review commit:

* added a cache to SearchIndexingSignalEnrichmentFacade in order to only evaluate "patterns" once for a given namespace
* removed copy&pasted unit tests in SearchIndexingSignalEnrichmentFacadeTest by adding another abstract test class AbstractCachingSignalEnrichmentFacadeTest
* added a unit test testing selection of JsonFieldSelectors based on different namespaces
* minor cleanup and formatting
* enhanced documentation about how to configure the indexed namespaces via system properties

Signed-off-by: Thomas Jäckle <thomas.jaeckle@beyonnex.io>

documentation/src/main/resources/pages/ditto/installation-operating.md

@@ -330,24 +330,31 @@ The default behavior of Ditto is to index the complete JSON of a thing, which in
 * Increased load on the search database, leading to performance degradation and increased database cost.
 * Only a few fields are ever used for searching.
 
-In Ditto *3.5.0*, there is now configuration to specify, by a namespace pattern, which fields will be included in the search database.
+Since Ditto *3.5.0*, there is a configuration to specify, by a namespace pattern, which fields will be included in the search database.
 
 To enable this functionality, there are two new options in the `thing-search.conf` configuration:
 
-```
+```hocon
 ditto {
-  ...
+  //...
   caching-signal-enrichment-facade-provider = org.eclipse.ditto.thingsearch.service.persistence.write.streaming.SearchIndexingSignalEnrichmentFacadeProvider
-  ...
+  //...
   search {
     namespace-search-include-fields = [
       {
-        namespace-pattern = "org.eclipse",
-        search-include-fields = [ "attributes", "features/info" ]
+        namespace-pattern = "org.eclipse.test"
+        search-include-fields = [
+           "attributes",
+           "features/info/properties",
+           "features/info/other"
+        ]
       },
       {
-        namespace-pattern = "org.eclipse.test",
-        search-include-fields = [ "attributes", "features/info/properties/", "features/info/other" ]
+        namespace-pattern = "org.eclipse*"
+        search-include-fields = [
+           "attributes",
+           "features/info"
+        ]
       }
     ]
   }
@@ -356,17 +363,30 @@ ditto {
 There is a new implementation of the caching signal enrichment facade provider that must be configured to enable this
 functionality.
 
-For each namespace pattern, only the selected fields are included in the search database. In the example above, for
-things in the "org.eclipse" namespace, only the "attributes" and "features/info" paths will be the only fields indexed
-in the search database. For things in the "org.eclipse.test" namespace, the fields indexed in the search database will
-only be "attributes", "features/info/properties", and "features/info/other".
+For each namespace pattern, only the selected fields are included in the search database. In the example above, for 
+things in the "org.eclipse.test" namespace, the fields indexed in the search database will
+only be "attributes", "features/info/properties", and "features/info/other".  
+Things matching the "org.eclipse*" namespace, only the "attributes" and "features/info" paths will be the only fields 
+indexed in the search database.
 
 Important notes: 
 * Ditto will use the namespace of the thing and match the FIRST namespace-pattern it encounters. So make sure any
   configured namespace-patterns are unique enough to match.
 * Ditto will automatically add the system-level fields it needs to operate, so no manual configuration of these is
   necessary.
 
+Example for configuring the same configuration via system properties for the `things-search` service:
+
+```shell
+-Dditto.search.namespace-search-include-fields.0.namespace-pattern=org.eclipse.test
+-Dditto.search.namespace-search-include-fields.0.search-include-fields.0=attributes
+-Dditto.search.namespace-search-include-fields.0.search-include-fields.1=features/info/properties
+-Dditto.search.namespace-search-include-fields.0.search-include-fields.2=features/info/other
+-Dditto.search.namespace-search-include-fields.1.namespace-pattern=org.eclipse*
+-Dditto.search.namespace-search-include-fields.1.search-include-fields.0=attributes
+-Dditto.search.namespace-search-include-fields.1.search-include-fields.1=features/info
+```
+
 ## Logging
 
 Gathering logs for a running Ditto installation can be achieved by:

internal/models/signalenrichment/src/main/java/org/eclipse/ditto/internal/models/signalenrichment/DittoCachingSignalEnrichmentFacade.java

@@ -28,11 +28,11 @@
 import org.eclipse.ditto.base.model.headers.DittoHeaders;
 import org.eclipse.ditto.base.model.signals.Signal;
 import org.eclipse.ditto.base.model.signals.WithResource;
-import org.eclipse.ditto.internal.utils.pekko.logging.DittoLoggerFactory;
-import org.eclipse.ditto.internal.utils.pekko.logging.ThreadSafeDittoLogger;
 import org.eclipse.ditto.internal.utils.cache.Cache;
 import org.eclipse.ditto.internal.utils.cache.CacheFactory;
 import org.eclipse.ditto.internal.utils.cache.config.CacheConfig;
+import org.eclipse.ditto.internal.utils.pekko.logging.DittoLoggerFactory;
+import org.eclipse.ditto.internal.utils.pekko.logging.ThreadSafeDittoLogger;
 import org.eclipse.ditto.json.JsonFactory;
 import org.eclipse.ditto.json.JsonFieldSelector;
 import org.eclipse.ditto.json.JsonObject;
@@ -97,7 +97,7 @@ public CompletionStage<JsonObject> retrieveThing(final ThingId thingId, final Li
 
         final DittoHeaders dittoHeaders = DittoHeaders.empty();
 
-        JsonFieldSelector fieldSelector = determineSelector(thingId.getNamespace());
+        final JsonFieldSelector fieldSelector = determineSelector(thingId.getNamespace());
 
         if (minAcceptableSeqNr < 0) {
             final var cacheKey =
@@ -450,7 +450,7 @@ private JsonObject enhanceJsonObject(final JsonObject jsonObject, final List<Thi
     }
 
     @Nullable
-    protected JsonFieldSelector determineSelector(String namespace) {
+    protected JsonFieldSelector determineSelector(final String namespace) {
         // By default, we do not return a field selector.
         return null;
     }

internal/models/signalenrichment/src/main/java/org/eclipse/ditto/internal/models/signalenrichment/SearchIndexingSignalEnrichmentFacade.java

@@ -12,25 +12,17 @@
  */
 package org.eclipse.ditto.internal.models.signalenrichment;
 
-import org.apache.pekko.japi.Pair;
-import org.eclipse.ditto.base.model.headers.DittoHeaders;
-import org.eclipse.ditto.internal.utils.cache.config.CacheConfig;
-import org.eclipse.ditto.internal.utils.pekko.logging.DittoLoggerFactory;
-import org.eclipse.ditto.internal.utils.pekko.logging.ThreadSafeDittoLogger;
-import org.eclipse.ditto.json.JsonFieldSelector;
-import org.eclipse.ditto.json.JsonObject;
-import org.eclipse.ditto.things.model.Thing;
-import org.eclipse.ditto.things.model.ThingId;
-import org.eclipse.ditto.things.model.signals.events.ThingEvent;
+import static org.eclipse.ditto.base.model.common.ConditionChecker.checkNotNull;
 
-import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.CompletionStage;
 import java.util.concurrent.Executor;
 import java.util.regex.Pattern;
 
-import static org.eclipse.ditto.base.model.common.ConditionChecker.checkNotNull;
+import org.apache.pekko.japi.Pair;
+import org.eclipse.ditto.internal.utils.cache.config.CacheConfig;
+import org.eclipse.ditto.json.JsonFieldSelector;
 
 /**
  * Extension of {@code DittoCachingSignalEnrichmentFacade} that allows a selected map of selected indexes grouped by
@@ -39,6 +31,7 @@
 public final class SearchIndexingSignalEnrichmentFacade extends DittoCachingSignalEnrichmentFacade {
 
     private final List<Pair<Pattern, JsonFieldSelector>> selectedIndexes;
+    private final Map<String, JsonFieldSelector> selectedIndexesCache;
 
     private SearchIndexingSignalEnrichmentFacade(
             final List<Pair<Pattern, JsonFieldSelector>> selectedIndexes,
@@ -49,7 +42,8 @@ private SearchIndexingSignalEnrichmentFacade(
 
         super(cacheLoaderFacade, cacheConfig, cacheLoaderExecutor, cacheNamePrefix);
 
-        this.selectedIndexes = Collections.unmodifiableList(selectedIndexes);
+        this.selectedIndexes = List.copyOf(selectedIndexes);
+        selectedIndexesCache = new HashMap<>();
     }
 
     /**
@@ -78,16 +72,15 @@ public static SearchIndexingSignalEnrichmentFacade newInstance(
     }
 
     @Override
-    protected JsonFieldSelector determineSelector(String namespace) {
+    protected JsonFieldSelector determineSelector(final String namespace) {
 
-        // We iterate through the list and return the first JsonFieldSelector that matches the namespace pattern.
-        for (final Pair<Pattern, JsonFieldSelector> pair : selectedIndexes) {
-
-            if (pair.first().matcher(namespace).matches()) {
-                return pair.second();
-            }
+        if (!selectedIndexesCache.containsKey(namespace)) {
+            // We iterate through the list and return the first JsonFieldSelector that matches the namespace pattern.
+            selectedIndexes.stream()
+                    .filter(pair -> pair.first().matcher(namespace).matches())
+                    .findFirst()
+                    .ifPresent(pair -> selectedIndexesCache.put(namespace, pair.second()));
         }
-
-        return super.determineSelector(namespace);
+        return selectedIndexesCache.get(namespace);
     }
 }