sigram commented on a change in pull request #1962:
URL: https://github.com/apache/lucene-solr/pull/1962#discussion_r512718408
##########
File path:
solr/core/src/java/org/apache/solr/core/ClusterEventProducerFactory.java
##########
@@ -0,0 +1,178 @@
+package org.apache.solr.core;
+
+import org.apache.solr.api.CustomContainerPlugins;
+import org.apache.solr.cluster.events.ClusterEvent;
+import org.apache.solr.cluster.events.ClusterEventListener;
+import org.apache.solr.cluster.events.ClusterEventProducer;
+import org.apache.solr.cluster.events.impl.DefaultClusterEventProducer;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * This class helps in handling the initial registration of plugin-based
listeners,
+ * when both the final {@link ClusterEventProducer} implementation and
listeners
+ * are configured using plugins.
+ */
+public class ClusterEventProducerFactory implements ClusterEventProducer {
+ private Map<ClusterEvent.EventType, Set<ClusterEventListener>>
initialListeners = new HashMap<>();
+ private CustomContainerPlugins.PluginRegistryListener initialPluginListener;
+ private final CoreContainer cc;
+ private boolean created = false;
+
+ public ClusterEventProducerFactory(CoreContainer cc) {
+ this.cc = cc;
+ initialPluginListener = new
CustomContainerPlugins.PluginRegistryListener() {
+ @Override
+ public void added(CustomContainerPlugins.ApiInfo plugin) {
+ if (plugin == null || plugin.getInstance() == null) {
+ return;
+ }
+ Object instance = plugin.getInstance();
+ if (instance instanceof ClusterEventListener) {
+ registerListener((ClusterEventListener) instance);
+ }
+ }
+
+ @Override
+ public void deleted(CustomContainerPlugins.ApiInfo plugin) {
+ if (plugin == null || plugin.getInstance() == null) {
+ return;
+ }
+ Object instance = plugin.getInstance();
+ if (instance instanceof ClusterEventListener) {
+ unregisterListener((ClusterEventListener) instance);
+ }
+ }
+
+ @Override
+ public void modified(CustomContainerPlugins.ApiInfo old,
CustomContainerPlugins.ApiInfo replacement) {
+ added(replacement);
+ deleted(old);
+ }
+ };
+ }
+
+ /**
+ * This method returns an initial plugin registry listener that helps to
capture the
+ * freshly loaded listener plugins before the final cluster event producer
is created.
+ * @return initial listener
+ */
+ public CustomContainerPlugins.PluginRegistryListener
getPluginRegistryListener() {
+ return initialPluginListener;
+ }
+
+ /**
+ * Create a {@link ClusterEventProducer} based on the current plugin
configurations.
+ * <p>NOTE: this method can only be called once because it has side-effects,
such as
+ * transferring the initially collected listeners to the resulting
producer's instance, and
+ * installing a {@link
org.apache.solr.api.CustomContainerPlugins.PluginRegistryListener}.
+ * Calling this method more than once will result in an exception.</p>
+ * @param plugins current plugin configurations
+ * @return configured instance of cluster event producer (with side-effects,
see above)
+ */
+ public ClusterEventProducer create(CustomContainerPlugins plugins) {
+ if (created) {
+ throw new RuntimeException("this factory can be called only once!");
+ }
+ final ClusterEventProducer clusterEventProducer;
+ CustomContainerPlugins.ApiInfo clusterEventProducerInfo =
plugins.getPlugin(ClusterEventProducer.PLUGIN_NAME);
+ if (clusterEventProducerInfo != null) {
+ // the listener in ClusterSingletons already registered it
+ clusterEventProducer = (ClusterEventProducer)
clusterEventProducerInfo.getInstance();
+ } else {
+ // create the default impl
+ clusterEventProducer = new DefaultClusterEventProducer(cc);
Review comment:
AFAIK there is no mechanism in plugins to require the presence of
another plugin. If we decide to make this functionality an opt-in then other
plugins that depend on this may silently fail to work properly because the user
forgot to specify this opt-in. This problem is not specific to this plugin, but
rather it's a lack of functionality in our whole plugin ecosystem.
Until we have a working mechanism to specify plugin requirements /
dependencies I think we have to provide sensible defaults, and a sensible
default is a working implementation, plus the ability to opt-out or change the
default impl. I'll add a mechanism to opt-out.
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