Introducing interfaces for new Batching API (#692)

* Initial Commit for fresh approach

* Batcher Implementation with blocking flush

* Introducing new Batching API interfaces

## What this PR contain
This contains interfaces required to perform batching with elements(`MutateRowsRequest.Entry` or `LogEntry`) object directly instead of RequestWrappers(`MutateRowsRequest` or `WriteLogEntriesRequest`).

* reverting batching/BatchingSettings

* Addressing the javadoc related comments

* Removed `@Override` annotation from Javadoc code sample

* removed access specifiers from sample code and some tiny language edits

* Updated generic types name for Batcher related interfaces

Have updated type names to increase user understanding by their name.
Address some sample changes according to feedback.

* Reverting type names and RequestBuilder#build

removed return statement

* Updated @param for types in batcher interfaces

Author: rahulKQL

gax/src/main/java/com/google/api/gax/batching/v2/Batcher.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2019 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.batching.v2;
+
+import com.google.api.core.ApiFuture;
+
+/**
+ * Represents a batching context where individual elements will be accumulated and flushed in a
+ * large batch request at some point in the future. The buffered elements can be flushed manually or
+ * when triggered by an internal threshold. This is intended to be used for high throughput
+ * scenarios at the cost of latency.
+ *
+ * @param <ElementT> The type of each individual element to be batched.
+ * @param <ElementResultT> The type of the result for each individual element.
+ */
+public interface Batcher<ElementT, ElementResultT> extends AutoCloseable {
+
+  /**
+   * Queues the passed in element to be sent at some point in the future.
+   *
+   * <p>The element will be sent as part of a larger batch request at some point in the future. The
+   * returned {@link ApiFuture} will be resolved once the result for the element has been extracted
+   * from the batch response.
+   *
+   * <p>Note: Cancelling returned result simply marks the future cancelled, It would not stop the
+   * batch request.
+   */
+  ApiFuture<ElementResultT> add(ElementT entry);
+
+  /**
+   * Synchronously sends any pending elements as a batch and waits for all outstanding batches to be
+   * complete.
+   */
+  void flush() throws InterruptedException;
+
+  /**
+   * Closes this Batcher by preventing new elements from being added and flushing the existing
+   * elements.
+   */
+  @Override
+  void close() throws InterruptedException;
+}

gax/src/main/java/com/google/api/gax/batching/v2/BatchingDescriptor.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2019 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.batching.v2;
+
+import com.google.api.core.SettableApiFuture;
+import java.util.List;
+
+/**
+ * An adapter that packs and unpacks the elements in and out of batch requests and responses.
+ *
+ * <p>This interface should be implemented by either a service specific client or autogenerated by
+ * gapic-generator.
+ *
+ * <p>Example implementation:
+ *
+ * <pre>{@code
+ * class ListDescriptor implements BatchingDescriptor<String, String, List<String>, List<String>> {
+ *
+ *   RequestBuilder<String, List<String>> newRequestBuilder(List<String> prototype) {
+ *     return new RequestBuilder<String, List<String>>() {
+ *
+ *       void add(String element) {
+ *         list.add(element);
+ *       }
+ *
+ *       List<String> build() {
+ *         return list.clone();
+ *       }
+ *     };
+ *   }
+ *
+ *   void splitResponse(List<String> callableResponse, List<SettableApiFuture<String>> batch) {
+ *     for (int i = 0; i < batchResponse.size(); i++) {
+ *       batch.get(i).set(batchResponse.get(i);
+ *     }
+ *   }
+ *
+ *   void splitException(Throwable throwable, List<SettableApiFuture<String>> batch) {
+ *     for (SettableApiFuture<String> result : batch) {
+ *       result.setException(throwable);
+ *     }
+ *   }
+ *
+ *   long countBytes(String element) {
+ *     return element.length();
+ *   }
+ * }
+ * }</pre>
+ *
+ * @param <ElementT> The type of each individual element to be batched
+ * @param <ElementResultT> The type of the result for each individual element
+ * @param <RequestT> The type of the request that will contain the accumulated elements
+ * @param <ResponseT> The type of the response that will be unpacked into individual element results
+ */
+public interface BatchingDescriptor<ElementT, ElementResultT, RequestT, ResponseT> {
+
+  /**
+   * Creates a new wrapper for the underlying request builder. It's used to pack the current batch
+   * request with elements.
+   */
+  RequestBuilder<ElementT, RequestT> newRequestBuilder(RequestT prototype);
+
+  /** Unpacks the batch response into individual elements results. */
+  void splitResponse(ResponseT batchResponse, List<SettableApiFuture<ElementResultT>> batch);
+
+  /** Unpacks the batch response error into individual element errors. */
+  void splitException(Throwable throwable, List<SettableApiFuture<ElementResultT>> batch);
+
+  /** Returns the size of the passed element object in bytes. */
+  long countBytes(ElementT element);
+}

gax/src/main/java/com/google/api/gax/batching/v2/RequestBuilder.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2019 Google LLC
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google LLC nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.google.api.gax.batching.v2;
+
+/**
+ * Adapter to pack individual elements into a larger batch request.
+ *
+ * <p>The implementation for this interface will be implemented by service specific client or auto
+ * generated by the gapic-generator.
+ *
+ * @param <ElementT> The type of each individual element to be batched.
+ * @param <RequestT> The type of the request that will contain the accumulated elements.
+ */
+public interface RequestBuilder<ElementT, RequestT> {
+
+  /** Adds element object into client specific batch request. */
+  void add(ElementT element);
+
+  /** Returns the collected elements as a single client specific batch request. */
+  RequestT build();
+}