[ML] Data Frame HLRC Preview API (#40258)

client/rest-high-level/src/main/java/org/elasticsearch/client/DataFrameClient.java

@@ -22,6 +22,8 @@ package org.elasticsearch.client;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.client.core.AcknowledgedResponse;
 import org.elasticsearch.client.dataframe.DeleteDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformResponse;
 import org.elasticsearch.client.dataframe.PutDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformResponse;
@@ -120,6 +122,45 @@ public final class DataFrameClient {
                 Collections.emptySet());
     }
 
+    /**
+     * Preview the result of a data frame transform
+     * <p>
+     * For additional info
+     * see <a href="https://www.TODO.com">Preview Data Frame transform documentation</a>
+     *
+     * @param request The preview data frame transform request
+     * @param options Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
+     * @return A response containing the results of the applied transform
+     * @throws IOException when there is a serialization issue sending the request or receiving the response
+     */
+    public PreviewDataFrameTransformResponse previewDataFrameTransform(PreviewDataFrameTransformRequest request, RequestOptions options)
+            throws IOException {
+        return restHighLevelClient.performRequestAndParseEntity(request,
+                DataFrameRequestConverters::previewDataFrameTransform,
+                options,
+                PreviewDataFrameTransformResponse::fromXContent,
+                Collections.emptySet());
+    }
+
+    /**
+     * Preview the result of a data frame transform asynchronously and notifies listener on completion
+     * <p>
+     * For additional info
+     * see <a href="https://www.TODO.com">Preview Data Frame transform documentation</a>
+     *
+     * @param request The preview data frame transform request
+     * @param options Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
+     * @param listener Listener to be notified upon request completion
+     */
+    public void previewDataFrameTransformAsync(PreviewDataFrameTransformRequest request, RequestOptions options,
+                                             ActionListener<PreviewDataFrameTransformResponse> listener) {
+        restHighLevelClient.performRequestAsyncAndParseEntity(request,
+                DataFrameRequestConverters::previewDataFrameTransform,
+                options,
+                PreviewDataFrameTransformResponse::fromXContent,
+                listener,
+                Collections.emptySet());
+    }
 
     /**
      * Start a data frame transform

client/rest-high-level/src/main/java/org/elasticsearch/client/DataFrameRequestConverters.java

@@ -23,6 +23,7 @@ import org.apache.http.client.methods.HttpDelete;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.client.methods.HttpPut;
 import org.elasticsearch.client.dataframe.DeleteDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.PutDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StopDataFrameTransformRequest;
@@ -84,4 +85,13 @@ final class DataFrameRequestConverters {
             }
             return request;
     }
+
+    static Request previewDataFrameTransform(PreviewDataFrameTransformRequest previewRequest) throws IOException {
+        String endpoint = new RequestConverters.EndpointBuilder()
+                .addPathPartAsIs("_data_frame", "transforms", "_preview")
+                .build();
+        Request request = new Request(HttpPost.METHOD_NAME, endpoint);
+        request.setEntity(createEntity(previewRequest, REQUEST_BODY_CONTENT_TYPE));
+        return request;
+    }
 }

client/rest-high-level/src/main/java/org/elasticsearch/client/dataframe/PreviewDataFrameTransformRequest.java

@@ -0,0 +1,85 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.client.dataframe;
+
+import org.elasticsearch.client.Validatable;
+import org.elasticsearch.client.ValidationException;
+import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfig;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.ToXContentObject;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+
+import java.io.IOException;
+import java.util.Objects;
+import java.util.Optional;
+
+public class PreviewDataFrameTransformRequest implements ToXContentObject, Validatable {
+
+    private final DataFrameTransformConfig config;
+
+    public PreviewDataFrameTransformRequest(DataFrameTransformConfig config) {
+        this.config = config;
+    }
+
+    public DataFrameTransformConfig getConfig() {
+        return config;
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, ToXContent.Params params) throws IOException {
+        return config.toXContent(builder, params);
+    }
+
+    @Override
+    public Optional<ValidationException> validate() {
+        ValidationException validationException = new ValidationException();
+        if (config == null) {
+            validationException.addValidationError("preview requires a non-null data frame config");
+            return Optional.of(validationException);
+        } else {
+            if (config.getSource() == null) {
+                validationException.addValidationError("data frame transform source cannot be null");
+            }
+        }
+
+        if (validationException.validationErrors().isEmpty()) {
+            return Optional.empty();
+        } else {
+            return Optional.of(validationException);
+        }
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(config);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == null) {
+            return false;
+        }
+        if (getClass() != obj.getClass()) {
+            return false;
+        }
+        PreviewDataFrameTransformRequest other = (PreviewDataFrameTransformRequest) obj;
+        return Objects.equals(config, other.config);
+    }
+}

client/rest-high-level/src/main/java/org/elasticsearch/client/dataframe/PreviewDataFrameTransformResponse.java

@@ -0,0 +1,68 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.client.dataframe;
+
+import org.elasticsearch.common.xcontent.XContentParser;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+
+public class PreviewDataFrameTransformResponse {
+
+    private static final String PREVIEW = "preview";
+
+    @SuppressWarnings("unchecked")
+    public static PreviewDataFrameTransformResponse fromXContent(final XContentParser parser) throws IOException {
+        Object previewDocs = parser.map().get(PREVIEW);
+        return new PreviewDataFrameTransformResponse((List<Map<String, Object>>) previewDocs);
+    }
+
+    private List<Map<String, Object>> docs;
+
+    public PreviewDataFrameTransformResponse(List<Map<String, Object>> docs) {
+        this.docs = docs;
+    }
+
+    public List<Map<String, Object>> getDocs() {
+        return docs;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == this) {
+            return true;
+        }
+
+        if (obj == null || obj.getClass() != getClass()) {
+            return false;
+        }
+
+        PreviewDataFrameTransformResponse other = (PreviewDataFrameTransformResponse) obj;
+        return Objects.equals(other.docs, docs);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hashCode(docs);
+    }
+
+}

client/rest-high-level/src/main/java/org/elasticsearch/client/dataframe/PutDataFrameTransformRequest.java

@@ -20,12 +20,14 @@
 package org.elasticsearch.client.dataframe;
 
 import org.elasticsearch.client.Validatable;
+import org.elasticsearch.client.ValidationException;
 import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfig;
 import org.elasticsearch.common.xcontent.ToXContentObject;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 
 import java.io.IOException;
 import java.util.Objects;
+import java.util.Optional;
 
 public class PutDataFrameTransformRequest implements ToXContentObject, Validatable {
 
@@ -39,6 +41,31 @@ public class PutDataFrameTransformRequest implements ToXContentObject, Validatab
         return config;
     }
 
+    @Override
+    public Optional<ValidationException> validate() {
+        ValidationException validationException = new ValidationException();
+        if (config == null) {
+            validationException.addValidationError("put requires a non-null data frame config");
+            return Optional.of(validationException);
+        } else {
+            if (config.getId() == null) {
+                validationException.addValidationError("data frame transform id cannot be null");
+            }
+            if (config.getSource() == null) {
+                validationException.addValidationError("data frame transform source cannot be null");
+            }
+            if (config.getDestination() == null) {
+                validationException.addValidationError("data frame transform destination cannot be null");
+            }
+        }
+
+        if (validationException.validationErrors().isEmpty()) {
+            return Optional.empty();
+        } else {
+            return Optional.of(validationException);
+        }
+    }
+
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         return config.toXContent(builder, params);

client/rest-high-level/src/main/java/org/elasticsearch/client/dataframe/transforms/DataFrameTransformConfig.java

@@ -77,9 +77,9 @@ public class DataFrameTransformConfig implements ToXContentObject {
                                     final String dest,
                                     final QueryConfig queryConfig,
                                     final PivotConfig pivotConfig) {
-        this.id = Objects.requireNonNull(id);
+        this.id = id;
-        this.source = Objects.requireNonNull(source);
+        this.source = source;
-        this.dest = Objects.requireNonNull(dest);
+        this.dest = dest;
         this.queryConfig = queryConfig;
         this.pivotConfig = pivotConfig;
     }
@@ -104,24 +104,16 @@ public class DataFrameTransformConfig implements ToXContentObject {
         return queryConfig;
     }
 
-    public boolean isValid() {
-        if (queryConfig != null && queryConfig.isValid() == false) {
-            return false;
-        }
-
-        if (pivotConfig == null || pivotConfig.isValid() == false) {
-            return false;
-        }
-
-        return true;
-    }
-
     @Override
     public XContentBuilder toXContent(final XContentBuilder builder, final Params params) throws IOException {
         builder.startObject();
-        builder.field(ID.getPreferredName(), id);
+        if (id != null) {
+            builder.field(ID.getPreferredName(), id);
+        }
         builder.field(SOURCE.getPreferredName(), source);
-        builder.field(DEST.getPreferredName(), dest);
+        if (dest != null) {
+            builder.field(DEST.getPreferredName(), dest);
+        }
         if (queryConfig != null) {
             builder.field(QUERY.getPreferredName(), queryConfig);
         }

client/rest-high-level/src/test/java/org/elasticsearch/client/DataFrameRequestConvertersTests.java

@@ -23,6 +23,7 @@ import org.apache.http.client.methods.HttpDelete;
 import org.apache.http.client.methods.HttpPost;
 import org.apache.http.client.methods.HttpPut;
 import org.elasticsearch.client.dataframe.DeleteDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.PutDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StopDataFrameTransformRequest;
@@ -122,4 +123,18 @@ public class DataFrameRequestConvertersTests extends ESTestCase {
             assertFalse(request.getParameters().containsKey("timeout"));
         }
     }
+
+    public void testPreviewDataFrameTransform() throws IOException {
+        PreviewDataFrameTransformRequest previewRequest = new PreviewDataFrameTransformRequest(
+                DataFrameTransformConfigTests.randomDataFrameTransformConfig());
+        Request request = DataFrameRequestConverters.previewDataFrameTransform(previewRequest);
+
+        assertEquals(HttpPost.METHOD_NAME, request.getMethod());
+        assertThat(request.getEndpoint(), equalTo("/_data_frame/transforms/_preview"));
+
+        try (XContentParser parser = createParser(JsonXContent.jsonXContent, request.getEntity().getContent())) {
+            DataFrameTransformConfig parsedConfig = DataFrameTransformConfig.PARSER.apply(parser, null);
+            assertThat(parsedConfig, equalTo(previewRequest.getConfig()));
+        }
+    }
 }

client/rest-high-level/src/test/java/org/elasticsearch/client/DataFrameTransformIT.java

@@ -20,8 +20,14 @@
 package org.elasticsearch.client;
 
 import org.elasticsearch.ElasticsearchStatusException;
+import org.elasticsearch.action.bulk.BulkRequest;
+import org.elasticsearch.action.bulk.BulkResponse;
+import org.elasticsearch.action.index.IndexRequest;
+import org.elasticsearch.action.support.WriteRequest;
 import org.elasticsearch.client.core.AcknowledgedResponse;
 import org.elasticsearch.client.dataframe.DeleteDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformResponse;
 import org.elasticsearch.client.dataframe.PutDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformResponse;
@@ -36,19 +42,29 @@ import org.elasticsearch.client.dataframe.transforms.pivot.TermsGroupSource;
 import org.elasticsearch.client.indices.CreateIndexRequest;
 import org.elasticsearch.client.indices.CreateIndexResponse;
 import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.index.query.MatchAllQueryBuilder;
 import org.elasticsearch.search.aggregations.AggregationBuilders;
 import org.elasticsearch.search.aggregations.AggregatorFactories;
+import org.junit.After;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
 
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.hasSize;
 
 public class DataFrameTransformIT extends ESRestHighLevelClientTestCase {
 
+    private List<String> transformsToClean = new ArrayList<>();
+
     private void createIndex(String indexName) throws IOException {
 
         XContentBuilder builder = jsonBuilder();
@@ -72,6 +88,58 @@ public class DataFrameTransformIT extends ESRestHighLevelClientTestCase {
         assertTrue(response.isAcknowledged());
     }
 
+    private void indexData(String indexName) throws IOException {
+        BulkRequest request = new BulkRequest();
+        {
+            Map<String, Object> doc = new HashMap<>();
+            doc.put("timestamp", "2019-03-10T12:00:00+00");
+            doc.put("user_id", "theresa");
+            doc.put("stars", 2);
+            request.add(new IndexRequest(indexName).source(doc, XContentType.JSON));
+
+            doc = new HashMap<>();
+            doc.put("timestamp", "2019-03-10T18:00:00+00");
+            doc.put("user_id", "theresa");
+            doc.put("stars", 3);
+            request.add(new IndexRequest(indexName).source(doc, XContentType.JSON));
+
+            doc = new HashMap<>();
+            doc.put("timestamp", "2019-03-10T12:00:00+00");
+            doc.put("user_id", "michel");
+            doc.put("stars", 5);
+            request.add(new IndexRequest(indexName).source(doc, XContentType.JSON));
+
+            doc = new HashMap<>();
+            doc.put("timestamp", "2019-03-10T18:00:00+00");
+            doc.put("user_id", "michel");
+            doc.put("stars", 3);
+            request.add(new IndexRequest(indexName).source(doc, XContentType.JSON));
+
+            doc = new HashMap<>();
+            doc.put("timestamp", "2019-03-11T12:00:00+00");
+            doc.put("user_id", "michel");
+            doc.put("stars", 3);
+            request.add(new IndexRequest(indexName).source(doc, XContentType.JSON));
+            request.setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE);
+        }
+        BulkResponse response = highLevelClient().bulk(request, RequestOptions.DEFAULT);
+        assertFalse(response.hasFailures());
+    }
+
+    @After
+    public void cleanUpTransforms() throws IOException {
+        for (String transformId : transformsToClean) {
+            highLevelClient().dataFrame().stopDataFrameTransform(new StopDataFrameTransformRequest(transformId), RequestOptions.DEFAULT);
+        }
+
+        for (String transformId : transformsToClean) {
+            highLevelClient().dataFrame().deleteDataFrameTransform(
+                    new DeleteDataFrameTransformRequest(transformId), RequestOptions.DEFAULT);
+        }
+
+        transformsToClean = new ArrayList<>();
+    }
+
     public void testCreateDelete() throws IOException {
         String sourceIndex = "transform-source";
         createIndex(sourceIndex);
@@ -120,6 +188,7 @@ public class DataFrameTransformIT extends ESRestHighLevelClientTestCase {
         AcknowledgedResponse ack = execute(new PutDataFrameTransformRequest(transform), client::putDataFrameTransform,
                 client::putDataFrameTransformAsync);
         assertTrue(ack.isAcknowledged());
+        transformsToClean.add(id);
 
         StartDataFrameTransformRequest startRequest = new StartDataFrameTransformRequest(id);
         StartDataFrameTransformResponse startResponse =
@@ -137,5 +206,35 @@ public class DataFrameTransformIT extends ESRestHighLevelClientTestCase {
         assertThat(stopResponse.getNodeFailures(), empty());
         assertThat(stopResponse.getTaskFailures(), empty());
     }
+
+    public void testPreview() throws IOException {
+        String sourceIndex = "transform-source";
+        createIndex(sourceIndex);
+        indexData(sourceIndex);
+
+        QueryConfig queryConfig = new QueryConfig(new MatchAllQueryBuilder());
+        GroupConfig groupConfig = new GroupConfig(Collections.singletonMap("reviewer", new TermsGroupSource("user_id")));
+        AggregatorFactories.Builder aggBuilder = new AggregatorFactories.Builder();
+        aggBuilder.addAggregator(AggregationBuilders.avg("avg_rating").field("stars"));
+        AggregationConfig aggConfig = new AggregationConfig(aggBuilder);
+        PivotConfig pivotConfig = new PivotConfig(groupConfig, aggConfig);
+
+        DataFrameTransformConfig transform = new DataFrameTransformConfig("test-preview", sourceIndex, null, queryConfig, pivotConfig);
+
+        DataFrameClient client = highLevelClient().dataFrame();
+        PreviewDataFrameTransformResponse preview = execute(new PreviewDataFrameTransformRequest(transform),
+                client::previewDataFrameTransform,
+                client::previewDataFrameTransformAsync);
+
+        List<Map<String, Object>> docs = preview.getDocs();
+        assertThat(docs, hasSize(2));
+        Optional<Map<String, Object>> theresa = docs.stream().filter(doc -> "theresa".equals(doc.get("reviewer"))).findFirst();
+        assertTrue(theresa.isPresent());
+        assertEquals(2.5d, (double)theresa.get().get("avg_rating"), 0.01d);
+
+        Optional<Map<String, Object>> michel = docs.stream().filter(doc -> "michel".equals(doc.get("reviewer"))).findFirst();
+        assertTrue(michel.isPresent());
+        assertEquals(3.6d, (double)michel.get().get("avg_rating"), 0.1d);
+    }
 }
 

client/rest-high-level/src/test/java/org/elasticsearch/client/dataframe/PreviewDataFrameTransformRequestTests.java

@@ -0,0 +1,81 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.client.dataframe;
+
+import org.elasticsearch.client.ValidationException;
+import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfig;
+import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfigTests;
+import org.elasticsearch.client.dataframe.transforms.QueryConfigTests;
+import org.elasticsearch.client.dataframe.transforms.pivot.PivotConfigTests;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.NamedXContentRegistry;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.search.SearchModule;
+import org.elasticsearch.test.AbstractXContentTestCase;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Optional;
+
+import static org.hamcrest.Matchers.containsString;
+
+public class PreviewDataFrameTransformRequestTests extends AbstractXContentTestCase<PreviewDataFrameTransformRequest> {
+    @Override
+    protected PreviewDataFrameTransformRequest createTestInstance() {
+        return new PreviewDataFrameTransformRequest(DataFrameTransformConfigTests.randomDataFrameTransformConfig());
+    }
+
+    @Override
+    protected PreviewDataFrameTransformRequest doParseInstance(XContentParser parser) throws IOException {
+        return new PreviewDataFrameTransformRequest(DataFrameTransformConfig.fromXContent(parser));
+    }
+
+    @Override
+    protected boolean supportsUnknownFields() {
+        return false;
+    }
+
+    @Override
+    protected NamedXContentRegistry xContentRegistry() {
+        SearchModule searchModule = new SearchModule(Settings.EMPTY, false, Collections.emptyList());
+        return new NamedXContentRegistry(searchModule.getNamedXContents());
+    }
+
+    public void testValidate() {
+        assertFalse(new PreviewDataFrameTransformRequest(DataFrameTransformConfigTests.randomDataFrameTransformConfig())
+                .validate().isPresent());
+        assertThat(new PreviewDataFrameTransformRequest(null).validate().get().getMessage(),
+                containsString("preview requires a non-null data frame config"));
+
+        // null id and destination is valid
+        DataFrameTransformConfig config = new DataFrameTransformConfig(null, "source", null,
+                QueryConfigTests.randomQueryConfig(), PivotConfigTests.randomPivotConfig());
+
+        assertFalse(new PreviewDataFrameTransformRequest(config).validate().isPresent());
+
+        // null source is not valid
+        config = new DataFrameTransformConfig(null, null, null,
+                QueryConfigTests.randomQueryConfig(), PivotConfigTests.randomPivotConfig());
+
+        Optional<ValidationException> error = new PreviewDataFrameTransformRequest(config).validate();
+        assertTrue(error.isPresent());
+        assertThat(error.get().getMessage(), containsString("data frame transform source cannot be null"));
+    }
+}

client/rest-high-level/src/test/java/org/elasticsearch/client/dataframe/PreviewDataFrameTransformResponseTests.java

@@ -0,0 +1,69 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.client.dataframe;
+
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.test.ESTestCase;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import static org.elasticsearch.test.AbstractXContentTestCase.xContentTester;
+
+public class PreviewDataFrameTransformResponseTests extends ESTestCase {
+
+    public void testFromXContent() throws IOException {
+        xContentTester(this::createParser,
+                this::createTestInstance,
+                this::toXContent,
+                PreviewDataFrameTransformResponse::fromXContent)
+                .supportsUnknownFields(true)
+                .randomFieldsExcludeFilter(path -> path.isEmpty() == false)
+                .test();
+    }
+
+    private PreviewDataFrameTransformResponse createTestInstance() {
+        int numDocs = randomIntBetween(5, 10);
+        List<Map<String, Object>> docs = new ArrayList<>(numDocs);
+        for (int i=0; i<numDocs; i++) {
+            int numFields = randomIntBetween(1, 4);
+            Map<String, Object> doc = new HashMap<>();
+            for (int j=0; j<numFields; j++) {
+                doc.put(randomAlphaOfLength(5), randomAlphaOfLength(5));
+            }
+            docs.add(doc);
+        }
+
+        return new PreviewDataFrameTransformResponse(docs);
+    }
+
+    private void toXContent(PreviewDataFrameTransformResponse response, XContentBuilder builder) throws IOException {
+        builder.startObject();
+        builder.startArray("preview");
+        for (Map<String, Object> doc : response.getDocs()) {
+            builder.map(doc);
+        }
+        builder.endArray();
+        builder.endObject();
+    }
+}

client/rest-high-level/src/test/java/org/elasticsearch/client/dataframe/PutDataFrameTransformRequestTests.java

@@ -19,8 +19,11 @@
 
 package org.elasticsearch.client.dataframe;
 
+import org.elasticsearch.client.ValidationException;
 import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfig;
 import org.elasticsearch.client.dataframe.transforms.DataFrameTransformConfigTests;
+import org.elasticsearch.client.dataframe.transforms.QueryConfigTests;
+import org.elasticsearch.client.dataframe.transforms.pivot.PivotConfigTests;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
 import org.elasticsearch.common.xcontent.XContentParser;
@@ -29,8 +32,29 @@ import org.elasticsearch.test.AbstractXContentTestCase;
 
 import java.io.IOException;
 import java.util.Collections;
+import java.util.Optional;
+
+import static org.hamcrest.Matchers.containsString;
 
 public class PutDataFrameTransformRequestTests extends AbstractXContentTestCase<PutDataFrameTransformRequest> {
+
+    public void testValidate() {
+        assertFalse(createTestInstance().validate().isPresent());
+
+        DataFrameTransformConfig config = new DataFrameTransformConfig(null, null, null,
+                QueryConfigTests.randomQueryConfig(), PivotConfigTests.randomPivotConfig());
+
+        Optional<ValidationException> error = new PutDataFrameTransformRequest(config).validate();
+        assertTrue(error.isPresent());
+        assertThat(error.get().getMessage(), containsString("data frame transform id cannot be null"));
+        assertThat(error.get().getMessage(), containsString("data frame transform source cannot be null"));
+        assertThat(error.get().getMessage(), containsString("data frame transform destination cannot be null"));
+
+        error = new PutDataFrameTransformRequest(null).validate();
+        assertTrue(error.isPresent());
+        assertThat(error.get().getMessage(), containsString("put requires a non-null data frame config"));
+    }
+
     @Override
     protected PutDataFrameTransformRequest createTestInstance() {
         return new PutDataFrameTransformRequest(DataFrameTransformConfigTests.randomDataFrameTransformConfig());

client/rest-high-level/src/test/java/org/elasticsearch/client/documentation/DataFrameTransformDocumentationIT.java

@@ -26,6 +26,8 @@ import org.elasticsearch.client.RequestOptions;
 import org.elasticsearch.client.RestHighLevelClient;
 import org.elasticsearch.client.core.AcknowledgedResponse;
 import org.elasticsearch.client.dataframe.DeleteDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformRequest;
+import org.elasticsearch.client.dataframe.PreviewDataFrameTransformResponse;
 import org.elasticsearch.client.dataframe.PutDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformRequest;
 import org.elasticsearch.client.dataframe.StartDataFrameTransformResponse;
@@ -356,6 +358,67 @@ public class DataFrameTransformDocumentationIT extends ESRestHighLevelClientTest
 
             assertTrue(latch.await(30L, TimeUnit.SECONDS));
         }
+    }
 
+    public void testPreview() throws IOException, InterruptedException {
+        createIndex("source-data");
+
+        RestHighLevelClient client = highLevelClient();
+
+        QueryConfig queryConfig = new QueryConfig(new MatchAllQueryBuilder());
+        GroupConfig groupConfig = new GroupConfig(Collections.singletonMap("reviewer", new TermsGroupSource("user_id")));
+        AggregatorFactories.Builder aggBuilder = new AggregatorFactories.Builder();
+        aggBuilder.addAggregator(AggregationBuilders.avg("avg_rating").field("stars"));
+        AggregationConfig aggConfig = new AggregationConfig(aggBuilder);
+        PivotConfig pivotConfig = new PivotConfig(groupConfig, aggConfig);
+
+        // tag::preview-data-frame-transform-request
+        DataFrameTransformConfig transformConfig =
+                new DataFrameTransformConfig(null,  // <1>
+                "source-data",
+                null,                               // <2>
+                queryConfig,
+                pivotConfig);
+
+        PreviewDataFrameTransformRequest request =
+            new PreviewDataFrameTransformRequest(transformConfig); // <3>
+        // end::preview-data-frame-transform-request
+
+        {
+            // tag::preview-data-frame-transform-execute
+            PreviewDataFrameTransformResponse response =
+                    client.dataFrame()
+                            .previewDataFrameTransform(request, RequestOptions.DEFAULT);
+            // end::preview-data-frame-transform-execute
+
+            assertNotNull(response.getDocs());
+        }
+        {
+            // tag::preview-data-frame-transform-execute-listener
+            ActionListener<PreviewDataFrameTransformResponse> listener =
+                new ActionListener<PreviewDataFrameTransformResponse>() {
+                    @Override
+                    public void onResponse(PreviewDataFrameTransformResponse response) {
+                        // <1>
+                    }
+
+                    @Override
+                    public void onFailure(Exception e) {
+                        // <2>
+                    }
+                };
+            // end::preview-data-frame-transform-execute-listener
+
+            // Replace the empty listener by a blocking listener in test
+            final CountDownLatch latch = new CountDownLatch(1);
+            listener = new LatchedActionListener<>(listener, latch);
+
+            // tag::preview-data-frame-transform-execute-async
+            client.dataFrame().previewDataFrameTransformAsync(
+                    request, RequestOptions.DEFAULT, listener);  // <1>
+            // end::preview-data-frame-transform-execute-async
+
+            assertTrue(latch.await(30L, TimeUnit.SECONDS));
+        }
     }
 }

docs/java-rest/high-level/dataframe/preview_data_frame.asciidoc

@@ -0,0 +1,32 @@
+--
+:api: preview-data-frame-transform
+:request: PreviewDataFrameTransformRequest
+:response: PreviewDataFrameTransformResponse
+--
+[id="{upid}-{api}"]
+=== Preview Data Frame Transform API
+
+The Preview Data Frame Transform API is used to preview the results of
+a {dataframe-transform}.
+
+The API accepts a +{request}+ object as a request and returns a +{response}+.
+
+[id="{upid}-{api}-request"]
+==== Preview Data Frame Request
+
+A +{request}+ takes a single argument: a valid data frame transform config.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-request]
+--------------------------------------------------
+<1> The transform Id may be null for the preview
+<2> The destination may be null for the preview
+<3> The configuration of the {dataframe-job} to preview
+
+include::../execution.asciidoc[]
+
+[id="{upid}-{api}-response"]
+==== Response
+
+The returned +{response}+ contains the preview documents

docs/java-rest/high-level/supported-apis.asciidoc

@@ -556,10 +556,12 @@ The Java High Level REST Client supports the following Data Frame APIs:
 
 * <<{upid}-put-data-frame-transform>>
 * <<{upid}-delete-data-frame-transform>>
+* <<{upid}-preview-data-frame-transform>>
 * <<{upid}-start-data-frame-transform>>
 * <<{upid}-stop-data-frame-transform>>
 
 include::dataframe/put_data_frame.asciidoc[]
 include::dataframe/delete_data_frame.asciidoc[]
+include::dataframe/preview_data_frame.asciidoc[]
 include::dataframe/start_data_frame.asciidoc[]
 include::dataframe/stop_data_frame.asciidoc[]

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/dataframe/action/PreviewDataFrameTransformAction.java

@@ -60,6 +60,7 @@ public class PreviewDataFrameTransformAction extends Action<PreviewDataFrameTran
             Map<String, Object> content = parser.map();
             // Destination and ID are not required for Preview, so we just supply our own
             content.put(DataFrameField.DESTINATION.getPreferredName(), "unused-transform-preview-index");
+            content.put(DataFrameField.ID.getPreferredName(), "transform-preview");
             try(XContentBuilder xContentBuilder = XContentFactory.jsonBuilder().map(content);
                 XContentParser newParser = XContentType.JSON
                     .xContent()

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/dataframe/action/PreviewDataFrameTransformActionRequestTests.java

@@ -6,10 +6,13 @@
 
 package org.elasticsearch.xpack.core.dataframe.action;
 
+import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.DeprecationHandler;
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
 import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.search.SearchModule;
 import org.elasticsearch.test.AbstractStreamableXContentTestCase;
 import org.elasticsearch.xpack.core.dataframe.action.PreviewDataFrameTransformAction.Request;
@@ -28,7 +31,7 @@ public class PreviewDataFrameTransformActionRequestTests extends AbstractStreama
     private NamedXContentRegistry namedXContentRegistry;
 
     @Before
-    public void registerAggregationNamedObjects() throws Exception {
+    public void registerAggregationNamedObjects() {
         // register aggregations as NamedWriteable
         SearchModule searchModule = new SearchModule(Settings.EMPTY, false, emptyList());
         namedWriteableRegistry = new NamedWriteableRegistry(searchModule.getNamedWriteables());
@@ -67,4 +70,24 @@ public class PreviewDataFrameTransformActionRequestTests extends AbstractStreama
         return new Request(config);
     }
 
+    public void testParsingOverwritesIdAndDestFields() throws IOException {
+        // id & dest fields will be set by the parser
+        BytesArray json = new BytesArray(
+                "{ " +
+                    "\"source\":\"foo\", " +
+                    "\"query\": {\"match_all\": {}}," +
+                    "\"pivot\": {" +
+                        "\"group_by\": {\"destination-field2\": {\"terms\": {\"field\": \"term-field\"}}}," +
+                        "\"aggs\": {\"avg_response\": {\"avg\": {\"field\": \"responsetime\"}}}" +
+                    "}" +
+                "}");
+
+        try (XContentParser parser = JsonXContent.jsonXContent
+                .createParser(xContentRegistry(), DeprecationHandler.THROW_UNSUPPORTED_OPERATION, json.streamInput())) {
+
+            Request request = Request.fromXContent(parser);
+            assertEquals("transform-preview", request.getConfig().getId());
+            assertEquals("unused-transform-preview-index", request.getConfig().getDestination());
+        }
+    }
 }

x-pack/plugin/data-frame/src/main/java/org/elasticsearch/xpack/dataframe/rest/action/RestStartDataFrameTransformAction.java

@@ -29,9 +29,7 @@ public class RestStartDataFrameTransformAction extends BaseRestHandler {
     protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
         String id = restRequest.param(DataFrameField.ID.getPreferredName());
         StartDataFrameTransformAction.Request request = new StartDataFrameTransformAction.Request(id);
-        if (restRequest.hasParam(DataFrameField.TIMEOUT.getPreferredName())) {
+        request.timeout(restRequest.paramAsTime(DataFrameField.TIMEOUT.getPreferredName(), AcknowledgedRequest.DEFAULT_ACK_TIMEOUT));
-            request.timeout(restRequest.paramAsTime(DataFrameField.TIMEOUT.getPreferredName(), AcknowledgedRequest.DEFAULT_ACK_TIMEOUT));
-        }
         return channel -> client.execute(StartDataFrameTransformAction.INSTANCE, request, new RestToXContentListener<>(channel));
     }