fix(migration): Use transactions! (#3266)

Signed-off-by: Vladislav Oleshko <vlad@dragonflydb.io>

src/server/cluster/outgoing_slot_migration.cc

@@ -16,6 +16,7 @@
 #include "server/engine_shard_set.h"
 #include "server/error.h"
 #include "server/journal/streamer.h"
+#include "server/main_service.h"
 #include "server/server_family.h"
 
 ABSL_FLAG(int, slot_migration_connection_timeout_ms, 2000, "Timeout for network operations");
@@ -33,7 +34,10 @@ class OutgoingMigration::SliceSlotMigration : private ProtocolClient {
       : ProtocolClient(server_context), streamer_(slice, std::move(slots), journal, &cntx_) {
   }
 
-  void Sync(const std::string& node_id, uint32_t shard_id) {
+  // Send DFLYMIGRATE FLOW
+  void PrepareFlow(const std::string& node_id) {
+    uint32_t shard_id = EngineShard::tlocal()->shard_id();
+
     VLOG(1) << "Connecting to source node_id " << node_id << " shard_id " << shard_id;
     auto timeout = absl::GetFlag(FLAGS_slot_migration_connection_timeout_ms) * 1ms;
     if (auto ec = ConnectAndAuth(timeout, &cntx_); ec) {
@@ -57,10 +61,18 @@ class OutgoingMigration::SliceSlotMigration : private ProtocolClient {
       cntx_.ReportError("Incorrect response for FLOW cmd");
       return;
     }
+  }
 
+  // Register db_slice and journal change listeners
+  void PrepareSync() {
     streamer_.Start(Sock());
   }
 
+  // Run restore streamer
+  void RunSync() {
+    streamer_.Run();
+  }
+
   void Cancel() {
     streamer_.Cancel();
   }
@@ -82,18 +94,17 @@ OutgoingMigration::OutgoingMigration(MigrationInfo info, ClusterFamily* cf, Serv
       migration_info_(std::move(info)),
       slot_migrations_(shard_set->size()),
       server_family_(sf),
-      cf_(cf) {
+      cf_(cf),
+      tx_(new Transaction{sf->service().FindCmd("DFLYCLUSTER")}) {
+  tx_->InitByArgs(0, {});
 }
 
 OutgoingMigration::~OutgoingMigration() {
   main_sync_fb_.JoinIfNeeded();
 
-  // Destroy each flow in its dedicated thread, because we could be the last owner of the db tables
+  // Destroy each flow in its dedicated thread, because we could be the last
-  shard_set->pool()->AwaitFiberOnAll([this](util::ProactorBase* pb) {
+  // owner of the db tables
-    if (const auto* shard = EngineShard::tlocal(); shard) {
+  OnAllShards([](auto& migration) { migration.reset(); });
-      slot_migrations_[shard->shard_id()].reset();
-    }
-  });
 }
 
 bool OutgoingMigration::ChangeState(MigrationState new_state) {
@@ -106,6 +117,15 @@ bool OutgoingMigration::ChangeState(MigrationState new_state) {
   return true;
 }
 
+void OutgoingMigration::OnAllShards(
+    std::function<void(std::unique_ptr<SliceSlotMigration>&)> func) {
+  shard_set->pool()->AwaitFiberOnAll([this, &func](util::ProactorBase* pb) {
+    if (const auto* shard = EngineShard::tlocal(); shard) {
+      func(slot_migrations_[shard->shard_id()]);
+    }
+  });
+}
+
 void OutgoingMigration::Finish(bool is_error) {
   VLOG(1) << "Finish outgoing migration for " << cf_->MyID() << " : "
           << migration_info_.node_info.id;
@@ -132,12 +152,9 @@ void OutgoingMigration::Finish(bool is_error) {
   }
 
   if (should_cancel_flows) {
-    shard_set->pool()->AwaitFiberOnAll([this](util::ProactorBase* pb) {
+    OnAllShards([](auto& migration) {
-      if (const auto* shard = EngineShard::tlocal(); shard) {
+      CHECK(migration != nullptr);
-        auto& flow = slot_migrations_[shard->shard_id()];
+      migration->Cancel();
-        CHECK(flow != nullptr);
-        flow->Cancel();
-      }
     });
   }
 }
@@ -161,8 +178,7 @@ void OutgoingMigration::SyncFb() {
 
     if (last_error_) {
       LOG(ERROR) << last_error_.Format();
-      // if error is happened on the previous attempt we wait for some time and try again
+      ThisFiber::SleepFor(1000ms);  // wait some time before next retry
-      ThisFiber::SleepFor(1000ms);
     }
 
     VLOG(2) << "Connecting to source";
@@ -195,27 +211,34 @@ void OutgoingMigration::SyncFb() {
       continue;
     }
 
-    shard_set->pool()->AwaitFiberOnAll([this](util::ProactorBase* pb) {
+    OnAllShards([this](auto& migration) {
-      if (auto* shard = EngineShard::tlocal(); shard) {
+      auto* shard = EngineShard::tlocal();
       server_family_->journal()->StartInThread();
-        slot_migrations_[shard->shard_id()] = std::make_unique<SliceSlotMigration>(
+      migration = std::make_unique<SliceSlotMigration>(
           &shard->db_slice(), server(), migration_info_.slot_ranges, server_family_->journal());
-      }
     });
 
     if (!ChangeState(MigrationState::C_SYNC)) {
       break;
     }
 
-    shard_set->pool()->AwaitFiberOnAll([this](util::ProactorBase* pb) {
+    OnAllShards([this](auto& migration) { migration->PrepareFlow(cf_->MyID()); });
-      if (auto* shard = EngineShard::tlocal(); shard) {
+    if (CheckFlowsForErrors()) {
-        auto& migration = slot_migrations_[shard->shard_id()];
+      LOG(WARNING) << "Preparation error detected, retrying outgoing migration";
-        CHECK(migration != nullptr);
+      continue;
-        migration->Sync(cf_->MyID(), shard->shard_id());
+    }
-        if (migration->GetError()) {
+
+    // Global transactional cut for migration to register db_slice and journal
+    // listeners
+    {
+      Transaction::Guard tg{tx_.get()};
+      OnAllShards([](auto& migration) { migration->PrepareSync(); });
+    }
+
+    OnAllShards([this](auto& migration) {
+      migration->RunSync();
+      if (migration->GetError())
         Finish(true);
-        }
-      }
     });
 
     if (CheckFlowsForErrors()) {
@@ -240,8 +263,8 @@ void OutgoingMigration::SyncFb() {
 }
 
 bool OutgoingMigration::FinalizeMigration(long attempt) {
-  // if it's not the 1st attempt and flows are work correctly we try to reconnect and ACK one more
+  // if it's not the 1st attempt and flows are work correctly we try to
-  // time
+  // reconnect and ACK one more time
   VLOG(1) << "FinalizeMigration for " << cf_->MyID() << " : " << migration_info_.node_info.id;
   if (attempt > 1) {
     if (CheckFlowsForErrors()) {
@@ -255,6 +278,9 @@ bool OutgoingMigration::FinalizeMigration(long attempt) {
       return false;
     }
   }
+
+  // Migration finalization has to be done via client pause because commands need to
+  // be blocked on coordinator level to avoid intializing transactions with stale cluster slot info
   // TODO implement blocking on migrated slots only
   bool is_block_active = true;
   auto is_pause_in_progress = [&is_block_active] { return is_block_active; };
@@ -270,14 +296,8 @@ bool OutgoingMigration::FinalizeMigration(long attempt) {
     pause_fb_opt->JoinIfNeeded();
   });
 
-  auto cb = [this, attempt](util::ProactorBase* pb) {
-    if (const auto* shard = EngineShard::tlocal(); shard) {
-      slot_migrations_[shard->shard_id()]->Finalize(attempt);
-    }
-  };
-
   VLOG(1) << "FINALIZE flows for " << cf_->MyID() << " : " << migration_info_.node_info.id;
-  shard_set->pool()->AwaitFiberOnAll(std::move(cb));
+  OnAllShards([attempt](auto& migration) { migration->Finalize(attempt); });
 
   auto cmd = absl::StrCat("DFLYMIGRATE ACK ", cf_->MyID(), " ", attempt);
   VLOG(1) << "send " << cmd;

src/server/cluster/outgoing_slot_migration.h

@@ -3,9 +3,12 @@
 //
 #pragma once
 
+#include <boost/smart_ptr/intrusive_ptr.hpp>
+
 #include "io/io.h"
 #include "server/cluster/cluster_defs.h"
 #include "server/protocol_client.h"
+#include "server/transaction.h"
 
 namespace dfly {
 class DbSlice;
@@ -75,6 +78,8 @@ class OutgoingMigration : private ProtocolClient {
 
   bool ChangeState(MigrationState new_state) ABSL_LOCKS_EXCLUDED(state_mu_);
 
+  void OnAllShards(std::function<void(std::unique_ptr<SliceSlotMigration>&)>);
+
  private:
   MigrationInfo migration_info_;
   std::vector<std::unique_ptr<SliceSlotMigration>> slot_migrations_;
@@ -87,6 +92,8 @@ class OutgoingMigration : private ProtocolClient {
   mutable util::fb2::Mutex state_mu_;
   MigrationState state_ ABSL_GUARDED_BY(state_mu_) = MigrationState::C_NO_STATE;
 
+  boost::intrusive_ptr<Transaction> tx_;
+
   // when migration is finished we need to store number of migrated keys
   // because new request can add or remove keys and we get incorrect statistic
   size_t keys_number_ = 0;

src/server/db_slice.cc

@@ -1089,19 +1089,7 @@ void DbSlice::ExpireAllIfNeeded() {
 }
 
 uint64_t DbSlice::RegisterOnChange(ChangeCallback cb) {
-  // TODO rewrite this logic to be more clear
+  return change_cb_.emplace_back(NextVersion(), std::move(cb)).first;
-  // this mutex lock is needed to check that this method is not called simultaneously with
-  // change_cb_ calls and journal_slice::change_cb_arr_ calls.
-  // It can be unlocked anytime because DbSlice::RegisterOnChange
-  // and journal_slice::RegisterOnChange calls without preemption
-  std::lock_guard lk(cb_mu_);
-
-  uint64_t ver = NextVersion();
-  change_cb_.emplace_back(ver, std::move(cb));
-  DCHECK(std::is_sorted(change_cb_.begin(), change_cb_.end(),
-                        [](auto& a, auto& b) { return a.first < b.first; }));
-
-  return ver;
 }
 
 void DbSlice::FlushChangeToEarlierCallbacks(DbIndex db_ind, Iterator it, uint64_t upper_bound) {
@@ -1125,14 +1113,10 @@ void DbSlice::FlushChangeToEarlierCallbacks(DbIndex db_ind, Iterator it, uint64_
 
 //! Unregisters the callback.
 void DbSlice::UnregisterOnChange(uint64_t id) {
-  lock_guard lk(cb_mu_);  // we need to wait until callback is finished before remove it
+  auto it = find_if(change_cb_.begin(), change_cb_.end(),
-  for (auto it = change_cb_.begin(); it != change_cb_.end(); ++it) {
+                    [id](const auto& cb) { return cb.first == id; });
-    if (it->first == id) {
+  CHECK(it != change_cb_.end());
   change_cb_.erase(it);
-      return;
-    }
-  }
-  LOG(DFATAL) << "Could not find " << id << " to unregister";
 }
 
 auto DbSlice::DeleteExpiredStep(const Context& cntx, unsigned count) -> DeleteExpiredStats {

src/server/db_slice.h

@@ -469,14 +469,6 @@ class DbSlice {
   void PerformDeletion(Iterator del_it, DbTable* table);
   void PerformDeletion(PrimeIterator del_it, DbTable* table);
 
-  void LockChangeCb() const {
-    return cb_mu_.lock_shared();
-  }
-
-  void UnlockChangeCb() const {
-    return cb_mu_.unlock_shared();
-  }
-
  private:
   void PreUpdate(DbIndex db_ind, Iterator it, std::string_view key);
   void PostUpdate(DbIndex db_ind, Iterator it, std::string_view key, size_t orig_size);
@@ -552,12 +544,6 @@ class DbSlice {
   // Used in temporary computations in Acquire/Release.
   mutable absl::flat_hash_set<uint64_t> uniq_fps_;
 
-  // To ensure correct data replication, we must serialize the buckets that each running command
-  // will modify, followed by serializing the command to the journal. We use a mutex to prevent
-  // interleaving between bucket and journal registrations, and the command execution with its
-  // journaling. LockChangeCb is called before the callback, and UnlockChangeCb is called after
-  // journaling is completed. Register to bucket and journal changes is also does without preemption
-  mutable util::fb2::SharedMutex cb_mu_;
   // ordered from the smallest to largest version.
   std::vector<std::pair<uint64_t, ChangeCallback>> change_cb_;
 

src/server/detail/save_stages_controller.cc

@@ -254,11 +254,7 @@ void SaveStagesController::SaveDfs() {
 
   // Save shard files.
   auto cb = [this](Transaction* t, EngineShard* shard) {
-    auto& db_slice = shard->db_slice();
-    // a hack to avoid deadlock in Transaction::RunCallback(...)
-    db_slice.UnlockChangeCb();
     SaveDfsSingle(shard);
-    db_slice.LockChangeCb();
     return OpStatus::OK;
   };
   trans_->ScheduleSingleHop(std::move(cb));
@@ -298,11 +294,7 @@ void SaveStagesController::SaveRdb() {
   }
 
   auto cb = [snapshot = snapshot.get()](Transaction* t, EngineShard* shard) {
-    // a hack to avoid deadlock in Transaction::RunCallback(...)
-    auto& db_slice = shard->db_slice();
-    db_slice.UnlockChangeCb();
     snapshot->StartInShard(shard);
-    db_slice.LockChangeCb();
     return OpStatus::OK;
   };
   trans_->ScheduleSingleHop(std::move(cb));

src/server/dflycmd.cc

@@ -71,32 +71,6 @@ std::string_view SyncStateName(DflyCmd::SyncState sync_state) {
   return "unsupported";
 }
 
-struct TransactionGuard {
-  static OpStatus ExitGuardCb(Transaction* t, EngineShard* shard) {
-    t->GetDbSlice(shard->shard_id()).SetExpireAllowed(true);
-    return OpStatus::OK;
-  };
-
-  explicit TransactionGuard(Transaction* t, bool disable_expirations = false) : t(t) {
-    t->Execute(
-        [disable_expirations](Transaction* t, EngineShard* shard) {
-          if (disable_expirations) {
-            t->GetDbSlice(shard->shard_id()).SetExpireAllowed(!disable_expirations);
-          }
-          return OpStatus::OK;
-        },
-        false);
-    VLOG(2) << "Transaction guard engaged";
-  }
-
-  ~TransactionGuard() {
-    VLOG(2) << "Releasing transaction guard";
-    t->Execute(ExitGuardCb, true);
-  }
-
-  Transaction* t;
-};
-
 OpStatus WaitReplicaFlowToCatchup(absl::Time end_time, shared_ptr<DflyCmd::ReplicaInfo> replica,
                                   EngineShard* shard) {
   // We don't want any writes to the journal after we send the `PING`,
@@ -299,7 +273,7 @@ void DflyCmd::Sync(CmdArgList args, ConnectionContext* cntx) {
 
   // Start full sync.
   {
-    TransactionGuard tg{cntx->transaction};
+    Transaction::Guard tg{cntx->transaction};
     AggregateStatus status;
 
     // Use explicit assignment for replica_ptr, because capturing structured bindings is C++20.
@@ -337,7 +311,7 @@ void DflyCmd::StartStable(CmdArgList args, ConnectionContext* cntx) {
     return;
 
   {
-    TransactionGuard tg{cntx->transaction};
+    Transaction::Guard tg{cntx->transaction};
     AggregateStatus status;
 
     auto cb = [this, &status, replica_ptr = replica_ptr](EngineShard* shard) {

src/server/journal/streamer.cc

@@ -201,6 +201,10 @@ void RestoreStreamer::Start(util::FiberSocketBase* dest, bool send_lsn) {
   snapshot_version_ = db_slice_->RegisterOnChange(std::move(db_cb));
 
   JournalStreamer::Start(dest, send_lsn);
+}
+
+void RestoreStreamer::Run() {
+  VLOG(1) << "RestoreStreamer run";
 
   PrimeTable::Cursor cursor;
   uint64_t last_yield = 0;

src/server/journal/streamer.h

@@ -77,6 +77,9 @@ class RestoreStreamer : public JournalStreamer {
   ~RestoreStreamer() override;
 
   void Start(util::FiberSocketBase* dest, bool send_lsn = false) override;
+
+  void Run();
+
   // Cancel() must be called if Start() is called
   void Cancel() override;
 

src/server/transaction.cc

@@ -127,6 +127,16 @@ cv_status Transaction::BatonBarrier::Wait(time_point tp) {
   return cv_status::no_timeout;
 }
 
+Transaction::Guard::Guard(Transaction* tx) : tx(tx) {
+  DCHECK(tx->cid_->opt_mask() & CO::GLOBAL_TRANS);
+  tx->Execute([](auto*, auto*) { return OpStatus::OK; }, false);
+}
+
+Transaction::Guard::~Guard() {
+  tx->Conclude();
+  tx->Refurbish();
+}
+
 Transaction::Transaction(const CommandId* cid) : cid_{cid} {
   InitTxTime();
   string_view cmd_name(cid_->name());
@@ -620,7 +630,6 @@ void Transaction::RunCallback(EngineShard* shard) {
 
   RunnableResult result;
   auto& db_slice = GetDbSlice(shard->shard_id());
-  db_slice.LockChangeCb();
   try {
     result = (*cb_ptr_)(this, shard);
 
@@ -658,10 +667,7 @@ void Transaction::RunCallback(EngineShard* shard) {
   // Log to journal only once the command finished running
   if ((coordinator_state_ & COORD_CONCLUDING) || (multi_ && multi_->concluding)) {
     LogAutoJournalOnShard(shard, result);
-    db_slice.UnlockChangeCb();
     MaybeInvokeTrackingCb();
-  } else {
-    db_slice.UnlockChangeCb();
   }
 }
 
@@ -1223,11 +1229,11 @@ OpStatus Transaction::RunSquashedMultiCb(RunnableType cb) {
 
   auto* shard = EngineShard::tlocal();
   auto& db_slice = GetDbSlice(shard->shard_id());
-  db_slice.LockChangeCb();
+
   auto result = cb(this, shard);
   db_slice.OnCbFinish();
+
   LogAutoJournalOnShard(shard, result);
-  db_slice.UnlockChangeCb();
   MaybeInvokeTrackingCb();
 
   DCHECK_EQ(result.flags, 0);  // if it's sophisticated, we shouldn't squash it

src/server/transaction.h

@@ -170,6 +170,14 @@ class Transaction {
     RAN_IMMEDIATELY = 1 << 7,  // Whether the shard executed immediately (during schedule)
   };
 
+  struct Guard {
+    explicit Guard(Transaction* tx);
+    ~Guard();
+
+   private:
+    Transaction* tx;
+  };
+
   explicit Transaction(const CommandId* cid);
 
   // Initialize transaction for squashing placed on a specific shard with a given parent tx