Extract tx::SingleNodeEngine from tx::MasterEngine

Summary:
No logic changes, just split `tx::MasterEngine` into
`tx::SingleNodeEngine` and `tx::MasterEngine`. This gives better
responsibility separation and is more appropriate now there is no
Start/Shutdown.

Reviewers: dgleich, teon.banek, buda

Reviewed By: dgleich, teon.banek

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1099

src/CMakeLists.txt

@@ -45,6 +45,7 @@ set(memgraph_src_files
     storage/vertex_accessor.cpp
     threading/thread.cpp
     transactions/engine_master.cpp
+    transactions/engine_single_node.cpp
     transactions/engine_worker.cpp
     utils/watchdog.cpp
 )

src/database/graph_db.cpp

@@ -11,6 +11,7 @@
 #include "storage/concurrent_id_mapper_master.hpp"
 #include "storage/concurrent_id_mapper_worker.hpp"
 #include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 #include "transactions/engine_worker.hpp"
 #include "utils/timer.hpp"
 
@@ -22,7 +23,7 @@ namespace fs = std::experimental::filesystem;
   properties_ = std::make_unique<type<GraphDbTypes::Property>>(__VA_ARGS__);
 
 GraphDb::GraphDb(Config config) : GraphDb(config, 0) {
-  tx_engine_ = std::make_unique<tx::MasterEngine>(&wal_);
+  tx_engine_ = std::make_unique<tx::SingleNodeEngine>(&wal_);
   counters_ = std::make_unique<database::SingleNodeCounters>();
   INIT_MAPPERS(storage::SingleNodeConcurrentIdMapper);
   Start();
@@ -31,9 +32,7 @@ GraphDb::GraphDb(Config config) : GraphDb(config, 0) {
 GraphDb::GraphDb(communication::messaging::System &system,
                  distributed::MasterCoordination &master, Config config)
     : GraphDb(config, 0) {
-  auto tx_engine = std::make_unique<tx::MasterEngine>(&wal_);
-  tx_engine->StartServer(system);
-  tx_engine_ = std::move(tx_engine);
+  tx_engine_ = std::make_unique<tx::MasterEngine>(system, &wal_);
   auto counters = std::make_unique<database::MasterCounters>(system);
   counters_ = std::move(counters);
   INIT_MAPPERS(storage::MasterConcurrentIdMapper, system);

src/database/graph_db.hpp

@@ -75,7 +75,7 @@ class GraphDb {
   };
 
   /** Single-node GraphDb ctor. */
-  GraphDb(Config config = Config{});
+  explicit GraphDb(Config config = Config{});
 
   /** Distributed master GraphDb ctor. */
   GraphDb(communication::messaging::System &system,

src/database/graph_db_accessor.cpp

@@ -10,7 +10,7 @@
 #include "utils/on_scope_exit.hpp"
 
 GraphDbAccessor::GraphDbAccessor(GraphDb &db)
-    : db_(db), transaction_(MasterEngine().Begin()) {}
+    : db_(db), transaction_(SingleNodeEngine().Begin()) {}
 
 GraphDbAccessor::~GraphDbAccessor() {
   if (!commited_ && !aborted_) {
@@ -24,18 +24,18 @@ tx::transaction_id_t GraphDbAccessor::transaction_id() const {
 
 void GraphDbAccessor::AdvanceCommand() {
   DCHECK(!commited_ && !aborted_) << "Accessor committed or aborted";
-  MasterEngine().Advance(transaction_->id_);
+  SingleNodeEngine().Advance(transaction_->id_);
 }
 
 void GraphDbAccessor::Commit() {
   DCHECK(!commited_ && !aborted_) << "Already aborted or commited transaction.";
-  MasterEngine().Commit(*transaction_);
+  SingleNodeEngine().Commit(*transaction_);
   commited_ = true;
 }
 
 void GraphDbAccessor::Abort() {
   DCHECK(!commited_ && !aborted_) << "Already aborted or commited transaction.";
-  MasterEngine().Abort(*transaction_);
+  SingleNodeEngine().Abort(*transaction_);
   aborted_ = true;
 }
 

src/database/graph_db_accessor.hpp

@@ -610,13 +610,15 @@ class GraphDbAccessor {
                            const RecordAccessor<Vertex> &vertex_accessor,
                            const Vertex *const vertex);
 
-  /** Casts the DB's engine to MasterEngine and returns it. If the DB's engine
-   * is RemoteEngine, this function will crash MG. */
-  tx::MasterEngine &MasterEngine() {
-    auto *master_engine =
-        dynamic_cast<tx::MasterEngine *>(db_.tx_engine_.get());
-    DCHECK(master_engine) << "Asked for MasterEngine on distributed worker";
-    return *master_engine;
+  /** Casts the DB's engine to SingleNodeEngine and returns it. If the DB's
+   * engine is RemoteEngine, this function will crash MG. It must be either
+   * SingleNodeEngine, or MasterEngine (which inherits it). */
+  tx::SingleNodeEngine &SingleNodeEngine() {
+    auto *single_node_engine =
+        dynamic_cast<tx::SingleNodeEngine *>(db_.tx_engine_.get());
+    DCHECK(single_node_engine)
+        << "Asked for SingleNodeEngine on distributed worker";
+    return *single_node_engine;
   }
 
   GraphDb &db_;

src/transactions/engine_master.cpp

@@ -9,127 +9,31 @@
 
 namespace tx {
 
-MasterEngine::MasterEngine(durability::WriteAheadLog *wal) : wal_(wal) {}
-
-Transaction *MasterEngine::Begin() {
-  std::lock_guard<SpinLock> guard(lock_);
-
-  transaction_id_t id{++counter_};
-  auto t = new Transaction(id, active_, *this);
-  active_.insert(id);
-  store_.emplace(id, t);
-  if (wal_) {
-    wal_->Emplace(database::StateDelta::TxBegin(id));
-  }
-  return t;
-}
-
-void MasterEngine::Advance(transaction_id_t id) {
-  std::lock_guard<SpinLock> guard(lock_);
-
-  auto it = store_.find(id);
-  DCHECK(it != store_.end())
-      << "Transaction::advance on non-existing transaction";
-
-  Transaction *t = it->second.get();
-  if (t->cid_ == std::numeric_limits<command_id_t>::max())
-    throw TransactionError(
-        "Reached maximum number of commands in this "
-        "transaction.");
-
-  t->cid_++;
-}
-
-void MasterEngine::Commit(const Transaction &t) {
-  std::lock_guard<SpinLock> guard(lock_);
-  clog_.set_committed(t.id_);
-  active_.remove(t.id_);
-  if (wal_) {
-    wal_->Emplace(database::StateDelta::TxCommit(t.id_));
-  }
-  store_.erase(store_.find(t.id_));
-}
-
-void MasterEngine::Abort(const Transaction &t) {
-  std::lock_guard<SpinLock> guard(lock_);
-  clog_.set_aborted(t.id_);
-  active_.remove(t.id_);
-  if (wal_) {
-    wal_->Emplace(database::StateDelta::TxAbort(t.id_));
-  }
-  store_.erase(store_.find(t.id_));
-}
-
-CommitLog::Info MasterEngine::Info(transaction_id_t tx) const {
-  return clog_.fetch_info(tx);
-}
-
-Snapshot MasterEngine::GlobalGcSnapshot() {
-  std::lock_guard<SpinLock> guard(lock_);
-
-  // No active transactions.
-  if (active_.size() == 0) {
-    auto snapshot_copy = active_;
-    snapshot_copy.insert(counter_ + 1);
-    return snapshot_copy;
-  }
-
-  // There are active transactions.
-  auto snapshot_copy = store_.find(active_.front())->second->snapshot();
-  snapshot_copy.insert(active_.front());
-  return snapshot_copy;
-}
-
-Snapshot MasterEngine::GlobalActiveTransactions() {
-  std::lock_guard<SpinLock> guard(lock_);
-  Snapshot active_transactions = active_;
-  return active_transactions;
-}
-
-bool MasterEngine::GlobalIsActive(transaction_id_t tx) const {
-  return clog_.is_active(tx);
-}
-
-tx::transaction_id_t MasterEngine::LocalLast() const { return counter_.load(); }
-
-void MasterEngine::LocalForEachActiveTransaction(
-    std::function<void(Transaction &)> f) {
-  std::lock_guard<SpinLock> guard(lock_);
-  for (auto transaction : active_) {
-    f(*store_.find(transaction)->second);
-  }
-}
-
-void MasterEngine::StartServer(communication::messaging::System &system) {
-  CHECK(!rpc_server_) << "Can't start a running server";
-  rpc_server_.emplace(system, "tx_engine");
-
-  rpc_server_->Register<SnapshotRpc>([this](const SnapshotReq &req) {
+MasterEngine::MasterEngine(communication::messaging::System &system,
+                           durability::WriteAheadLog *wal)
+    : SingleNodeEngine(wal), rpc_server_(system, kTransactionEngineRpc) {
+  rpc_server_.Register<SnapshotRpc>([this](const SnapshotReq &req) {
     // It is guaranteed that the Worker will not be requesting this for a
     // transaction that's done, and that there are no race conditions here.
-    auto found = store_.find(req.member);
-    DCHECK(found != store_.end())
-        << "Can't return snapshot for an inactive transaction";
-    return std::make_unique<SnapshotRes>(found->second->snapshot());
+    return std::make_unique<SnapshotRes>(GetSnapshot(req.member));
   });
 
-  rpc_server_->Register<GcSnapshotRpc>(
+  rpc_server_.Register<GcSnapshotRpc>(
       [this](const communication::messaging::Message &) {
         return std::make_unique<SnapshotRes>(GlobalGcSnapshot());
       });
 
-  rpc_server_->Register<ClogInfoRpc>([this](const ClogInfoReq &req) {
+  rpc_server_.Register<ClogInfoRpc>([this](const ClogInfoReq &req) {
     return std::make_unique<ClogInfoRes>(Info(req.member));
   });
 
-  rpc_server_->Register<ActiveTransactionsRpc>(
+  rpc_server_.Register<ActiveTransactionsRpc>(
       [this](const communication::messaging::Message &) {
         return std::make_unique<SnapshotRes>(GlobalActiveTransactions());
       });
 
-  rpc_server_->Register<IsActiveRpc>([this](const IsActiveReq &req) {
+  rpc_server_.Register<IsActiveRpc>([this](const IsActiveReq &req) {
     return std::make_unique<IsActiveRes>(GlobalIsActive(req.member));
   });
 }
-
 }  // namespace tx

src/transactions/engine_master.hpp

@@ -1,89 +1,23 @@
 #pragma once
 
-#include <atomic>
-#include <experimental/optional>
-#include <unordered_map>
-
 #include "communication/messaging/distributed.hpp"
 #include "communication/rpc/rpc.hpp"
-#include "durability/wal.hpp"
-#include "threading/sync/spinlock.hpp"
-#include "transactions/commit_log.hpp"
-#include "transactions/engine.hpp"
-#include "transactions/transaction.hpp"
-#include "utils/exceptions.hpp"
+#include "transactions/engine_single_node.hpp"
 
 namespace tx {
 
-/** Indicates an error in transaction handling (currently
- * only command id overflow). */
-class TransactionError : public utils::BasicException {
- public:
-  using utils::BasicException::BasicException;
-};
-
-/**
- * A transaction engine that contains everything necessary for transactional
- * handling. Used for single-node Memgraph deployments and for the master in a
- * distributed system.
- */
-class MasterEngine : public Engine {
+/** Distributed master transaction engine. Has complete engine functionality and
+ * exposes an RPC server to be used by distributed Workers. */
+class MasterEngine : public SingleNodeEngine {
  public:
   /**
    * @param wal - Optional. If present, the Engine will write tx
    * Begin/Commit/Abort atomically (while under lock).
    */
-  MasterEngine(durability::WriteAheadLog *wal = nullptr);
-
-  /**
-   * Begins a transaction and returns a pointer to
-   * it's object.
-   *
-   * The transaction object is owned by this engine.
-   * It will be released when the transaction gets
-   * committted or aborted.
-   */
-  Transaction *Begin();
-
-  /**
-   * Advances the command on the transaction with the
-   * given id.
-   *
-   * @param id - Transation id. That transaction must
-   * be currently active.
-   */
-  void Advance(transaction_id_t id);
-
-  /** Comits the given transaction. Deletes the transaction object, it's not
-   * valid after this function executes. */
-  void Commit(const Transaction &t);
-
-  /** Aborts the given transaction. Deletes the transaction object, it's not
-   * valid after this function executes. */
-  void Abort(const Transaction &t);
-
-  CommitLog::Info Info(transaction_id_t tx) const override;
-  Snapshot GlobalGcSnapshot() override;
-  Snapshot GlobalActiveTransactions() override;
-  bool GlobalIsActive(transaction_id_t tx) const override;
-  tx::transaction_id_t LocalLast() const override;
-  void LocalForEachActiveTransaction(
-      std::function<void(Transaction &)> f) override;
-
-  /** Starts the RPC server of the master transactional engine. */
-  void StartServer(communication::messaging::System &system);
+  MasterEngine(communication::messaging::System &system,
+               durability::WriteAheadLog *wal = nullptr);
 
  private:
-  std::atomic<transaction_id_t> counter_{0};
-  CommitLog clog_;
-  std::unordered_map<transaction_id_t, std::unique_ptr<Transaction>> store_;
-  Snapshot active_;
-  SpinLock lock_;
-  // Optional. If present, the Engine will write tx Begin/Commit/Abort
-  // atomically (while under lock).
-  durability::WriteAheadLog *wal_{nullptr};
-
-  // Optional RPC server, only used in distributed, not in single_node.
-  std::experimental::optional<communication::rpc::Server> rpc_server_;
+  communication::rpc::Server rpc_server_;
 };
 }  // namespace tx

src/transactions/engine_rpc_messages.hpp

@@ -8,6 +8,8 @@
 
 namespace tx {
 
+const std::string kTransactionEngineRpc = "transaction_engine_rpc";
+
 RPC_SINGLE_MEMBER_MESSAGE(SnapshotReq, transaction_id_t)
 RPC_SINGLE_MEMBER_MESSAGE(SnapshotRes, Snapshot)
 RPC_NO_MEMBER_MESSAGE(GcSnapshotReq)

src/transactions/engine_single_node.cpp

@@ -0,0 +1,113 @@
+#include <limits>
+#include <mutex>
+
+#include "glog/logging.h"
+
+#include "database/state_delta.hpp"
+#include "transactions/engine_rpc_messages.hpp"
+#include "transactions/engine_single_node.hpp"
+
+namespace tx {
+
+SingleNodeEngine::SingleNodeEngine(durability::WriteAheadLog *wal)
+    : wal_(wal) {}
+
+Transaction *SingleNodeEngine::Begin() {
+  std::lock_guard<SpinLock> guard(lock_);
+
+  transaction_id_t id{++counter_};
+  auto t = new Transaction(id, active_, *this);
+  active_.insert(id);
+  store_.emplace(id, t);
+  if (wal_) {
+    wal_->Emplace(database::StateDelta::TxBegin(id));
+  }
+  return t;
+}
+
+void SingleNodeEngine::Advance(transaction_id_t id) {
+  std::lock_guard<SpinLock> guard(lock_);
+
+  auto it = store_.find(id);
+  DCHECK(it != store_.end())
+      << "Transaction::advance on non-existing transaction";
+
+  Transaction *t = it->second.get();
+  if (t->cid_ == std::numeric_limits<command_id_t>::max())
+    throw TransactionError(
+        "Reached maximum number of commands in this "
+        "transaction.");
+
+  t->cid_++;
+}
+
+void SingleNodeEngine::Commit(const Transaction &t) {
+  std::lock_guard<SpinLock> guard(lock_);
+  clog_.set_committed(t.id_);
+  active_.remove(t.id_);
+  if (wal_) {
+    wal_->Emplace(database::StateDelta::TxCommit(t.id_));
+  }
+  store_.erase(store_.find(t.id_));
+}
+
+void SingleNodeEngine::Abort(const Transaction &t) {
+  std::lock_guard<SpinLock> guard(lock_);
+  clog_.set_aborted(t.id_);
+  active_.remove(t.id_);
+  if (wal_) {
+    wal_->Emplace(database::StateDelta::TxAbort(t.id_));
+  }
+  store_.erase(store_.find(t.id_));
+}
+
+CommitLog::Info SingleNodeEngine::Info(transaction_id_t tx) const {
+  return clog_.fetch_info(tx);
+}
+
+Snapshot SingleNodeEngine::GlobalGcSnapshot() {
+  std::lock_guard<SpinLock> guard(lock_);
+
+  // No active transactions.
+  if (active_.size() == 0) {
+    auto snapshot_copy = active_;
+    snapshot_copy.insert(counter_ + 1);
+    return snapshot_copy;
+  }
+
+  // There are active transactions.
+  auto snapshot_copy = store_.find(active_.front())->second->snapshot();
+  snapshot_copy.insert(active_.front());
+  return snapshot_copy;
+}
+
+Snapshot SingleNodeEngine::GlobalActiveTransactions() {
+  std::lock_guard<SpinLock> guard(lock_);
+  Snapshot active_transactions = active_;
+  return active_transactions;
+}
+
+bool SingleNodeEngine::GlobalIsActive(transaction_id_t tx) const {
+  return clog_.is_active(tx);
+}
+
+tx::transaction_id_t SingleNodeEngine::LocalLast() const {
+  return counter_.load();
+}
+
+void SingleNodeEngine::LocalForEachActiveTransaction(
+    std::function<void(Transaction &)> f) {
+  std::lock_guard<SpinLock> guard(lock_);
+  for (auto transaction : active_) {
+    f(*store_.find(transaction)->second);
+  }
+}
+
+Snapshot SingleNodeEngine::GetSnapshot(tx::transaction_id_t tx_id) {
+  std::lock_guard<SpinLock> guard(lock_);
+  auto found = store_.find(tx_id);
+  DCHECK(found != store_.end())
+      << "Can't return snapshot for an inactive transaction";
+  return found->second->snapshot();
+}
+}  // namespace tx

src/transactions/engine_single_node.hpp

@@ -0,0 +1,83 @@
+#pragma once
+
+#include <atomic>
+#include <experimental/optional>
+#include <unordered_map>
+
+#include "communication/messaging/distributed.hpp"
+#include "communication/rpc/rpc.hpp"
+#include "durability/wal.hpp"
+#include "threading/sync/spinlock.hpp"
+#include "transactions/commit_log.hpp"
+#include "transactions/engine.hpp"
+#include "transactions/transaction.hpp"
+#include "utils/exceptions.hpp"
+
+namespace tx {
+
+/** Indicates an error in transaction handling (currently
+ * only command id overflow). */
+class TransactionError : public utils::BasicException {
+ public:
+  using utils::BasicException::BasicException;
+};
+
+/** Single-node deployment transaction engine. Has complete functionality. */
+class SingleNodeEngine : public Engine {
+ public:
+  /**
+   * @param wal - Optional. If present, the Engine will write tx
+   * Begin/Commit/Abort atomically (while under lock).
+   */
+  explicit SingleNodeEngine(durability::WriteAheadLog *wal = nullptr);
+
+  /**
+   * Begins a transaction and returns a pointer to
+   * it's object.
+   *
+   * The transaction object is owned by this engine.
+   * It will be released when the transaction gets
+   * committted or aborted.
+   */
+  Transaction *Begin();
+
+  /**
+   * Advances the command on the transaction with the
+   * given id.
+   *
+   * @param id - Transation id. That transaction must
+   * be currently active.
+   */
+  void Advance(transaction_id_t id);
+
+  /** Comits the given transaction. Deletes the transaction object, it's not
+   * valid after this function executes. */
+  void Commit(const Transaction &t);
+
+  /** Aborts the given transaction. Deletes the transaction object, it's not
+   * valid after this function executes. */
+  void Abort(const Transaction &t);
+
+  CommitLog::Info Info(transaction_id_t tx) const override;
+  Snapshot GlobalGcSnapshot() override;
+  Snapshot GlobalActiveTransactions() override;
+  bool GlobalIsActive(transaction_id_t tx) const override;
+  tx::transaction_id_t LocalLast() const override;
+  void LocalForEachActiveTransaction(
+      std::function<void(Transaction &)> f) override;
+
+ protected:
+  // Exposed for MasterEngine. Transaction for tx_id must be alive.
+  Snapshot GetSnapshot(tx::transaction_id_t tx_id);
+
+ private:
+  std::atomic<transaction_id_t> counter_{0};
+  CommitLog clog_;
+  std::unordered_map<transaction_id_t, std::unique_ptr<Transaction>> store_;
+  Snapshot active_;
+  SpinLock lock_;
+  // Optional. If present, the Engine will write tx Begin/Commit/Abort
+  // atomically (while under lock).
+  durability::WriteAheadLog *wal_{nullptr};
+};
+}  // namespace tx

src/transactions/engine_worker.cpp

@@ -11,7 +11,7 @@ static const auto kRpcTimeout = 100ms;
 
 WorkerEngine::WorkerEngine(communication::messaging::System &system,
                            const io::network::NetworkEndpoint &endpoint)
-    : rpc_client_(system, endpoint, "tx_engine") {}
+    : rpc_client_(system, endpoint, kTransactionEngineRpc) {}
 
 Transaction *WorkerEngine::LocalBegin(transaction_id_t tx_id) {
   auto accessor = active_.access();

src/transactions/engine_worker.hpp

@@ -13,7 +13,8 @@
 
 namespace tx {
 
-/** A transactional engine for the worker in a distributed system. */
+/** Distributed worker transaction engine. Connects to a MasterEngine (single
+ * source of truth) to obtain transactional info. Caches most info locally. */
 class WorkerEngine : public Engine {
  public:
   WorkerEngine(communication::messaging::System &system,

src/transactions/transaction.hpp

@@ -26,6 +26,7 @@ class Transaction {
   }
 
  private:
+  friend class SingleNodeEngine;
   friend class MasterEngine;
   friend class WorkerEngine;
 

tests/benchmark/mvcc.cpp

@@ -4,7 +4,7 @@
 
 #include "mvcc/record.hpp"
 #include "mvcc/version_list.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 class Prop : public mvcc::Record<Prop> {
  public:
@@ -19,7 +19,7 @@ class Prop : public mvcc::Record<Prop> {
 void MvccMix(benchmark::State &state) {
   while (state.KeepRunning()) {
     state.PauseTiming();
-    tx::MasterEngine engine;
+    tx::SingleNodeEngine engine;
     auto t1 = engine.Begin();
     mvcc::VersionList<Prop> version_list(*t1, 0);
 

tests/unit/database_key_index.cpp

@@ -4,7 +4,7 @@
 #include "database/graph_db_accessor.hpp"
 #include "database/graph_db_datatypes.hpp"
 #include "storage/vertex.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 #include "mvcc_gc_common.hpp"
 
@@ -15,7 +15,7 @@ TEST(LabelsIndex, UniqueInsert) {
   KeyIndex<GraphDbTypes::Label, Vertex> index;
   GraphDb db;
   GraphDbAccessor dba(db);
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   auto t1 = engine.Begin();
   mvcc::VersionList<Vertex> vlist(*t1, 0);
   engine.Commit(*t1);
@@ -43,7 +43,7 @@ TEST(LabelsIndex, UniqueFilter) {
   GraphDb db;
   KeyIndex<GraphDbTypes::Label, Vertex> index;
   GraphDbAccessor dba(db);
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
 
   auto t1 = engine.Begin();
   mvcc::VersionList<Vertex> vlist1(*t1, 0);
@@ -83,7 +83,7 @@ TEST(LabelsIndex, Refresh) {
   KeyIndex<GraphDbTypes::Label, Vertex> index;
   GraphDb db;
   GraphDbAccessor access(db);
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
 
   // add two vertices to  database
   auto t1 = engine.Begin();

tests/unit/database_label_property_index.cpp

@@ -4,7 +4,7 @@
 #include "database/graph_db_accessor.hpp"
 #include "database/graph_db_datatypes.hpp"
 #include "database/indexes/label_property_index.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 #include "mvcc_gc_common.hpp"
 
@@ -44,7 +44,7 @@ class LabelPropertyIndexComplexTest : public ::testing::Test {
   LabelPropertyIndex index;
   LabelPropertyIndex::Key *key;
 
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   tx::Transaction *t{nullptr};
 
   mvcc::VersionList<Vertex> *vlist;

tests/unit/distributed_serialization.cpp

@@ -10,7 +10,7 @@
 #include "storage/edge.hpp"
 #include "storage/property_value_store.hpp"
 #include "storage/vertex.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 using namespace GraphDbTypes;
 
@@ -118,7 +118,7 @@ TEST(DistributedSerialization, VertexProperties) {
 
 class DistributedSerializationMvcc : public ::testing::Test {
  protected:
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   tx::Transaction *tx = engine.Begin();
   mvcc::VersionList<Vertex> v1_vlist{*tx, 0};
   Vertex &v1 = *v1_vlist.Oldest();

tests/unit/mvcc.cpp

@@ -5,13 +5,13 @@
 #include "mvcc/version.hpp"
 #include "mvcc/version_list.hpp"
 #include "threading/sync/lock_timeout_exception.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 #include "transactions/transaction.hpp"
 
 #include "mvcc_gc_common.hpp"
 
 TEST(MVCC, Deadlock) {
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
 
   auto t0 = engine.Begin();
   mvcc::VersionList<Prop> version_list1(*t0, 0);
@@ -31,7 +31,7 @@ TEST(MVCC, Deadlock) {
 TEST(MVCC, UpdateDontDelete) {
   std::atomic<int> count{0};
   {
-    tx::MasterEngine engine;
+    tx::SingleNodeEngine engine;
     auto t1 = engine.Begin();
     mvcc::VersionList<DestrCountRec> version_list(*t1, 0, count);
     engine.Commit(*t1);
@@ -55,7 +55,7 @@ TEST(MVCC, UpdateDontDelete) {
 
 // Check that we get the oldest record.
 TEST(MVCC, Oldest) {
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   auto t1 = engine.Begin();
   mvcc::VersionList<Prop> version_list(*t1, 0);
   auto first = version_list.Oldest();

tests/unit/mvcc_find_update_common.hpp

@@ -5,13 +5,13 @@
 #include "mvcc/version.hpp"
 #include "mvcc/version_list.hpp"
 #include "threading/sync/lock_timeout_exception.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 #include "transactions/transaction.hpp"
 
 class TestClass : public mvcc::Record<TestClass> {
  public:
   // constructs first version, size should be 0
-  TestClass(int &version_list_size) : version_list_size_(version_list_size) {
+  explicit TestClass(int &version_list_size) : version_list_size_(version_list_size) {
     ++version_list_size_;
   }
   TestClass *CloneData() { return new TestClass(version_list_size_); }
@@ -58,7 +58,7 @@ class Mvcc : public ::testing::Test {
   }
   // variable where number of versions is stored
   int version_list_size = 0;
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   tx::Transaction *t1 = engine.Begin();
   mvcc::VersionList<TestClass> version_list{*t1, 0, version_list_size};
   TestClass *v1 = nullptr;

tests/unit/mvcc_gc.cpp

@@ -12,13 +12,13 @@
 #include "mvcc/version_list.hpp"
 #include "storage/garbage_collector.hpp"
 #include "storage/vertex.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 #include "mvcc_gc_common.hpp"
 
 class MvccGcTest : public ::testing::Test {
  protected:
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
 
  private:
   tx::Transaction *t0 = engine.Begin();
@@ -116,7 +116,7 @@ TEST_F(MvccGcTest, OldestTransactionSnapshot) {
  */
 TEST(GarbageCollector, GcClean) {
   ConcurrentMap<int64_t, mvcc::VersionList<DestrCountRec> *> collection;
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   DeferredDeleter<DestrCountRec> deleter;
   DeferredDeleter<mvcc::VersionList<DestrCountRec>> vlist_deleter;
   GarbageCollector<decltype(collection), DestrCountRec> gc(collection, deleter,

tests/unit/mvcc_gc_common.hpp

@@ -1,7 +1,7 @@
 #pragma once
 
 #include "mvcc/record.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 
 /**
  * @brief - Empty class which inherits from mvcc:Record.
@@ -18,7 +18,7 @@ class Prop : public mvcc::Record<Prop> {
  */
 class DestrCountRec : public mvcc::Record<DestrCountRec> {
  public:
-  DestrCountRec(std::atomic<int> &count) : count_(count) {}
+  explicit DestrCountRec(std::atomic<int> &count) : count_(count) {}
   DestrCountRec *CloneData() { return new DestrCountRec(count_); }
   ~DestrCountRec() { ++count_; }
 
@@ -29,7 +29,7 @@ class DestrCountRec : public mvcc::Record<DestrCountRec> {
 // helper function for creating a GC snapshot
 // if given a nullptr it makes a GC snapshot like there
 // are no active transactions
-auto GcSnapshot(tx::MasterEngine &engine, tx::Transaction *t) {
+auto GcSnapshot(tx::SingleNodeEngine &engine, tx::Transaction *t) {
   if (t != nullptr) {
     tx::Snapshot gc_snap = t->snapshot();
     gc_snap.insert(t->id_);

tests/unit/transaction_engine_distributed.cpp

@@ -16,12 +16,10 @@ class WorkerEngineTest : public testing::Test {
   const std::string local{"127.0.0.1"};
 
   System master_system_{local, 0};
-  MasterEngine master_;
+  MasterEngine master_{master_system_};
 
   System worker_system_{local, 0};
   WorkerEngine worker_{worker_system_, master_system_.endpoint()};
-
-  void SetUp() override { master_.StartServer(master_system_); }
 };
 
 TEST_F(WorkerEngineTest, LocalBegin) {

tests/unit/transaction_engine_single_node.cpp

@@ -4,11 +4,11 @@
 #include <vector>
 
 #include "data_structures/concurrent/concurrent_set.hpp"
-#include "transactions/engine_master.hpp"
+#include "transactions/engine_single_node.hpp"
 #include "transactions/transaction.hpp"
 
 TEST(Engine, GcSnapshot) {
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   ASSERT_EQ(engine.GlobalGcSnapshot(), tx::Snapshot({1}));
 
   std::vector<tx::Transaction *> transactions;
@@ -36,7 +36,7 @@ TEST(Engine, GcSnapshot) {
 }
 
 TEST(Engine, Advance) {
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
 
   auto t0 = engine.Begin();
   auto t1 = engine.Begin();
@@ -49,7 +49,7 @@ TEST(Engine, Advance) {
 }
 
 TEST(Engine, ConcurrentBegin) {
-  tx::MasterEngine engine;
+  tx::SingleNodeEngine engine;
   std::vector<std::thread> threads;
   ConcurrentSet<tx::transaction_id_t> tx_ids;
   for (int i = 0; i < 10; ++i) {