Fix a flaky unit test:testMultiFieldsKnnIndex, which was failing due …

…to inconsistent merge behaviors

Signed-off-by: Navneet Verma <[email protected]>

CHANGELOG.md

@@ -21,6 +21,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Infrastructure
 ### Documentation
 ### Maintenance
+* Fix a flaky unit test:testMultiFieldsKnnIndex, which was failing due to inconsistent merge behaviors [#1924](https://github.com/opensearch-project/k-NN/pull/1924)
 ### Refactoring
 * Introduce KNNVectorValues interface to iterate on different types of Vector values during indexing and search [#1897](https://github.com/opensearch-project/k-NN/pull/1897)
 * Clean up parsing for query [#1824](https://github.com/opensearch-project/k-NN/pull/1824)

src/test/java/org/opensearch/knn/index/codec/KNNCodecTestCase.java

@@ -137,28 +137,27 @@ public void testMultiFieldsKnnIndex(Codec codec) throws Exception {
         Document doc = new Document();
         doc.add(vectorField);
         writer.addDocument(doc);
-        writer.close();
+        // ensuring the refresh happens, to create the segment and hnsw file
+        writer.flush();
 
         /**
          * Add doc with field "my_vector"
          */
-        IndexWriterConfig iwc1 = newIndexWriterConfig();
-        iwc1.setMergeScheduler(new SerialMergeScheduler());
-        iwc1.setCodec(ACTUAL_CODEC);
-        writer = new RandomIndexWriter(random(), dir, iwc1);
         float[] array1 = { 6.0f, 14.0f };
         VectorField vectorField1 = new VectorField("my_vector", array1, sampleFieldType);
         Document doc1 = new Document();
         doc1.add(vectorField1);
         writer.addDocument(doc1);
+        // ensuring the refresh happens, to create the segment and hnsw file
+        writer.flush();
         IndexReader reader = writer.getReader();
         writer.close();
         ResourceWatcherService resourceWatcherService = createDisabledResourceWatcherService();
         NativeMemoryLoadStrategy.IndexLoadStrategy.initialize(resourceWatcherService);
         List<String> hnswfiles = Arrays.stream(dir.listAll()).filter(x -> x.contains("hnsw")).collect(Collectors.toList());
 
         // there should be 2 hnsw index files created. one for test_vector and one for my_vector
-        assertEquals(hnswfiles.size(), 2);
+        assertEquals(2, hnswfiles.size());
         assertEquals(hnswfiles.stream().filter(x -> x.contains("test_vector")).collect(Collectors.toList()).size(), 1);
         assertEquals(hnswfiles.stream().filter(x -> x.contains("my_vector")).collect(Collectors.toList()).size(), 1);