Closes #830 - Extend EUM server to handle non existing beacon fields as empty values

components/inspectit-ocelot-eum-server/src/main/java/rocks/inspectit/oce/eum/server/beacon/processor/RegexReplacementBeaconProcessor.java

@@ -60,20 +60,25 @@ public Beacon process(Beacon beacon) {
     }
 
     private String deriveTag(RegexDerivedTag tag, String input) {
-        if (input == null) {
-            return null;
+        String value = input;
+        if (value == null) {
+            if (tag.isNullAsEmpty()) {
+                value = "";
+            } else {
+                return null;
+            }
         }
 
         Pattern regex = tag.getRegex();
         if (regex != null) {
             try {
-                return regexReplaceAll(input, regex, tag.getReplacement(), tag.isKeepIfNoMatch());
+                return regexReplaceAll(value, regex, tag.getReplacement(), tag.isKeepIfNoMatch());
             } catch (Exception ex) {
-                log.error("Could not derive beacon tag value <{}> of input string <{}>!", tag.getTagName(), input, tag.getReplacement(), ex);
+                log.error("Could not derive beacon tag value <{}> of input string <{}>!", tag.getTagName(), value, tag.getReplacement(), ex);
                 return null;
             }
         } else {
-            return input;
+            return value;
         }
     }
 
@@ -149,6 +154,11 @@ private static class RegexDerivedTag {
          */
         boolean keepIfNoMatch;
 
+        /**
+         * Specify whether the input field should be considered as an empty string if it does not exists.
+         */
+        boolean nullAsEmpty;
+
         private static RegexDerivedTag fromSettings(String tagName, BeaconTagSettings settings) {
             Pattern pattern = null;
             String regex = settings.getRegex();
@@ -161,6 +171,7 @@ private static RegexDerivedTag fromSettings(String tagName, BeaconTagSettings se
                     .regex(pattern)
                     .replacement(settings.getReplacement())
                     .keepIfNoMatch(settings.isKeepNoMatch())
+                    .nullAsEmpty(settings.isNullAsEmpty())
                     .build();
         }
     }

components/inspectit-ocelot-eum-server/src/main/java/rocks/inspectit/oce/eum/server/configuration/model/BeaconTagSettings.java

@@ -47,4 +47,9 @@ public class BeaconTagSettings {
     @Builder.Default
     private String replacement = "";
 
+    /**
+     * Specify whether the input field should be considered as an empty string if it does not exists.
+     */
+    @Builder.Default
+    private boolean nullAsEmpty = false;
 }

components/inspectit-ocelot-eum-server/src/test/java/rocks/inspectit/oce/eum/server/beacon/processor/RegexReplacementBeaconProcessorTest.java

@@ -139,5 +139,26 @@ void nullAttribute() {
                     entry("value", "Hello World")
             );
         }
+
+        @Test
+        void nullAttributeAsEmpty() {
+            BeaconTagSettings first = BeaconTagSettings.builder().input("not-existing").regex("^$").replacement("one").nullAsEmpty(true).build();
+
+            Map<String, BeaconTagSettings> beaconTags = new LinkedHashMap<>();
+            beaconTags.put("first", first);
+
+            EumServerConfiguration conf = new EumServerConfiguration();
+            conf.setTags(new EumTagsSettings());
+            conf.getTags().setBeacon(beaconTags);
+
+            RegexReplacementBeaconProcessor processor = new RegexReplacementBeaconProcessor(conf);
+
+            Beacon result = processor.process(Beacon.of(ImmutableMap.of("value", "Hello World")));
+
+            assertThat(result.toMap()).containsOnly(
+                    entry("value", "Hello World"),
+                    entry("first", "one")
+            );
+        }
     }
 }