[breaking] [jvm-packages] Remove rabit check point.

- Add `numBoostedRound` to jvm packages
- Remove rabit checkpoint version. [breaking]
- Change the starting version of training continuation in JVM [breaking].

The last item is a bit more subtle, the change aligns JVM packages with Python
packages. After this PR, the second training phrase counts iteration from 0 instead of
from the previous starting iteration.

include/xgboost/c_api.h

@@ -1308,24 +1308,6 @@ XGB_DLL int XGBoosterSerializeToBuffer(BoosterHandle handle, bst_ulong *out_len,
 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. Currently the
  * support is experimental, function signature may change in the future without

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/ExternalCheckpointManagerSuite.scala

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014-2022 by Contributors
+ Copyright (c) 2014-2023 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -51,13 +51,13 @@ class ExternalCheckpointManagerSuite extends AnyFunSuite with TmpFolderPerSuite
  var files = FileSystem.get(sc.hadoopConfiguration).listStatus(new Path(tmpPath))
  assert(files.length == 1)
  assert(files.head.getPath.getName == "4.model")
- assert(manager.loadCheckpointAsScalaBooster().getVersion == 4)
+ assert(manager.loadCheckpointAsScalaBooster().getNumBoostedRound == 4)
 
  manager.updateCheckpoint(model8._booster)
  files = FileSystem.get(sc.hadoopConfiguration).listStatus(new Path(tmpPath))
  assert(files.length == 1)
  assert(files.head.getPath.getName == "8.model")
- assert(manager.loadCheckpointAsScalaBooster().getVersion == 8)
+ assert(manager.loadCheckpointAsScalaBooster().getNumBoostedRound == 8)
  }
 
  test("test cleanUpHigherVersions") {

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Booster.java

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014-2022 by Contributors
+ Copyright (c) 2014-2023 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -808,14 +808,6 @@ private String[] getDumpInfo(boolean withStats) throws XGBoostError {
  return modelInfos[0];
  }
 
- public int getVersion() {
- return this.version;
- }
-
- public void setVersion(int version) {
- this.version = version;
- }
-
  /**
  * Save model into raw byte array. Currently it's using the deprecated format as
  * default, which will be changed into `ubj` in future releases.
@@ -841,29 +833,6 @@ public byte[] toByteArray(String format) throws XGBoostError {
  return bytes[0];
  }
 
- /**
- * Load the booster model from thread-local rabit checkpoint.
- * This is only used in distributed training.
- * @return the stored version number of the checkpoint.
- * @throws XGBoostError
- */
- int loadRabitCheckpoint() throws XGBoostError {
- int[] out = new int[1];
- XGBoostJNI.checkCall(XGBoostJNI.XGBoosterLoadRabitCheckpoint(this.handle, out));
- version = out[0];
- return version;
- }
-
- /**
- * Save the booster model into thread-local rabit checkpoint and increment the version.
- * This is only used in distributed training.
- * @throws XGBoostError
- */
- void saveRabitCheckpoint() throws XGBoostError {
- XGBoostJNI.checkCall(XGBoostJNI.XGBoosterSaveRabitCheckpoint(this.handle));
- version += 1;
- }
-
  /**
  * Get number of model features.
  * @return the number of features.
@@ -874,6 +843,11 @@ public long getNumFeature() throws XGBoostError {
  XGBoostJNI.checkCall(XGBoostJNI.XGBoosterGetNumFeature(this.handle, numFeature));
  return numFeature[0];
  }
+ public int getNumBoostedRound() throws XGBoostError {
+ int[] numRound = new int[1];
+ XGBoostJNI.checkCall(XGBoostJNI.XGBoosterGetNumBoostedRound(this.handle, numRound));
+ return numRound[0];
+ }
 
  /**
  * Internal initialization function.

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/ExternalCheckpointManager.java

@@ -1,3 +1,18 @@
+/*
+ Copyright (c) 2014-2023 by Contributors
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
 package ml.dmlc.xgboost4j.java;
 
 import java.io.IOException;
@@ -56,7 +71,6 @@ public Booster loadCheckpointAsBooster() throws IOException, XGBoostError {
  InputStream in = fs.open(new Path(checkpointPath));
  logger.info("loaded checkpoint from " + checkpointPath);
  Booster booster = XGBoost.loadModel(in);
- booster.setVersion(latestVersion);
  return booster;
  } else {
  return null;
@@ -66,12 +80,12 @@ public Booster loadCheckpointAsBooster() throws IOException, XGBoostError {
  public void updateCheckpoint(Booster boosterToCheckpoint) throws IOException, XGBoostError {
  List<String> prevModelPaths = getExistingVersions().stream()
  .map(this::getPath).collect(Collectors.toList());
- String eventualPath = getPath(boosterToCheckpoint.getVersion());
+ String eventualPath = getPath(boosterToCheckpoint.getNumBoostedRound());
  String tempPath = eventualPath + "-" + UUID.randomUUID();
  try (OutputStream out = fs.create(new Path(tempPath), true)) {
  boosterToCheckpoint.saveModel(out);
  fs.rename(new Path(tempPath), new Path(eventualPath));
- logger.info("saving checkpoint with version " + boosterToCheckpoint.getVersion());
+ logger.info("saving checkpoint with version " + boosterToCheckpoint.getNumBoostedRound());
  prevModelPaths.stream().forEach(path -> {
  try {
  fs.delete(new Path(path), true);

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoost.java

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014,2021 by Contributors
+ Copyright (c) 2014-2023 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -203,7 +203,6 @@ public static Booster trainAndSaveCheckpoint(
  booster = new Booster(params, allMats);
  booster.setFeatureNames(dtrain.getFeatureNames());
  booster.setFeatureTypes(dtrain.getFeatureTypes());
- booster.loadRabitCheckpoint();
  } else {
  // Start training on an existing booster
  booster.setParams(params);
@@ -217,18 +216,11 @@ public static Booster trainAndSaveCheckpoint(
  boolean max_direction = false;
 
  // begin to train
- for (int iter = booster.getVersion() / 2; iter < numRounds; iter++) {
- if (booster.getVersion() % 2 == 0) {
- if (obj != null) {
- booster.update(dtrain, obj);
- } else {
- booster.update(dtrain, iter);
- }
- saveCheckpoint(booster, iter, checkpointIterations, ecm);
- booster.saveRabitCheckpoint();
- }
+ for (int iter = 0; iter < numRounds; iter++) {
+ booster.update(dtrain, iter);
+ saveCheckpoint(booster, iter, checkpointIterations, ecm);
 
- //evaluation
+ // evaluation
  if (evalMats.length > 0) {
  float[] metricsOut = new float[evalMats.length];
  String evalInfo;
@@ -285,7 +277,6 @@ public static Booster trainAndSaveCheckpoint(
  Communicator.communicatorPrint(evalInfo + '\n');
  }
  }
- booster.saveRabitCheckpoint();
  }
  return booster;
  }

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java

@@ -140,10 +140,11 @@ public final static native int XGBoosterDumpModelExWithFeatures(
  public final static native int XGBoosterGetAttrNames(long handle, String[][] out_strings);
  public final static native int XGBoosterGetAttr(long handle, String key, String[] out_string);
  public final static native int XGBoosterSetAttr(long handle, String key, String value);
- public final static native int XGBoosterLoadRabitCheckpoint(long handle, int[] out_version);
- public final static native int XGBoosterSaveRabitCheckpoint(long handle);
+
  public final static native int XGBoosterGetNumFeature(long handle, long[] feature);
 
+ public final static native int XGBoosterGetNumBoostedRound(long handle, int[] rounds);
+
  // communicator functions
  public final static native int CommunicatorInit(String[] args);
  public final static native int CommunicatorFinalize();

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/Booster.scala

@@ -326,7 +326,7 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
  @throws(classOf[XGBoostError])
  def getNumFeature: Long = booster.getNumFeature
 
- def getVersion: Int = booster.getVersion
+ def getNumBoostedRound: Long = booster.getNumBoostedRound
 
  /**
  * Save model into a raw byte array. Available options are "json", "ubj" and "deprecated".

jvm-packages/xgboost4j/src/native/xgboost4j.cpp

@@ -984,33 +984,6 @@ JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_XGBoosterSetAttr
  return ret;
 }
 
-/*
- * Class: ml_dmlc_xgboost4j_java_XGBoostJNI
- * Method: XGBoosterLoadRabitCheckpoint
- * Signature: (J[I)I
- */
-JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_XGBoosterLoadRabitCheckpoint
- (JNIEnv *jenv , jclass jcls, jlong jhandle, jintArray jout) {
- BoosterHandle handle = (BoosterHandle) jhandle;
- int version;
- int ret = XGBoosterLoadRabitCheckpoint(handle, &version);
- JVM_CHECK_CALL(ret);
- jint jversion = version;
- jenv->SetIntArrayRegion(jout, 0, 1, &jversion);
- return ret;
-}
-
-/*
- * Class: ml_dmlc_xgboost4j_java_XGBoostJNI
- * Method: XGBoosterSaveRabitCheckpoint
- * Signature: (J)I
- */
-JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_XGBoosterSaveRabitCheckpoint
- (JNIEnv *jenv, jclass jcls, jlong jhandle) {
- BoosterHandle handle = (BoosterHandle) jhandle;
- return XGBoosterSaveRabitCheckpoint(handle);
-}
-
 /*
  * Class: ml_dmlc_xgboost4j_java_XGBoostJNI
  * Method: XGBoosterGetNumFeature
@@ -1027,6 +1000,17 @@ JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_XGBoosterGetNumFea
  return ret;
 }
 
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_XGBoosterGetNumBoostedRound(
+ JNIEnv *jenv, jclass, jlong jhandle, jintArray jout) {
+ BoosterHandle handle = (BoosterHandle)jhandle;
+ std::int32_t n_rounds{0};
+ auto ret = XGBoosterBoostedRounds(handle, &n_rounds);
+ JVM_CHECK_CALL(ret);
+ jint jn_rounds = n_rounds;
+ jenv->SetIntArrayRegion(jout, 0, 1, &jn_rounds);
+ return ret;
+}
+
 /*
  * Class: ml_dmlc_xgboost4j_java_XGBoostJNI
  * Method: CommunicatorInit

jvm-packages/xgboost4j/src/test/java/ml/dmlc/xgboost4j/java/BoosterImplTest.java

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014-2022 by Contributors
+ Copyright (c) 2014-2023 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -16,7 +16,6 @@
 package ml.dmlc.xgboost4j.java;
 
 import junit.framework.TestCase;
-import org.junit.Assert;
 import org.junit.Test;
 
 import java.io.ByteArrayInputStream;
@@ -31,7 +30,7 @@
 
 /**
  * test cases for Booster Inplace Predict
- * 
+ *
  * @author hzx and Sovrn
  */
 public class BoosterImplTest {
@@ -845,14 +844,12 @@ public void testTrainFromExistingModel() throws XGBoostError, IOException {
  float tempBoosterError = eval.eval(tempBooster.predict(testMat, true, 0), testMat);
 
  // Save tempBooster to bytestream and load back
- int prevVersion = tempBooster.getVersion();
  ByteArrayInputStream in = new ByteArrayInputStream(tempBooster.toByteArray());
  tempBooster = XGBoost.loadModel(in);
  in.close();
- tempBooster.setVersion(prevVersion);
 
  // Continue training using tempBooster
- round = 4;
+ round = 2;
  Booster booster2 = XGBoost.train(trainMat, paramMap, round, watches, null, null, null, 0, tempBooster);
  float booster2error = eval.eval(booster2.predict(testMat, true, 0), testMat);
  TestCase.assertTrue(booster1error == booster2error);

src/c_api/c_api.cc

@@ -1430,36 +1430,13 @@ XGB_DLL int XGBoosterUnserializeFromBuffer(BoosterHandle handle,
  API_END();
 }
 
-XGB_DLL int XGBoosterLoadRabitCheckpoint(BoosterHandle handle,
- int* version) {
- API_BEGIN();
- CHECK_HANDLE();
- auto* bst = static_cast<Learner*>(handle);
- xgboost_CHECK_C_ARG_PTR(version);
- *version = rabit::LoadCheckPoint();
- if (*version != 0) {
- bst->Configure();
- }
- API_END();
-}
-
-XGB_DLL int XGBoosterSaveRabitCheckpoint(BoosterHandle handle) {
- API_BEGIN();
- CHECK_HANDLE();
- auto *learner = static_cast<Learner *>(handle);
- learner->Configure();
- rabit::CheckPoint();
- API_END();
-}
-
-XGB_DLL int XGBoosterSlice(BoosterHandle handle, int begin_layer,
- int end_layer, int step,
+XGB_DLL int XGBoosterSlice(BoosterHandle handle, int begin_layer, int end_layer, int step,
  BoosterHandle *out) {
  API_BEGIN();
  CHECK_HANDLE();
  xgboost_CHECK_C_ARG_PTR(out);
 
- auto* learner = static_cast<Learner*>(handle);
+ auto *learner = static_cast<Learner *>(handle);
  bool out_of_bound = false;
  auto p_out = learner->Slice(begin_layer, end_layer, step, &out_of_bound);
  if (out_of_bound) {