Continued development

18 files changed, 186 insertions, 145 deletions

diff --git a/include/framework/DynamicExtension.h b/include/framework/DynamicExtension.h
index 1886234..c35bb93 100644
--- a/include/framework/DynamicExtension.h
+++ b/include/framework/DynamicExtension.h
@@ -15,7 +15,6 @@
 
 #include "framework/interface/Scheduler.h"
 #include "framework/reconstruction/ReconstructionPolicy.h"
-#include "framework/scheduling/FIFOScheduler.h"
 #include "framework/scheduling/SerialScheduler.h"
 
 #include "framework/structure/ExtensionStructure.h"
@@ -28,7 +27,7 @@ namespace de {
 
 template <ShardInterface ShardType, QueryInterface<ShardType> QueryType,
           DeletePolicy D = DeletePolicy::TAGGING,
-          SchedulerInterface SchedType = de::FIFOScheduler>
+          SchedulerInterface SchedType = de::SerialScheduler>
 class DynamicExtension {
 private:
   /* convenience typedefs for commonly used types within the class */
@@ -76,10 +75,12 @@ public:
    *        performing compactions and flushes, etc.
    */
   DynamicExtension(ReconPolicyType *recon_policy, size_t buffer_low_watermark,
-                   size_t buffer_high_watermark, size_t memory_budget = 0,
+                   size_t buffer_high_watermark = 0, size_t memory_budget = 0,
                    size_t thread_cnt = 16)
       : m_max_delete_prop(1), m_sched(memory_budget, thread_cnt),
-        m_buffer(new Buffer(buffer_low_watermark, buffer_high_watermark)),
+        m_buffer(new Buffer(buffer_low_watermark, (buffer_high_watermark == 0)
+                                                      ? buffer_low_watermark
+                                                      : buffer_high_watermark)),
         m_core_cnt(thread_cnt), m_next_core(0), m_epoch_cnt(0),
         m_recon_policy(recon_policy) {
 
@@ -165,7 +166,7 @@ public:
       auto view = m_buffer->get_buffer_view();
 
       auto epoch = get_active_epoch();
-      if (epoch->get_structure()->tagged_delete(rec)) {
+      if (epoch->get_mutable_structure()->tagged_delete(rec)) {
         end_job(epoch);
         return 1;
       }
@@ -303,13 +304,14 @@ public:
 
     auto epoch = get_active_epoch();
     auto vers = epoch->get_structure();
-    std::vector<ShardType *> shards;
-
-    if (vers->get_levels().size() > 0) {
-      for (int i = vers->get_levels().size() - 1; i >= 0; i--) {
-        if (vers->get_levels()[i] &&
-            vers->get_levels()[i]->get_record_count() > 0) {
-          shards.emplace_back(vers->get_levels()[i]->get_combined_shard());
+    std::vector<const ShardType *> shards;
+
+    if (vers->get_level_vector().size() > 0) {
+      for (int i = vers->get_level_vector().size() - 1; i >= 0; i--) {
+        if (vers->get_level_vector()[i] &&
+            vers->get_level_vector()[i]->get_record_count() > 0) {
+          shards.emplace_back(
+              vers->get_level_vector()[i]->get_combined_shard());
         }
       }
     }
@@ -358,7 +360,8 @@ public:
    */
   bool validate_tombstone_proportion() {
     auto epoch = get_active_epoch();
-    auto t = epoch->get_structure()->validate_tombstone_proportion();
+    auto t = epoch->get_structure()->validate_tombstone_proportion(
+        m_max_delete_prop);
     end_job(epoch);
     return t;
   }
@@ -370,7 +373,6 @@ public:
   void print_scheduler_statistics() const { m_sched.print_statistics(); }
 
 private:
-  ReconPolicyType const *m_recon_policy;
   double m_max_delete_prop;
 
   SchedType m_sched;
@@ -380,6 +382,8 @@ private:
   std::atomic<int> m_next_core;
   std::atomic<size_t> m_epoch_cnt;
 
+  ReconPolicyType const *m_recon_policy;
+
   alignas(64) std::atomic<bool> m_reconstruction_scheduled;
 
   std::atomic<epoch_ptr> m_next_epoch;
@@ -547,39 +551,36 @@ private:
     auto args = (ReconstructionArgs<ShardType, QueryType> *)arguments;
 
     ((DynamicExtension *)args->extension)->SetThreadAffinity();
-    Structure *vers = args->epoch->get_structure();
+    Structure *vers = args->epoch->get_mutable_structure();
 
-    for (size_t i=0; i<args->tasks.size(); i++) {
-      vers->perform_reconstruction(args->tasks[i]);
-    }
+    ReconstructionTask flush_task;
+    flush_task.type = ReconstructionType::Invalid;
 
-    /*
-     * we'll grab the buffer AFTER doing the internal reconstruction, so we
-     * can flush as many records as possible in one go. The reconstruction
-     * was done so as to make room for the full buffer anyway, so there's
-     * no real benefit to doing this first.
-     */
-    auto buffer_view = args->epoch->get_buffer();
-    size_t new_head = buffer_view.get_tail();
+    for (size_t i = 0; i < args->tasks.size(); i++) {
+      if (args->tasks[i].sources.size() > 0 &&
+          args->tasks[i].sources[0] == buffer_shid) {
+        flush_task = args->tasks[i];
+        continue;
+      }
 
-    /*
-     * if performing a compaction, don't flush the buffer, as
-     * there is no guarantee that any necessary reconstructions
-     * will free sufficient space in L0 to support a flush
-     */
-    if (!args->compaction) {
-      vers->flush_buffer(std::move(buffer_view));
+      vers->perform_reconstruction(args->tasks[i]);
     }
 
-    args->result.set_value(true);
+    if (flush_task.type != ReconstructionType::Invalid) {
+      /*
+       * we'll grab the buffer AFTER doing the internal reconstruction, so we
+       * can flush as many records as possible in one go. The reconstruction
+       * was done so as to make room for the full buffer anyway, so there's
+       * no real benefit to doing this first.
+       */
+      auto buffer_view = args->epoch->get_buffer();
+      size_t new_head = buffer_view.get_tail();
 
-    /*
-     * Compactions occur on an epoch _before_ it becomes active,
-     * and as a result the active epoch should _not_ be advanced as
-     * part of a compaction
-     */
-    if (!args->compaction) {
+      vers->perform_flush(flush_task, std::move(buffer_view));
+      args->result.set_value(true);
       ((DynamicExtension *)args->extension)->advance_epoch(new_head);
+    } else {
+      args->result.set_value(true);
     }
 
     ((DynamicExtension *)args->extension)
@@ -660,10 +661,12 @@ private:
     args->tasks = m_recon_policy->get_reconstruction_tasks(
         epoch, m_buffer->get_high_watermark());
     args->extension = this;
-    args->compaction = false;
-    /* NOTE: args is deleted by the reconstruction job, so shouldn't be freed
-     * here */
+    args->tasks.add_reconstruction(m_recon_policy->get_flush_task(epoch));
 
+    /* 
+     * NOTE: args is deleted by the reconstruction job, so shouldn't be freed
+     * here 
+     */
     m_sched.schedule_job(reconstruction, 0, args, RECONSTRUCTION);
   }
 
@@ -691,7 +694,8 @@ private:
     return m_buffer->append(rec, ts);
   }
 
-#ifdef _GNU_SOURCE
+//#ifdef _GNU_SOURCE
+#if 0
   void SetThreadAffinity() {
     if constexpr (std::same_as<SchedType, SerialScheduler>) {
       return;
diff --git a/include/framework/interface/Shard.h b/include/framework/interface/Shard.h
index bd980c0..fb5ce1a 100644
--- a/include/framework/interface/Shard.h
+++ b/include/framework/interface/Shard.h
@@ -14,7 +14,7 @@ namespace de {
 
 template <typename SHARD>
 concept ShardInterface = RecordInterface<typename SHARD::RECORD> &&
-    requires(SHARD shard, const std::vector<SHARD *> &shard_vector, bool b,
+    requires(SHARD shard, const std::vector<const SHARD *> &shard_vector, bool b,
              BufferView<typename SHARD::RECORD> bv,
              typename SHARD::RECORD rec) {
   /* construct a shard from a vector of shards of the same type */
diff --git a/include/framework/reconstruction/BSMPolicy.h b/include/framework/reconstruction/BSMPolicy.h
index 9138bd1..4ceca9a 100644
--- a/include/framework/reconstruction/BSMPolicy.h
+++ b/include/framework/reconstruction/BSMPolicy.h
@@ -16,17 +16,17 @@
 
 namespace de {
 template <ShardInterface ShardType, QueryInterface<ShardType> QueryType>
-class BSMPolicy : ReconstructionPolicy<ShardType, QueryType> {
+class BSMPolicy : public ReconstructionPolicy<ShardType, QueryType> {
   typedef std::vector<std::shared_ptr<InternalLevel<ShardType, QueryType>>>
       LevelVector;
 
 public:
-  BSMPolicy(size_t scale_factor, size_t buffer_size)
-      : m_scale_factor(scale_factor), m_buffer_size(buffer_size) {}
+  BSMPolicy(size_t buffer_size)
+      : m_scale_factor(2), m_buffer_size(buffer_size) {}
 
   ReconstructionVector
-  get_reconstruction_tasks(Epoch<ShardType, QueryType> *epoch,
-                           size_t incoming_reccnt) override {
+  get_reconstruction_tasks(const Epoch<ShardType, QueryType> *epoch,
+                           size_t incoming_reccnt) const override {
     ReconstructionVector reconstructions;
     auto levels = epoch->get_structure()->get_level_vector();
 
@@ -44,7 +44,7 @@ public:
     task.type = ReconstructionType::Merge;
 
     for (level_index i = target_level; i > source_level; i--) {
-      if (i < levels.size()) {
+      if (i < (level_index)levels.size()) {
         task.add_shard({i, all_shards_idx}, levels[i]->get_record_count());
       }
     }
@@ -54,17 +54,17 @@ public:
   }
 
   ReconstructionTask
-  get_flush_task(Epoch<ShardType, QueryType> *epoch) override {
+  get_flush_task(const Epoch<ShardType, QueryType> *epoch) const override {
     return ReconstructionTask{
         {{buffer_shid}}, 0, m_buffer_size, ReconstructionType::Flush};
   }
 
 private:
-  level_index find_reconstruction_target(LevelVector &levels) {
+  level_index find_reconstruction_target(LevelVector &levels) const {
     level_index target_level = 0;
 
-    for (size_t i = 0; i < (level_index)levels.size(); i++) {
-      if (levels[i].get_record_count() + 1 <= capacity(i)) {
+    for (level_index i = 0; i < (level_index)levels.size(); i++) {
+      if (levels[i]->get_record_count() + 1 <= capacity(i)) {
         target_level = i;
         break;
       }
@@ -73,7 +73,7 @@ private:
     return target_level;
   }
 
-  inline size_t capacity(level_index level) {
+  inline size_t capacity(level_index level) const {
     return m_buffer_size * pow(m_scale_factor, level + 1);
   }
 
diff --git a/include/framework/reconstruction/LevelingPolicy.h b/include/framework/reconstruction/LevelingPolicy.h
index 00f2cff..8bf5645 100644
--- a/include/framework/reconstruction/LevelingPolicy.h
+++ b/include/framework/reconstruction/LevelingPolicy.h
@@ -16,7 +16,7 @@
 
 namespace de {
 template <ShardInterface ShardType, QueryInterface<ShardType> QueryType>
-class LevelingPolicy : ReconstructionPolicy<ShardType, QueryType> {
+class LevelingPolicy : public ReconstructionPolicy<ShardType, QueryType> {
   typedef std::vector<std::shared_ptr<InternalLevel<ShardType, QueryType>>>
       LevelVector;
 
@@ -25,8 +25,8 @@ public:
       : m_scale_factor(scale_factor), m_buffer_size(buffer_size) {}
 
   ReconstructionVector
-  get_reconstruction_tasks(Epoch<ShardType, QueryType> *epoch,
-                           size_t incoming_reccnt) override {
+  get_reconstruction_tasks(const Epoch<ShardType, QueryType> *epoch,
+                           size_t incoming_reccnt) const override {
     ReconstructionVector reconstructions;
     auto levels = epoch->get_structure()->get_level_vector();
 
@@ -41,7 +41,7 @@ public:
 
     for (level_index i = target_level; i > source_level; i--) {
       size_t target_reccnt =
-          (i < levels.size()) ? levels[i]->get_record_count() : 0;
+          (i < (level_index)levels.size()) ? levels[i]->get_record_count() : 0;
       size_t total_reccnt = levels[i - 1]->get_record_count() + target_reccnt;
 
       reconstructions.add_reconstruction(i - 1, i, total_reccnt,
@@ -52,29 +52,29 @@ public:
   }
 
   ReconstructionTask
-  get_flush_task(Epoch<ShardType, QueryType> *epoch) override {
+  get_flush_task(const Epoch<ShardType, QueryType> *epoch) const override {
     return ReconstructionTask{
-        {{buffer_shid}}, 0, m_buffer_size, ReconstructionType::Flush};
+        {{buffer_shid}}, 0, m_buffer_size, ReconstructionType::Merge};
   }
 
 private:
-  level_index find_reconstruction_target(LevelVector &levels) {
+  level_index find_reconstruction_target(LevelVector &levels) const {
     level_index target_level = 0;
     size_t incoming_records = m_buffer_size;
 
-    for (size_t i = 0; i < (level_index)levels.size(); i++) {
-      if (levels[i].get_record_count() + incoming_records < capacity(i)) {
+    for (level_index i = 0; i < (level_index)levels.size(); i++) {
+      if (levels[i]->get_record_count() + incoming_records < capacity(i)) {
         target_level = i;
         break;
       }
 
-      incoming_records = levels[i].get_record_count();
+      incoming_records = levels[i]->get_record_count();
     }
 
     return target_level;
   }
 
-  inline size_t capacity(level_index level) {
+  inline size_t capacity(level_index level) const {
     return m_buffer_size * pow(m_scale_factor, level + 1);
   }
 
diff --git a/include/framework/reconstruction/ReconstructionPolicy.h b/include/framework/reconstruction/ReconstructionPolicy.h
index 976091e..aa213df 100644
--- a/include/framework/reconstruction/ReconstructionPolicy.h
+++ b/include/framework/reconstruction/ReconstructionPolicy.h
@@ -23,8 +23,8 @@ class ReconstructionPolicy {
 
 public:
   ReconstructionPolicy() {}
-  virtual ReconstructionVector get_reconstruction_tasks(Epoch<ShardType, QueryType> *epoch, 
-                                                        size_t incoming_reccnt) = 0;
-  virtual ReconstructionTask get_flush_task(Epoch<ShardType, QueryType> *epoch) = 0;
+  virtual ReconstructionVector get_reconstruction_tasks(const Epoch<ShardType, QueryType> *epoch, 
+                                                        size_t incoming_reccnt) const = 0;
+  virtual ReconstructionTask get_flush_task(const Epoch<ShardType, QueryType> *epoch) const = 0;
   };
 }
diff --git a/include/framework/reconstruction/TieringPolicy.h b/include/framework/reconstruction/TieringPolicy.h
index 120bcb5..71fe9ec 100644
--- a/include/framework/reconstruction/TieringPolicy.h
+++ b/include/framework/reconstruction/TieringPolicy.h
@@ -16,15 +16,15 @@
 
 namespace de {
 template <ShardInterface ShardType, QueryInterface<ShardType> QueryType>
-class TieringPolicy : ReconstructionPolicy<ShardType, QueryType> {
+class TieringPolicy : public ReconstructionPolicy<ShardType, QueryType> {
   typedef std::vector<std::shared_ptr<InternalLevel<ShardType, QueryType>>> LevelVector;
 public:
   TieringPolicy(size_t scale_factor, size_t buffer_size)
       : m_scale_factor(scale_factor), m_buffer_size(buffer_size) {}
 
   ReconstructionVector
-  get_reconstruction_tasks(Epoch<ShardType, QueryType> *epoch,
-                           size_t incoming_reccnt) override {
+  get_reconstruction_tasks(const Epoch<ShardType, QueryType> *epoch,
+                           size_t incoming_reccnt) const override {
     ReconstructionVector reconstructions;
     auto levels = epoch->get_structure()->get_level_vector();
 
@@ -39,7 +39,7 @@ public:
 
     for (level_index i = target_level; i > source_level; i--) {
       size_t target_reccnt =
-          (i < levels.size()) ? levels[i]->get_record_count() : 0;
+          (i < (level_index)levels.size()) ? levels[i]->get_record_count() : 0;
       size_t total_reccnt = levels[i - 1]->get_record_count() + target_reccnt;
 
       reconstructions.add_reconstruction(i - 1, i, total_reccnt,
@@ -50,17 +50,17 @@ public:
   }
 
   ReconstructionTask
-  get_flush_task(Epoch<ShardType, QueryType> *epoch) override {
+  get_flush_task(const Epoch<ShardType, QueryType> *epoch) const override {
     return ReconstructionTask{
         {{buffer_shid}}, 0, m_buffer_size, ReconstructionType::Flush};
   }
 
 private:
-  level_index find_reconstruction_target(LevelVector &levels) {
+  level_index find_reconstruction_target(LevelVector &levels) const {
     level_index target_level = 0;
 
-    for (size_t i = 0; i < (level_index) levels.size(); i++) {
-      if (levels[i].get_shard_count() + 1 <= capacity()) {
+    for (level_index i = 0; i < (level_index) levels.size(); i++) {
+      if (levels[i]->get_shard_count() + 1 <= capacity()) {
         target_level = i;
         break;
       }
@@ -69,7 +69,7 @@ private:
     return target_level;
   }
 
-  inline size_t capacity() {
+  inline size_t capacity() const {
     return m_scale_factor;
   }
 
diff --git a/include/framework/scheduling/Epoch.h b/include/framework/scheduling/Epoch.h
index 303ab2f..95c64ea 100644
--- a/include/framework/scheduling/Epoch.h
+++ b/include/framework/scheduling/Epoch.h
@@ -54,11 +54,13 @@ public:
   Epoch &operator=(const Epoch &) = delete;
   Epoch &operator=(Epoch &&) = delete;
 
-  size_t get_epoch_number() { return m_epoch_number; }
+  size_t get_epoch_number() const { return m_epoch_number; }
 
-  Structure *get_structure() { return m_structure; }
+  const Structure *get_structure() const { return m_structure; }
 
-  BufView get_buffer() { return m_buffer->get_buffer_view(m_buffer_head); }
+  Structure *get_mutable_structure() { return m_structure; }
+
+  BufView get_buffer() const { return m_buffer->get_buffer_view(m_buffer_head); }
 
   /*
    * Returns a new Epoch object that is a copy of this one. The new object
diff --git a/include/framework/structure/ExtensionStructure.h b/include/framework/structure/ExtensionStructure.h
index 9b7ae87..3bb8a0b 100644
--- a/include/framework/structure/ExtensionStructure.h
+++ b/include/framework/structure/ExtensionStructure.h
@@ -147,14 +147,14 @@ public:
 
   inline void perform_reconstruction(ReconstructionTask task) {
     /* perform the reconstruction itself */
-    std::vector<ShardType *> shards;
+    std::vector<const ShardType *> shards;
     for (ShardID shid : task.sources) {
-      assert(shid.level_idx < m_levels.size());
+      assert(shid.level_idx < (level_index) m_levels.size());
       assert(shid.shard_idx >= -1);
 
       /* if unspecified, push all shards into the vector */
       if (shid.shard_idx == all_shards_idx) {
-        for (size_t i = 0; i < m_levels[shid.level_idx].get_shard_count();
+        for (size_t i = 0; i < m_levels[shid.level_idx]->get_shard_count();
              i++) {
           if (m_levels[shid.level_idx]->get_shard(i)) {
             shards.push_back(m_levels[shid.level_idx]->get_shard(i));
@@ -165,7 +165,7 @@ public:
       }
     }
 
-    auto new_shard = Shard(shards);
+    auto new_shard = new ShardType(shards);
 
     /*
      * Remove all of the shards processed by the operation
@@ -181,10 +181,11 @@ public:
     /*
      * Append the new shard to the target level
      */
-    if (task.target < m_levels.size()) {
-      m_levels[task.target]->append_shard(new_shard);
-    } else {
-      m_levels.push_back();
+    if (task.target < (level_index)m_levels.size()) {
+      m_levels[task.target]->append(std::shared_ptr<ShardType>(new_shard));
+    } else { /* grow the structure if needed */
+      m_levels.push_back(std::make_shared<InternalLevel<ShardType, QueryType>>(task.target));
+      m_levels[task.target]->append(std::shared_ptr<ShardType>(new_shard));
     }
   }
 
@@ -197,12 +198,20 @@ public:
      * the buffer itself. Given that we're unlikely to actually use policies
      * like that, we'll leave this as low priority.
      */
-    ShardType *buffer_shard = new ShardType(buffer);
-    if (task.type == ReconstructionType::Append) {
-      m_levels[0]->append(std::shared_ptr(buffer_shard));
+
+    /* insert the first level, if needed */
+    if (m_levels.size() == 0) {
+      m_levels.push_back(
+          std::make_shared<InternalLevel<ShardType, QueryType>>(0));
+    }
+
+    ShardType *buffer_shard = new ShardType(std::move(buffer));
+    if (task.type == ReconstructionType::Append || m_levels[0]->get_shard_count() == 0) {
+      m_levels[0]->append(std::shared_ptr<ShardType>(buffer_shard));
     } else {
-      std::vector<ShardType *> shards;
-      for (size_t i = 0; i < m_levels[0].size(); i++) {
+      std::vector<const ShardType *> shards;
+      for (level_index i = 0; i < (level_index)m_levels[0]->get_shard_count();
+           i++) {
         if (m_levels[0]->get_shard(i)) {
           shards.push_back(m_levels[0]->get_shard(i));
         }
@@ -210,7 +219,7 @@ public:
         shards.push_back(buffer_shard);
         ShardType *new_shard = new ShardType(shards);
         m_levels[0]->truncate();
-        m_levels[0]->append(std::shared_ptr(new_shard));
+        m_levels[0]->append(std::shared_ptr<ShardType>(new_shard));
       }
     }
   }
@@ -243,6 +252,32 @@ public:
 
   LevelVector const &get_level_vector() const { return m_levels; }
 
+  
+    /*
+     * Validate that no level in the structure exceeds its maximum tombstone
+     * capacity. This is used to trigger preemptive compactions at the end of
+     * the reconstruction process.
+     */
+    bool validate_tombstone_proportion(double max_delete_prop) const {
+      long double ts_prop;
+      for (size_t i = 0; i < m_levels.size(); i++) {
+        if (m_levels[i]) {
+          ts_prop = (long double)m_levels[i]->get_tombstone_count() /
+                    (long double)m_levels[i]->get_record_count();
+          if (ts_prop > (long double)max_delete_prop) {
+            return false;
+          }
+        }
+      }
+
+      return true;
+    }
+
+    bool validate_tombstone_proportion(level_index level, double max_delete_prop) const {
+        long double ts_prop =  (long double) m_levels[level]->get_tombstone_count() / (long double) m_levels[level]->get_record_count();
+        return ts_prop <= (long double) max_delete_prop;
+    }
+
 private:
   std::atomic<size_t> m_refcnt;
   LevelVector m_levels;
diff --git a/include/framework/structure/InternalLevel.h b/include/framework/structure/InternalLevel.h
index 8cfcd49..c9d1749 100644
--- a/include/framework/structure/InternalLevel.h
+++ b/include/framework/structure/InternalLevel.h
@@ -40,12 +40,12 @@ public:
    *
    * No changes are made to this level.
    */
-  ShardType *get_combined_shard() {
+  ShardType *get_combined_shard() const {
     if (m_shards.size() == 0) {
       return nullptr;
     }
 
-    std::vector<ShardType *> shards;
+    std::vector<const ShardType *> shards;
     for (auto shard : m_shards) {
       if (shard)
         shards.emplace_back(shard.get());
@@ -57,7 +57,7 @@ public:
   void get_local_queries(
       std::vector<std::pair<ShardID, ShardType *>> &shards,
       std::vector<typename QueryType::LocalQuery *> &local_queries,
-      typename QueryType::Parameters *query_parms) {
+      typename QueryType::Parameters *query_parms) const {
     for (size_t i = 0; i < m_shards.size(); i++) {
       if (m_shards[i]) {
         auto local_query =
@@ -68,7 +68,7 @@ public:
     }
   }
 
-  bool check_tombstone(size_t shard_stop, const RecordType &rec) {
+  bool check_tombstone(size_t shard_stop, const RecordType &rec) const {
     if (m_shards.size() == 0)
       return false;
 
@@ -100,7 +100,7 @@ public:
     return false;
   }
 
-  ShardType *get_shard(size_t idx) {
+  const ShardType *get_shard(size_t idx) const {
     if (idx >= m_shards.size()) {
       return nullptr;
     }
@@ -108,9 +108,9 @@ public:
     return m_shards[idx].get();
   }
 
-  size_t get_shard_count() { return m_shards.size(); }
+  size_t get_shard_count() const { return m_shards.size(); }
 
-  size_t get_record_count() {
+  size_t get_record_count() const {
     size_t cnt = 0;
     for (size_t i = 0; i < m_shards.size(); i++) {
       if (m_shards[i]) {
@@ -121,7 +121,7 @@ public:
     return cnt;
   }
 
-  size_t get_tombstone_count() {
+  size_t get_tombstone_count() const {
     size_t res = 0;
     for (size_t i = 0; i < m_shards.size(); ++i) {
       if (m_shards[i]) {
@@ -131,7 +131,7 @@ public:
     return res;
   }
 
-  size_t get_aux_memory_usage() {
+  size_t get_aux_memory_usage() const {
     size_t cnt = 0;
     for (size_t i = 0; i < m_shards.size(); i++) {
       if (m_shards[i]) {
@@ -142,7 +142,7 @@ public:
     return cnt;
   }
 
-  size_t get_memory_usage() {
+  size_t get_memory_usage() const {
     size_t cnt = 0;
     for (size_t i = 0; i < m_shards.size(); i++) {
       if (m_shards[i]) {
@@ -153,7 +153,7 @@ public:
     return cnt;
   }
 
-  double get_tombstone_prop() {
+  double get_tombstone_prop() const {
     size_t tscnt = 0;
     size_t reccnt = 0;
     for (size_t i = 0; i < m_shards.size(); i++) {
@@ -166,10 +166,10 @@ public:
     return (double)tscnt / (double)(tscnt + reccnt);
   }
 
-  std::shared_ptr<InternalLevel> clone() {
+  std::shared_ptr<InternalLevel> clone() const {
     auto new_level = std::make_shared<InternalLevel>(m_level_no);
     for (size_t i = 0; i < m_shards.size(); i++) {
-      new_level->m_shards[i] = m_shards[i];
+      new_level->append(m_shards[i]);
     }
 
     return new_level;
@@ -181,7 +181,7 @@ public:
     m_shards.erase(m_shards.begin() + shard);
   }
 
-  bool append(std::shared_ptr<ShardType> shard) {
+  void append(std::shared_ptr<ShardType> shard) {
     m_shards.emplace_back(shard);
   }
 
diff --git a/include/framework/structure/MutableBuffer.h b/include/framework/structure/MutableBuffer.h
index 625b04b..0eae73d 100644
--- a/include/framework/structure/MutableBuffer.h
+++ b/include/framework/structure/MutableBuffer.h
@@ -97,15 +97,15 @@ public:
     return true;
   }
 
-  size_t get_record_count() { return m_tail.load() - m_head.load().head_idx; }
+  size_t get_record_count() const { return m_tail.load() - m_head.load().head_idx; }
 
-  size_t get_capacity() { return m_cap; }
+  size_t get_capacity() const { return m_cap; }
 
-  bool is_full() { return get_record_count() >= m_hwm; }
+  bool is_full() const { return get_record_count() >= m_hwm; }
 
-  bool is_at_low_watermark() { return get_record_count() >= m_lwm; }
+  bool is_at_low_watermark() const { return get_record_count() >= m_lwm; }
 
-  size_t get_tombstone_count() { return m_tscnt.load(); }
+  size_t get_tombstone_count() const { return m_tscnt.load(); }
 
   bool delete_record(const R &rec) {
     return get_buffer_view().delete_record(rec);
@@ -115,9 +115,9 @@ public:
     return get_buffer_view().check_tombstone(rec);
   }
 
-  size_t get_memory_usage() { return m_cap * sizeof(Wrapped<R>); }
+  size_t get_memory_usage() const { return m_cap * sizeof(Wrapped<R>); }
 
-  size_t get_aux_memory_usage() {
+  size_t get_aux_memory_usage() const {
     return m_tombstone_filter->get_memory_usage();
   }
 
@@ -200,7 +200,7 @@ public:
     m_lwm = lwm;
   }
 
-  size_t get_low_watermark() { return m_lwm; }
+  size_t get_low_watermark() const { return m_lwm; }
 
   void set_high_watermark(size_t hwm) {
     assert(hwm > m_lwm);
@@ -208,9 +208,9 @@ public:
     m_hwm = hwm;
   }
 
-  size_t get_high_watermark() { return m_hwm; }
+  size_t get_high_watermark() const { return m_hwm; }
 
-  size_t get_tail() { return m_tail.load(); }
+  size_t get_tail() const { return m_tail.load(); }
 
   /*
    * Note: this returns the available physical storage capacity,
@@ -220,7 +220,7 @@ public:
    * but a buggy framework implementation may violate the
    * assumption.
    */
-  size_t get_available_capacity() {
+  size_t get_available_capacity() const {
     if (m_old_head.load().refcnt == 0) {
       return m_cap - (m_tail.load() - m_head.load().head_idx);
     }
diff --git a/include/shard/Alias.h b/include/shard/Alias.h
index 8fe70a5..15b0884 100644
--- a/include/shard/Alias.h
+++ b/include/shard/Alias.h
@@ -70,7 +70,7 @@ public:
         }
     }
 
-    Alias(std::vector<Alias*> const &shards)
+    Alias(std::vector<const Alias*> const &shards)
         : m_data(nullptr)
         , m_alias(nullptr)
         , m_total_weight(0)
@@ -146,15 +146,15 @@ public:
     }
 
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return 0;
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         return (m_bf) ? m_bf->memory_usage() : 0;
     }
 
-    W get_total_weight() {
+    W get_total_weight() const {
         return m_total_weight;
     }
 
diff --git a/include/shard/FSTrie.h b/include/shard/FSTrie.h
index 4e51037..d720aad 100644
--- a/include/shard/FSTrie.h
+++ b/include/shard/FSTrie.h
@@ -82,7 +82,7 @@ public:
         delete[] temp_buffer;
     }
 
-    FSTrie(std::vector<FSTrie*> const &shards) 
+    FSTrie(std::vector<const FSTrie*> const &shards) 
         : m_data(nullptr)
         , m_reccnt(0)
         , m_alloc_size(0)
@@ -181,11 +181,11 @@ public:
     }
 
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return m_fst->getMemoryUsage();
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         return m_alloc_size;
     }
 
diff --git a/include/shard/ISAMTree.h b/include/shard/ISAMTree.h
index 64c0b2b..f6b525f 100644
--- a/include/shard/ISAMTree.h
+++ b/include/shard/ISAMTree.h
@@ -65,7 +65,7 @@ public:
     }
   }
 
-  ISAMTree(std::vector<ISAMTree *> const &shards)
+  ISAMTree(std::vector<const ISAMTree *> const &shards)
       : m_bf(nullptr), m_isam_nodes(nullptr), m_root(nullptr), m_reccnt(0),
         m_tombstone_cnt(0), m_internal_node_cnt(0), m_deleted_cnt(0),
         m_alloc_size(0) {
@@ -93,7 +93,7 @@ public:
     delete m_bf;
   }
 
-  Wrapped<R> *point_lookup(const R &rec, bool filter = false) {
+  Wrapped<R> *point_lookup(const R &rec, bool filter = false) const {
     if (filter && !m_bf->lookup(rec)) {
       return nullptr;
     }
diff --git a/include/shard/LoudsPatricia.h b/include/shard/LoudsPatricia.h
index 3452839..fe0c30e 100644
--- a/include/shard/LoudsPatricia.h
+++ b/include/shard/LoudsPatricia.h
@@ -82,7 +82,7 @@ public:
         delete[] temp_buffer;
     }
 
-    LoudsPatricia(std::vector<LoudsPatricia*> &shards) 
+    LoudsPatricia(std::vector<const LoudsPatricia*> &shards) 
         : m_data(nullptr)
         , m_reccnt(0)
         , m_alloc_size(0)
@@ -178,11 +178,11 @@ public:
     }
 
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return m_louds->size();
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         return m_alloc_size;
     }
 
diff --git a/include/shard/PGM.h b/include/shard/PGM.h
index 7d1f492..5b39ab4 100644
--- a/include/shard/PGM.h
+++ b/include/shard/PGM.h
@@ -111,7 +111,7 @@ public:
         }
     }
 
-    PGM(std::vector<PGM*> const &shards)
+    PGM(std::vector<const PGM*> const &shards)
         : m_data(nullptr)
         , m_bf(nullptr)
         , m_reccnt(0)
@@ -190,7 +190,7 @@ public:
         delete m_bf;
     }
 
-    Wrapped<R> *point_lookup(const R &rec, bool filter=false) {
+    Wrapped<R> *point_lookup(const R &rec, bool filter=false) const {
         size_t idx = get_lower_bound(rec.key);
         if (idx >= m_reccnt) {
             return nullptr;
@@ -223,11 +223,11 @@ public:
     }
 
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return m_pgm.size_in_bytes();
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         return (m_bf) ? m_bf->memory_usage() : 0;
     }
 
diff --git a/include/shard/TrieSpline.h b/include/shard/TrieSpline.h
index 9d8c3bb..7f4d4e5 100644
--- a/include/shard/TrieSpline.h
+++ b/include/shard/TrieSpline.h
@@ -124,7 +124,7 @@ public:
         }
     }
 
-    TrieSpline(std::vector<TrieSpline*> const &shards) 
+    TrieSpline(std::vector<const TrieSpline*> const &shards) 
         : m_reccnt(0)
         , m_tombstone_cnt(0)
         , m_alloc_size(0)
@@ -229,7 +229,7 @@ public:
         delete m_bf;
     }
 
-    Wrapped<R> *point_lookup(const R &rec, bool filter=false) {
+    Wrapped<R> *point_lookup(const R &rec, bool filter=false) const {
         if (filter && m_bf && !m_bf->lookup(rec)) {
             return nullptr;
         }
@@ -266,11 +266,11 @@ public:
     }
 
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return m_ts.GetSize();
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         return (m_bf) ? m_bf->memory_usage() : 0;
     }
 
diff --git a/include/shard/VPTree.h b/include/shard/VPTree.h
index 477db5c..7130efe 100644
--- a/include/shard/VPTree.h
+++ b/include/shard/VPTree.h
@@ -86,7 +86,7 @@ public:
         }
     }
 
-    VPTree(std::vector<VPTree*> shards) 
+    VPTree(std::vector<const VPTree*> shards) 
     : m_reccnt(0), m_tombstone_cnt(0), m_node_cnt(0), m_root(nullptr) {
 
         size_t attemp_reccnt = 0;
@@ -174,11 +174,11 @@ public:
         return m_data + idx;
     }
 
-    size_t get_memory_usage() {
+    size_t get_memory_usage() const {
         return m_node_cnt * sizeof(vpnode) + m_reccnt * sizeof(R*);
     }
 
-    size_t get_aux_memory_usage() {
+    size_t get_aux_memory_usage() const {
         // FIXME: need to return the size of the unordered_map
         return 0;
     }
diff --git a/include/util/SortedMerge.h b/include/util/SortedMerge.h
index b0a3215..c41a7ae 100644
--- a/include/util/SortedMerge.h
+++ b/include/util/SortedMerge.h
@@ -51,7 +51,7 @@ struct merge_info {
  */
 template <RecordInterface R, ShardInterface S>
 static std::vector<Cursor<Wrapped<R>>>
-build_cursor_vec(std::vector<S *> const &shards, size_t *reccnt,
+build_cursor_vec(std::vector<const S *> const &shards, size_t *reccnt,
                  size_t *tscnt) {
   std::vector<Cursor<Wrapped<R>>> cursors;
   cursors.reserve(shards.size());