diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
index 8dfa82f..1f12a1f 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java
@@ -45,6 +45,7 @@ public class DataStore {
 
 	private static final Pattern EXTRACT_TAGS_PATTERN = Pattern.compile(REGEX_STORAGE_FILE);
 
+	// to be guarded by itself
 	private final List<Doc> docIdToDoc = new ArrayList<>();
 
 	private final Map<String, Map<String, IntList>> keyToValueToDocId = new ConcurrentHashMap<>();
@@ -70,13 +71,18 @@ public class DataStore {
 
 		});
 		trimIntLists();
-		((ArrayList<Doc>)docIdToDoc).trimToSize();
+		synchronized (docIdToDoc) {			
+			((ArrayList<Doc>)docIdToDoc).trimToSize();
+		}
 	}
 
 	private void cacheTagToFileMapping(final Tags tags, final Path path) {
 
-		final int docId = docIdToDoc.size();
-		docIdToDoc.add(new Doc(tags, path));
+		final int docId;
+		synchronized (docIdToDoc) {			
+			docId = docIdToDoc.size();
+			docIdToDoc.add(new Doc(tags, path));
+		}
 
 		for (final String key : tags.getKeys()) {
 			final Map<String, IntList> valueToDocIds = keyToValueToDocId.computeIfAbsent(key, k -> new ConcurrentHashMap<>());
@@ -231,23 +237,35 @@ public class DataStore {
 
 	private IntList executeQuery(final String query) {
 		final long start = System.nanoTime();
-		final Expression expression = QueryLanguageParser.parse(query);
-		final ExpressionToDocIdVisitor visitor = new ExpressionToDocIdVisitor(keyToValueToDocId,
-				new AllDocIds(docIdToDoc));
-		final IntList docIdsList = expression.visit(visitor);
-		EXECUTE_QUERY_LOGGER.debug("executeQuery({}) took {}ms returned {} results " , query, (System.nanoTime() - start) / 1_000_000.0, docIdsList.size());
-		return docIdsList;
+		synchronized (docIdToDoc) {
+			final Expression expression = QueryLanguageParser.parse(query);
+			final ExpressionToDocIdVisitor visitor = new ExpressionToDocIdVisitor(
+					keyToValueToDocId, new AllDocIds(docIdToDoc));
+			final IntList docIdsList = expression.visit(visitor);
+			EXECUTE_QUERY_LOGGER.debug(
+					"executeQuery({}) took {}ms returned {} results ", query,
+					(System.nanoTime() - start) / 1_000_000.0,
+					docIdsList.size());
+			return docIdsList;
+		}
 	}
 
 	private List<Doc> mapDocIdsToDocs(final IntList docIdsList) {
 		final List<Doc> result = new ArrayList<>(docIdsList.size());
 
+		synchronized (docIdToDoc) {	
 		final int[] intDocIds = docIdsList.toArray();
-		for (int i = 0; i < intDocIds.length; i++) {
-			final int docId = intDocIds[i];
-
-			final Doc doc = docIdToDoc.get(docId);
-			result.add(doc);
+			for (int i = 0; i < intDocIds.length; i++) {
+				final int docId = intDocIds[i];
+	
+				final Doc doc = docIdToDoc.get(docId);
+				
+				if (!doc.getTags().getValue("pod").equals("vadtrans01")){
+					System.out.println();
+				}
+				
+				result.add(doc);
+			}
 		}
 		return result;
 	}