JSON configuration IO. (#5111)

* Add saving/loading JSON configuration.
* Implement Python pickle interface with new IO routines.
* Basic tests for training continuation.

include/xgboost/c_api.h

@@ -461,9 +461,69 @@ XGB_DLL int XGBoosterLoadModelFromBuffer(BoosterHandle handle,
  * \param out_dptr the argument to hold the output data pointer
  * \return 0 when success, -1 when failure happens
  */
-XGB_DLL int XGBoosterGetModelRaw(BoosterHandle handle,
-                                 bst_ulong *out_len,
+XGB_DLL int XGBoosterGetModelRaw(BoosterHandle handle, bst_ulong *out_len,
                                  const char **out_dptr);
+
+/*!
+ * \brief Memory snapshot based serialization method.  Saves everything states
+ * into buffer.
+ *
+ * \param handle handle
+ * \param out_len the argument to hold the output length
+ * \param out_dptr the argument to hold the output data pointer
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterSerializeToBuffer(BoosterHandle handle, bst_ulong *out_len,
+                                       const char **out_dptr);
+/*!
+ * \brief Memory snapshot based serialization method.  Loads the buffer returned
+ *        from `XGBoosterSerializeToBuffer'.
+ *
+ * \param handle handle
+ * \param buf pointer to the buffer
+ * \param len the length of the buffer
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterUnserializeFromBuffer(BoosterHandle handle,
+                                           const void *buf, bst_ulong len);
+
+/*!
+ * \brief Initialize the booster from rabit checkpoint.
+ *  This is used in distributed training API.
+ * \param handle handle
+ * \param version The output version of the model.
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterLoadRabitCheckpoint(BoosterHandle handle,
+                                         int* version);
+
+/*!
+ * \brief Save the current checkpoint to rabit.
+ * \param handle handle
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterSaveRabitCheckpoint(BoosterHandle handle);
+
+
+/*!
+ * \brief Save XGBoost's internal configuration into a JSON document.
+ * \param handle handle to Booster object.
+ * \param out_str A valid pointer to array of characters.  The characters array is
+ *                allocated and managed by XGBoost, while pointer to that array needs to
+ *                be managed by caller.
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterSaveJsonConfig(BoosterHandle handle, bst_ulong *out_len,
+                                    char const **out_str);
+/*!
+ * \brief Load XGBoost's internal configuration from a JSON document.
+ * \param handle handle to Booster object.
+ * \param json_parameters string representation of a JSON document.
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterLoadJsonConfig(BoosterHandle handle,
+                                    char const *json_parameters);
+
 /*!
  * \brief dump model, return array of strings representing model dump
  * \param handle handle
@@ -570,25 +630,4 @@ XGB_DLL int XGBoosterSetAttr(BoosterHandle handle,
 XGB_DLL int XGBoosterGetAttrNames(BoosterHandle handle,
                                   bst_ulong* out_len,
                                   const char*** out);
-
-// --- Distributed training API----
-// NOTE: functions in rabit/c_api.h will be also available in libxgboost.so
-/*!
- * \brief Initialize the booster from rabit checkpoint.
- *  This is used in distributed training API.
- * \param handle handle
- * \param version The output version of the model.
- * \return 0 when success, -1 when failure happens
- */
-XGB_DLL int XGBoosterLoadRabitCheckpoint(
-    BoosterHandle handle,
-    int* version);
-
-/*!
- * \brief Save the current checkpoint to rabit.
- * \param handle handle
- * \return 0 when success, -1 when failure happens
- */
-XGB_DLL int XGBoosterSaveRabitCheckpoint(BoosterHandle handle);
-
 #endif  // XGBOOST_C_API_H_

include/xgboost/gbm.h

@@ -32,7 +32,7 @@ struct LearnerModelParam;
 /*!
  * \brief interface of gradient boosting model.
  */
-class GradientBooster : public Model {
+class GradientBooster : public Model, public Configurable {
  protected:
   GenericParameter const* generic_param_;
 

include/xgboost/learner.h

@@ -45,7 +45,7 @@ class Json;
  *
  *  \endcode
  */
-class Learner : public Model, public rabit::Serializable {
+class Learner : public Model, public Configurable, public rabit::Serializable {
  public:
   /*! \brief virtual destructor */
   ~Learner() override;
@@ -53,16 +53,6 @@ class Learner : public Model, public rabit::Serializable {
    * \brief Configure Learner based on set parameters.
    */
   virtual void Configure() = 0;
-  /*!
-   * \brief load model from stream
-   * \param fi input stream.
-   */
-  void Load(dmlc::Stream* fi) override = 0;
-  /*!
-   * \brief save model to stream.
-   * \param fo output stream
-   */
-  void Save(dmlc::Stream* fo) const override = 0;
   /*!
    * \brief update the model for one iteration
    *  With the specified objective function.
@@ -110,6 +100,13 @@ class Learner : public Model, public rabit::Serializable {
                        bool pred_contribs = false,
                        bool approx_contribs = false,
                        bool pred_interactions = false) = 0;
+
+  void LoadModel(Json const& in) override = 0;
+  void SaveModel(Json* out) const override = 0;
+
+  virtual void LoadModel(dmlc::Stream* fi) = 0;
+  virtual void SaveModel(dmlc::Stream* fo) const = 0;
+
   /*!
    * \brief Set multiple parameters at once.
    *

include/xgboost/parameter.h

@@ -99,6 +99,7 @@ struct XGBoostParameter : public dmlc::Parameter<Type> {
       return unknown;
     }
   }
+  bool GetInitialised() const { return static_cast<bool>(this->initialised_); }
 };
 }  // namespace xgboost