Add implementation of synchronous replication (#7)

This implements the initial version of synchronous replication.
Currently, only one replica is supported and that isn't configurable.

To run the main instance use the following command:
```
./memgraph \
    --main \
    --data-directory main-data \
    --storage-properties-on-edges \
    --storage-wal-enabled \
    --storage-snapshot-interval-sec 300
```

To run the replica instance use the following command:
```
./memgraph \
    --replica \
    --data-directory replica-data \
    --storage-properties-on-edges \
    --bolt-port 7688
```

You can then write/read data to Bolt port 7687 (the main instance) and also you
can read the data from the replica instance using Bolt port 7688.

NOTE: The main instance *must* be started without any data and the replica
*must* be started before any data is added to the main instance.

* Add basic synchronous replication test
* Using RWLock for replication stuff

Co-authored-by: Matej Ferencevic <matej.ferencevic@memgraph.io>
Co-authored-by: Antonio Andelic <antonio.andelic@memgraph.io>

src/storage/v2/CMakeLists.txt

@@ -10,5 +10,23 @@ set(storage_v2_src_files
     vertex_accessor.cpp
     storage.cpp)
 
+if(MG_ENTERPRISE)
+    define_add_lcp(add_lcp_storage lcp_storage_cpp_files generated_lcp_storage_files)
+
+    add_lcp_storage(replication/rpc.lcp SLK_SERIALIZE)
+
+    add_custom_target(generate_lcp_storage DEPENDS ${generated_lcp_storage_files})
+
+    set(storage_v2_src_files
+        ${storage_v2_src_files}
+        replication/slk.cpp
+        ${lcp_storage_cpp_files})
+endif()
+
 add_library(mg-storage-v2 STATIC ${storage_v2_src_files})
 target_link_libraries(mg-storage-v2 Threads::Threads mg-utils glog gflags)
+
+if(MG_ENTERPRISE)
+    add_dependencies(mg-storage-v2 generate_lcp_storage)
+    target_link_libraries(mg-storage-v2 mg-rpc mg-slk)
+endif()

src/storage/v2/property_value.hpp

@@ -25,7 +25,15 @@ class PropertyValueException : public utils::BasicException {
 class PropertyValue {
  public:
   /// A value type, each type corresponds to exactly one C++ type.
-  enum class Type : unsigned char { Null, Bool, Int, Double, String, List, Map };
+  enum class Type : uint8_t {
+    Null = 0,
+    Bool = 1,
+    Int = 2,
+    Double = 3,
+    String = 4,
+    List = 5,
+    Map = 6,
+  };
 
   static bool AreComparableTypes(Type a, Type b) {
     return (a == b) || (a == Type::Int && b == Type::Double) ||

src/storage/v2/replication/.gitignore

@@ -0,0 +1,2 @@
+# autogenerated files
+rpc.hpp

src/storage/v2/replication/replication.hpp

@@ -0,0 +1,82 @@
+#pragma once
+
+#include "rpc/client.hpp"
+#include "storage/v2/config.hpp"
+#include "storage/v2/delta.hpp"
+#include "storage/v2/durability/wal.hpp"
+#include "storage/v2/id_types.hpp"
+#include "storage/v2/mvcc.hpp"
+#include "storage/v2/name_id_mapper.hpp"
+#include "storage/v2/property_value.hpp"
+#include "storage/v2/replication/rpc.hpp"
+#include "storage/v2/replication/serialization.hpp"
+
+namespace storage::replication {
+
+class ReplicationClient {
+ public:
+  ReplicationClient(NameIdMapper *name_id_mapper, Config::Items items,
+                    const io::network::Endpoint &endpoint, bool use_ssl)
+      : name_id_mapper_(name_id_mapper),
+        items_(items),
+        rpc_context_(use_ssl),
+        rpc_client_(endpoint, &rpc_context_) {}
+
+  class Handler {
+   private:
+    friend class ReplicationClient;
+
+    /// @throw rpc::RpcFailedException
+    explicit Handler(ReplicationClient *self)
+        : self_(self), stream_(self_->rpc_client_.Stream<AppendDeltasRpc>()) {}
+
+   public:
+    /// @throw rpc::RpcFailedException
+    void AppendDelta(const Delta &delta, const Vertex &vertex,
+                     uint64_t final_commit_timestamp) {
+      Encoder encoder(stream_.GetBuilder());
+      EncodeDelta(&encoder, self_->name_id_mapper_, self_->items_, delta,
+                  vertex, final_commit_timestamp);
+    }
+
+    /// @throw rpc::RpcFailedException
+    void AppendDelta(const Delta &delta, const Edge &edge,
+                     uint64_t final_commit_timestamp) {
+      Encoder encoder(stream_.GetBuilder());
+      EncodeDelta(&encoder, self_->name_id_mapper_, delta, edge,
+                  final_commit_timestamp);
+    }
+
+    /// @throw rpc::RpcFailedException
+    void AppendTransactionEnd(uint64_t final_commit_timestamp) {
+      Encoder encoder(stream_.GetBuilder());
+      EncodeTransactionEnd(&encoder, final_commit_timestamp);
+    }
+
+    /// @throw rpc::RpcFailedException
+    void AppendOperation(durability::StorageGlobalOperation operation,
+                         LabelId label, const std::set<PropertyId> &properties,
+                         uint64_t timestamp) {
+      Encoder encoder(stream_.GetBuilder());
+      EncodeOperation(&encoder, self_->name_id_mapper_, operation, label,
+                      properties, timestamp);
+    }
+
+    /// @throw rpc::RpcFailedException
+    void Finalize() { stream_.AwaitResponse(); }
+
+   private:
+    ReplicationClient *self_;
+    rpc::Client::StreamHandler<AppendDeltasRpc> stream_;
+  };
+
+  Handler ReplicateTransaction() { return Handler(this); }
+
+ private:
+  NameIdMapper *name_id_mapper_;
+  Config::Items items_;
+  communication::ClientContext rpc_context_;
+  rpc::Client rpc_client_;
+};
+
+}  // namespace storage::replication

src/storage/v2/replication/rpc.lcp

@@ -0,0 +1,32 @@
+#>cpp
+#pragma once
+
+#include <cstring>
+#include <cstdint>
+
+#include "rpc/messages.hpp"
+#include "slk/serialization.hpp"
+#include "slk/streams.hpp"
+cpp<#
+
+;; TODO(mferencevic): Change namespace to `storage::replication` once LCP is
+;; updated to support such namespaces.
+(lcp:namespace storage)
+
+(lcp:define-rpc append-deltas
+  ;; The actual deltas are sent as additional data using the RPC client's
+  ;; streaming API for additional data.
+  (:request ())
+  (:response
+    ((success :bool)
+     (term :uint64_t))))
+
+(lcp:define-rpc heartbeat
+  (:request
+    ((leader-id :uint16_t)
+     (term :uint64_t)))
+  (:response
+    ((success :bool)
+     (term :uint64_t))))
+
+(lcp:pop-namespace) ;; storage

src/storage/v2/replication/serialization.hpp

@@ -0,0 +1,124 @@
+#pragma once
+
+#include "slk/streams.hpp"
+#include "storage/v2/durability/serialization.hpp"
+#include "storage/v2/replication/slk.hpp"
+#include "utils/cast.hpp"
+
+namespace storage::replication {
+
+class Encoder final : public durability::BaseEncoder {
+ public:
+  explicit Encoder(slk::Builder *builder) : builder_(builder) {}
+
+  void WriteMarker(durability::Marker marker) override {
+    slk::Save(marker, builder_);
+  }
+
+  void WriteBool(bool value) override {
+    WriteMarker(durability::Marker::TYPE_BOOL);
+    slk::Save(value, builder_);
+  }
+
+  void WriteUint(uint64_t value) override {
+    WriteMarker(durability::Marker::TYPE_INT);
+    slk::Save(value, builder_);
+  }
+
+  void WriteDouble(double value) override {
+    WriteMarker(durability::Marker::TYPE_DOUBLE);
+    slk::Save(value, builder_);
+  }
+
+  void WriteString(const std::string_view &value) override {
+    WriteMarker(durability::Marker::TYPE_STRING);
+    slk::Save(value, builder_);
+  }
+
+  void WritePropertyValue(const PropertyValue &value) override {
+    WriteMarker(durability::Marker::TYPE_PROPERTY_VALUE);
+    slk::Save(value, builder_);
+  }
+
+ private:
+  slk::Builder *builder_;
+};
+
+class Decoder final : public durability::BaseDecoder {
+ public:
+  explicit Decoder(slk::Reader *reader) : reader_(reader) {}
+
+  std::optional<durability::Marker> ReadMarker() override {
+    durability::Marker marker;
+    slk::Load(&marker, reader_);
+    return marker;
+  }
+
+  std::optional<bool> ReadBool() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_BOOL)
+      return std::nullopt;
+    bool value;
+    slk::Load(&value, reader_);
+    return value;
+  }
+
+  std::optional<uint64_t> ReadUint() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_INT)
+      return std::nullopt;
+    uint64_t value;
+    slk::Load(&value, reader_);
+    return value;
+  }
+
+  std::optional<double> ReadDouble() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_DOUBLE)
+      return std::nullopt;
+    double value;
+    slk::Load(&value, reader_);
+    return value;
+  }
+
+  std::optional<std::string> ReadString() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_STRING)
+      return std::nullopt;
+    std::string value;
+    slk::Load(&value, reader_);
+    return std::move(value);
+  }
+
+  std::optional<PropertyValue> ReadPropertyValue() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_PROPERTY_VALUE)
+      return std::nullopt;
+    PropertyValue value;
+    slk::Load(&value, reader_);
+    return std::move(value);
+  }
+
+  bool SkipString() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_STRING)
+      return false;
+    std::string value;
+    slk::Load(&value, reader_);
+    return true;
+  }
+
+  bool SkipPropertyValue() override {
+    if (const auto marker = ReadMarker();
+        !marker || marker != durability::Marker::TYPE_PROPERTY_VALUE)
+      return false;
+    PropertyValue value;
+    slk::Load(&value, reader_);
+    return true;
+  }
+
+ private:
+  slk::Reader *reader_;
+};
+
+}  // namespace storage::replication

src/storage/v2/replication/slk.cpp

@@ -0,0 +1,161 @@
+#include "storage/v2/replication/slk.hpp"
+
+#include <type_traits>
+
+#include "utils/cast.hpp"
+
+namespace slk {
+
+void Save(const storage::Gid &gid, slk::Builder *builder) {
+  slk::Save(gid.AsUint(), builder);
+}
+
+void Load(storage::Gid *gid, slk::Reader *reader) {
+  uint64_t value;
+  slk::Load(&value, reader);
+  *gid = storage::Gid::FromUint(value);
+}
+
+void Save(const storage::PropertyValue::Type &type, slk::Builder *builder) {
+  slk::Save(utils::UnderlyingCast(type), builder);
+}
+
+void Load(storage::PropertyValue::Type *type, slk::Reader *reader) {
+  using PVTypeUnderlyingType =
+      std::underlying_type_t<storage::PropertyValue::Type>;
+  PVTypeUnderlyingType value;
+  slk::Load(&value, reader);
+  bool valid;
+  switch (value) {
+    case utils::UnderlyingCast(storage::PropertyValue::Type::Null):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::Bool):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::Int):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::Double):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::String):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::List):
+    case utils::UnderlyingCast(storage::PropertyValue::Type::Map):
+      valid = true;
+      break;
+    default:
+      valid = false;
+      break;
+  }
+  if (!valid)
+    throw slk::SlkDecodeException(
+        "Trying to load unknown storage::PropertyValue!");
+  *type = static_cast<storage::PropertyValue::Type>(value);
+}
+
+void Save(const storage::PropertyValue &value, slk::Builder *builder) {
+  switch (value.type()) {
+    case storage::PropertyValue::Type::Null:
+      slk::Save(storage::PropertyValue::Type::Null, builder);
+      return;
+    case storage::PropertyValue::Type::Bool:
+      slk::Save(storage::PropertyValue::Type::Bool, builder);
+      slk::Save(value.ValueBool(), builder);
+      return;
+    case storage::PropertyValue::Type::Int:
+      slk::Save(storage::PropertyValue::Type::Int, builder);
+      slk::Save(value.ValueInt(), builder);
+      return;
+    case storage::PropertyValue::Type::Double:
+      slk::Save(storage::PropertyValue::Type::Double, builder);
+      slk::Save(value.ValueDouble(), builder);
+      return;
+    case storage::PropertyValue::Type::String:
+      slk::Save(storage::PropertyValue::Type::String, builder);
+      slk::Save(value.ValueString(), builder);
+      return;
+    case storage::PropertyValue::Type::List: {
+      slk::Save(storage::PropertyValue::Type::List, builder);
+      const auto &values = value.ValueList();
+      size_t size = values.size();
+      slk::Save(size, builder);
+      for (const auto &v : values) {
+        slk::Save(v, builder);
+      }
+      return;
+    }
+    case storage::PropertyValue::Type::Map: {
+      slk::Save(storage::PropertyValue::Type::Map, builder);
+      const auto &map = value.ValueMap();
+      size_t size = map.size();
+      slk::Save(size, builder);
+      for (const auto &kv : map) {
+        slk::Save(kv, builder);
+      }
+      return;
+    }
+  }
+}
+
+void Load(storage::PropertyValue *value, slk::Reader *reader) {
+  storage::PropertyValue::Type type;
+  slk::Load(&type, reader);
+  switch (type) {
+    case storage::PropertyValue::Type::Null:
+      *value = storage::PropertyValue();
+      return;
+    case storage::PropertyValue::Type::Bool: {
+      bool v;
+      slk::Load(&v, reader);
+      *value = storage::PropertyValue(v);
+      return;
+    }
+    case storage::PropertyValue::Type::Int: {
+      int64_t v;
+      slk::Load(&v, reader);
+      *value = storage::PropertyValue(v);
+      return;
+    }
+    case storage::PropertyValue::Type::Double: {
+      double v;
+      slk::Load(&v, reader);
+      *value = storage::PropertyValue(v);
+      return;
+    }
+    case storage::PropertyValue::Type::String: {
+      std::string v;
+      slk::Load(&v, reader);
+      *value = storage::PropertyValue(std::move(v));
+      return;
+    }
+    case storage::PropertyValue::Type::List: {
+      size_t size;
+      slk::Load(&size, reader);
+      std::vector<storage::PropertyValue> list(size);
+      for (size_t i = 0; i < size; ++i) {
+        slk::Load(&list[i], reader);
+      }
+      *value = storage::PropertyValue(std::move(list));
+      return;
+    }
+    case storage::PropertyValue::Type::Map: {
+      size_t size;
+      slk::Load(&size, reader);
+      std::map<std::string, storage::PropertyValue> map;
+      for (size_t i = 0; i < size; ++i) {
+        std::pair<std::string, storage::PropertyValue> kv;
+        slk::Load(&kv, reader);
+        map.insert(kv);
+      }
+      *value = storage::PropertyValue(std::move(map));
+      return;
+    }
+  }
+}
+
+void Save(const storage::durability::Marker &marker, slk::Builder *builder) {
+  slk::Save(utils::UnderlyingCast(marker), builder);
+}
+
+void Load(storage::durability::Marker *marker, slk::Reader *reader) {
+  using PVTypeUnderlyingType =
+      std::underlying_type_t<storage::PropertyValue::Type>;
+  PVTypeUnderlyingType value;
+  slk::Load(&value, reader);
+  *marker = static_cast<storage::durability::Marker>(value);
+}
+
+}  // namespace slk

src/storage/v2/replication/slk.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "slk/serialization.hpp"
+#include "storage/v2/durability/marker.hpp"
+#include "storage/v2/id_types.hpp"
+#include "storage/v2/property_value.hpp"
+
+namespace slk {
+
+void Save(const storage::Gid &gid, slk::Builder *builder);
+void Load(storage::Gid *gid, slk::Reader *reader);
+
+void Save(const storage::PropertyValue &value, slk::Builder *builder);
+void Load(storage::PropertyValue *value, slk::Reader *reader);
+
+void Save(const storage::durability::Marker &marker, slk::Builder *builder);
+void Load(storage::durability::Marker *marker, slk::Reader *reader);
+
+}  // namespace slk

src/storage/v2/storage.cpp

@@ -1,7 +1,9 @@
 #include "storage/v2/storage.hpp"
 
 #include <algorithm>
+#include <atomic>
 #include <memory>
+#include <mutex>
 
 #include <gflags/gflags.h>
 #include <glog/logging.h>
@@ -10,9 +12,20 @@
 #include "storage/v2/durability/paths.hpp"
 #include "storage/v2/durability/snapshot.hpp"
 #include "storage/v2/mvcc.hpp"
+#include "utils/rw_lock.hpp"
+#include "utils/spin_lock.hpp"
 #include "utils/stat.hpp"
 #include "utils/uuid.hpp"
 
+#ifdef MG_ENTERPRISE
+#include "storage/v2/replication/rpc.hpp"
+#endif
+
+#ifdef MG_ENTERPRISE
+DEFINE_bool(main, false, "Set to true to be the main");
+DEFINE_bool(replica, false, "Set to true to be the replica");
+#endif
+
 namespace storage {
 
 auto AdvanceToVisibleVertex(utils::SkipList<Vertex>::Iterator it,
@@ -401,12 +414,28 @@ Storage::Storage(Config config)
     gc_runner_.Run("Storage GC", config_.gc.interval,
                    [this] { this->CollectGarbage(); });
   }
+
+#ifdef MG_ENTERPRISE
+  // For testing purposes until we can define the instance type from
+  // a query.
+  if (FLAGS_main) {
+    SetReplicationState(ReplicationState::MAIN);
+  } else if (FLAGS_replica) {
+    SetReplicationState(ReplicationState::REPLICA);
+  }
+#endif
 }
 
 Storage::~Storage() {
   if (config_.gc.type == Config::Gc::Type::PERIODIC) {
     gc_runner_.Stop();
   }
+#ifdef MG_ENTERPRISE
+  if (replication_server_) {
+    replication_server_->Shutdown();
+    replication_server_->AwaitShutdown();
+  }
+#endif
   wal_file_ = std::nullopt;
   if (config_.durability.snapshot_wal_mode !=
       Config::Durability::SnapshotWalMode::DISABLED) {
@@ -467,6 +496,29 @@ VertexAccessor Storage::Accessor::CreateVertex() {
                         &storage_->constraints_, config_);
 }
 
+#ifdef MG_ENTERPRISE
+VertexAccessor Storage::Accessor::CreateVertex(storage::Gid gid) {
+  // NOTE: When we update the next `vertex_id_` here we perform a RMW
+  // (read-modify-write) operation that ISN'T atomic! But, that isn't an issue
+  // because this function is only called from the replication delta applier
+  // that runs single-threadedly and while this instance is set-up to apply
+  // threads (it is the replica), it is guaranteed that no other writes are
+  // possible.
+  storage_->vertex_id_.store(
+      std::max(storage_->vertex_id_.load(std::memory_order_acquire),
+               gid.AsUint() + 1),
+      std::memory_order_release);
+  auto acc = storage_->vertices_.access();
+  auto delta = CreateDeleteObjectDelta(&transaction_);
+  auto [it, inserted] = acc.insert(Vertex{gid, delta});
+  CHECK(inserted) << "The vertex must be inserted here!";
+  CHECK(it != acc.end()) << "Invalid Vertex accessor!";
+  delta->prev.Set(&*it);
+  return VertexAccessor(&*it, &transaction_, &storage_->indices_,
+                        &storage_->constraints_, config_);
+}
+#endif
+
 std::optional<VertexAccessor> Storage::Accessor::FindVertex(Gid gid,
                                                             View view) {
   auto acc = storage_->vertices_.access();
@@ -625,6 +677,84 @@ Result<EdgeAccessor> Storage::Accessor::CreateEdge(VertexAccessor *from,
                       &storage_->indices_, &storage_->constraints_, config_);
 }
 
+#ifdef MG_ENTERPRISE
+Result<EdgeAccessor> Storage::Accessor::CreateEdge(VertexAccessor *from,
+                                                   VertexAccessor *to,
+                                                   EdgeTypeId edge_type,
+                                                   storage::Gid gid) {
+  CHECK(from->transaction_ == to->transaction_)
+      << "VertexAccessors must be from the same transaction when creating "
+         "an edge!";
+  CHECK(from->transaction_ == &transaction_)
+      << "VertexAccessors must be from the same transaction in when "
+         "creating an edge!";
+
+  auto from_vertex = from->vertex_;
+  auto to_vertex = to->vertex_;
+
+  // Obtain the locks by `gid` order to avoid lock cycles.
+  std::unique_lock<utils::SpinLock> guard_from(from_vertex->lock,
+                                               std::defer_lock);
+  std::unique_lock<utils::SpinLock> guard_to(to_vertex->lock, std::defer_lock);
+  if (from_vertex->gid < to_vertex->gid) {
+    guard_from.lock();
+    guard_to.lock();
+  } else if (from_vertex->gid > to_vertex->gid) {
+    guard_to.lock();
+    guard_from.lock();
+  } else {
+    // The vertices are the same vertex, only lock one.
+    guard_from.lock();
+  }
+
+  if (!PrepareForWrite(&transaction_, from_vertex))
+    return Error::SERIALIZATION_ERROR;
+  if (from_vertex->deleted) return Error::DELETED_OBJECT;
+
+  if (to_vertex != from_vertex) {
+    if (!PrepareForWrite(&transaction_, to_vertex))
+      return Error::SERIALIZATION_ERROR;
+    if (to_vertex->deleted) return Error::DELETED_OBJECT;
+  }
+
+  // NOTE: When we update the next `edge_id_` here we perform a RMW
+  // (read-modify-write) operation that ISN'T atomic! But, that isn't an issue
+  // because this function is only called from the replication delta applier
+  // that runs single-threadedly and while this instance is set-up to apply
+  // threads (it is the replica), it is guaranteed that no other writes are
+  // possible.
+  storage_->edge_id_.store(
+      std::max(storage_->edge_id_.load(std::memory_order_acquire),
+               gid.AsUint() + 1),
+      std::memory_order_release);
+
+  EdgeRef edge(gid);
+  if (config_.properties_on_edges) {
+    auto acc = storage_->edges_.access();
+    auto delta = CreateDeleteObjectDelta(&transaction_);
+    auto [it, inserted] = acc.insert(Edge(gid, delta));
+    CHECK(inserted) << "The edge must be inserted here!";
+    CHECK(it != acc.end()) << "Invalid Edge accessor!";
+    edge = EdgeRef(&*it);
+    delta->prev.Set(&*it);
+  }
+
+  CreateAndLinkDelta(&transaction_, from_vertex, Delta::RemoveOutEdgeTag(),
+                     edge_type, to_vertex, edge);
+  from_vertex->out_edges.emplace_back(edge_type, to_vertex, edge);
+
+  CreateAndLinkDelta(&transaction_, to_vertex, Delta::RemoveInEdgeTag(),
+                     edge_type, from_vertex, edge);
+  to_vertex->in_edges.emplace_back(edge_type, from_vertex, edge);
+
+  // Increment edge count.
+  storage_->edge_count_.fetch_add(1, std::memory_order_acq_rel);
+
+  return EdgeAccessor(edge, edge_type, from_vertex, to_vertex, &transaction_,
+                      &storage_->indices_, &storage_->constraints_, config_);
+}
+#endif
+
 Result<bool> Storage::Accessor::DeleteEdge(EdgeAccessor *edge) {
   CHECK(edge->transaction_ == &transaction_)
       << "EdgeAccessor must be from the same transaction as the storage "
@@ -743,7 +873,16 @@ EdgeTypeId Storage::Accessor::NameToEdgeType(const std::string_view &name) {
 
 void Storage::Accessor::AdvanceCommand() { ++transaction_.command_id; }
 
+#ifdef MG_ENTERPRISE
 utils::BasicResult<ConstraintViolation, void> Storage::Accessor::Commit() {
+  return Commit(std::nullopt);
+}
+
+utils::BasicResult<ConstraintViolation, void> Storage::Accessor::Commit(
+    std::optional<uint64_t> desired_commit_timestamp) {
+#else
+utils::BasicResult<ConstraintViolation, void> Storage::Accessor::Commit() {
+#endif
   CHECK(is_transaction_active_) << "The transaction is already terminated!";
   CHECK(!transaction_.must_abort) << "The transaction can't be committed!";
 
@@ -780,7 +919,17 @@ utils::BasicResult<ConstraintViolation, void> Storage::Accessor::Commit() {
 
     {
       std::unique_lock<utils::SpinLock> engine_guard(storage_->engine_lock_);
+#ifdef MG_ENTERPRISE
+      if (!desired_commit_timestamp) {
+        commit_timestamp = storage_->timestamp_++;
+      } else {
+        commit_timestamp = *desired_commit_timestamp;
+        storage_->timestamp_ =
+            std::max(storage_->timestamp_, *desired_commit_timestamp + 1);
+      }
+#else
       commit_timestamp = storage_->timestamp_++;
+#endif
 
       // Before committing and validating vertices against unique constraints,
       // we have to update unique constraints with the vertices that are going
@@ -1223,7 +1372,15 @@ Transaction Storage::CreateTransaction() {
   {
     std::lock_guard<utils::SpinLock> guard(engine_lock_);
     transaction_id = transaction_id_++;
+#ifdef MG_ENTERPRISE
+    if (replication_state_.load() != ReplicationState::REPLICA) {
+      start_timestamp = timestamp_++;
+    } else {
+      start_timestamp = timestamp_;
+    }
+#else
     start_timestamp = timestamp_++;
+#endif
   }
   return {transaction_id, start_timestamp};
 }
@@ -1490,6 +1647,20 @@ void Storage::AppendToWal(const Transaction &transaction,
   // A single transaction will always be contained in a single WAL file.
   auto current_commit_timestamp =
       transaction.commit_timestamp->load(std::memory_order_acquire);
+
+#ifdef MG_ENTERPRISE
+  std::shared_lock<utils::RWLock> replication_guard(replication_lock_);
+  std::optional<replication::ReplicationClient::Handler> stream;
+  if (replication_client_) {
+    try {
+      stream.emplace(replication_client_->ReplicateTransaction());
+    } catch (const rpc::RpcFailedException &) {
+      LOG(FATAL) << "Couldn't replicate data!";
+    }
+  }
+  replication_guard.unlock();
+#endif
+
   // Helper lambda that traverses the delta chain on order to find the first
   // delta that should be processed and then appends all discovered deltas.
   auto find_and_apply_deltas = [&](const auto *delta, const auto &parent,
@@ -1505,6 +1676,15 @@ void Storage::AppendToWal(const Transaction &transaction,
     while (true) {
       if (filter(delta->action)) {
         wal_file_->AppendDelta(*delta, parent, final_commit_timestamp);
+#ifdef MG_ENTERPRISE
+        if (stream) {
+          try {
+            stream->AppendDelta(*delta, parent, final_commit_timestamp);
+          } catch (const rpc::RpcFailedException &) {
+            LOG(FATAL) << "Couldn't replicate data!";
+          }
+        }
+#endif
       }
       auto prev = delta->prev.Get();
       if (prev.type != PreviousPtr::Type::DELTA) break;
@@ -1637,6 +1817,17 @@ void Storage::AppendToWal(const Transaction &transaction,
   wal_file_->AppendTransactionEnd(final_commit_timestamp);
 
   FinalizeWalFile();
+
+#ifdef MG_ENTERPRISE
+  if (stream) {
+    try {
+      stream->AppendTransactionEnd(final_commit_timestamp);
+      stream->Finalize();
+    } catch (const rpc::RpcFailedException &) {
+      LOG(FATAL) << "Couldn't replicate data!";
+    }
+  }
+#endif
 }
 
 void Storage::AppendToWal(durability::StorageGlobalOperation operation,
@@ -1645,7 +1836,438 @@ void Storage::AppendToWal(durability::StorageGlobalOperation operation,
   if (!InitializeWalFile()) return;
   wal_file_->AppendOperation(operation, label, properties,
                              final_commit_timestamp);
+#ifdef MG_ENTERPRISE
+  std::shared_lock<utils::RWLock> replication_guard(replication_lock_);
+  if (replication_client_) {
+    auto stream = replication_client_->ReplicateTransaction();
+    try {
+      stream.AppendOperation(operation, label, properties,
+                             final_commit_timestamp);
+      stream.Finalize();
+    } catch (const rpc::RpcFailedException &) {
+      LOG(FATAL) << "Couldn't replicate data!";
+    }
+  }
+  replication_guard.unlock();
+#endif
   FinalizeWalFile();
 }
 
+#ifdef MG_ENTERPRISE
+void Storage::ConfigureReplica() {
+  // Create RPC server.
+  // TODO(mferencevic): Add support for SSL.
+  replication_server_context_.emplace();
+  // NOTE: The replication server must have a single thread for processing
+  // because there is no need for more processing threads - each replica can
+  // have only a single main server. Also, the single-threaded guarantee
+  // simplifies the rest of the implementation.
+  // TODO(mferencevic): Make endpoint configurable.
+  replication_server_.emplace(io::network::Endpoint{"127.0.0.1", 10000},
+                              &*replication_server_context_,
+                              /* workers_count = */ 1);
+  replication_server_->Register<AppendDeltasRpc>([this](auto *req_reader,
+                                                        auto *res_builder) {
+    AppendDeltasReq req;
+    slk::Load(&req, req_reader);
+
+    DLOG(INFO) << "Received AppendDeltasRpc:";
+
+    replication::Decoder decoder(req_reader);
+
+    auto edge_acc = edges_.access();
+    auto vertex_acc = vertices_.access();
+
+    std::optional<std::pair<uint64_t, storage::Storage::Accessor>>
+        commit_timestamp_and_accessor;
+    auto get_transaction =
+        [this, &commit_timestamp_and_accessor](uint64_t commit_timestamp) {
+          if (!commit_timestamp_and_accessor) {
+            commit_timestamp_and_accessor.emplace(commit_timestamp, Access());
+          } else if (commit_timestamp_and_accessor->first != commit_timestamp) {
+            throw utils::BasicException("Received more than one transaction!");
+          }
+          return &commit_timestamp_and_accessor->second;
+        };
+
+    bool transaction_complete = false;
+    for (uint64_t i = 0; !transaction_complete; ++i) {
+      uint64_t timestamp;
+      durability::WalDeltaData delta;
+
+      try {
+        timestamp = ReadWalDeltaHeader(&decoder);
+        DLOG(INFO) << "   Delta " << i;
+        DLOG(INFO) << "       Timestamp " << timestamp;
+        delta = ReadWalDeltaData(&decoder);
+      } catch (const slk::SlkReaderException &) {
+        throw utils::BasicException("Missing data!");
+      } catch (const durability::RecoveryFailure &) {
+        throw utils::BasicException("Invalid data!");
+      }
+
+      switch (delta.type) {
+        case durability::WalDeltaData::Type::VERTEX_CREATE: {
+          DLOG(INFO) << "       Create vertex "
+                     << delta.vertex_create_delete.gid.AsUint();
+          auto transaction = get_transaction(timestamp);
+          transaction->CreateVertex(delta.vertex_create_delete.gid);
+          break;
+        }
+        case durability::WalDeltaData::Type::VERTEX_DELETE: {
+          DLOG(INFO) << "       Delete vertex "
+                     << delta.vertex_create_delete.gid.AsUint();
+          auto transaction = get_transaction(timestamp);
+          auto vertex = transaction->FindVertex(delta.vertex_create_delete.gid,
+                                                storage::View::NEW);
+          if (!vertex) throw utils::BasicException("Invalid transaction!");
+          auto ret = transaction->DeleteVertex(&*vertex);
+          if (ret.HasError() || !ret.GetValue())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::VERTEX_ADD_LABEL: {
+          DLOG(INFO) << "       Vertex "
+                     << delta.vertex_add_remove_label.gid.AsUint()
+                     << " add label " << delta.vertex_add_remove_label.label;
+          auto transaction = get_transaction(timestamp);
+          auto vertex = transaction->FindVertex(
+              delta.vertex_add_remove_label.gid, storage::View::NEW);
+          if (!vertex) throw utils::BasicException("Invalid transaction!");
+          auto ret = vertex->AddLabel(
+              transaction->NameToLabel(delta.vertex_add_remove_label.label));
+          if (ret.HasError() || !ret.GetValue())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::VERTEX_REMOVE_LABEL: {
+          DLOG(INFO) << "       Vertex "
+                     << delta.vertex_add_remove_label.gid.AsUint()
+                     << " remove label " << delta.vertex_add_remove_label.label;
+          auto transaction = get_transaction(timestamp);
+          auto vertex = transaction->FindVertex(
+              delta.vertex_add_remove_label.gid, storage::View::NEW);
+          if (!vertex) throw utils::BasicException("Invalid transaction!");
+          auto ret = vertex->RemoveLabel(
+              transaction->NameToLabel(delta.vertex_add_remove_label.label));
+          if (ret.HasError() || !ret.GetValue())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::VERTEX_SET_PROPERTY: {
+          DLOG(INFO) << "       Vertex "
+                     << delta.vertex_edge_set_property.gid.AsUint()
+                     << " set property "
+                     << delta.vertex_edge_set_property.property << " to "
+                     << delta.vertex_edge_set_property.value;
+          auto transaction = get_transaction(timestamp);
+          auto vertex = transaction->FindVertex(
+              delta.vertex_edge_set_property.gid, storage::View::NEW);
+          if (!vertex) throw utils::BasicException("Invalid transaction!");
+          auto ret =
+              vertex->SetProperty(transaction->NameToProperty(
+                                      delta.vertex_edge_set_property.property),
+                                  delta.vertex_edge_set_property.value);
+          if (ret.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::EDGE_CREATE: {
+          DLOG(INFO) << "       Create edge "
+                     << delta.edge_create_delete.gid.AsUint() << " of type "
+                     << delta.edge_create_delete.edge_type << " from vertex "
+                     << delta.edge_create_delete.from_vertex.AsUint()
+                     << " to vertex "
+                     << delta.edge_create_delete.to_vertex.AsUint();
+          auto transaction = get_transaction(timestamp);
+          auto from_vertex = transaction->FindVertex(
+              delta.edge_create_delete.from_vertex, storage::View::NEW);
+          if (!from_vertex) throw utils::BasicException("Invalid transaction!");
+          auto to_vertex = transaction->FindVertex(
+              delta.edge_create_delete.to_vertex, storage::View::NEW);
+          if (!to_vertex) throw utils::BasicException("Invalid transaction!");
+          auto edge = transaction->CreateEdge(
+              &*from_vertex, &*to_vertex,
+              transaction->NameToEdgeType(delta.edge_create_delete.edge_type),
+              delta.edge_create_delete.gid);
+          if (edge.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::EDGE_DELETE: {
+          DLOG(INFO) << "       Delete edge "
+                     << delta.edge_create_delete.gid.AsUint() << " of type "
+                     << delta.edge_create_delete.edge_type << " from vertex "
+                     << delta.edge_create_delete.from_vertex.AsUint()
+                     << " to vertex "
+                     << delta.edge_create_delete.to_vertex.AsUint();
+          auto transaction = get_transaction(timestamp);
+          auto from_vertex = transaction->FindVertex(
+              delta.edge_create_delete.from_vertex, storage::View::NEW);
+          if (!from_vertex) throw utils::BasicException("Invalid transaction!");
+          auto to_vertex = transaction->FindVertex(
+              delta.edge_create_delete.to_vertex, storage::View::NEW);
+          if (!to_vertex) throw utils::BasicException("Invalid transaction!");
+          auto edges = from_vertex->OutEdges(
+              storage::View::NEW,
+              {transaction->NameToEdgeType(delta.edge_create_delete.edge_type)},
+              &*to_vertex);
+          if (edges.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          if (edges->size() != 1)
+            throw utils::BasicException("Invalid transaction!");
+          auto &edge = (*edges)[0];
+          auto ret = transaction->DeleteEdge(&edge);
+          if (ret.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+        case durability::WalDeltaData::Type::EDGE_SET_PROPERTY: {
+          DLOG(INFO) << "       Edge "
+                     << delta.vertex_edge_set_property.gid.AsUint()
+                     << " set property "
+                     << delta.vertex_edge_set_property.property << " to "
+                     << delta.vertex_edge_set_property.value;
+
+          if (!config_.items.properties_on_edges)
+            throw utils::BasicException(
+                "Can't set properties on edges because properties on edges "
+                "are disabled!");
+
+          auto transaction = get_transaction(timestamp);
+
+          // The following block of code effectively implements `FindEdge` and
+          // yields an accessor that is only valid for managing the edge's
+          // properties.
+          auto edge = edge_acc.find(delta.vertex_edge_set_property.gid);
+          if (edge == edge_acc.end())
+            throw utils::BasicException("Invalid transaction!");
+          // The edge visibility check must be done here manually because we
+          // don't allow direct access to the edges through the public API.
+          {
+            bool is_visible = true;
+            Delta *delta = nullptr;
+            {
+              std::lock_guard<utils::SpinLock> guard(edge->lock);
+              is_visible = !edge->deleted;
+              delta = edge->delta;
+            }
+            ApplyDeltasForRead(&transaction->transaction_, delta, View::NEW,
+                               [&is_visible](const Delta &delta) {
+                                 switch (delta.action) {
+                                   case Delta::Action::ADD_LABEL:
+                                   case Delta::Action::REMOVE_LABEL:
+                                   case Delta::Action::SET_PROPERTY:
+                                   case Delta::Action::ADD_IN_EDGE:
+                                   case Delta::Action::ADD_OUT_EDGE:
+                                   case Delta::Action::REMOVE_IN_EDGE:
+                                   case Delta::Action::REMOVE_OUT_EDGE:
+                                     break;
+                                   case Delta::Action::RECREATE_OBJECT: {
+                                     is_visible = true;
+                                     break;
+                                   }
+                                   case Delta::Action::DELETE_OBJECT: {
+                                     is_visible = false;
+                                     break;
+                                   }
+                                 }
+                               });
+            if (!is_visible)
+              throw utils::BasicException("Invalid transaction!");
+          }
+          EdgeRef edge_ref(&*edge);
+          // Here we create an edge accessor that we will use to get the
+          // properties of the edge. The accessor is created with an invalid
+          // type and invalid from/to pointers because we don't know them
+          // here, but that isn't an issue because we won't use that part of
+          // the API here.
+          auto ea = EdgeAccessor{edge_ref,
+                                 EdgeTypeId::FromUint(0UL),
+                                 nullptr,
+                                 nullptr,
+                                 &transaction->transaction_,
+                                 &indices_,
+                                 &constraints_,
+                                 config_.items};
+
+          auto ret =
+              ea.SetProperty(transaction->NameToProperty(
+                                 delta.vertex_edge_set_property.property),
+                             delta.vertex_edge_set_property.value);
+          if (ret.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          break;
+        }
+
+        case durability::WalDeltaData::Type::TRANSACTION_END: {
+          DLOG(INFO) << "       Transaction end";
+          if (!commit_timestamp_and_accessor ||
+              commit_timestamp_and_accessor->first != timestamp)
+            throw utils::BasicException("Invalid data!");
+          auto ret = commit_timestamp_and_accessor->second.Commit(
+              commit_timestamp_and_accessor->first);
+          if (ret.HasError())
+            throw utils::BasicException("Invalid transaction!");
+          commit_timestamp_and_accessor = std::nullopt;
+          transaction_complete = true;
+          break;
+        }
+
+        case durability::WalDeltaData::Type::LABEL_INDEX_CREATE: {
+          DLOG(INFO) << "       Create label index on :"
+                     << delta.operation_label.label;
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          if (!CreateIndex(NameToLabel(delta.operation_label.label)))
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::LABEL_INDEX_DROP: {
+          DLOG(INFO) << "       Drop label index on :"
+                     << delta.operation_label.label;
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          if (!DropIndex(NameToLabel(delta.operation_label.label)))
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::LABEL_PROPERTY_INDEX_CREATE: {
+          DLOG(INFO) << "       Create label+property index on :"
+                     << delta.operation_label_property.label << " ("
+                     << delta.operation_label_property.property << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          if (!CreateIndex(
+                  NameToLabel(delta.operation_label_property.label),
+                  NameToProperty(delta.operation_label_property.property)))
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::LABEL_PROPERTY_INDEX_DROP: {
+          DLOG(INFO) << "       Drop label+property index on :"
+                     << delta.operation_label_property.label << " ("
+                     << delta.operation_label_property.property << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          if (!DropIndex(
+                  NameToLabel(delta.operation_label_property.label),
+                  NameToProperty(delta.operation_label_property.property)))
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::EXISTENCE_CONSTRAINT_CREATE: {
+          DLOG(INFO) << "       Create existence constraint on :"
+                     << delta.operation_label_property.label << " ("
+                     << delta.operation_label_property.property << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          auto ret = CreateExistenceConstraint(
+              NameToLabel(delta.operation_label_property.label),
+              NameToProperty(delta.operation_label_property.property));
+          if (!ret.HasValue() || !ret.GetValue())
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::EXISTENCE_CONSTRAINT_DROP: {
+          DLOG(INFO) << "       Drop existence constraint on :"
+                     << delta.operation_label_property.label << " ("
+                     << delta.operation_label_property.property << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          if (!DropExistenceConstraint(
+                  NameToLabel(delta.operation_label_property.label),
+                  NameToProperty(delta.operation_label_property.property)))
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::UNIQUE_CONSTRAINT_CREATE: {
+          std::stringstream ss;
+          utils::PrintIterable(ss, delta.operation_label_properties.properties);
+          DLOG(INFO) << "       Create unique constraint on :"
+                     << delta.operation_label_properties.label << " ("
+                     << ss.str() << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          std::set<PropertyId> properties;
+          for (const auto &prop : delta.operation_label_properties.properties) {
+            properties.emplace(NameToProperty(prop));
+          }
+          auto ret = CreateUniqueConstraint(
+              NameToLabel(delta.operation_label_properties.label), properties);
+          if (!ret.HasValue() ||
+              ret.GetValue() != UniqueConstraints::CreationStatus::SUCCESS)
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+        case durability::WalDeltaData::Type::UNIQUE_CONSTRAINT_DROP: {
+          std::stringstream ss;
+          utils::PrintIterable(ss, delta.operation_label_properties.properties);
+          DLOG(INFO) << "       Drop unique constraint on :"
+                     << delta.operation_label_properties.label << " ("
+                     << ss.str() << ")";
+          if (commit_timestamp_and_accessor)
+            throw utils::BasicException("Invalid transaction!");
+          std::set<PropertyId> properties;
+          for (const auto &prop : delta.operation_label_properties.properties) {
+            properties.emplace(NameToProperty(prop));
+          }
+          auto ret = DropUniqueConstraint(
+              NameToLabel(delta.operation_label_properties.label), properties);
+          if (ret != UniqueConstraints::DeletionStatus::SUCCESS)
+            throw utils::BasicException("Invalid transaction!");
+          transaction_complete = true;
+          break;
+        }
+      }
+    }
+
+    if (commit_timestamp_and_accessor)
+      throw utils::BasicException("Invalid data!");
+
+    AppendDeltasRes res;
+    slk::Save(res, res_builder);
+  });
+  replication_server_->Start();
+}
+
+void Storage::ConfigureMain() {
+  replication_client_.emplace(&name_id_mapper_, config_.items,
+                              io::network::Endpoint{"127.0.0.1", 10000}, false);
+}
+
+void Storage::SetReplicationState(const ReplicationState state) {
+  if (replication_state_.load(std::memory_order_acquire) == state) {
+    return;
+  }
+
+  std::unique_lock<utils::RWLock> replication_guard(replication_lock_);
+
+  replication_server_.reset();
+  replication_server_context_.reset();
+  replication_client_.reset();
+
+  switch (state) {
+    case ReplicationState::MAIN:
+      ConfigureMain();
+      break;
+    case ReplicationState::REPLICA:
+      ConfigureReplica();
+      break;
+    case ReplicationState::NONE:
+    default:
+      break;
+  }
+
+  replication_state_.store(state, std::memory_order_release);
+}
+#endif
+
 }  // namespace storage

src/storage/v2/storage.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <atomic>
 #include <filesystem>
 #include <optional>
 #include <shared_mutex>
@@ -22,6 +23,13 @@
 #include "utils/skip_list.hpp"
 #include "utils/synchronized.hpp"
 
+#ifdef MG_ENTERPRISE
+#include "rpc/server.hpp"
+#include "storage/v2/replication/replication.hpp"
+#include "storage/v2/replication/rpc.hpp"
+#include "storage/v2/replication/serialization.hpp"
+#endif
+
 namespace storage {
 
 // The storage is based on this paper:
@@ -159,6 +167,10 @@ struct StorageInfo {
   uint64_t disk_usage;
 };
 
+#ifdef MG_ENTERPRISE
+enum class ReplicationState : uint8_t { NONE, MAIN, REPLICA };
+#endif
+
 class Storage final {
  public:
   /// @throw std::system_error
@@ -305,6 +317,19 @@ class Storage final {
     void Abort();
 
    private:
+#ifdef MG_ENTERPRISE
+    /// @throw std::bad_alloc
+    VertexAccessor CreateVertex(storage::Gid gid);
+
+    /// @throw std::bad_alloc
+    Result<EdgeAccessor> CreateEdge(VertexAccessor *from, VertexAccessor *to,
+                                    EdgeTypeId edge_type, storage::Gid gid);
+
+    /// @throw std::bad_alloc
+    utils::BasicResult<ConstraintViolation, void> Commit(
+        std::optional<uint64_t> desired_commit_timestamp);
+#endif
+
     Storage *storage_;
     std::shared_lock<utils::RWLock> storage_guard_;
     Transaction transaction_;
@@ -379,6 +404,10 @@ class Storage final {
 
   StorageInfo GetInfo() const;
 
+#ifdef MG_ENTERPRISE
+  void SetReplicationState(ReplicationState state);
+#endif
+
  private:
   Transaction CreateTransaction();
 
@@ -395,6 +424,11 @@ class Storage final {
                    const std::set<PropertyId> &properties,
                    uint64_t final_commit_timestamp);
 
+#ifdef MG_ENTERPRISE
+  void ConfigureReplica();
+  void ConfigureMain();
+#endif
+
   // Main storage lock.
   //
   // Accessors take a shared lock when starting, so it is possible to block
@@ -467,6 +501,16 @@ class Storage final {
 
   std::optional<durability::WalFile> wal_file_;
   uint64_t wal_unsynced_transactions_{0};
+
+  // Replication
+#ifdef MG_ENTERPRISE
+  utils::RWLock replication_lock_{utils::RWLock::Priority::WRITE};
+  std::optional<communication::ServerContext> replication_server_context_;
+  std::optional<rpc::Server> replication_server_;
+  // TODO(mferencevic): Add support for multiple clients.
+  std::optional<replication::ReplicationClient> replication_client_;
+  std::atomic<ReplicationState> replication_state_{ReplicationState::NONE};
+#endif
 };
 
 }  // namespace storage

tests/unit/CMakeLists.txt

@@ -234,6 +234,10 @@ target_link_libraries(${test_prefix}storage_v2_property_store mg-storage-v2 fmt)
 add_unit_test(storage_v2_wal_file.cpp)
 target_link_libraries(${test_prefix}storage_v2_wal_file mg-storage-v2 fmt)
 
+if (MG_ENTERPRISE)
+add_unit_test(storage_v2_replication.cpp)
+target_link_libraries(${test_prefix}storage_v2_replication mg-storage-v2 fmt)
+endif()
 
 # Test mg-auth
 
@@ -246,10 +250,11 @@ endif()
 # Test mg-slk
 
 if (MG_ENTERPRISE)
-## TODO: REPLACE single-node-ha
-#add_unit_test(slk_advanced.cpp)
-#target_link_libraries(${test_prefix}slk_advanced mg-single-node-ha mg-kvstore-dummy)
+add_unit_test(slk_advanced.cpp)
+target_link_libraries(${test_prefix}slk_advanced mg-storage-v2)
+endif()
 
+if (MG_ENTERPRISE)
 add_unit_test(slk_core.cpp)
 target_link_libraries(${test_prefix}slk_core mg-slk glog gflags fmt)
 

tests/unit/slk_advanced.cpp

@@ -1,6 +1,6 @@
 #include <gtest/gtest.h>
 
-#include "storage/common/types/slk.hpp"
+#include "storage/v2/replication/slk.hpp"
 
 #include "slk_common.hpp"
 

tests/unit/storage_v2_replication.cpp

@@ -0,0 +1,252 @@
+#include <chrono>
+#include <thread>
+
+#include <fmt/format.h>
+#include <gmock/gmock-generated-matchers.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <storage/v2/property_value.hpp>
+#include <storage/v2/storage.hpp>
+
+using testing::UnorderedElementsAre;
+
+TEST(ReplicationTest, BasicSynchronousReplicationTest) {
+  std::filesystem::path storage_directory{
+      std::filesystem::temp_directory_path() /
+      "MG_test_unit_storage_v2_replication"};
+
+  storage::Storage main_store(
+      {.items = {.properties_on_edges = true},
+       .durability = {
+           .storage_directory = storage_directory,
+           .snapshot_wal_mode = storage::Config::Durability::SnapshotWalMode::
+               PERIODIC_SNAPSHOT_WITH_WAL,
+       }});
+  main_store.SetReplicationState(storage::ReplicationState::MAIN);
+
+  storage::Storage replica_store(
+      {.items = {.properties_on_edges = true},
+       .durability = {
+           .storage_directory = storage_directory,
+           .snapshot_wal_mode = storage::Config::Durability::SnapshotWalMode::
+               PERIODIC_SNAPSHOT_WITH_WAL,
+       }});
+  replica_store.SetReplicationState(storage::ReplicationState::REPLICA);
+
+  // vertex create
+  // vertex add label
+  // vertex set property
+  const auto *vertex_label = "vertex_label";
+  const auto *vertex_property = "vertex_property";
+  const auto *vertex_property_value = "vertex_property_value";
+  std::optional<storage::Gid> vertex_gid;
+  {
+    auto acc = main_store.Access();
+    auto v = acc.CreateVertex();
+    vertex_gid.emplace(v.Gid());
+    ASSERT_TRUE(v.AddLabel(main_store.NameToLabel(vertex_label)).HasValue());
+    ASSERT_TRUE(v.SetProperty(main_store.NameToProperty(vertex_property),
+                              storage::PropertyValue(vertex_property_value))
+                    .HasValue());
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  {
+    auto acc = replica_store.Access();
+    const auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    const auto labels = v->Labels(storage::View::OLD);
+    ASSERT_TRUE(labels.HasValue());
+    ASSERT_EQ(labels->size(), 1);
+    ASSERT_THAT(*labels,
+                UnorderedElementsAre(replica_store.NameToLabel(vertex_label)));
+    const auto properties = v->Properties(storage::View::OLD);
+    ASSERT_TRUE(properties.HasValue());
+    ASSERT_EQ(properties->size(), 1);
+    ASSERT_THAT(*properties,
+                UnorderedElementsAre(std::make_pair(
+                    replica_store.NameToProperty(vertex_property),
+                    storage::PropertyValue(vertex_property_value))));
+
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  // vertex remove label
+  {
+    auto acc = main_store.Access();
+    auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    ASSERT_TRUE(
+        v->RemoveLabel(main_store.NameToLabel(vertex_label)).HasValue());
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  {
+    auto acc = replica_store.Access();
+    const auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    const auto labels = v->Labels(storage::View::OLD);
+    ASSERT_TRUE(labels.HasValue());
+    ASSERT_EQ(labels->size(), 0);
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  // vertex delete
+  {
+    auto acc = main_store.Access();
+    auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    ASSERT_TRUE(acc.DeleteVertex(&*v).HasValue());
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  {
+    auto acc = replica_store.Access();
+    const auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_FALSE(v);
+    vertex_gid.reset();
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  // edge create
+  // edge set property
+  const auto *edge_type = "edge_type";
+  const auto *edge_property = "edge_property";
+  const auto *edge_property_value = "edge_property_value";
+  std::optional<storage::Gid> edge_gid;
+  {
+    auto acc = main_store.Access();
+    auto v = acc.CreateVertex();
+    vertex_gid.emplace(v.Gid());
+    auto edge = acc.CreateEdge(&v, &v, main_store.NameToEdgeType(edge_type));
+    ASSERT_TRUE(edge.HasValue());
+    ASSERT_TRUE(edge->SetProperty(main_store.NameToProperty(edge_property),
+                                  storage::PropertyValue(edge_property_value))
+                    .HasValue());
+    edge_gid.emplace(edge->Gid());
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  const auto find_edge =
+      [&](const auto &edges,
+          const storage::Gid edge_gid) -> std::optional<storage::EdgeAccessor> {
+    for (const auto &edge : edges) {
+      if (edge.Gid() == edge_gid) {
+        return edge;
+      }
+    }
+    return std::nullopt;
+  };
+
+  {
+    auto acc = replica_store.Access();
+    const auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    const auto out_edges = v->OutEdges(storage::View::OLD);
+    ASSERT_TRUE(out_edges.HasValue());
+    const auto edge = find_edge(*out_edges, *edge_gid);
+    ASSERT_EQ(edge->EdgeType(), replica_store.NameToEdgeType(edge_type));
+    const auto properties = edge->Properties(storage::View::OLD);
+    ASSERT_TRUE(properties.HasValue());
+    ASSERT_EQ(properties->size(), 1);
+    ASSERT_THAT(*properties, UnorderedElementsAre(std::make_pair(
+                                 replica_store.NameToProperty(edge_property),
+                                 storage::PropertyValue(edge_property_value))));
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  // delete edge
+  {
+    auto acc = main_store.Access();
+    auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    auto out_edges = v->OutEdges(storage::View::OLD);
+    auto edge = find_edge(*out_edges, *edge_gid);
+    ASSERT_TRUE(edge);
+    ASSERT_TRUE(acc.DeleteEdge(&*edge).HasValue());
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  {
+    auto acc = replica_store.Access();
+    const auto v = acc.FindVertex(*vertex_gid, storage::View::OLD);
+    ASSERT_TRUE(v);
+    const auto out_edges = v->OutEdges(storage::View::OLD);
+    ASSERT_TRUE(out_edges.HasValue());
+    ASSERT_FALSE(find_edge(*out_edges, *edge_gid));
+    ASSERT_FALSE(acc.Commit().HasError());
+  }
+
+  // label index create
+  // label property index create
+  // existence constraint create
+  // unique constriant create
+  const auto *label = "label";
+  const auto *property = "property";
+  const auto *property_extra = "property_extra";
+  {
+    ASSERT_TRUE(main_store.CreateIndex(main_store.NameToLabel(label)));
+    ASSERT_TRUE(main_store.CreateIndex(main_store.NameToLabel(label),
+                                       main_store.NameToProperty(property)));
+    ASSERT_FALSE(
+        main_store
+            .CreateExistenceConstraint(main_store.NameToLabel(label),
+                                       main_store.NameToProperty(property))
+            .HasError());
+    ASSERT_FALSE(
+        main_store
+            .CreateUniqueConstraint(main_store.NameToLabel(label),
+                                    {main_store.NameToProperty(property),
+                                     main_store.NameToProperty(property_extra)})
+            .HasError());
+  }
+
+  {
+    const auto indices = replica_store.ListAllIndices();
+    ASSERT_THAT(indices.label,
+                UnorderedElementsAre(replica_store.NameToLabel(label)));
+    ASSERT_THAT(indices.label_property,
+                UnorderedElementsAre(
+                    std::make_pair(replica_store.NameToLabel(label),
+                                   replica_store.NameToProperty(property))));
+
+    const auto constraints = replica_store.ListAllConstraints();
+    ASSERT_THAT(constraints.existence,
+                UnorderedElementsAre(
+                    std::make_pair(replica_store.NameToLabel(label),
+                                   replica_store.NameToProperty(property))));
+    ASSERT_THAT(constraints.unique,
+                UnorderedElementsAre(std::make_pair(
+                    replica_store.NameToLabel(label),
+                    std::set{replica_store.NameToProperty(property),
+                             replica_store.NameToProperty(property_extra)})));
+  }
+
+  // label index drop
+  // label property index drop
+  // existence constraint drop
+  // unique constriant drop
+  {
+    ASSERT_TRUE(main_store.DropIndex(main_store.NameToLabel(label)));
+    ASSERT_TRUE(main_store.DropIndex(main_store.NameToLabel(label),
+                                     main_store.NameToProperty(property)));
+    ASSERT_TRUE(main_store.DropExistenceConstraint(
+        main_store.NameToLabel(label), main_store.NameToProperty(property)));
+    ASSERT_EQ(main_store.DropUniqueConstraint(
+                  main_store.NameToLabel(label),
+                  {main_store.NameToProperty(property),
+                   main_store.NameToProperty(property_extra)}),
+              storage::UniqueConstraints::DeletionStatus::SUCCESS);
+  }
+
+  {
+    const auto indices = replica_store.ListAllIndices();
+    ASSERT_EQ(indices.label.size(), 0);
+    ASSERT_EQ(indices.label_property.size(), 0);
+
+    const auto constraints = replica_store.ListAllConstraints();
+    ASSERT_EQ(constraints.existence.size(), 0);
+    ASSERT_EQ(constraints.unique.size(), 0);
+  }
+}