Adapted the namespace regex and prohibit the use of the empty namespace.

Add more specific exception for invalid persistence ids.
Add unit tests.

Signed-off-by: Stefan Maute <stefan.maute@bosch-si.com>

model/base/src/main/java/org/eclipse/ditto/model/base/entity/Entity.java

@@ -35,7 +35,7 @@ public interface Entity<T extends Revision<T>> extends Jsonifiable.WithFieldSele
     /**
      * The regex pattern for a Namespace.
      */
-    String NAMESPACE_REGEX = "(?<ns>|(?:(?:[a-zA-Z]\\w*+)(?:\\.[a-zA-Z]\\w*+)*+))";
+    String NAMESPACE_REGEX = "(?<ns>(?:(?:[a-zA-Z]\\w*+)(?:\\.[a-zA-Z]\\w*+)*+))";
 
     /**
      * The regex pattern for an Entity Name. Has to be conform to

services/utils/persistence/src/main/java/org/eclipse/ditto/services/utils/persistence/mongo/suffixes/NamespaceSuffixCollectionNames.java

@@ -63,10 +63,8 @@ public String getSuffixFromPersistenceId(final String persistenceId) {
 
         final String[] persistenceIdSplitByColons = persistenceId.split(":");
 
-        if (persistenceIdSplitByColons.length < 3) {
-            throw new IllegalStateException(
-                    String.format("Persistence id <%s> is not in the expected format of <prefix:namespace:name>",
-                            persistenceId));
+        if (persistenceIdSplitByColons.length < 2) {
+            throw new PersistenceIdInvalidException(persistenceId);
         }
 
         final String prefix = persistenceIdSplitByColons[0];
@@ -106,4 +104,13 @@ static String doValidateMongoCharacters(final String input) {
         }
         return escaped;
     }
+
+    public static final class PersistenceIdInvalidException extends RuntimeException {
+
+        private static final long serialVersionUID = -4789912839628096316L;
+
+        private  PersistenceIdInvalidException(final String persistenceId){
+            super(String.format("Persistence id <%s> is not in the expected format of <prefix:namespace:name>", persistenceId));
+        }
+    }
 }

services/utils/persistence/src/test/java/org/eclipse/ditto/services/utils/persistence/mongo/suffixes/NamespaceSuffixCollectionNamesTest.java

@@ -26,7 +26,7 @@ public class NamespaceSuffixCollectionNamesTest {
     @Before
     public void setup() {
         NamespaceSuffixCollectionNames.resetConfig();
-        this.sut = new NamespaceSuffixCollectionNames();
+        sut = new NamespaceSuffixCollectionNames();
     }
 
     @Test
@@ -91,4 +91,23 @@ public void validateMongoCharactersThrowsNullpointerExceptionWithoutConfig() {
 
         sut.getSuffixFromPersistenceId(persistenceId);
     }
+
+    @Test(expected = NamespaceSuffixCollectionNames.PersistenceIdInvalidException.class)
+    public void getSuffixFromPersistenceIdThrowsIllegalStateException() {
+        NamespaceSuffixCollectionNames.setConfig(new SuffixBuilderConfig(Collections.singletonList("thing")));
+        final String persistenceId = "thing:::::::";
+
+        sut.getSuffixFromPersistenceId(persistenceId);
+    }
+
+    @Test
+    public void getSuffixFromPersistenceIdWithNamespaceAndOnlyColons() {
+        NamespaceSuffixCollectionNames.setConfig(new SuffixBuilderConfig(Collections.singletonList("thing")));
+        final String persistenceId = "thing:org.eclipse.ditto:::::::";
+
+        final String suffix = sut.getSuffixFromPersistenceId(persistenceId);
+
+        final String expectedSuffix = "org.eclipse.ditto";
+        assertThat(suffix).isEqualTo(expectedSuffix);
+    }
 }