[MOD-13819] Add two new metrics for tiered index observability: frontend buffer size and HNSW main thread insertion

src/VecSim/algorithms/hnsw/hnsw_tiered.h

@@ -94,6 +94,11 @@ class TieredHNSWIndex : public VecSimTieredIndex<DataType, DistType> {
     // associated swap jobs.
     std::mutex idToRepairJobsGuard;
 
+    // Counter for vectors inserted directly into HNSW by the main thread (bypassing flat buffer).
+    // This happens in WriteInPlace mode or when the flat buffer is full.
+    // Not atomic since it's only accessed from the main thread.
+    size_t directHNSWInsertions{0};
+
     void executeInsertJob(HNSWInsertJob *job);
     void executeRepairJob(HNSWRepairJob *job);
 
@@ -211,6 +216,7 @@ class TieredHNSWIndex : public VecSimTieredIndex<DataType, DistType> {
     // needed.
     VecSimIndexDebugInfo debugInfo() const override;
     VecSimIndexBasicInfo basicInfo() const override;
+    VecSimIndexStatsInfo statisticInfo() const override;
     VecSimDebugInfoIterator *debugInfoIterator() const override;
     VecSimBatchIterator *newBatchIterator(const void *queryBlob,
                                           VecSimQueryParams *queryParams) const override {
@@ -729,6 +735,8 @@ int TieredHNSWIndex<DataType, DistType>::addVector(const void *blob, labelType l
         this->lockMainIndexGuard();
         hnsw_index->addVector(storage_blob.get(), label);
         this->unlockMainIndexGuard();
+        // Track direct insertion to HNSW (bypassing flat buffer)
+        ++this->directHNSWInsertions;
         return ret;
     }
     if (this->frontendIndex->indexSize() >= this->flatBufferLimit) {
@@ -746,6 +754,8 @@ int TieredHNSWIndex<DataType, DistType>::addVector(const void *blob, labelType l
             // index.
             auto storage_blob = this->frontendIndex->preprocessForStorage(blob);
             this->insertVectorToHNSW<false>(hnsw_index, label, storage_blob.get());
+            // Track direct insertion to HNSW (flat buffer was full)
+            ++this->directHNSWInsertions;
             return ret;
         }
         // Otherwise, we fall back to the "regular" insertion into the flat buffer
@@ -1151,6 +1161,13 @@ void TieredHNSWIndex<DataType, DistType>::TieredHNSW_BatchIterator::filter_irrel
     results.resize(cur_end - results.begin());
 }
 
+template <typename DataType, typename DistType>
+VecSimIndexStatsInfo TieredHNSWIndex<DataType, DistType>::statisticInfo() const {
+    auto stats = VecSimTieredIndex<DataType, DistType>::statisticInfo();
+    stats.directHNSWInsertions = this->directHNSWInsertions;
+    return stats;
+}
+
 template <typename DataType, typename DistType>
 VecSimIndexDebugInfo TieredHNSWIndex<DataType, DistType>::debugInfo() const {
     auto info = VecSimTieredIndex<DataType, DistType>::debugInfo();

src/VecSim/vec_sim_common.h

@@ -342,6 +342,9 @@ typedef struct {
     size_t memory;
     size_t numberOfMarkedDeleted; // The number of vectors that are marked as deleted (HNSW/tiered
                                   // only).
+    size_t directHNSWInsertions;  // Count of vectors inserted directly into HNSW by main thread
+                                  // (bypassing flat buffer). Tiered HNSW only.
+    size_t flatBufferSize;        // Current flat buffer size. Tiered indexes only.
 } VecSimIndexStatsInfo;
 
 typedef struct {

src/VecSim/vec_sim_index.h

@@ -198,6 +198,8 @@ struct VecSimIndexAbstract : public VecSimIndexInterface {
         return VecSimIndexStatsInfo{
             .memory = this->getAllocationSize(),
             .numberOfMarkedDeleted = 0,
+            .directHNSWInsertions = 0,
+            .flatBufferSize = 0,
         };
     }
 

src/VecSim/vec_sim_tiered_index.h

@@ -320,6 +320,8 @@ VecSimIndexStatsInfo VecSimTieredIndex<DataType, DistType>::statisticInfo() cons
     auto stats = VecSimIndexStatsInfo{
         .memory = this->getAllocationSize(),
         .numberOfMarkedDeleted = this->getNumMarkedDeleted(),
+        .directHNSWInsertions = 0, // Base tiered index returns 0; TieredHNSWIndex overrides
+        .flatBufferSize = this->frontendIndex->indexSize(),
     };
 
     return stats;

tests/unit/test_hnsw_tiered.cpp

@@ -2761,6 +2761,9 @@ TYPED_TEST(HNSWTieredIndexTest, testInfo) {
     EXPECT_EQ(info.tieredInfo.backgroundIndexing, false);
     EXPECT_EQ(info.tieredInfo.bufferLimit, 1000);
     EXPECT_EQ(info.tieredInfo.specificTieredBackendInfo.hnswTieredInfo.pendingSwapJobsThreshold, 1);
+    // Verify new tiered-specific stats
+    EXPECT_EQ(stats.flatBufferSize, 0);
+    EXPECT_EQ(stats.directHNSWInsertions, 0);
 
     // Validate that Static info returns the right restricted info as well.
     VecSimIndexBasicInfo s_info = VecSimIndex_BasicInfo(tiered_index);
@@ -2787,6 +2790,9 @@ TYPED_TEST(HNSWTieredIndexTest, testInfo) {
                                           info.tieredInfo.frontendCommonInfo.memory);
     EXPECT_EQ(info.commonInfo.memory, stats.memory);
     EXPECT_EQ(info.tieredInfo.backgroundIndexing, true);
+    // Vector is in flat buffer, no direct insertions yet
+    EXPECT_EQ(stats.flatBufferSize, 1);
+    EXPECT_EQ(stats.directHNSWInsertions, 0);
 
     mock_thread_pool.thread_iteration();
     info = tiered_index->debugInfo();
@@ -2803,6 +2809,9 @@ TYPED_TEST(HNSWTieredIndexTest, testInfo) {
                                           info.tieredInfo.frontendCommonInfo.memory);
     EXPECT_EQ(info.commonInfo.memory, stats.memory);
     EXPECT_EQ(info.tieredInfo.backgroundIndexing, false);
+    // Vector moved from flat buffer to HNSW by background thread
+    EXPECT_EQ(stats.flatBufferSize, 0);
+    EXPECT_EQ(stats.directHNSWInsertions, 0);
 
     if (TypeParam::isMulti()) {
         GenerateAndAddVector<TEST_DATA_T>(tiered_index, dim, 1, 1);
@@ -2839,6 +2848,74 @@ TYPED_TEST(HNSWTieredIndexTest, testInfo) {
     EXPECT_EQ(info.tieredInfo.backgroundIndexing, false);
 }
 
+TYPED_TEST(HNSWTieredIndexTest, testDirectHNSWInsertionsStats) {
+    // Test that directHNSWInsertions counter is incremented when flat buffer is full.
+    size_t dim = 4;
+    size_t buffer_limit = 5;
+    HNSWParams params = {.type = TypeParam::get_index_type(),
+                         .dim = dim,
+                         .metric = VecSimMetric_L2,
+                         .multi = TypeParam::isMulti()};
+    VecSimParams hnsw_params = CreateParams(params);
+    auto mock_thread_pool = tieredIndexMock();
+
+    // Create index with small buffer limit
+    auto *tiered_index =
+        this->CreateTieredHNSWIndex(hnsw_params, mock_thread_pool, 1, buffer_limit);
+
+    // Fill the flat buffer
+    for (size_t i = 0; i < buffer_limit; i++) {
+        GenerateAndAddVector<TEST_DATA_T>(tiered_index, dim, i, i);
+    }
+
+    VecSimIndexStatsInfo stats = tiered_index->statisticInfo();
+    EXPECT_EQ(stats.flatBufferSize, buffer_limit);
+    EXPECT_EQ(stats.directHNSWInsertions, 0);
+
+    // Add more vectors - these should go directly to HNSW
+    size_t extra_vectors = 3;
+    for (size_t i = buffer_limit; i < buffer_limit + extra_vectors; i++) {
+        GenerateAndAddVector<TEST_DATA_T>(tiered_index, dim, i, i);
+    }
+
+    stats = tiered_index->statisticInfo();
+    EXPECT_EQ(stats.flatBufferSize, buffer_limit);
+    EXPECT_EQ(stats.directHNSWInsertions, extra_vectors);
+
+    // Drain the flat buffer by starting threads and waiting for them to finish
+    mock_thread_pool.init_threads();
+    mock_thread_pool.thread_pool_join();
+
+    stats = tiered_index->statisticInfo();
+    EXPECT_EQ(stats.flatBufferSize, 0);
+    // Direct insertions counter should be preserved
+    EXPECT_EQ(stats.directHNSWInsertions, extra_vectors);
+
+    // Test write-in-place mode: vectors should go directly to HNSW even when buffer is not full
+    VecSim_SetWriteMode(VecSim_WriteInPlace);
+
+    size_t write_in_place_vectors = 4;
+    size_t label_offset = buffer_limit + extra_vectors;
+    for (size_t i = 0; i < write_in_place_vectors; i++) {
+        GenerateAndAddVector<TEST_DATA_T>(tiered_index, dim, label_offset + i, label_offset + i);
+    }
+
+    stats = tiered_index->statisticInfo();
+    // Flat buffer should still be empty (vectors went directly to HNSW)
+    EXPECT_EQ(stats.flatBufferSize, 0);
+    // Direct insertions counter should include the write-in-place vectors
+    EXPECT_EQ(stats.directHNSWInsertions, extra_vectors + write_in_place_vectors);
+
+    // Verify all vectors are in the backend index
+    VecSimIndexDebugInfo info = tiered_index->debugInfo();
+    EXPECT_EQ(info.tieredInfo.backendCommonInfo.indexSize,
+              buffer_limit + extra_vectors + write_in_place_vectors);
+    EXPECT_EQ(info.tieredInfo.frontendCommonInfo.indexSize, 0);
+
+    // Reset to async mode
+    VecSim_SetWriteMode(VecSim_WriteAsync);
+}
+
 TYPED_TEST(HNSWTieredIndexTest, testInfoIterator) {
     // Create TieredHNSW index instance with a mock queue.
     size_t dim = 4;