Don't split input data in federated mode (#8279)

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

plugin/federated/federated_communicator.h

@@ -170,6 +170,12 @@ class FederatedCommunicator : public Communicator {
    */
   bool IsDistributed() const override { return true; }
 
+  /**
+   * \brief Get if the communicator is federated.
+   * \return True.
+   */
+  bool IsFederated() const override { return true; }
+
   /**
    * \brief Perform in-place allreduce.
    * \param send_receive_buffer Buffer for both sending and receiving data.

src/c_api/c_api.cc

@@ -208,16 +208,12 @@ XGB_DLL int XGBGetGlobalConfig(const char** json_str) {
 XGB_DLL int XGDMatrixCreateFromFile(const char *fname, int silent, DMatrixHandle *out) {
   API_BEGIN();
   bool load_row_split = false;
-#if defined(XGBOOST_USE_FEDERATED)
-  LOG(CONSOLE) << "XGBoost federated mode detected, not splitting data among workers";
-#else
-  if (collective::IsDistributed()) {
-    LOG(CONSOLE) << "XGBoost distributed mode detected, "
-                 << "will split data among workers";
+  if (collective::IsFederated()) {
+    LOG(CONSOLE) << "XGBoost federated mode detected, not splitting data among workers";
+  } else if (collective::IsDistributed()) {
+    LOG(CONSOLE) << "XGBoost distributed mode detected, will split data among workers";
     load_row_split = true;
   }
-#endif
-
   xgboost_CHECK_C_ARG_PTR(fname);
   xgboost_CHECK_C_ARG_PTR(out);
   *out = new std::shared_ptr<DMatrix>(DMatrix::Load(fname, silent != 0, load_row_split));

src/collective/communicator-inl.h

@@ -88,6 +88,13 @@ inline int GetWorldSize() { return Communicator::Get()->GetWorldSize(); }
  */
 inline bool IsDistributed() { return Communicator::Get()->IsDistributed(); }
 
+/*!
+ * \brief Get if the communicator is federated.
+ *
+ * \return True if the communicator is federated.
+ */
+inline bool IsFederated() { return Communicator::Get()->IsFederated(); }
+
 /*!
  * \brief Print the message to the communicator.
  *

src/collective/communicator.h

@@ -78,6 +78,9 @@ class Communicator {
   /** @brief Whether the communicator is running in distributed mode. */
   virtual bool IsDistributed() const = 0;
 
+  /** @brief Whether the communicator is running in federated mode. */
+  virtual bool IsFederated() const = 0;
+
   /**
    * @brief Combines values from all processes and distributes the result back to all processes.
    *

src/collective/noop_communicator.h

@@ -16,6 +16,7 @@ class NoOpCommunicator : public Communicator {
  public:
   NoOpCommunicator() : Communicator(1, 0) {}
   bool IsDistributed() const override { return false; }
+  bool IsFederated() const override { return false; }
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override {}
   void Broadcast(void *send_receive_buffer, std::size_t size, int root) override {}

src/collective/rabit_communicator.h

@@ -53,6 +53,8 @@ class RabitCommunicator : public Communicator {
 
   bool IsDistributed() const override { return rabit::IsDistributed(); }
 
+  bool IsFederated() const override { return false; }
+
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override {
     switch (data_type) {