set<String> getAllCoreNames() {
+    return transientDescriptors.keySet();
+  }
+  
+  @Override
+  public Set<String> getLoadedCoreNames() {
+    return transientCores.keySet();
+  }
+
+  // Remove a core from the internal structures, presumably it 
+  // being closed. If the core is re-opened, it will be readded by CoreContainer.
+  @Override
+  public SolrCore removeCore(String name) {
+    return transientCores.remove(name);
+  }
+
+  // Get the core associated with the name. Return null if you don't want this core to be used.
+  @Override
+  public SolrCore getCore(String name) {
+    return transientCores.get(name);
+  }
+
+  @Override
+  public boolean containsCore(String name) {
+    return transientCores.containsKey(name);
+  }
+
+  // These methods allow the implementation to maintain control over the core descriptors.
+
+
+  // This method will only be called during core discovery at startup.
+  @Override
+  public void addTransientDescriptor(String rawName, CoreDescriptor cd) {
+    transientDescriptors.put(rawName, cd);
+  }
+
+  // This method is used when opening cores and the like. If you want to change a core's descriptor, override this
+  // method and return the current core descriptor.
+  @Override
+  public CoreDescriptor getTransientDescriptor(String name) {
+    return transientDescriptors.get(name);
+  }
+
+  @Override
+  public CoreDescriptor removeTransientDescriptor(String name) {
+    return transientDescriptors.remove(name);
+  }
+
+  @Override
+  public List<String> getNamesForCore(SolrCore core) {
+    List<String> ret = new ArrayList<>();
+    for (Map.Entry<String, SolrCore> entry : transientCores.entrySet()) {
+      if (core == entry.getValue()) {
+        ret.add(entry.getKey());
+      }
+    }
+    return ret;
+  }
+
+  /**
+   * Must be called in order to free resources!
+   */
+  @Override
+  public void close() {
+    deleteObserver(this.observer);
+  }
+
+
+  // For custom implementations to communicate arbitrary information as necessary.
+  @Override
+  public int getStatus(String coreName) { return 0; } //no_op for default handler.
+
+  @Override
+  public void setStatus(String coreName, int status) {} //no_op for default handler.
+
+}

solr/core/src/java/org/apache/solr/core/TransientSolrCoreCacheFactory.java

@@ -0,0 +1,85 @@
+/*
+ * 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.core;
+
+import java.lang.invoke.MethodHandles;
+import java.util.Collections;
+import java.util.Locale;
+
+import com.google.common.collect.ImmutableMap;
+import org.apache.solr.util.plugin.PluginInfoInitialized;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * An interface that allows custom transient caches to be maintained with different implementations
+ */
+public abstract class TransientSolrCoreCacheFactory {
+  private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
+  private CoreContainer coreContainer = null;
+
+  public abstract TransientSolrCoreCache getTransientSolrCoreCache();
+  /**
+   * Create a new TransientSolrCoreCacheFactory instance
+   *
+   * @param loader a SolrResourceLoader used to find the TransientSolrCacheFactory classes
+   * @param coreContainer CoreContainer that encloses all the Solr cores.              
+   * @return a new, initialized TransientSolrCoreCache instance
+   */
+
+  public static TransientSolrCoreCacheFactory newInstance(SolrResourceLoader loader, CoreContainer coreContainer) {
+    PluginInfo info = coreContainer.getConfig().getTransientCachePluginInfo();
+    if (info == null) { // definition not in our solr.xml file, use default
+      info = DEFAULT_TRANSIENT_SOLR_CACHE_INFO;
+    }
+
+    try {
+      // According to the docs, this returns a TransientSolrCoreCacheFactory with the default c'tor
+      TransientSolrCoreCacheFactory tccf = loader.findClass(info.className, TransientSolrCoreCacheFactory.class).newInstance(); 
+      
+      // OK, now we call it's init method.
+      if (PluginInfoInitialized.class.isAssignableFrom(tccf.getClass()))
+        PluginInfoInitialized.class.cast(tccf).init(info);
+      tccf.setCoreContainer(coreContainer);
+      return tccf;
+    } catch (Exception e) {
+      // Many things could cuse this, bad solrconfig, mis-typed class name, whatever. However, this should not
+      // keep the enclosing coreContainer from instantiating, so log an error and continue.
+      log.error(String.format(Locale.ROOT, "Error instantiating TransientSolrCoreCacheFactory class [%s]: %s",
+          info.className, e.getMessage()));
+      return null;
+    }
+
+  }
+  public static final PluginInfo DEFAULT_TRANSIENT_SOLR_CACHE_INFO =
+      new PluginInfo("transientSolrCoreCacheFactory",
+          ImmutableMap.of("class", TransientSolrCoreCacheFactoryDefault.class.getName(), 
+              "name", TransientSolrCoreCacheFactory.class.getName()),
+          null, Collections.<PluginInfo>emptyList());
+
+
+  // Need this because the plugin framework doesn't require a PluginINfo in the init method, don't see a way to
+  // pass additional parameters and we need this when we create the transient core cache, it's _really_ important.
+  public void setCoreContainer(CoreContainer coreContainer) {
+    this.coreContainer = coreContainer;
+  }
+
+  public CoreContainer getCoreContainer() {
+    return coreContainer;
+  }
+}

solr/core/src/java/org/apache/solr/core/TransientSolrCoreCacheFactoryDefault.java

@@ -0,0 +1,31 @@
+/*
+ * 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.core;
+
+public class TransientSolrCoreCacheFactoryDefault extends TransientSolrCoreCacheFactory {
+
+  TransientSolrCoreCache transientSolrCoreCache = null;
+
+  @Override
+  public TransientSolrCoreCache getTransientSolrCoreCache() {
+    if (transientSolrCoreCache == null) {
+      transientSolrCoreCache = new TransientSolrCoreCacheDefault(getCoreContainer());
+    }
+
+    return transientSolrCoreCache;
+  }
+}

solr/core/src/test-files/solr/solr.xml

@@ -31,6 +31,11 @@
     <int name="connTimeout">${connTimeout:15000}</int>
   </shardHandlerFactory>
 
+  <transientCoreCacheFactory name="transientCoreCacheFactory" class="TransientSolrCoreCacheFactoryDefault">
+    <int name="transientCacheSize">4</int>
+  </transientCoreCacheFactory>
+
+
   <solrcloud>
     <str name="host">127.0.0.1</str>
     <int name="hostPort">${hostPort:8983}</int>

solr/core/src/test/org/apache/solr/cloud/ZkControllerTest.java

@@ -29,6 +29,8 @@ import org.apache.solr.common.util.Utils;
 import org.apache.solr.core.CloudConfig;
 import org.apache.solr.core.CoreContainer;
 import org.apache.solr.core.CoreDescriptor;
+import org.apache.solr.core.SolrXmlConfig;
+import org.apache.solr.core.TransientSolrCoreCache;
 import org.apache.solr.handler.admin.CoreAdminHandler;
 import org.apache.solr.handler.component.HttpShardHandlerFactory;
 import org.apache.solr.update.UpdateShardHandler;
@@ -327,7 +329,7 @@ public class ZkControllerTest extends SolrTestCaseJ4 {
   private static class MockCoreContainer extends CoreContainer {
     UpdateShardHandler updateShardHandler = new UpdateShardHandler(UpdateShardHandlerConfig.DEFAULT);
     public MockCoreContainer() {
-      super((Object)null);
+      super(SolrXmlConfig.fromString(null, "<solr/>"));
       this.shardHandlerFactory = new HttpShardHandlerFactory();
       this.coreAdminHandler = new CoreAdminHandler();
     }
@@ -346,5 +348,10 @@ public class ZkControllerTest extends SolrTestCaseJ4 {
       super.shutdown();
     }
     
+    @Override
+    public TransientSolrCoreCache getTransientCacheHandler() {
+      return transientSolrCoreCache;
+    }
+
   }
 }

solr/core/src/test/org/apache/solr/core/TestCoreDiscovery.java

@@ -60,12 +60,12 @@ public class TestCoreDiscovery extends SolrTestCaseJ4 {
     setMeUp(null);
   }
 
-  private Properties makeCorePropFile(String name, boolean isLazy, boolean loadOnStartup, String... extraProps) {
+  private Properties makeCorePropFile(String name, boolean isTransient, boolean loadOnStartup, String... extraProps) {
     Properties props = new Properties();
     props.put(CoreDescriptor.CORE_NAME, name);
     props.put(CoreDescriptor.CORE_SCHEMA, "schema-tiny.xml");
     props.put(CoreDescriptor.CORE_CONFIG, "solrconfig-minimal.xml");
-    props.put(CoreDescriptor.CORE_TRANSIENT, Boolean.toString(isLazy));
+    props.put(CoreDescriptor.CORE_TRANSIENT, Boolean.toString(isTransient));
     props.put(CoreDescriptor.CORE_LOADONSTARTUP, Boolean.toString(loadOnStartup));
     props.put(CoreDescriptor.CORE_DATADIR, "${core.dataDir:stuffandnonsense}");
 
@@ -140,7 +140,7 @@ public class TestCoreDiscovery extends SolrTestCaseJ4 {
     try {
 
       TestLazyCores.checkInCores(cc, "core1");
-      TestLazyCores.checkNotInCores(cc, "lazy1", "core2", "collection1");
+      TestLazyCores.checkNotInCores(cc, "lazy1", "core2");
 
       // force loading of core2 and lazy1 by getting them from the CoreContainer
       try (SolrCore core1 = cc.getCore("core1");
@@ -463,4 +463,5 @@ public class TestCoreDiscovery extends SolrTestCaseJ4 {
     NodeConfig absConfig = SolrXmlConfig.fromString(loader, "<solr><str name=\"coreRootDirectory\">/absolute</str></solr>");
     assertThat(absConfig.getCoreRootDirectory().toString(), not(containsString(solrHomeDirectory.getAbsolutePath())));
   }
+  
 }

solr/core/src/test/org/apache/solr/core/TestLazyCores.java

@@ -38,6 +38,7 @@ import org.apache.solr.handler.admin.CoreAdminHandler;
 import org.apache.solr.request.LocalSolrQueryRequest;
 import org.apache.solr.request.SolrQueryRequest;
 import org.apache.solr.response.SolrQueryResponse;
+import org.apache.solr.servlet.SolrDispatchFilter;
 import org.apache.solr.update.AddUpdateCommand;
 import org.apache.solr.update.CommitUpdateCommand;
 import org.apache.solr.update.UpdateHandler;
@@ -83,13 +84,13 @@ public class TestLazyCores extends SolrTestCaseJ4 {
   private CoreContainer init() throws Exception {
     solrHomeDirectory = createTempDir().toFile();
     
+    copyXmlToHome(solrHomeDirectory.getAbsoluteFile(), "solr.xml");
     for (int idx = 1; idx < 10; ++idx) {
       copyMinConf(new File(solrHomeDirectory, "collection" + idx));
     }
 
-    SolrResourceLoader loader = new SolrResourceLoader(solrHomeDirectory.toPath());
+    NodeConfig cfg = SolrDispatchFilter.loadNodeConfig(solrHomeDirectory.toPath(), null);
-    NodeConfig config = new NodeConfig.NodeConfigBuilder("testNode", loader).setTransientCacheSize(4).build();
+    return createCoreContainer(cfg, testCores);
-    return createCoreContainer(config, testCores);
   }
   
   @Test
@@ -188,7 +189,7 @@ public class TestLazyCores extends SolrTestCaseJ4 {
           , "//result[@numFound='0']"
       );
 
-      checkInCores(cc, "collection4");
+      checkInCores(cc, "collection1", "collection2", "collection4", "collection5");
 
       core4.close();
       collection1.close();
@@ -454,11 +455,14 @@ public class TestLazyCores extends SolrTestCaseJ4 {
   // 1> produce errors as appropriate when the config or schema files are foo'd
   // 2> "self heal". That is, if the problem is corrected can the core be reloaded and used?
   // 3> that OK cores can be searched even when some cores failed to load.
+  // 4> that having no solr.xml entry for transient chache handler correctly uses the default.
   @Test
   public void testBadConfigsGenerateErrors() throws Exception {
     final CoreContainer cc = initGoodAndBad(Arrays.asList("core1", "core2"),
         Arrays.asList("badSchema1", "badSchema2"),
         Arrays.asList("badConfig1", "badConfig2"));
+    
+    
     try {
       // first, did the two good cores load successfully?
       checkInCores(cc, "core1", "core2");
@@ -491,8 +495,9 @@ public class TestLazyCores extends SolrTestCaseJ4 {
       copyGoodConf("badSchema1", "schema-tiny.xml", "schema.xml");
       copyGoodConf("badSchema2", "schema-tiny.xml", "schema.xml");
 
+      
       // This should force a reload of the cores.
-      SolrCore bc1 = cc.getCore("badConfig1");
+      SolrCore bc1 = cc.getCore("badConfig1");;
       SolrCore bc2 = cc.getCore("badConfig2");
       SolrCore bs1 = cc.getCore("badSchema1");
       SolrCore bs2 = cc.getCore("badSchema2");
@@ -635,16 +640,46 @@ public class TestLazyCores extends SolrTestCaseJ4 {
   }
 
   public static void checkNotInCores(CoreContainer cc, String... nameCheck) {
-    Collection<String> names = cc.getCoreNames();
+    Collection<String> loadedNames = cc.getCoreNames();
     for (String name : nameCheck) {
-      assertFalse("core " + name + " was found in the list of cores", names.contains(name));
+      assertFalse("core " + name + " was found in the list of cores", loadedNames.contains(name));
+    }
+    
+    // There was a problem at one point exacerbated by the poor naming conventions. So parallel to loaded cores, there
+    // should be the ability to get the core _names_ that are loaded as well as all the core names _possible_
+    //
+    // the names above should only contain loaded core names. Every name in names should be in allNames, but none of 
+    // the names in nameCheck should be loaded and thus should not be in names.
+    
+    Collection<String> allNames = cc.getAllCoreNames();
+    // Every core, loaded or not should be in the accumulated coredescriptors:
+    List<CoreDescriptor> descriptors = cc.getCoreDescriptors();
+
+    assertEquals("There should be as many coreDescriptors as coreNames", allNames.size(), descriptors.size());
+    for (CoreDescriptor desc : descriptors) {
+      assertTrue("Name should have a corresponding descriptor", allNames.contains(desc.getName()));
+    }
+    
+    // First check that all loaded cores are in allNames.
+    for (String name : loadedNames) {                                                                                        
+      assertTrue("Loaded core " + name + " should have been found in the list of all possible core names",
+          allNames.contains(name));
+    }
+
+    for (String name : nameCheck) {
+      assertTrue("Not-currently-loaded core " + name + " should have been found in the list of all possible core names",
+          allNames.contains(name));
     }
   }
 
   public static void checkInCores(CoreContainer cc, String... nameCheck) {
-    Collection<String> names = cc.getCoreNames();
+    Collection<String> loadedNames = cc.getCoreNames();
+    
+    assertEquals("There whould be exactly as many loaded cores as loaded names returned. ", 
+        loadedNames.size(), nameCheck.length);
+    
     for (String name : nameCheck) {
-      assertTrue("core " + name + " was not found in the list of cores", names.contains(name));
+      assertTrue("core " + name + " was not found in the list of cores", loadedNames.contains(name));
     }
   }
 

solr/test-framework/src/java/org/apache/solr/SolrTestCaseJ4.java

@@ -2002,8 +2002,7 @@ public abstract class SolrTestCaseJ4 extends LuceneTestCase {
     FileUtils.copyFile(new File(top, "solrconfig.snippet.randomindexconfig.xml"), new File(subHome, "solrconfig.snippet.randomindexconfig.xml"));
   }
 
-  // Creates minimal full setup, including the old solr.xml file that used to be hard coded in ConfigSolrXmlOld
+  // Creates minimal full setup, including solr.xml
-  // TODO: remove for 5.0
   public static void copyMinFullSetup(File dstRoot) throws IOException {
     if (! dstRoot.exists()) {
       assertTrue("Failed to make subdirectory ", dstRoot.mkdirs());
@@ -2013,6 +2012,15 @@ public abstract class SolrTestCaseJ4 extends LuceneTestCase {
     copyMinConf(dstRoot);
   }
 
+  // Just copies the file indicated to the tmp home directory naming it "solr.xml"
+  public static void copyXmlToHome(File dstRoot, String fromFile) throws IOException {
+    if (! dstRoot.exists()) {
+      assertTrue("Failed to make subdirectory ", dstRoot.mkdirs());
+    }
+    File xmlF = new File(SolrTestCaseJ4.TEST_HOME(), fromFile);
+    FileUtils.copyFile(xmlF, new File(dstRoot, "solr.xml"));
+    
+  }
   // Creates a consistent configuration, _including_ solr.xml at dstRoot. Creates collection1/conf and copies
   // the stock files in there.
 
@@ -2020,7 +2028,6 @@ public abstract class SolrTestCaseJ4 extends LuceneTestCase {
     if (!dstRoot.exists()) {
       assertTrue("Failed to make subdirectory ", dstRoot.mkdirs());
     }
-
     FileUtils.copyFile(new File(SolrTestCaseJ4.TEST_HOME(), "solr.xml"), new File(dstRoot, "solr.xml"));
 
     File subHome = new File(dstRoot, collection + File.separator + "conf");