feat(replication): Use a ring buffer with messages to serve replication. (#1835)

* feat(replication): Use a ring buffer with messages to serve replication.

* Fix libraries dep graph

* Address PR feedback

* nits

* add a comment

* Lower the default log length

src/server/CMakeLists.txt

@@ -17,6 +17,7 @@ endif()
 add_library(dfly_transaction db_slice.cc malloc_stats.cc engine_shard_set.cc blocking_controller.cc
             common.cc journal/journal.cc journal/types.cc journal/journal_slice.cc
             server_state.cc table.cc  top_keys.cc transaction.cc
+            serializer_commons.cc journal/serializer.cc journal/executor.cc journal/streamer.cc
             ${TX_LINUX_SRCS}
             )
 cxx_link(dfly_transaction dfly_core strings_lib)
@@ -38,7 +39,6 @@ add_library(dragonfly_lib channel_store.cc command_registry.cc
             detail/snapshot_storage.cc
             set_family.cc stream_family.cc string_family.cc
             zset_family.cc version.cc bitops_family.cc container_utils.cc io_utils.cc
-            serializer_commons.cc journal/serializer.cc journal/executor.cc journal/streamer.cc
             top_keys.cc multi_command_squasher.cc hll_family.cc cluster/cluster_config.cc
             cluster/cluster_family.cc acl/user.cc acl/user_registry.cc acl/acl_family.cc
             acl/validator.cc acl/helpers.cc)

src/server/journal/executor.cc

@@ -20,7 +20,7 @@ namespace dfly {
 namespace {
 // Build a CmdData from parts passed to absl::StrCat.
 template <typename... Ts> journal::ParsedEntry::CmdData BuildFromParts(Ts... parts) {
-  vector<string> raw_parts{absl::StrCat(forward<Ts>(parts))...};
+  vector<string> raw_parts{absl::StrCat(std::forward<Ts>(parts))...};
 
   auto cmd_str = accumulate(raw_parts.begin(), raw_parts.end(), std::string{});
   auto buf = make_unique<char[]>(cmd_str.size());
@@ -28,8 +28,8 @@ template <typename... Ts> journal::ParsedEntry::CmdData BuildFromParts(Ts... par
 
   CmdArgVec slice_parts{};
   size_t start = 0;
-  for (auto part : raw_parts) {
+  for (const auto& part : raw_parts) {
-    slice_parts.push_back(MutableSlice{buf.get() + start, part.size()});
+    slice_parts.emplace_back(buf.get() + start, part.size());
     start += part.size();
   }
 
@@ -38,8 +38,9 @@ template <typename... Ts> journal::ParsedEntry::CmdData BuildFromParts(Ts... par
 }  // namespace
 
 JournalExecutor::JournalExecutor(Service* service)
-    : service_{service}, reply_builder_{facade::ReplyMode::NONE}, conn_context_{nullptr, nullptr,
+    : service_{service},
-                                                                                &reply_builder_} {
+      reply_builder_{facade::ReplyMode::NONE},
+      conn_context_{nullptr, nullptr, &reply_builder_} {
   conn_context_.is_replicating = true;
   conn_context_.journal_emulated = true;
 }

src/server/journal/executor.h

@@ -16,7 +16,7 @@ class Service;
 // JournalExecutor allows executing journal entries.
 class JournalExecutor {
  public:
-  JournalExecutor(Service* service);
+  explicit JournalExecutor(Service* service);
   ~JournalExecutor();
 
   JournalExecutor(JournalExecutor&&) = delete;

src/server/journal/journal.cc

@@ -96,6 +96,14 @@ bool Journal::HasRegisteredCallbacks() const {
   return journal_slice.HasRegisteredCallbacks();
 }
 
+bool Journal::IsLSNInBuffer(LSN lsn) const {
+  return journal_slice.IsLSNInBuffer(lsn);
+}
+
+std::string_view Journal::GetEntry(LSN lsn) const {
+  return journal_slice.GetEntry(lsn);
+}
+
 LSN Journal::GetLsn() const {
   return journal_slice.cur_lsn();
 }

src/server/journal/journal.h

@@ -39,6 +39,9 @@ class Journal {
   void UnregisterOnChange(uint32_t id);
   bool HasRegisteredCallbacks() const;
 
+  bool IsLSNInBuffer(LSN lsn) const;
+  std::string_view GetEntry(LSN lsn) const;
+
   /*
   void AddCmd(TxId txid, Op opcode, Span args) {
     OpArgs(txid, opcode, args);

src/server/journal/journal_slice.cc

@@ -5,12 +5,19 @@
 #include "server/journal/journal_slice.h"
 
 #include <absl/container/inlined_vector.h>
+#include <absl/flags/flag.h>
+#include <absl/strings/escaping.h>
 #include <absl/strings/str_cat.h>
 #include <fcntl.h>
 
 #include <filesystem>
 
+#include "base/function2.hpp"
 #include "base/logging.h"
+#include "server/journal/serializer.h"
+
+ABSL_FLAG(int, shard_repl_backlog_len, 1 << 10,
+          "The length of the circular replication log per shard");
 
 namespace dfly {
 namespace journal {
@@ -34,12 +41,6 @@ string ShardName(std::string_view base, unsigned index) {
     CHECK(!__ec$) << "Error: " << __ec$ << " " << __ec$.message() << " for " << #x; \
   } while (false)
 
-struct JournalSlice::RingItem {
-  LSN lsn;
-  TxId txid;
-  Op opcode;
-};
-
 JournalSlice::JournalSlice() {
 }
 
@@ -48,11 +49,11 @@ JournalSlice::~JournalSlice() {
 }
 
 void JournalSlice::Init(unsigned index) {
-  // if (ring_buffer_)  // calling this function multiple times is allowed and it's a no-op.
+  if (ring_buffer_)  // calling this function multiple times is allowed and it's a no-op.
-  //   return;
+    return;
 
   slice_index_ = index;
-  // ring_buffer_.emplace(128);  // TODO: to make it configurable
+  ring_buffer_.emplace(absl::GetFlag(FLAGS_shard_repl_backlog_len));
 }
 
 #if 0
@@ -116,20 +117,49 @@ error_code JournalSlice::Close() {
 }
 #endif
 
+bool JournalSlice::IsLSNInBuffer(LSN lsn) const {
+  DCHECK(ring_buffer_);
+
+  if (ring_buffer_->empty()) {
+    return false;
+  }
+  return (*ring_buffer_)[0].lsn <= lsn && lsn <= ((*ring_buffer_)[ring_buffer_->size() - 1].lsn);
+}
+
+std::string_view JournalSlice::GetEntry(LSN lsn) const {
+  DCHECK(ring_buffer_ && IsLSNInBuffer(lsn));
+  auto start = (*ring_buffer_)[0].lsn;
+  DCHECK((*ring_buffer_)[lsn - start].lsn == lsn);
+  return (*ring_buffer_)[lsn - start].data;
+}
+
 void JournalSlice::AddLogRecord(const Entry& entry, bool await) {
-  // DCHECK(ring_buffer_);
+  DCHECK(ring_buffer_);
-  if (entry.opcode != Op::NOOP) {
+
-    lsn_++;
+  JournalItem dummy;
-// TODO: This is preparation for AOC style journaling, currently unused.
+  JournalItem* item;
+  if (entry.opcode == Op::NOOP) {
+    item = &dummy;
+    item->lsn = -1;
+    item->opcode = entry.opcode;
+    item->data = "";
+  } else {
+    // GetTail gives a pointer to a new tail entry in the buffer, possibly overriding the last entry
+    // if the buffer is full.
+    item = ring_buffer_->GetTail(true);
+    item->opcode = entry.opcode;
+    item->lsn = lsn_++;
+
+    io::BufSink buf_sink{&ring_serialize_buf_};
+    JournalWriter writer{&buf_sink};
+    writer.Write(entry);
+
+    item->data = io::View(ring_serialize_buf_.InputBuffer());
+    ring_serialize_buf_.Clear();
+    VLOG(2) << "Writing item [" << item->lsn << "]: " << entry.ToString();
+  }
+
 #if 0
-    RingItem item;
-    item.lsn = prev_lsn;
-
-    item.opcode = entry.opcode;
-    item.txid = entry.txid;
-    VLOG(1) << "Writing item [" << item.lsn << "]: " << entry.ToString();
-    ring_buffer_->EmplaceOrOverride(move(item));
-
     if (shard_file_) {
       string line = absl::StrCat(item.lsn, " ", entry.txid, " ", entry.opcode, "\n");
       error_code ec = shard_file_->Write(io::Buffer(line), file_offset_, 0);
@@ -137,15 +167,15 @@ void JournalSlice::AddLogRecord(const Entry& entry, bool await) {
       file_offset_ += line.size();
     }
 #endif
-  }
 
+  // TODO: Remove the callbacks, replace with notifiers
   {
     std::shared_lock lk(cb_mu_);
     DVLOG(2) << "AddLogRecord: run callbacks for " << entry.ToString()
              << " num callbacks: " << change_cb_arr_.size();
 
     for (const auto& k_v : change_cb_arr_) {
-      k_v.second(entry, await);
+      k_v.second(*item, await);
     }
   }
 }

src/server/journal/journal_slice.h

@@ -45,19 +45,28 @@ class JournalSlice {
 
   void AddLogRecord(const Entry& entry, bool await);
 
+  // Register a callback that will be called every time a new entry is
+  // added to the journal.
+  // The callback receives the entry and a boolean that indicates whether
+  // awaiting (to apply backpressure) is allowed.
   uint32_t RegisterOnChange(ChangeCallback cb);
   void UnregisterOnChange(uint32_t);
+
   bool HasRegisteredCallbacks() const {
     std::shared_lock lk(cb_mu_);
     return !change_cb_arr_.empty();
   }
 
- private:
+  /// Returns whether the journal entry with this LSN is available
-  struct RingItem;
+  /// from the buffer.
+  bool IsLSNInBuffer(LSN lsn) const;
+  std::string_view GetEntry(LSN lsn) const;
 
+ private:
   // std::string shard_path_;
   // std::unique_ptr<LinuxFile> shard_file_;
-  // std::optional<base::RingBuffer<RingItem>> ring_buffer_;
+  std::optional<base::RingBuffer<JournalItem>> ring_buffer_;
+  base::IoBuf ring_serialize_buf_;
 
   mutable util::SharedMutex cb_mu_;
   std::vector<std::pair<uint32_t, ChangeCallback>> change_cb_arr_ ABSL_GUARDED_BY(cb_mu_);

src/server/journal/serializer.cc

@@ -4,8 +4,11 @@
 
 #include "server/journal/serializer.h"
 
+#include <system_error>
+
 #include "base/io_buf.h"
 #include "base/logging.h"
+#include "glog/logging.h"
 #include "io/io.h"
 #include "server/common.h"
 #include "server/error.h"

src/server/journal/streamer.cc

@@ -10,12 +10,12 @@ using namespace util;
 void JournalStreamer::Start(io::Sink* dest) {
   using namespace journal;
   write_fb_ = fb2::Fiber("journal_stream", &JournalStreamer::WriterFb, this, dest);
-  journal_cb_id_ = journal_->RegisterOnChange([this](const Entry& entry, bool allow_await) {
+  journal_cb_id_ = journal_->RegisterOnChange([this](const JournalItem& item, bool allow_await) {
-    if (entry.opcode == Op::NOOP) {
+    if (item.opcode == Op::NOOP) {
-      // No recode to write, just await if data was written so consumer will read the data.
+      // No record to write, just await if data was written so consumer will read the data.
       return AwaitIfWritten();
     }
-    writer_.Write(entry);
+    Write(io::Buffer(item.data));
     NotifyWritten(allow_await);
   });
 }

src/server/journal/streamer.h

@@ -40,7 +40,6 @@ class JournalStreamer : protected BufferedStreamerBase {
   journal::Journal* journal_;
 
   Fiber write_fb_{};
-  JournalWriter writer_{this};
 };
 
 }  // namespace dfly

src/server/journal/types.h

@@ -69,7 +69,13 @@ struct ParsedEntry : public EntryBase {
   std::string ToString() const;
 };
 
-using ChangeCallback = std::function<void(const Entry&, bool await)>;
+struct JournalItem {
+  LSN lsn;
+  Op opcode;
+  std::string data;
+};
+
+using ChangeCallback = std::function<void(const JournalItem&, bool await)>;
 
 }  // namespace journal
 }  // namespace dfly

src/server/rdb_load.cc

@@ -4,6 +4,8 @@
 
 #include "server/rdb_load.h"
 
+#include "absl/strings/escaping.h"
+
 extern "C" {
 
 #include "redis/intset.h"
@@ -2147,8 +2149,18 @@ error_code RdbLoaderBase::HandleJournalBlob(Service* service) {
   while (done < num_entries) {
     journal::ParsedEntry entry{};
     SET_OR_RETURN(journal_reader_.ReadEntry(), entry);
+    done++;
+
+    // EXEC entries are just for preserving atomicity of transactions. We don't create
+    // transactions and we don't care about atomicity when we're loading an RDB, so skip them.
+    // Currently rdb_save also filters those records out, but we filter them additionally here
+    // for better forward compatibility if we decide to change that.
+    if (entry.opcode == journal::Op::EXEC)
+      continue;
+
+    if (entry.cmd.cmd_args.empty())
+      return RdbError(errc::rdb_file_corrupted);
 
-    if (!entry.cmd.cmd_args.empty()) {
     if (absl::EqualsIgnoreCase(facade::ToSV(entry.cmd.cmd_args[0]), "FLUSHALL") ||
         absl::EqualsIgnoreCase(facade::ToSV(entry.cmd.cmd_args[0]), "FLUSHDB")) {
       // Applying a flush* operation in the middle of a load can cause out-of-sync deletions of
@@ -2156,11 +2168,9 @@ error_code RdbLoaderBase::HandleJournalBlob(Service* service) {
       // By returning an error we are effectively restarting the replication.
       return RdbError(errc::unsupported_operation);
     }
-    }
 
+    VLOG(1) << "Executing item: " << entry.ToString();
     ex.Execute(entry.dbid, entry.cmd);
-    VLOG(1) << "Reading item: " << entry.ToString();
-    done++;
   }
 
   return std::error_code{};

src/server/rdb_save.cc

@@ -733,15 +733,10 @@ io::Bytes RdbSerializer::PrepareFlush() {
   return mem_buf_.InputBuffer();
 }
 
-error_code RdbSerializer::WriteJournalEntry(const journal::Entry& entry) {
+error_code RdbSerializer::WriteJournalEntry(std::string_view serialized_entry) {
-  io::BufSink buf_sink{&journal_mem_buf_};
-  JournalWriter writer{&buf_sink};
-  writer.Write(entry);
-
   RETURN_ON_ERR(WriteOpcode(RDB_OPCODE_JOURNAL_BLOB));
   RETURN_ON_ERR(SaveLen(1));
-  RETURN_ON_ERR(SaveString(io::View(journal_mem_buf_.InputBuffer())));
+  RETURN_ON_ERR(SaveString(serialized_entry));
-  journal_mem_buf_.Clear();
   return error_code{};
 }
 

src/server/rdb_save.h

@@ -158,7 +158,7 @@ class RdbSerializer {
   }
 
   // Write journal entry as an embedded journal blob.
-  std::error_code WriteJournalEntry(const journal::Entry& entry);
+  std::error_code WriteJournalEntry(std::string_view entry);
 
   std::error_code SendJournalOffset(uint64_t journal_offset);
 
@@ -188,7 +188,6 @@ class RdbSerializer {
   void AllocateCompressorOnce();
 
   base::IoBuf mem_buf_;
-  base::IoBuf journal_mem_buf_;
   std::string tmp_str_;
   base::PODArray<uint8_t> tmp_buf_;
   DbIndex last_entry_db_index_ = kInvalidDbId;

src/server/snapshot.cc

@@ -260,14 +260,13 @@ void SliceSnapshot::OnDbChange(DbIndex db_index, const DbSlice::ChangeReq& req)
 // transaction.
 // OnJournalEntry registers for changes in journal, the journal change function signature is
 // (const journal::Entry& entry, bool await) In snapshot flow we dont use the await argument.
-void SliceSnapshot::OnJournalEntry(const journal::Entry& entry, bool unused_await_arg) {
+void SliceSnapshot::OnJournalEntry(const journal::JournalItem& item, bool unused_await_arg) {
-  // We ignore non payload entries like EXEC because we have no transactional ordering during
+  // We ignore EXEC and NOOP entries because we they have no meaning during
-  // LOAD phase on replica.
+  // the LOAD phase on replica.
-  if (!entry.HasPayload()) {
+  if (item.opcode == journal::Op::NOOP || item.opcode == journal::Op::EXEC)
     return;
-  }
 
-  serializer_->WriteJournalEntry(entry);
+  serializer_->WriteJournalEntry(item.data);
 
   // This is the only place that flushes in streaming mode
   // once the iterate buckets fiber finished.

src/server/snapshot.h

@@ -92,7 +92,7 @@ class SliceSnapshot {
   void OnDbChange(DbIndex db_index, const DbSlice::ChangeReq& req);
 
   // Journal listener
-  void OnJournalEntry(const journal::Entry& entry, bool unused_await_arg);
+  void OnJournalEntry(const journal::JournalItem& item, bool unused_await_arg);
 
   // Close dest channel if not closed yet.
   void CloseRecordChannel();