Common interface for collective communication (#8057)

* implement broadcast for federated communicator

* implement allreduce

* add communicator factory

* add device adapter

* add device communicator to factory

* add rabit communicator

* add rabit communicator to the factory

* add nccl device communicator

* add synchronize to device communicator

* add back print and getprocessorname

* add python wrapper and c api

* clean up types

* fix non-gpu build

* try to fix ci

* fix std::size_t

* portable string compare ignore case

* c style size_t

* fix lint errors

* cross platform setenv

* fix memory leak

* fix lint errors

* address review feedback

* add python test for rabit communicator

* fix failing gtest

* use json to configure communicators

* fix lint error

* get rid of factories

* fix cpu build

* fix include

* fix python import

* don't export collective.py yet

* skip collective communicator pytest on windows

* add review feedback

* update documentation

* remove mpi communicator type

* fix tests

* shutdown the communicator separately

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

src/c_api/c_api.cc

@@ -22,6 +22,7 @@
 
 #include "c_api_error.h"
 #include "c_api_utils.h"
+#include "../collective/communicator.h"
 #include "../common/io.h"
 #include "../common/charconv.h"
 #include "../data/adapter.h"
@@ -1370,6 +1371,62 @@ XGB_DLL int XGBoosterFeatureScore(BoosterHandle handle, char const *json_config,
   API_END();
 }
 
+using xgboost::collective::Communicator;
+
+XGB_DLL int XGCommunicatorInit(char const* json_config) {
+  API_BEGIN();
+  Json config { Json::Load(StringView{json_config}) };
+  Communicator::Init(config);
+  API_END();
+}
+
+XGB_DLL int XGCommunicatorFinalize(void) {
+  API_BEGIN();
+  Communicator::Finalize();
+  API_END();
+}
+
+XGB_DLL int XGCommunicatorGetRank(void) {
+  return Communicator::Get()->GetRank();
+}
+
+XGB_DLL int XGCommunicatorGetWorldSize(void) {
+  return Communicator::Get()->GetWorldSize();
+}
+
+XGB_DLL int XGCommunicatorIsDistributed(void) {
+  return Communicator::Get()->IsDistributed();
+}
+
+XGB_DLL int XGCommunicatorPrint(char const *message) {
+  API_BEGIN();
+  Communicator::Get()->Print(message);
+  API_END();
+}
+
+XGB_DLL int XGCommunicatorGetProcessorName(char const **name_str) {
+  API_BEGIN();
+  auto& local = *GlobalConfigAPIThreadLocalStore::Get();
+  local.ret_str = Communicator::Get()->GetProcessorName();
+  *name_str = local.ret_str.c_str();
+  API_END();
+}
+
+XGB_DLL int XGCommunicatorBroadcast(void *send_receive_buffer, size_t size, int root) {
+  API_BEGIN();
+  Communicator::Get()->Broadcast(send_receive_buffer, size, root);
+  API_END();
+}
+
+XGB_DLL int XGCommunicatorAllreduce(void *send_receive_buffer, size_t count, int enum_dtype,
+                                    int enum_op) {
+  API_BEGIN();
+  Communicator::Get()->AllReduce(
+      send_receive_buffer, count, static_cast<xgboost::collective::DataType>(enum_dtype),
+      static_cast<xgboost::collective::Operation>(enum_op));
+  API_END();
+}
+
 #if defined(XGBOOST_USE_FEDERATED)
 XGB_DLL int XGBRunFederatedServer(int port, int world_size, char const *server_key_path,
                                   char const *server_cert_path, char const *client_cert_path) {

src/collective/communicator.cc

@@ -0,0 +1,59 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#include "communicator.h"
+
+#include "rabit_communicator.h"
+
+#if defined(XGBOOST_USE_FEDERATED)
+#include "../../plugin/federated/federated_communicator.h"
+#endif
+
+namespace xgboost {
+namespace collective {
+
+thread_local std::unique_ptr<Communicator> Communicator::communicator_{};
+thread_local CommunicatorType Communicator::type_{};
+
+void Communicator::Init(Json const& config) {
+  if (communicator_) {
+    LOG(FATAL) << "Communicator can only be initialized once.";
+  }
+
+  auto type = GetTypeFromEnv();
+  auto const arg = GetTypeFromConfig(config);
+  if (arg != CommunicatorType::kUnknown) {
+    type = arg;
+  }
+  if (type == CommunicatorType::kUnknown) {
+    // Default to Rabit if unspecified.
+    type = CommunicatorType::kRabit;
+  }
+  type_ = type;
+  switch (type) {
+    case CommunicatorType::kRabit: {
+      communicator_.reset(RabitCommunicator::Create(config));
+      break;
+    }
+    case CommunicatorType::kFederated: {
+#if defined(XGBOOST_USE_FEDERATED)
+      communicator_.reset(FederatedCommunicator::Create(config));
+#else
+      LOG(FATAL) << "XGBoost is not compiled with Federated Learning support.";
+#endif
+      break;
+    }
+    case CommunicatorType::kUnknown:
+      LOG(FATAL) << "Unknown communicator type.";
+  }
+}
+
+#ifndef XGBOOST_USE_CUDA
+void Communicator::Finalize() {
+  communicator_->Shutdown();
+  communicator_.reset(nullptr);
+}
+#endif
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/communicator.cu

@@ -0,0 +1,41 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#include "communicator.h"
+#include "device_communicator.cuh"
+#include "device_communicator_adapter.cuh"
+#ifdef XGBOOST_USE_NCCL
+#include "nccl_device_communicator.cuh"
+#endif
+
+namespace xgboost {
+namespace collective {
+
+thread_local int Communicator::device_ordinal_{-1};
+thread_local std::unique_ptr<DeviceCommunicator> Communicator::device_communicator_{};
+
+void Communicator::Finalize() {
+  communicator_->Shutdown();
+  communicator_.reset(nullptr);
+  device_ordinal_ = -1;
+  device_communicator_.reset(nullptr);
+}
+
+DeviceCommunicator* Communicator::GetDevice(int device_ordinal) {
+  if (!device_communicator_ || device_ordinal_ != device_ordinal) {
+    device_ordinal_ = device_ordinal;
+#ifdef XGBOOST_USE_NCCL
+    if (type_ != CommunicatorType::kFederated) {
+      device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal, Get()));
+    } else {
+      device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal, Get()));
+    }
+#else
+    device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal, Get()));
+#endif
+  }
+  return device_communicator_.get();
+}
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/communicator.h

@@ -0,0 +1,218 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#pragma once
+#include <xgboost/json.h>
+#include <xgboost/logging.h>
+
+#include <memory>
+#include <string>
+
+namespace xgboost {
+namespace collective {
+
+/** @brief Defines the integral and floating data types. */
+enum class DataType {
+  kInt8 = 0,
+  kUInt8 = 1,
+  kInt32 = 2,
+  kUInt32 = 3,
+  kInt64 = 4,
+  kUInt64 = 5,
+  kFloat = 6,
+  kDouble = 7
+};
+
+/** @brief Get the size of the data type. */
+inline std::size_t GetTypeSize(DataType data_type) {
+  std::size_t size{0};
+  switch (data_type) {
+    case DataType::kInt8:
+      size = sizeof(std::int8_t);
+      break;
+    case DataType::kUInt8:
+      size = sizeof(std::uint8_t);
+      break;
+    case DataType::kInt32:
+      size = sizeof(std::int32_t);
+      break;
+    case DataType::kUInt32:
+      size = sizeof(std::uint32_t);
+      break;
+    case DataType::kInt64:
+      size = sizeof(std::int64_t);
+      break;
+    case DataType::kUInt64:
+      size = sizeof(std::uint64_t);
+      break;
+    case DataType::kFloat:
+      size = sizeof(float);
+      break;
+    case DataType::kDouble:
+      size = sizeof(double);
+      break;
+    default:
+      LOG(FATAL) << "Unknown data type.";
+  }
+  return size;
+}
+
+/** @brief Defines the reduction operation. */
+enum class Operation { kMax = 0, kMin = 1, kSum = 2 };
+
+class DeviceCommunicator;
+
+enum class CommunicatorType { kUnknown, kRabit, kFederated };
+
+/** \brief Case-insensitive string comparison. */
+inline int CompareStringsCaseInsensitive(const char *s1, const char *s2) {
+#ifdef _MSC_VER
+  return _stricmp(s1, s2);
+#else   // _MSC_VER
+  return strcasecmp(s1, s2);
+#endif  // _MSC_VER
+}
+
+/**
+ * @brief A communicator class that handles collective communication.
+ */
+class Communicator {
+ public:
+  /**
+   * @brief Initialize the communicator. This can only be done once.
+   *
+   * @param config JSON configuration for the communicator.
+   */
+  static void Init(Json const &config);
+
+  /** @brief Finalize the communicator. */
+  static void Finalize();
+
+  /** @brief Get the communicator instance. */
+  static Communicator *Get() { return communicator_.get(); }
+
+#if defined(XGBOOST_USE_CUDA)
+  /**
+   * @brief Get the device communicator.
+   *
+   * @param device_ordinal ID of the device.
+   * @return An instance of device communicator.
+   */
+  static DeviceCommunicator *GetDevice(int device_ordinal);
+#endif
+
+  virtual ~Communicator() = default;
+
+  /** @brief Get the total number of processes. */
+  int GetWorldSize() const { return world_size_; }
+
+  /** @brief Get the rank of the current processes. */
+  int GetRank() const { return rank_; }
+
+  /** @brief Whether the communicator is running in distributed mode. */
+  virtual bool IsDistributed() const = 0;
+
+  /**
+   * @brief Combines values from all processes and distributes the result back to all processes.
+   *
+   * @param send_receive_buffer Buffer storing the data.
+   * @param count               Number of elements in the buffer.
+   * @param data_type           Data type stored in the buffer.
+   * @param op                  The operation to perform.
+   */
+  virtual void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
+                         Operation op) = 0;
+
+  /**
+   * @brief Broadcasts a message from the process with rank `root` to all other processes of the
+   * group.
+   *
+   * @param send_receive_buffer Buffer storing the data.
+   * @param size                Size of the data in bytes.
+   * @param root                Rank of broadcast root.
+   */
+  virtual void Broadcast(void *send_receive_buffer, std::size_t size, int root) = 0;
+
+  /**
+   * @brief Gets the name of the processor.
+   */
+  virtual std::string GetProcessorName() = 0;
+
+  /**
+   * @brief Prints the message.
+   */
+  virtual void Print(std::string const &message) = 0;
+
+  /** @brief Get the communicator type from environment variables. Visible for testing. */
+  static CommunicatorType GetTypeFromEnv() {
+    auto *env = std::getenv("XGBOOST_COMMUNICATOR");
+    if (env != nullptr) {
+      return StringToType(env);
+    } else {
+      return CommunicatorType::kUnknown;
+    }
+  }
+
+  /** @brief Get the communicator type from runtime configuration. Visible for testing. */
+  static CommunicatorType GetTypeFromConfig(Json const &config) {
+    auto const &j_upper = config["XGBOOST_COMMUNICATOR"];
+    if (IsA<String const>(j_upper)) {
+      return StringToType(get<String const>(j_upper).c_str());
+    }
+    auto const &j_lower = config["xgboost_communicator"];
+    if (IsA<String const>(j_lower)) {
+      return StringToType(get<String const>(j_lower).c_str());
+    }
+    return CommunicatorType::kUnknown;
+  }
+
+ protected:
+  /**
+   * @brief Construct a new communicator.
+   *
+   * @param world_size Total number of processes.
+   * @param rank       Rank of the current process.
+   */
+  Communicator(int world_size, int rank) : world_size_(world_size), rank_(rank) {
+    if (world_size < 1) {
+      LOG(FATAL) << "World size " << world_size << " is less than 1.";
+    }
+    if (rank < 0) {
+      LOG(FATAL) << "Rank " << rank << " is less than 0.";
+    }
+    if (rank >= world_size) {
+      LOG(FATAL) << "Rank " << rank << " is greater than world_size - 1: " << world_size - 1 << ".";
+    }
+  }
+
+  /**
+   * @brief Shuts down the communicator.
+   */
+  virtual void Shutdown() = 0;
+
+ private:
+  static CommunicatorType StringToType(char const *str) {
+    CommunicatorType result = CommunicatorType::kUnknown;
+    if (!CompareStringsCaseInsensitive("rabit", str)) {
+      result = CommunicatorType::kRabit;
+    } else if (!CompareStringsCaseInsensitive("federated", str)) {
+      result = CommunicatorType::kFederated;
+    } else {
+      LOG(FATAL) << "Unknown communicator type " << str;
+    }
+    return result;
+  }
+
+  static thread_local std::unique_ptr<Communicator> communicator_;
+  static thread_local CommunicatorType type_;
+#if defined(XGBOOST_USE_CUDA)
+  static thread_local int device_ordinal_;
+  static thread_local std::unique_ptr<DeviceCommunicator> device_communicator_;
+#endif
+
+  int const world_size_;
+  int const rank_;
+};
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/device_communicator.cuh

@@ -0,0 +1,42 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#pragma once
+#include <vector>
+
+#include "../common/device_helpers.cuh"
+
+namespace xgboost {
+namespace collective {
+
+/**
+ * @brief Collective communicator for device buffers.
+ */
+class DeviceCommunicator {
+ public:
+  virtual ~DeviceCommunicator() = default;
+
+  /**
+   * @brief Sum values from all processes and distribute the result back to all processes.
+   * @param send_receive_buffer Buffer storing the data.
+   * @param count               Number of elements in the buffer.
+   */
+  virtual void AllReduceSum(double *send_receive_buffer, int count) = 0;
+
+  /**
+   * @brief Gather variable-length values from all processes.
+   * @param send_buffer Buffer storing the input data.
+   * @param length_bytes Length in bytes of the input data.
+   * @param segments Size of each segment.
+   * @param receive_buffer Buffer storing the output data.
+   */
+  virtual void AllGatherV(void const *send_buffer, size_t length_bytes,
+                          std::vector<size_t> *segments,
+                          dh::caching_device_vector<char> *receive_buffer) = 0;
+
+  /** @brief Synchronize device operations. */
+  virtual void Synchronize() = 0;
+};
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/device_communicator_adapter.cuh

@@ -0,0 +1,76 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#pragma once
+
+#include "communicator.h"
+#include "device_communicator.cuh"
+
+namespace xgboost {
+namespace collective {
+
+class DeviceCommunicatorAdapter : public DeviceCommunicator {
+ public:
+  DeviceCommunicatorAdapter(int device_ordinal, Communicator *communicator)
+      : device_ordinal_{device_ordinal}, communicator_{communicator} {
+    if (device_ordinal_ < 0) {
+      LOG(FATAL) << "Invalid device ordinal: " << device_ordinal_;
+    }
+    if (communicator_ == nullptr) {
+      LOG(FATAL) << "Communicator cannot be null.";
+    }
+  }
+
+  ~DeviceCommunicatorAdapter() override = default;
+
+  void AllReduceSum(double *send_receive_buffer, int count) override {
+    dh::safe_cuda(cudaSetDevice(device_ordinal_));
+    auto size = count * sizeof(double);
+    host_buffer_.reserve(size);
+    dh::safe_cuda(cudaMemcpy(host_buffer_.data(), send_receive_buffer, size, cudaMemcpyDefault));
+    communicator_->AllReduce(host_buffer_.data(), count, DataType::kDouble, Operation::kSum);
+    dh::safe_cuda(cudaMemcpy(send_receive_buffer, host_buffer_.data(), size, cudaMemcpyDefault));
+  }
+
+  void AllGatherV(void const *send_buffer, size_t length_bytes, std::vector<std::size_t> *segments,
+                  dh::caching_device_vector<char> *receive_buffer) override {
+    dh::safe_cuda(cudaSetDevice(device_ordinal_));
+    int const world_size = communicator_->GetWorldSize();
+    int const rank = communicator_->GetRank();
+
+    segments->clear();
+    segments->resize(world_size, 0);
+    segments->at(rank) = length_bytes;
+    communicator_->AllReduce(segments->data(), segments->size(), DataType::kUInt64,
+                             Operation::kMax);
+    auto total_bytes = std::accumulate(segments->cbegin(), segments->cend(), 0UL);
+    receive_buffer->resize(total_bytes);
+
+    host_buffer_.reserve(total_bytes);
+    size_t offset = 0;
+    for (int32_t i = 0; i < world_size; ++i) {
+      size_t as_bytes = segments->at(i);
+      if (i == rank) {
+        dh::safe_cuda(cudaMemcpy(host_buffer_.data() + offset, send_buffer, segments->at(rank),
+                                 cudaMemcpyDefault));
+      }
+      communicator_->Broadcast(host_buffer_.data() + offset, as_bytes, i);
+      offset += as_bytes;
+    }
+    dh::safe_cuda(cudaMemcpy(receive_buffer->data().get(), host_buffer_.data(), total_bytes,
+                             cudaMemcpyDefault));
+  }
+
+  void Synchronize() override {
+    // Noop.
+  }
+
+ private:
+  int const device_ordinal_;
+  Communicator *communicator_;
+  /// Host buffer used to call communicator functions.
+  std::vector<char> host_buffer_{};
+};
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/nccl_device_communicator.cuh

@@ -0,0 +1,149 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#pragma once
+
+#include "../common/device_helpers.cuh"
+#include "communicator.h"
+#include "device_communicator.cuh"
+
+namespace xgboost {
+namespace collective {
+
+class NcclDeviceCommunicator : public DeviceCommunicator {
+ public:
+  NcclDeviceCommunicator(int device_ordinal, Communicator *communicator)
+      : device_ordinal_{device_ordinal}, communicator_{communicator} {
+    if (device_ordinal_ < 0) {
+      LOG(FATAL) << "Invalid device ordinal: " << device_ordinal_;
+    }
+    if (communicator_ == nullptr) {
+      LOG(FATAL) << "Communicator cannot be null.";
+    }
+
+    int32_t const rank = communicator_->GetRank();
+    int32_t const world = communicator_->GetWorldSize();
+
+    std::vector<uint64_t> uuids(world * kUuidLength, 0);
+    auto s_uuid = xgboost::common::Span<uint64_t>{uuids.data(), uuids.size()};
+    auto s_this_uuid = s_uuid.subspan(rank * kUuidLength, kUuidLength);
+    GetCudaUUID(s_this_uuid);
+
+    // TODO(rongou): replace this with allgather.
+    communicator_->AllReduce(uuids.data(), uuids.size(), DataType::kUInt64, Operation::kSum);
+
+    std::vector<xgboost::common::Span<uint64_t, kUuidLength>> converted(world);
+    size_t j = 0;
+    for (size_t i = 0; i < uuids.size(); i += kUuidLength) {
+      converted[j] = xgboost::common::Span<uint64_t, kUuidLength>{uuids.data() + i, kUuidLength};
+      j++;
+    }
+
+    auto iter = std::unique(converted.begin(), converted.end());
+    auto n_uniques = std::distance(converted.begin(), iter);
+
+    CHECK_EQ(n_uniques, world)
+        << "Multiple processes within communication group running on same CUDA "
+        << "device is not supported. " << PrintUUID(s_this_uuid) << "\n";
+
+    nccl_unique_id_ = GetUniqueId();
+    dh::safe_nccl(ncclCommInitRank(&nccl_comm_, world, nccl_unique_id_, rank));
+    dh::safe_cuda(cudaStreamCreate(&cuda_stream_));
+  }
+
+  ~NcclDeviceCommunicator() override {
+    dh::safe_cuda(cudaStreamDestroy(cuda_stream_));
+    ncclCommDestroy(nccl_comm_);
+    if (xgboost::ConsoleLogger::ShouldLog(xgboost::ConsoleLogger::LV::kDebug)) {
+      LOG(CONSOLE) << "======== NCCL Statistics========";
+      LOG(CONSOLE) << "AllReduce calls: " << allreduce_calls_;
+      LOG(CONSOLE) << "AllReduce total MiB communicated: " << allreduce_bytes_ / 1048576;
+    }
+  }
+
+  void AllReduceSum(double *send_receive_buffer, int count) override {
+    dh::safe_cuda(cudaSetDevice(device_ordinal_));
+    dh::safe_nccl(ncclAllReduce(send_receive_buffer, send_receive_buffer, count, ncclDouble,
+                                ncclSum, nccl_comm_, cuda_stream_));
+    allreduce_bytes_ += count * sizeof(double);
+    allreduce_calls_ += 1;
+  }
+
+  void AllGatherV(void const *send_buffer, size_t length_bytes, std::vector<std::size_t> *segments,
+                  dh::caching_device_vector<char> *receive_buffer) override {
+    dh::safe_cuda(cudaSetDevice(device_ordinal_));
+    int const world_size = communicator_->GetWorldSize();
+    int const rank = communicator_->GetRank();
+
+    segments->clear();
+    segments->resize(world_size, 0);
+    segments->at(rank) = length_bytes;
+    communicator_->AllReduce(segments->data(), segments->size(), DataType::kUInt64,
+                             Operation::kMax);
+    auto total_bytes = std::accumulate(segments->cbegin(), segments->cend(), 0UL);
+    receive_buffer->resize(total_bytes);
+
+    size_t offset = 0;
+    dh::safe_nccl(ncclGroupStart());
+    for (int32_t i = 0; i < world_size; ++i) {
+      size_t as_bytes = segments->at(i);
+      dh::safe_nccl(ncclBroadcast(send_buffer, receive_buffer->data().get() + offset, as_bytes,
+                                  ncclChar, i, nccl_comm_, cuda_stream_));
+      offset += as_bytes;
+    }
+    dh::safe_nccl(ncclGroupEnd());
+  }
+
+  void Synchronize() override {
+    dh::safe_cuda(cudaSetDevice(device_ordinal_));
+    dh::safe_cuda(cudaStreamSynchronize(cuda_stream_));
+  }
+
+ private:
+  static constexpr std::size_t kUuidLength =
+      sizeof(std::declval<cudaDeviceProp>().uuid) / sizeof(uint64_t);
+
+  void GetCudaUUID(xgboost::common::Span<uint64_t, kUuidLength> const &uuid) const {
+    cudaDeviceProp prob{};
+    dh::safe_cuda(cudaGetDeviceProperties(&prob, device_ordinal_));
+    std::memcpy(uuid.data(), static_cast<void *>(&(prob.uuid)), sizeof(prob.uuid));
+  }
+
+  static std::string PrintUUID(xgboost::common::Span<uint64_t, kUuidLength> const &uuid) {
+    std::stringstream ss;
+    for (auto v : uuid) {
+      ss << std::hex << v;
+    }
+    return ss.str();
+  }
+
+  /**
+   * \fn  ncclUniqueId GetUniqueId()
+   *
+   * \brief Gets the Unique ID from NCCL to be used in setting up interprocess
+   * communication
+   *
+   * \return the Unique ID
+   */
+  ncclUniqueId GetUniqueId() {
+    static const int kRootRank = 0;
+    ncclUniqueId id;
+    if (communicator_->GetRank() == kRootRank) {
+      dh::safe_nccl(ncclGetUniqueId(&id));
+    }
+    communicator_->Broadcast(static_cast<void *>(&id), sizeof(ncclUniqueId),
+                             static_cast<int>(kRootRank));
+    return id;
+  }
+
+  int const device_ordinal_;
+  Communicator *communicator_;
+  ncclComm_t nccl_comm_{};
+  cudaStream_t cuda_stream_{};
+  ncclUniqueId nccl_unique_id_{};
+  size_t allreduce_bytes_{0};  // Keep statistics of the number of bytes communicated.
+  size_t allreduce_calls_{0};  // Keep statistics of the number of reduce calls.
+};
+
+}  // namespace collective
+}  // namespace xgboost

src/collective/rabit_communicator.h

@@ -0,0 +1,120 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#pragma once
+#include <rabit/rabit.h>
+
+#include <string>
+#include <vector>
+
+#include "communicator.h"
+#include "xgboost/json.h"
+
+namespace xgboost {
+namespace collective {
+
+class RabitCommunicator : public Communicator {
+ public:
+  static Communicator *Create(Json const &config) {
+    std::vector<std::string> args_str;
+    for (auto &items : get<Object const>(config)) {
+      switch (items.second.GetValue().Type()) {
+        case xgboost::Value::ValueKind::kString: {
+          args_str.push_back(items.first + "=" + get<String const>(items.second));
+          break;
+        }
+        case xgboost::Value::ValueKind::kInteger: {
+          args_str.push_back(items.first + "=" + std::to_string(get<Integer const>(items.second)));
+          break;
+        }
+        case xgboost::Value::ValueKind::kBoolean: {
+          if (get<Boolean const>(items.second)) {
+            args_str.push_back(items.first + "=1");
+          } else {
+            args_str.push_back(items.first + "=0");
+          }
+          break;
+        }
+        default:
+          break;
+      }
+    }
+    std::vector<char *> args;
+    for (auto &key_value : args_str) {
+      args.push_back(&key_value[0]);
+    }
+    if (!rabit::Init(static_cast<int>(args.size()), &args[0])) {
+      LOG(FATAL) << "Failed to initialize Rabit";
+    }
+    return new RabitCommunicator(rabit::GetWorldSize(), rabit::GetRank());
+  }
+
+  RabitCommunicator(int world_size, int rank) : Communicator(world_size, rank) {}
+
+  bool IsDistributed() const override { return rabit::IsDistributed(); }
+
+  void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
+                 Operation op) override {
+    switch (data_type) {
+      case DataType::kInt8:
+        DoAllReduce<char>(send_receive_buffer, count, op);
+        break;
+      case DataType::kUInt8:
+        DoAllReduce<unsigned char>(send_receive_buffer, count, op);
+        break;
+      case DataType::kInt32:
+        DoAllReduce<std::int32_t>(send_receive_buffer, count, op);
+        break;
+      case DataType::kUInt32:
+        DoAllReduce<std::uint32_t>(send_receive_buffer, count, op);
+        break;
+      case DataType::kInt64:
+        DoAllReduce<std::int64_t>(send_receive_buffer, count, op);
+        break;
+      case DataType::kUInt64:
+        DoAllReduce<std::uint64_t>(send_receive_buffer, count, op);
+        break;
+      case DataType::kFloat:
+        DoAllReduce<float>(send_receive_buffer, count, op);
+        break;
+      case DataType::kDouble:
+        DoAllReduce<double>(send_receive_buffer, count, op);
+        break;
+      default:
+        LOG(FATAL) << "Unknown data type";
+    }
+  }
+
+  void Broadcast(void *send_receive_buffer, std::size_t size, int root) override {
+    rabit::Broadcast(send_receive_buffer, size, root);
+  }
+
+  std::string GetProcessorName() override { return rabit::GetProcessorName(); }
+
+  void Print(const std::string &message) override { rabit::TrackerPrint(message); }
+
+ protected:
+  void Shutdown() override {
+    rabit::Finalize();
+  }
+
+ private:
+  template <typename DType>
+  void DoAllReduce(void *send_receive_buffer, std::size_t count, Operation op) {
+    switch (op) {
+      case Operation::kMax:
+        rabit::Allreduce<rabit::op::Max, DType>(static_cast<DType *>(send_receive_buffer), count);
+        break;
+      case Operation::kMin:
+        rabit::Allreduce<rabit::op::Min, DType>(static_cast<DType *>(send_receive_buffer), count);
+        break;
+      case Operation::kSum:
+        rabit::Allreduce<rabit::op::Sum, DType>(static_cast<DType *>(send_receive_buffer), count);
+        break;
+      default:
+        LOG(FATAL) << "Unknown allreduce operation";
+    }
+  }
+};
+}  // namespace collective
+}  // namespace xgboost

src/common/io.h

@@ -12,6 +12,7 @@
 #include <rabit/rabit.h>
 #include <string>
 #include <cstring>
+#include <fstream>
 
 #include "common.h"
 
@@ -111,6 +112,22 @@ inline std::string ReadAll(dmlc::Stream* fi, PeekableInStream* fp) {
   }
   return buffer;
 }
+
+/**
+ * \brief Read the whole file content into a string.
+ */
+inline std::string ReadAll(std::string const &path) {
+  std::ifstream stream(path);
+  if (!stream.is_open()) {
+    LOG(FATAL) << "Could not open file " << path;
+  }
+  std::string content{std::istreambuf_iterator<char>(stream), std::istreambuf_iterator<char>()};
+  if (content.empty()) {
+    LOG(FATAL) << "Empty file " << path;
+  }
+  return content;
+}
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_IO_H_