Code review comment: it will be used only with Write which already does the PDone.in

Code under review:
public boolean start() throws IOException {
      restClient = source.spec.getConnectionConfiguration().createClient();

      String query = source.spec.getQuery() != null ? source.spec.getQuery().get() : null;
      if (query == null) {
        query = "{\"query\": { \"match_all\": {} }}";
      }
      if ((source.backendVersion >= 5) && source.numSlices != null && source.numSlices > 1) {
        
        String sliceQuery =
            String.format("\"slice\": {\"id\": %s,\"max\": %s}", source.sliceId, source.numSlices);
        query = query.replaceFirst("\\{", "{" + sliceQuery + ",");
      }
      String endPoint =
          String.format(
              "/%s/%s/_search",
              source.spec.getConnectionConfiguration().getIndex(),
              source.spec.getConnectionConfiguration().getType());
      Map<String, String> params = new HashMap<>();
      params.put("scroll", source.spec.getScrollKeepalive());
      if (source.backendVersion == 2) {
        params.put("size", String.valueOf(source.spec.getBatchSize()));
        if (source.shardPreference != null) {
          params.put("preference", "_shards:" + source.shardPreference);
        }
      }
      HttpEntity queryEntity = new NStringEntity(query, ContentType.APPLICATION_JSON);
      Request request = new Request("GET", endPoint);
      request.addParameters(params);
      request.setEntity(queryEntity);
      Response response = restClient.performRequest(request);
      JsonNode searchResult = parseResponse(response.getEntity());
      updateScrollId(searchResult);
      return readNextBatchAndReturnFirstDocument(searchResult);
    }

    private void updateScrollId(JsonNode searchResult) {
      scrollId = searchResult.path("_scroll_id").asText();
    }

    @Override
    public boolean advance() throws IOException {
      if (batchIterator.hasNext()) {
        current = batchIterator.next();
        return true;
      } else {
        String requestBody =
            String.format(
                "{\"scroll\" : \"%s\",\"scroll_id\" : \"%s\"}",
                source.spec.getScrollKeepalive(), scrollId);
        HttpEntity scrollEntity = new NStringEntity(requestBody, ContentType.APPLICATION_JSON);
        Request request = new Request("GET", "/_search/scroll");
        request.addParameters(Collections.emptyMap());
        request.setEntity(scrollEntity);
        Response response = restClient.performRequest(request);
        JsonNode searchResult = parseResponse(response.getEntity());
        updateScrollId(searchResult);
        return readNextBatchAndReturnFirstDocument(searchResult);
      }
    }

    private boolean readNextBatchAndReturnFirstDocument(JsonNode searchResult) {
      
      JsonNode hits = searchResult.path("hits").path("hits");
      if (hits.size() == 0) {
        current = null;
        batchIterator = null;
        return false;
      }
      
      List<String> batch = new ArrayList<>();
      boolean withMetadata = source.spec.isWithMetadata();
      for (JsonNode hit : hits) {
        if (withMetadata) {
          batch.add(hit.toString());
        } else {
          String document = hit.path("_source").toString();
          batch.add(document);
        }
      }
      batchIterator = batch.listIterator();
      current = batchIterator.next();
      return true;
    }

    @Override
    public String getCurrent() throws NoSuchElementException {
      if (current == null) {
        throw new NoSuchElementException();
      }
      return current;
    }

    @Override
    public void close() throws IOException {
      
      String requestBody = String.format("{\"scroll_id\" : [\"%s\"]}", scrollId);
      HttpEntity entity = new NStringEntity(requestBody, ContentType.APPLICATION_JSON);
      try {
        Request request = new Request("DELETE", "/_search/scroll");
        request.addParameters(Collections.emptyMap());
        request.setEntity(entity);
        restClient.performRequest(request);
      } finally {
        if (restClient != null) {
          restClient.close();
        }
      }
    }

    @Override
    public BoundedSource<String> getCurrentSource() {
      return source;
    }
  }
  /**
   * A POJO encapsulating a configuration for retry behavior when issuing requests to ES. A retry
   * will be attempted until the maxAttempts or maxDuration is exceeded, whichever comes first, for
   * 429 TOO_MANY_REQUESTS error.
   */
  @AutoValue
  public abstract static class RetryConfiguration implements Serializable {
    @VisibleForTesting
    static final RetryPredicate DEFAULT_RETRY_PREDICATE = new DefaultRetryPredicate();

    abstract int getMaxAttempts();

    abstract Duration getMaxDuration();

    abstract RetryPredicate getRetryPredicate();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract ElasticsearchIO.RetryConfiguration.Builder setMaxAttempts(int maxAttempts);

      abstract ElasticsearchIO.RetryConfiguration.Builder setMaxDuration(Duration maxDuration);

      abstract ElasticsearchIO.RetryConfiguration.Builder setRetryPredicate(
          RetryPredicate retryPredicate);

      abstract ElasticsearchIO.RetryConfiguration build();
    }

    /**
     * Creates RetryConfiguration for {@link ElasticsearchIO} with provided maxAttempts,
     * maxDurations and exponential backoff based retries.
     *
     * @param maxAttempts max number of attempts.
     * @param maxDuration maximum duration for retries.
     * @return {@link RetryConfiguration} object with provided settings.
     */
    public static RetryConfiguration create(int maxAttempts, Duration maxDuration) {
      checkArgument(maxAttempts > 0, "maxAttempts must be greater than 0");
      checkArgument(
          maxDuration != null && maxDuration.isLongerThan(Duration.ZERO),
          "maxDuration must be greater than 0");
      return new AutoValue_ElasticsearchIO_RetryConfiguration.Builder()
          .setMaxAttempts(maxAttempts)
          .setMaxDuration(maxDuration)
          .setRetryPredicate(DEFAULT_RETRY_PREDICATE)
          .build();
    }

    
    @VisibleForTesting
    RetryConfiguration withRetryPredicate(RetryPredicate predicate) {
      checkArgument(predicate != null, "predicate must be provided");
      return builder().setRetryPredicate(predicate).build();
    }

    /**
     * An interface used to control if we retry the Elasticsearch call when a {@link Response} is
     * obtained. If {@link RetryPredicate
     * the requests to the Elasticsearch server if the {@link RetryConfiguration} permits it.
     */
    @FunctionalInterface
    interface RetryPredicate extends Predicate<HttpEntity>, Serializable {}

    /**
     * This is the default predicate used to test if a failed ES operation should be retried. A
     * retry will be attempted until the maxAttempts or maxDuration is exceeded, whichever comes
     * first, for TOO_MANY_REQUESTS(429) error.
     */
    @VisibleForTesting
    static class DefaultRetryPredicate implements RetryPredicate {

      private int errorCode;

      DefaultRetryPredicate(int code) {
        this.errorCode = code;
      }

      DefaultRetryPredicate() {
        this(429);
      }

      /** Returns true if the response has the error code for any mutation. */
      private static boolean errorCodePresent(HttpEntity responseEntity, int errorCode) {
        try {
          JsonNode json = parseResponse(responseEntity);
          if (json.path("errors").asBoolean()) {
            for (JsonNode item : json.path("items")) {
              if (item.findValue("status").asInt() == errorCode) {
                return true;
              }
            }
          }
        } catch (IOException e) {
          LOG.warn("Could not extract error codes from responseEntity {}", responseEntity);
        }
        return false;
      }

      @Override
      public boolean test(HttpEntity responseEntity) {
        return errorCodePresent(responseEntity, errorCode);
      }
    }
  }

  /** A {@link PTransform} converting docs to their Bulk API counterparts. */
  @AutoValue
  public abstract static class DocToBulk
      extends PTransform<PCollection<String>, PCollection<String>> {

    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
    private static final int DEFAULT_RETRY_ON_CONFLICT = 5; 

    static {
      SimpleModule module = new SimpleModule();
      module.addSerializer(DocumentMetadata.class, new DocumentMetadataSerializer());
      OBJECT_MAPPER.registerModule(module);
    }

    abstract @Nullable ConnectionConfiguration getConnectionConfiguration();

    abstract Write.@Nullable FieldValueExtractFn getIdFn();

    abstract Write.@Nullable FieldValueExtractFn getIndexFn();

    abstract Write.@Nullable FieldValueExtractFn getRoutingFn();

    abstract Write.@Nullable FieldValueExtractFn getTypeFn();

    abstract Write.@Nullable FieldValueExtractFn getDocVersionFn();

    abstract @Nullable String getDocVersionType();

    abstract @Nullable String getUpsertScript();

    abstract @Nullable Boolean getUsePartialUpdate();

    abstract Write.@Nullable BooleanFieldValueExtractFn getIsDeleteFn();

    abstract @Nullable Integer getBackendVersion();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract Builder setConnectionConfiguration(ConnectionConfiguration connectionConfiguration);

      abstract Builder setIdFn(Write.FieldValueExtractFn idFunction);

      abstract Builder setIndexFn(Write.FieldValueExtractFn indexFn);

      abstract Builder setRoutingFn(Write.FieldValueExtractFn routingFunction);

      abstract Builder setTypeFn(Write.FieldValueExtractFn typeFn);

      abstract Builder setDocVersionFn(Write.FieldValueExtractFn docVersionFn);

      abstract Builder setDocVersionType(String docVersionType);

      abstract Builder setIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn);

      abstract Builder setUsePartialUpdate(Boolean usePartialUpdate);

      abstract Builder setUpsertScript(String source);

      abstract Builder setBackendVersion(Integer assumedBackendVersion);

      abstract DocToBulk build();
    }

    /**
     * Provide the Elasticsearch connection configuration object. Only required if
     * withBackendVersion was not used i.e. getBackendVersion() returns null.
     *
     * @param connectionConfiguration the Elasticsearch {@link ConnectionConfiguration} object
     * @return the {@link DocToBulk} with connection configuration set
     */
    public DocToBulk withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");
      return builder().setConnectionConfiguration(connectionConfiguration).build();
    }

    /**
     * Provide a function to extract the id from the document. This id will be used as the document
     * id in Elasticsearch. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param idFn to extract the document ID
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withIdFn(Write.FieldValueExtractFn idFn) {
      checkArgument(idFn != null, "idFn must not be null");
      return builder().setIdFn(idFn).build();
    }

    /**
     * Provide a function to extract the target index from the document allowing for dynamic
     * document routing. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param indexFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withIndexFn(Write.FieldValueExtractFn indexFn) {
      checkArgument(indexFn != null, "indexFn must not be null");
      return builder().setIndexFn(indexFn).build();
    }

    /**
     * Provide a function to extract the target routing from the document allowing for dynamic
     * document routing. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param routingFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withRoutingFn(Write.FieldValueExtractFn routingFn) {
      checkArgument(routingFn != null, "routingFn must not be null");
      return builder().setRoutingFn(routingFn).build();
    }

    /**
     * Provide a function to extract the target type from the document allowing for dynamic document
     * routing. Should the function throw an Exception then the batch will fail and the exception
     * propagated. Users are encouraged to consider carefully if multipe types are a sensible model
     * <a
     * href="https:
     * discussed in this blog</a>.
     *
     * @param typeFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withTypeFn(Write.FieldValueExtractFn typeFn) {
      checkArgument(typeFn != null, "typeFn must not be null");
      return builder().setTypeFn(typeFn).build();
    }

    /**
     * Provide an instruction to control whether partial updates or inserts (default) are issued to
     * Elasticsearch.
     *
     * @param usePartialUpdate set to true to issue partial updates
     * @return the {@link DocToBulk} with the partial update control set
     */
    public DocToBulk withUsePartialUpdate(boolean usePartialUpdate) {
      return builder().setUsePartialUpdate(usePartialUpdate).build();
    }

    /**
     * Whether to use scripted updates and what script to use.
     *
     * @param source set to the value of the script source, painless lang
     * @return the {@link DocToBulk} with the scripted updates set
     */
    public DocToBulk withUpsertScript(String source) {
      return builder().setUsePartialUpdate(false).setUpsertScript(source).build();
    }

    /**
     * Provide a function to extract the doc version from the document. This version number will be
     * used as the document version in Elasticsearch. Should the function throw an Exception then
     * the batch will fail and the exception propagated. Incompatible with update operations and
     * should only be used with withUsePartialUpdate(false)
     *
     * @param docVersionFn to extract the document version
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withDocVersionFn(Write.FieldValueExtractFn docVersionFn) {
      checkArgument(docVersionFn != null, "docVersionFn must not be null");
      return builder().setDocVersionFn(docVersionFn).build();
    }

    /**
     * Provide a function to extract the target operation either upsert or delete from the document
     * fields allowing dynamic bulk operation decision. While using withIsDeleteFn, it should be
     * taken care that the document's id extraction is defined using the withIdFn function or else
     * IllegalArgumentException is thrown. Should the function throw an Exception then the batch
     * will fail and the exception propagated.
     *
     * @param isDeleteFn set to true for deleting the specific document
     * @return the {@link Write} with the function set
     */
    public DocToBulk withIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn) {
      checkArgument(isDeleteFn != null, "deleteFn is required");
      return builder().setIsDeleteFn(isDeleteFn).build();
    }

    /**
     * Provide a function to extract the doc version from the document. This version number will be
     * used as the document version in Elasticsearch. Should the function throw an Exception then
     * the batch will fail and the exception propagated. Incompatible with update operations and
     * should only be used with withUsePartialUpdate(false)
     *
     * @param docVersionType the version type to use, one of {@value ElasticsearchIO
     * @return the {@link DocToBulk} with the doc version type set
     */
    public DocToBulk withDocVersionType(String docVersionType) {
      checkArgument(
          VERSION_TYPES.contains(docVersionType),
          "docVersionType must be one of " + "%s",
          String.join(", ", VERSION_TYPES));
      return builder().setDocVersionType(docVersionType).build();
    }

    /**
     * Use to set explicitly which version of Elasticsearch the destination cluster is running.
     * Providing this hint means there is no need for setting {@link
     * DocToBulk
     *
     * @param backendVersion the major version number of the version of Elasticsearch being run in
     *     the cluster where documents will be indexed.
     * @return the {@link DocToBulk} with the Elasticsearch major version number set
     */
    public DocToBulk withBackendVersion(int backendVersion) {
      checkArgument(
          VALID_CLUSTER_VERSIONS.contains(backendVersion),
          "Backend version may only be one of " + "%s",
          String.join(", ", VERSION_TYPES));
      return builder().setBackendVersion(backendVersion).build();
    }

    @Override
    public PCollection<String> expand(PCollection<String> docs) {
      ConnectionConfiguration connectionConfiguration = getConnectionConfiguration();
      Integer backendVersion = getBackendVersion();
      Write.FieldValueExtractFn idFn = getIdFn();
      Write.BooleanFieldValueExtractFn isDeleteFn = getIsDeleteFn();
      checkState(
          (backendVersion != null || connectionConfiguration != null),
          "withBackendVersion() or withConnectionConfiguration() is required");
      checkArgument(
          isDeleteFn == null || idFn != null,
          "Id needs to be specified by withIdFn for delete operation");

      return docs.apply(ParDo.of(new DocToBulkFn(this)));
    }

    
    private static class DocumentMetadata implements Serializable {
      final String index;
      final String type;
      final String id;
      final Integer retryOnConflict;
      final String routing;
      final Integer backendVersion;
      final String version;
      final String versionType;

      DocumentMetadata(
          String index,
          String type,
          String id,
          Integer retryOnConflict,
          String routing,
          Integer backendVersion,
          String version,
          String versionType) {
        this.index = index;
        this.id = id;
        this.type = type;
        this.retryOnConflict = retryOnConflict;
        this.routing = routing;
        this.backendVersion = backendVersion;
        this.version = version;
        this.versionType = versionType;
      }
    }

    private static class DocumentMetadataSerializer extends StdSerializer<DocumentMetadata> {
      private DocumentMetadataSerializer() {
        super(DocumentMetadata.class);
      }

      @Override
      public void serialize(DocumentMetadata value, JsonGenerator gen, SerializerProvider provider)
          throws IOException {
        gen.writeStartObject();
        if (value.index != null) {
          gen.writeStringField("_index", value.index);
        }
        if (value.type != null) {
          gen.writeStringField("_type", value.type);
        }
        if (value.id != null) {
          gen.writeStringField("_id", value.id);
        }
        if (value.routing != null) {
          gen.writeStringField("routing", value.routing);
        }
        if (value.retryOnConflict != null && value.backendVersion <= 6) {
          gen.writeNumberField("_retry_on_conflict", value.retryOnConflict);
        }
        if (value.retryOnConflict != null && value.backendVersion >= 7) {
          gen.writeNumberField("retry_on_conflict", value.retryOnConflict);
        }
        if (value.version != null) {
          gen.writeStringField("version", value.version);
        }
        if (value.versionType != null) {
          gen.writeStringField("version_type", value.versionType);
        }
        gen.writeEndObject();
      }
    }

    @VisibleForTesting
    static String createBulkApiEntity(DocToBulk spec, String document, int backendVersion)
        throws IOException {
      String documentMetadata = "{}";
      boolean isDelete = false;
      if (spec.getIndexFn() != null || spec.getTypeFn() != null || spec.getIdFn() != null) {
        
        JsonNode parsedDocument = OBJECT_MAPPER.readTree(document);
        documentMetadata = getDocumentMetadata(spec, parsedDocument, backendVersion);
        if (spec.getIsDeleteFn() != null) {
          isDelete = spec.getIsDeleteFn().apply(parsedDocument);
        }
      }

      if (isDelete) {
        
        return String.format("{ \"delete\" : %s }%n", documentMetadata);
      } else {
        
        
        if (spec.getUsePartialUpdate()) {
          return String.format(
              "{ \"update\" : %s }%n{ \"doc\" : %s, " + "\"doc_as_upsert\" : true }%n",
              documentMetadata, document);
        } else if (spec.getUpsertScript() != null) {
          return String.format(
              "{ \"update\" : %s }%n{ \"script\" : {\"source\": \"%s\", "
                  + "\"params\": %s}, \"upsert\" : %s }%n",
              documentMetadata, spec.getUpsertScript(), document, document);
        } else {
          return String.format("{ \"index\" : %s }%n%s%n", documentMetadata, document);
        }
      }
    }

    private static String lowerCaseOrNull(String input) {
      return input == null ? null : input.toLowerCase();
    }

    /**
     * Extracts the components that comprise the document address from the document using the {@link
     * Write.FieldValueExtractFn} configured. This allows any or all of the index, type and document
     * id to be controlled on a per document basis. If none are provided then an empty default of
     * {@code {}} is returned. Sanitization of the index is performed, automatically lower-casing
     * the value as required by Elasticsearch.
     *
     * @param parsedDocument the json from which the index, type and id may be extracted
     * @return the document address as JSON or the default
     * @throws IOException if the document cannot be parsed as JSON
     */
    private static String getDocumentMetadata(
        DocToBulk spec, JsonNode parsedDocument, int backendVersion) throws IOException {
      DocumentMetadata metadata =
          new DocumentMetadata(
              spec.getIndexFn() != null
                  ? lowerCaseOrNull(spec.getIndexFn().apply(parsedDocument))
                  : null,
              spec.getTypeFn() != null ? spec.getTypeFn().apply(parsedDocument) : null,
              spec.getIdFn() != null ? spec.getIdFn().apply(parsedDocument) : null,
              (spec.getUsePartialUpdate()
                      || (spec.getUpsertScript() != null && !spec.getUpsertScript().isEmpty()))
                  ? DEFAULT_RETRY_ON_CONFLICT
                  : null,
              spec.getRoutingFn() != null ? spec.getRoutingFn().apply(parsedDocument) : null,
              backendVersion,
              spec.getDocVersionFn() != null ? spec.getDocVersionFn().apply(parsedDocument) : null,
              spec.getDocVersionType());
      return OBJECT_MAPPER.writeValueAsString(metadata);
    }

    /** {@link DoFn} to for the {@link DocToBulk} transform. */
    @VisibleForTesting
    static class DocToBulkFn extends DoFn<String, String> {
      private final DocToBulk spec;
      private int backendVersion;

      public DocToBulkFn(DocToBulk spec) {
        this.spec = spec;
      }

      @Setup
      public void setup() throws IOException {
        ConnectionConfiguration connectionConfiguration = spec.getConnectionConfiguration();
        if (spec.getBackendVersion() == null) {
          backendVersion = ElasticsearchIO.getBackendVersion(connectionConfiguration);
        } else {
          backendVersion = spec.getBackendVersion();
        }
      }

      @ProcessElement
      public void processElement(ProcessContext c) throws IOException {
        c.output(createBulkApiEntity(spec, c.element(), backendVersion));
      }
    }
  }

  /**
   * A {@link PTransform} convenience wrapper for doing both document to bulk API serialization as
   * well as batching those Bulk API entities and writing them to an Elasticsearch cluster. This
   * class is effectively a thin proxy for DocToBulk->BulkIO all-in-one for convenience and backward
   * compatibility.
   */
  @AutoValue
  public abstract static class Write extends PTransform<PCollection<String>, PDone> {
    public interface FieldValueExtractFn extends SerializableFunction<JsonNode, String> {}

    public interface BooleanFieldValueExtractFn extends SerializableFunction<JsonNode, Boolean> {}

    public abstract DocToBulk getDocToBulk();

    public abstract BulkIO getBulkIO();

    abstract Builder writeBuilder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract Builder setDocToBulk(DocToBulk docToBulk);

      abstract Builder setBulkIO(BulkIO bulkIO);

      abstract Write build();
    }

    
    /** Refer to {@link DocToBulk
    public Write withIdFn(FieldValueExtractFn idFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withIdFn(idFn)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withIndexFn(FieldValueExtractFn indexFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withIndexFn(indexFn)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withRoutingFn(FieldValueExtractFn routingFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withRoutingFn(routingFn)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withTypeFn(FieldValueExtractFn typeFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withTypeFn(typeFn)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withDocVersionFn(FieldValueExtractFn docVersionFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withDocVersionFn(docVersionFn)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withDocVersionType(String docVersionType) {
      return writeBuilder().setDocToBulk(getDocToBulk().withDocVersionType(docVersionType)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withUsePartialUpdate(boolean usePartialUpdate) {
      return writeBuilder()
          .setDocToBulk(getDocToBulk().withUsePartialUpdate(usePartialUpdate))
          .build();
    }

    /** Refer to {@link DocToBulk
    public Write withUpsertScript(String source) {
      return writeBuilder().setDocToBulk(getDocToBulk().withUpsertScript(source)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withBackendVersion(int backendVersion) {
      return writeBuilder().setDocToBulk(getDocToBulk().withBackendVersion(backendVersion)).build();
    }

    /** Refer to {@link DocToBulk
    public Write withIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn) {
      return writeBuilder().setDocToBulk(getDocToBulk().withIsDeleteFn(isDeleteFn)).build();
    }
    

    /** Refer to {@link BulkIO
    public Write withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");

      return writeBuilder()
          .setDocToBulk(getDocToBulk().withConnectionConfiguration(connectionConfiguration))
          .setBulkIO(getBulkIO().withConnectionConfiguration(connectionConfiguration))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withMaxBatchSize(long batchSize) {
      return writeBuilder().setBulkIO(getBulkIO().withMaxBatchSize(batchSize)).build();
    }

    /** Refer to {@link BulkIO
    public Write withMaxBatchSizeBytes(long batchSizeBytes) {
      return writeBuilder().setBulkIO(getBulkIO().withMaxBatchSizeBytes(batchSizeBytes)).build();
    }

    /** Refer to {@link BulkIO
    public Write withRetryConfiguration(RetryConfiguration retryConfiguration) {
      return writeBuilder()
          .setBulkIO(getBulkIO().withRetryConfiguration(retryConfiguration))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withIgnoreVersionConflicts(boolean ignoreVersionConflicts) {
      return writeBuilder()
          .setBulkIO(getBulkIO().withIgnoreVersionConflicts(ignoreVersionConflicts))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withUseStatefulBatches(boolean useStatefulBatches) {
      return writeBuilder()
          .setBulkIO(getBulkIO().withUseStatefulBatches(useStatefulBatches))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withMaxBufferingDuration(Duration maxBufferingDuration) {
      return writeBuilder()
          .setBulkIO(getBulkIO().withMaxBufferingDuration(maxBufferingDuration))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withMaxParallelRquestsPerWindow(int maxParallelRquestsPerWindow) {
      return writeBuilder()
          .setBulkIO(getBulkIO().withMaxParallelRequestsPerWindow(maxParallelRquestsPerWindow))
          .build();
    }

    /** Refer to {@link BulkIO
    public Write withAllowableResponseErrors(@Nullable Set<String> allowableResponseErrors) {
      if (allowableResponseErrors == null) {
        allowableResponseErrors = new HashSet<>();
      }

      return writeBuilder()
          .setBulkIO(getBulkIO().withAllowableResponseErrors(allowableResponseErrors))
          .build();
    }

    @Override
    public PDone expand(PCollection<String> input) {
      input.apply(getDocToBulk()).apply(getBulkIO());
      return PDone.in(input.getPipeline());
    }
  }

  /** A {@link PTransform} writing data to Elasticsearch. */
  @AutoValue
  public abstract static class BulkIO extends PTransform<PCollection<String>, PDone> {
    @VisibleForTesting
    static final String RETRY_ATTEMPT_LOG = "Error writing to Elasticsearch. Retry attempt[%d]";

    @VisibleForTesting
    static final String RETRY_FAILED_LOG =
        "Error writing to ES after %d attempt(s). No more attempts allowed";

    abstract @Nullable ConnectionConfiguration getConnectionConfiguration();

    abstract long getMaxBatchSize();

    abstract long getMaxBatchSizeBytes();

    abstract @Nullable Duration getMaxBufferingDuration();

    abstract boolean getUseStatefulBatches();

    abstract int getMaxParallelRequestsPerWindow();

    abstract @Nullable RetryConfiguration getRetryConfiguration();

    abstract @Nullable Set<String> getAllowedResponseErrors();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract Builder setConnectionConfiguration(ConnectionConfiguration connectionConfiguration);

      abstract Builder setMaxBatchSize(long maxBatchSize);

      abstract Builder setMaxBatchSizeBytes(long maxBatchSizeBytes);

      abstract Builder setRetryConfiguration(RetryConfiguration retryConfiguration);

      abstract Builder setAllowedResponseErrors(Set<String> allowedResponseErrors);

      abstract Builder setMaxBufferingDuration(Duration maxBufferingDuration);

      abstract Builder setUseStatefulBatches(boolean useStatefulBatches);

      abstract Builder setMaxParallelRequestsPerWindow(int maxParallelRequestsPerWindow);

      abstract BulkIO build();
    }

    /**
     * Provide the Elasticsearch connection configuration object.
     *
     * @param connectionConfiguration the Elasticsearch {@link ConnectionConfiguration} object
     * @return the {@link BulkIO} with connection configuration set
     */
    public BulkIO withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");

      return builder().setConnectionConfiguration(connectionConfiguration).build();
    }

    /**
     * Provide a maximum size in number of documents for the batch see bulk API
     * (https:
     * docs (like Elasticsearch bulk size advice). See
     * https:
     * execution engine, size of bundles may vary, this sets the maximum size. Change this if you
     * need to have smaller ElasticSearch bulks.
     *
     * @param batchSize maximum batch size in number of documents
     * @return the {@link BulkIO} with connection batch size set
     */
    public BulkIO withMaxBatchSize(long batchSize) {
      checkArgument(batchSize > 0, "batchSize must be > 0, but was %s", batchSize);
      return builder().setMaxBatchSize(batchSize).build();
    }

    /**
     * Provide a maximum size in bytes for the batch see bulk API
     * (https:
     * (like Elasticsearch bulk size advice). See
     * https:
     * execution engine, size of bundles may vary, this sets the maximum size. Change this if you
     * need to have smaller ElasticSearch bulks.
     *
     * @param batchSizeBytes maximum batch size in bytes
     * @return the {@link BulkIO} with connection batch size in bytes set
     */
    public BulkIO withMaxBatchSizeBytes(long batchSizeBytes) {
      checkArgument(batchSizeBytes > 0, "batchSizeBytes must be > 0, but was %s", batchSizeBytes);
      return builder().setMaxBatchSizeBytes(batchSizeBytes).build();
    }

    /**
     * Provides configuration to retry a failed batch call to Elasticsearch. A batch is considered
     * as failed if the underlying {@link RestClient} surfaces 429 HTTP status code as error for one
     * or more of the items in the {@link Response}. Users should consider that retrying might
     * compound the underlying problem which caused the initial failure. Users should also be aware
     * that once retrying is exhausted the error is surfaced to the runner which <em>may</em> then
     * opt to retry the current bundle in entirety or abort if the max number of retries of the
     * runner is completed. Retrying uses an exponential backoff algorithm, with minimum backoff of
     * 5 seconds and then surfacing the error once the maximum number of retries or maximum
     * configuration duration is exceeded.
     *
     * <p>Example use:
     *
     * <pre>{@code
     * ElasticsearchIO.write()
     *   .withRetryConfiguration(ElasticsearchIO.RetryConfiguration.create(10, Duration.standardMinutes(3))
     *   ...
     * }</pre>
     *
     * @param retryConfiguration the rules which govern the retry behavior
     * @return the {@link BulkIO} with retrying configured
     */
    public BulkIO withRetryConfiguration(RetryConfiguration retryConfiguration) {
      checkArgument(retryConfiguration != null, "retryConfiguration is required");
      return builder().setRetryConfiguration(retryConfiguration).build();
    }

    /**
     * Whether or not to suppress version conflict errors in a Bulk API response. This can be useful
     * if your use case involves using external version types.
     *
     * @param ignoreVersionConflicts true to suppress version conflicts, false to surface version
     *     conflict errors.
     * @return the {@link BulkIO} with version conflict handling configured
     */
    public BulkIO withIgnoreVersionConflicts(boolean ignoreVersionConflicts) {
      Set<String> allowedResponseErrors = getAllowedResponseErrors();
      if (allowedResponseErrors == null) {
        allowedResponseErrors = new HashSet<>();
      }
      if (ignoreVersionConflicts) {
        allowedResponseErrors.add(VERSION_CONFLICT_ERROR);
      }

      return builder().setAllowedResponseErrors(allowedResponseErrors).build();
    }

    /**
     * Provide a set of textual error types which can be contained in Bulk API response
     * items[].error.type field. Any element in @param allowableResponseErrorTypes will suppress
     * errors of the same type in Bulk responses.
     *
     * <p>See also
     * https:
     *
     * @param allowableResponseErrorTypes
     * @return the {@link BulkIO} with allowable response errors set
     */
    public BulkIO withAllowableResponseErrors(@Nullable Set<String> allowableResponseErrorTypes) {
      if (allowableResponseErrorTypes == null) {
        allowableResponseErrorTypes = new HashSet<>();
      }

      return builder().setAllowedResponseErrors(allowableResponseErrorTypes).build();
    }

    /**
     * If using {@link BulkIO
     * time before buffered elements are emitted to Elasticsearch as a Bulk API request. If this
     * config is not set, Bulk requests will not be issued until {@link BulkIO
     * number of documents have been buffered. This may result in higher latency in particular if
     * your max batch size is set to a large value and your pipeline input is low volume.
     *
     * @param maxBufferingDuration the maximum duration to wait before sending any buffered
     *     documents to Elasticsearch, regardless of maxBatchSize.
     * @return the {@link BulkIO} with maximum buffering duration set
     */
    public BulkIO withMaxBufferingDuration(Duration maxBufferingDuration) {
      LOG.warn(
          "Use of withMaxBufferingDuration requires withUseStatefulBatches(true). "
              + "Setting that automatically.");
      return builder()
          .setUseStatefulBatches(true)
          .setMaxBufferingDuration(maxBufferingDuration)
          .build();
    }

    /**
     * Whether or not to use Stateful Processing to ensure bulk requests have the desired number of
     * entities i.e. as close to the maxBatchSize as possible. By default without this feature
     * enabled, Bulk requests will not contain more than maxBatchSize entities, but the lower bound
     * of batch size is determined by Beam Runner bundle sizes, which may be as few as 1.
     *
     * @param useStatefulBatches true enables the use of Stateful Processing to ensure that batches
     *     are as close to the maxBatchSize as possible.
     * @return the {@link BulkIO} with Stateful Processing enabled or disabled
     */
    public BulkIO withUseStatefulBatches(boolean useStatefulBatches) {
      return builder().setUseStatefulBatches(useStatefulBatches).build();
    }

    /**
     * When using {@link BulkIO
     * batches are maintained per-key-per-window. If data is globally windowed and this
     * configuration is set to 1, there will only ever be 1 request in flight. Having only a single
     * request in flight can be beneficial for ensuring an Elasticsearch cluster is not overwhelmed
     * by parallel requests, but may not work for all use cases. If this number is less than the
     * number of maximum workers in your pipeline, the IO work may not be distributed across all
     * workers.
     *
     * @param maxParallelRequestsPerWindow the maximum number of parallel bulk requests for a window
     *     of data
     * @return the {@link BulkIO} with maximum parallel bulk requests per window set
     */
    public BulkIO withMaxParallelRequestsPerWindow(int maxParallelRequestsPerWindow) {
      checkArgument(
          maxParallelRequestsPerWindow > 0, "parameter value must be positive " + "a integer");
      return builder().setMaxParallelRequestsPerWindow(maxParallelRequestsPerWindow).build();
    }

    @Override
    public PDone expand(PCollection<String> input) {
      ConnectionConfiguration connectionConfiguration = getConnectionConfiguration();
      checkState(connectionConfiguration != null, "withConnectionConfiguration() is required");

      if (getUseStatefulBatches()) {
        GroupIntoBatches<Integer, String> groupIntoBatches =
            GroupIntoBatches.ofSize(getMaxBatchSize());

        if (getMaxBufferingDuration() != null) {
          groupIntoBatches = groupIntoBatches.withMaxBufferingDuration(getMaxBufferingDuration());
        }
        input
            .apply(ParDo.of(new AssignShardFn<>(getMaxParallelRequestsPerWindow())))
            .apply(groupIntoBatches)
            .apply(
                "Remove key no longer needed",
                MapElements.into(TypeDescriptors.iterables(TypeDescriptors.strings()))
                    .via(KV::getValue))
            .apply(ParDo.of(new BulkIOFn(this)));
      } else {

        input
            .apply(
                "Make elements iterable",
                MapElements.into(TypeDescriptors.iterables(TypeDescriptors.strings()))
                    .via(Collections::singletonList))
            .apply(ParDo.of(new BulkIOFn(this)));
      }

      return PDone.in(input.getPipeline());
    }

    /** {@link DoFn} to for the {@link BulkIO} transform. */
    @VisibleForTesting
    static class BulkIOFn extends DoFn<Iterable<String>, Void> {
      private static final Duration RETRY_INITIAL_BACKOFF = Duration.standardSeconds(5);

      private transient FluentBackoff retryBackoff;

      protected BulkIO spec;
      private transient RestClient restClient;
      protected ArrayList<String> batch;
      long currentBatchSizeBytes;

      @VisibleForTesting
      BulkIOFn(BulkIO bulkSpec) {
        this.spec = bulkSpec;
      }

      @Setup
      public void setup() throws IOException {
        ConnectionConfiguration connectionConfiguration = spec.getConnectionConfiguration();

        restClient = connectionConfiguration.createClient();

        retryBackoff =
            FluentBackoff.DEFAULT.withMaxRetries(0).withInitialBackoff(RETRY_INITIAL_BACKOFF);

        if (spec.getRetryConfiguration() != null) {
          retryBackoff =
              FluentBackoff.DEFAULT
                  .withInitialBackoff(RETRY_INITIAL_BACKOFF)
                  .withMaxRetries(spec.getRetryConfiguration().getMaxAttempts() - 1)
                  .withMaxCumulativeBackoff(spec.getRetryConfiguration().getMaxDuration());
        }
      }

      @StartBundle
      public void startBundle(StartBundleContext context) {
        batch = new ArrayList<>();
        currentBatchSizeBytes = 0;
      }

      @FinishBundle
      public void finishBundle(FinishBundleContext context)
          throws IOException, InterruptedException {
        flushBatch();
      }

      @ProcessElement
      public void processElement(@Element @NonNull Iterable<String> bulkApiEntities)
          throws Exception {
        for (String bulkApiEntity : bulkApiEntities) {
          addAndMaybeFlush(bulkApiEntity);
        }
      }

      protected void addAndMaybeFlush(String bulkApiEntity)
          throws IOException, InterruptedException {
        batch.add(bulkApiEntity);
        currentBatchSizeBytes += bulkApiEntity.getBytes(StandardCharsets.UTF_8).length;

        if (batch.size() >= spec.getMaxBatchSize()
            || currentBatchSizeBytes >= spec.getMaxBatchSizeBytes()) {
          flushBatch();
        }
      }

      private void flushBatch() throws IOException, InterruptedException {
        if (batch.isEmpty()) {
          return;
        }

        LOG.info(
            "ElasticsearchIO batch size: {}, batch size bytes: {}",
            batch.size(),
            currentBatchSizeBytes);

        StringBuilder bulkRequest = new StringBuilder();
        for (String json : batch) {
          bulkRequest.append(json);
        }

        batch.clear();
        currentBatchSizeBytes = 0L;

        Response response = null;
        HttpEntity responseEntity = null;

        
        
        
        
        String endPoint = spec.getConnectionConfiguration().getBulkEndPoint();

        HttpEntity requestBody =
            new NStringEntity(bulkRequest.toString(), ContentType.APPLICATION_JSON);
        try {
          Request request = new Request("POST", endPoint);
          request.addParameters(Collections.emptyMap());
          request.setEntity(requestBody);
          response = restClient.performRequest(request);
          responseEntity = new BufferedHttpEntity(response.getEntity());
        } catch (java.io.IOException ex) {
          if (spec.getRetryConfiguration() == null) {
            throw ex;
          }
          LOG.error("Caught ES timeout, retrying", ex);
        }

        if (spec.getRetryConfiguration() != null
            && (response == null
                || responseEntity == null
                || spec.getRetryConfiguration().getRetryPredicate().test(responseEntity))) {
          if (responseEntity != null
              && spec.getRetryConfiguration().getRetryPredicate().test(responseEntity)) {
            LOG.warn("ES Cluster is responding with HTP 429 - TOO_MANY_REQUESTS.");
          }
          responseEntity = handleRetry("POST", endPoint, Collections.emptyMap(), requestBody);
        }
        checkForErrors(responseEntity, spec.getAllowedResponseErrors());
      }

      /** retry request based on retry configuration policy. */
      private HttpEntity handleRetry(
          String method, String endpoint, Map<String, String> params, HttpEntity requestBody)
          throws IOException, InterruptedException {
        Response response;
        HttpEntity responseEntity;
        Sleeper sleeper = Sleeper.DEFAULT;
        BackOff backoff = retryBackoff.backoff();
        int attempt = 0;
        
        while (BackOffUtils.next(sleeper, backoff)) {
          LOG.warn(String.format(RETRY_ATTEMPT_LOG, ++attempt));
          try {
            Request request = new Request(method, endpoint);
            request.addParameters(params);
            request.setEntity(requestBody);
            response = restClient.performRequest(request);
            responseEntity = new BufferedHttpEntity(response.getEntity());
          } catch (java.io.IOException ex) {
            LOG.error("Caught ES timeout, retrying", ex);
            continue;
          }
          
          if (!Objects.requireNonNull(spec.getRetryConfiguration())
              .getRetryPredicate()
              .test(responseEntity)) {
            return responseEntity;
          } else {
            LOG.warn("ES Cluster is responding with HTP 429 - TOO_MANY_REQUESTS.");
          }
        }
        throw new IOException(String.format(RETRY_FAILED_LOG, attempt));
      }

      @Teardown
      public void closeClient() throws IOException {
        if (restClient != null) {
          restClient.close();
        }
      }
    }
  }

  static int getBackendVersion(ConnectionConfiguration connectionConfiguration) {
    try (RestClient restClient = connectionConfiguration.createClient()) {
      Request request = new Request("GET", "");
      Response response = restClient.performRequest(request);
      JsonNode jsonNode = parseResponse(response.getEntity());
      int backendVersion =
          Integer.parseInt(jsonNode.path("version").path("number").asText().substring(0, 1));
      checkArgument(
          (VALID_CLUSTER_VERSIONS.contains(backendVersion)),
          "The Elasticsearch version to connect to is %s.x. "
              + "This version of the ElasticsearchIO is only compatible with "
              + "Elasticsearch v7.x, v6.x, v5.x and v2.x",
          backendVersion);
      return backendVersion;

    } catch (IOException e) {
      throw new IllegalArgumentException("Cannot get Elasticsearch version", e);
    }
  }
}

Line commented on: return PDone.in(input.getPipeline());

Revised code:
public boolean start() throws IOException {
      restClient = source.spec.getConnectionConfiguration().createClient();

      String query = source.spec.getQuery() != null ? source.spec.getQuery().get() : null;
      if (query == null) {
        query = "{\"query\": { \"match_all\": {} }}";
      }
      if ((source.backendVersion >= 5) && source.numSlices != null && source.numSlices > 1) {
        
        String sliceQuery =
            String.format("\"slice\": {\"id\": %s,\"max\": %s}", source.sliceId, source.numSlices);
        query = query.replaceFirst("\\{", "{" + sliceQuery + ",");
      }
      String endPoint =
          String.format(
              "/%s/%s/_search",
              source.spec.getConnectionConfiguration().getIndex(),
              source.spec.getConnectionConfiguration().getType());
      Map<String, String> params = new HashMap<>();
      params.put("scroll", source.spec.getScrollKeepalive());
      if (source.backendVersion == 2) {
        params.put("size", String.valueOf(source.spec.getBatchSize()));
        if (source.shardPreference != null) {
          params.put("preference", "_shards:" + source.shardPreference);
        }
      }
      HttpEntity queryEntity = new NStringEntity(query, ContentType.APPLICATION_JSON);
      Request request = new Request("GET", endPoint);
      request.addParameters(params);
      request.setEntity(queryEntity);
      Response response = restClient.performRequest(request);
      JsonNode searchResult = parseResponse(response.getEntity());
      updateScrollId(searchResult);
      return readNextBatchAndReturnFirstDocument(searchResult);
    }

    private void updateScrollId(JsonNode searchResult) {
      scrollId = searchResult.path("_scroll_id").asText();
    }

    @Override
    public boolean advance() throws IOException {
      if (batchIterator.hasNext()) {
        current = batchIterator.next();
        return true;
      } else {
        String requestBody =
            String.format(
                "{\"scroll\" : \"%s\",\"scroll_id\" : \"%s\"}",
                source.spec.getScrollKeepalive(), scrollId);
        HttpEntity scrollEntity = new NStringEntity(requestBody, ContentType.APPLICATION_JSON);
        Request request = new Request("GET", "/_search/scroll");
        request.addParameters(Collections.emptyMap());
        request.setEntity(scrollEntity);
        Response response = restClient.performRequest(request);
        JsonNode searchResult = parseResponse(response.getEntity());
        updateScrollId(searchResult);
        return readNextBatchAndReturnFirstDocument(searchResult);
      }
    }

    private boolean readNextBatchAndReturnFirstDocument(JsonNode searchResult) {
      
      JsonNode hits = searchResult.path("hits").path("hits");
      if (hits.size() == 0) {
        current = null;
        batchIterator = null;
        return false;
      }
      
      List<String> batch = new ArrayList<>();
      boolean withMetadata = source.spec.isWithMetadata();
      for (JsonNode hit : hits) {
        if (withMetadata) {
          batch.add(hit.toString());
        } else {
          String document = hit.path("_source").toString();
          batch.add(document);
        }
      }
      batchIterator = batch.listIterator();
      current = batchIterator.next();
      return true;
    }

    @Override
    public String getCurrent() throws NoSuchElementException {
      if (current == null) {
        throw new NoSuchElementException();
      }
      return current;
    }

    @Override
    public void close() throws IOException {
      
      String requestBody = String.format("{\"scroll_id\" : [\"%s\"]}", scrollId);
      HttpEntity entity = new NStringEntity(requestBody, ContentType.APPLICATION_JSON);
      try {
        Request request = new Request("DELETE", "/_search/scroll");
        request.addParameters(Collections.emptyMap());
        request.setEntity(entity);
        restClient.performRequest(request);
      } finally {
        if (restClient != null) {
          restClient.close();
        }
      }
    }

    @Override
    public BoundedSource<String> getCurrentSource() {
      return source;
    }
  }
  /**
   * A POJO encapsulating a configuration for retry behavior when issuing requests to ES. A retry
   * will be attempted until the maxAttempts or maxDuration is exceeded, whichever comes first, for
   * 429 TOO_MANY_REQUESTS error.
   */
  @AutoValue
  public abstract static class RetryConfiguration implements Serializable {
    @VisibleForTesting
    static final RetryPredicate DEFAULT_RETRY_PREDICATE = new DefaultRetryPredicate();

    abstract int getMaxAttempts();

    abstract Duration getMaxDuration();

    abstract RetryPredicate getRetryPredicate();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract ElasticsearchIO.RetryConfiguration.Builder setMaxAttempts(int maxAttempts);

      abstract ElasticsearchIO.RetryConfiguration.Builder setMaxDuration(Duration maxDuration);

      abstract ElasticsearchIO.RetryConfiguration.Builder setRetryPredicate(
          RetryPredicate retryPredicate);

      abstract ElasticsearchIO.RetryConfiguration build();
    }

    /**
     * Creates RetryConfiguration for {@link ElasticsearchIO} with provided maxAttempts,
     * maxDurations and exponential backoff based retries.
     *
     * @param maxAttempts max number of attempts.
     * @param maxDuration maximum duration for retries.
     * @return {@link RetryConfiguration} object with provided settings.
     */
    public static RetryConfiguration create(int maxAttempts, Duration maxDuration) {
      checkArgument(maxAttempts > 0, "maxAttempts must be greater than 0");
      checkArgument(
          maxDuration != null && maxDuration.isLongerThan(Duration.ZERO),
          "maxDuration must be greater than 0");
      return new AutoValue_ElasticsearchIO_RetryConfiguration.Builder()
          .setMaxAttempts(maxAttempts)
          .setMaxDuration(maxDuration)
          .setRetryPredicate(DEFAULT_RETRY_PREDICATE)
          .build();
    }

    
    @VisibleForTesting
    RetryConfiguration withRetryPredicate(RetryPredicate predicate) {
      checkArgument(predicate != null, "predicate must be provided");
      return builder().setRetryPredicate(predicate).build();
    }

    /**
     * An interface used to control if we retry the Elasticsearch call when a {@link Response} is
     * obtained. If {@link RetryPredicate
     * the requests to the Elasticsearch server if the {@link RetryConfiguration} permits it.
     */
    @FunctionalInterface
    interface RetryPredicate extends Predicate<HttpEntity>, Serializable {}

    /**
     * This is the default predicate used to test if a failed ES operation should be retried. A
     * retry will be attempted until the maxAttempts or maxDuration is exceeded, whichever comes
     * first, for TOO_MANY_REQUESTS(429) error.
     */
    @VisibleForTesting
    static class DefaultRetryPredicate implements RetryPredicate {

      private int errorCode;

      DefaultRetryPredicate(int code) {
        this.errorCode = code;
      }

      DefaultRetryPredicate() {
        this(429);
      }

      /** Returns true if the response has the error code for any mutation. */
      private static boolean errorCodePresent(HttpEntity responseEntity, int errorCode) {
        try {
          JsonNode json = parseResponse(responseEntity);
          if (json.path("errors").asBoolean()) {
            for (JsonNode item : json.path("items")) {
              if (item.findValue("status").asInt() == errorCode) {
                return true;
              }
            }
          }
        } catch (IOException e) {
          LOG.warn("Could not extract error codes from responseEntity {}", responseEntity);
        }
        return false;
      }

      @Override
      public boolean test(HttpEntity responseEntity) {
        return errorCodePresent(responseEntity, errorCode);
      }
    }
  }

  /** A {@link PTransform} converting docs to their Bulk API counterparts. */
  @AutoValue
  public abstract static class DocToBulk
      extends PTransform<PCollection<String>, PCollection<String>> {

    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
    private static final int DEFAULT_RETRY_ON_CONFLICT = 5; 

    static {
      SimpleModule module = new SimpleModule();
      module.addSerializer(DocumentMetadata.class, new DocumentMetadataSerializer());
      OBJECT_MAPPER.registerModule(module);
    }

    abstract @Nullable ConnectionConfiguration getConnectionConfiguration();

    abstract Write.@Nullable FieldValueExtractFn getIdFn();

    abstract Write.@Nullable FieldValueExtractFn getIndexFn();

    abstract Write.@Nullable FieldValueExtractFn getRoutingFn();

    abstract Write.@Nullable FieldValueExtractFn getTypeFn();

    abstract Write.@Nullable FieldValueExtractFn getDocVersionFn();

    abstract @Nullable String getDocVersionType();

    abstract @Nullable String getUpsertScript();

    abstract @Nullable Boolean getUsePartialUpdate();

    abstract Write.@Nullable BooleanFieldValueExtractFn getIsDeleteFn();

    abstract @Nullable Integer getBackendVersion();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract Builder setConnectionConfiguration(ConnectionConfiguration connectionConfiguration);

      abstract Builder setIdFn(Write.FieldValueExtractFn idFunction);

      abstract Builder setIndexFn(Write.FieldValueExtractFn indexFn);

      abstract Builder setRoutingFn(Write.FieldValueExtractFn routingFunction);

      abstract Builder setTypeFn(Write.FieldValueExtractFn typeFn);

      abstract Builder setDocVersionFn(Write.FieldValueExtractFn docVersionFn);

      abstract Builder setDocVersionType(String docVersionType);

      abstract Builder setIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn);

      abstract Builder setUsePartialUpdate(Boolean usePartialUpdate);

      abstract Builder setUpsertScript(String source);

      abstract Builder setBackendVersion(Integer assumedBackendVersion);

      abstract DocToBulk build();
    }

    /**
     * Provide the Elasticsearch connection configuration object. Only required if
     * withBackendVersion was not used i.e. getBackendVersion() returns null.
     *
     * @param connectionConfiguration the Elasticsearch {@link ConnectionConfiguration} object
     * @return the {@link DocToBulk} with connection configuration set
     */
    public DocToBulk withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");
      return builder().setConnectionConfiguration(connectionConfiguration).build();
    }

    /**
     * Provide a function to extract the id from the document. This id will be used as the document
     * id in Elasticsearch. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param idFn to extract the document ID
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withIdFn(Write.FieldValueExtractFn idFn) {
      checkArgument(idFn != null, "idFn must not be null");
      return builder().setIdFn(idFn).build();
    }

    /**
     * Provide a function to extract the target index from the document allowing for dynamic
     * document routing. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param indexFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withIndexFn(Write.FieldValueExtractFn indexFn) {
      checkArgument(indexFn != null, "indexFn must not be null");
      return builder().setIndexFn(indexFn).build();
    }

    /**
     * Provide a function to extract the target routing from the document allowing for dynamic
     * document routing. Should the function throw an Exception then the batch will fail and the
     * exception propagated.
     *
     * @param routingFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withRoutingFn(Write.FieldValueExtractFn routingFn) {
      checkArgument(routingFn != null, "routingFn must not be null");
      return builder().setRoutingFn(routingFn).build();
    }

    /**
     * Provide a function to extract the target type from the document allowing for dynamic document
     * routing. Should the function throw an Exception then the batch will fail and the exception
     * propagated. Users are encouraged to consider carefully if multipe types are a sensible model
     * <a
     * href="https:
     * discussed in this blog</a>.
     *
     * @param typeFn to extract the destination index from
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withTypeFn(Write.FieldValueExtractFn typeFn) {
      checkArgument(typeFn != null, "typeFn must not be null");
      return builder().setTypeFn(typeFn).build();
    }

    /**
     * Provide an instruction to control whether partial updates or inserts (default) are issued to
     * Elasticsearch.
     *
     * @param usePartialUpdate set to true to issue partial updates
     * @return the {@link DocToBulk} with the partial update control set
     */
    public DocToBulk withUsePartialUpdate(boolean usePartialUpdate) {
      return builder().setUsePartialUpdate(usePartialUpdate).build();
    }

    /**
     * Whether to use scripted updates and what script to use.
     *
     * @param source set to the value of the script source, painless lang
     * @return the {@link DocToBulk} with the scripted updates set
     */
    public DocToBulk withUpsertScript(String source) {
      if (getBackendVersion() == null || getBackendVersion() == 2) {
        LOG.warn("Painless scripts are not supported on Elasticsearch clusters before version 5.0");
      }
      return builder().setUsePartialUpdate(false).setUpsertScript(source).build();
    }

    /**
     * Provide a function to extract the doc version from the document. This version number will be
     * used as the document version in Elasticsearch. Should the function throw an Exception then
     * the batch will fail and the exception propagated. Incompatible with update operations and
     * should only be used with withUsePartialUpdate(false)
     *
     * @param docVersionFn to extract the document version
     * @return the {@link DocToBulk} with the function set
     */
    public DocToBulk withDocVersionFn(Write.FieldValueExtractFn docVersionFn) {
      checkArgument(docVersionFn != null, "docVersionFn must not be null");
      return builder().setDocVersionFn(docVersionFn).build();
    }

    /**
     * Provide a function to extract the target operation either upsert or delete from the document
     * fields allowing dynamic bulk operation decision. While using withIsDeleteFn, it should be
     * taken care that the document's id extraction is defined using the withIdFn function or else
     * IllegalArgumentException is thrown. Should the function throw an Exception then the batch
     * will fail and the exception propagated.
     *
     * @param isDeleteFn set to true for deleting the specific document
     * @return the {@link Write} with the function set
     */
    public DocToBulk withIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn) {
      checkArgument(isDeleteFn != null, "deleteFn is required");
      return builder().setIsDeleteFn(isDeleteFn).build();
    }

    /**
     * Provide a function to extract the doc version from the document. This version number will be
     * used as the document version in Elasticsearch. Should the function throw an Exception then
     * the batch will fail and the exception propagated. Incompatible with update operations and
     * should only be used with withUsePartialUpdate(false)
     *
     * @param docVersionType the version type to use, one of {@value VERSION_TYPES}
     * @return the {@link DocToBulk} with the doc version type set
     */
    public DocToBulk withDocVersionType(String docVersionType) {
      checkArgument(
          VERSION_TYPES.contains(docVersionType),
          "docVersionType must be one of " + "%s",
          String.join(", ", VERSION_TYPES));
      return builder().setDocVersionType(docVersionType).build();
    }

    /**
     * Use to set explicitly which version of Elasticsearch the destination cluster is running.
     * Providing this hint means there is no need for setting {@link
     * DocToBulk
     *
     * <p>Note: if the value of @param backendVersion differs from the version the destination
     * cluster is running, behavior is undefined and likely to yield errors.
     *
     * @param backendVersion the major version number of the version of Elasticsearch being run in
     *     the cluster where documents will be indexed.
     * @return the {@link DocToBulk} with the Elasticsearch major version number set
     */
    public DocToBulk withBackendVersion(int backendVersion) {
      checkArgument(
          VALID_CLUSTER_VERSIONS.contains(backendVersion),
          "Backend version may only be one of " + "%s",
          String.join(", ", VERSION_TYPES));
      return builder().setBackendVersion(backendVersion).build();
    }

    @Override
    public PCollection<String> expand(PCollection<String> docs) {
      ConnectionConfiguration connectionConfiguration = getConnectionConfiguration();
      Integer backendVersion = getBackendVersion();
      Write.FieldValueExtractFn idFn = getIdFn();
      Write.BooleanFieldValueExtractFn isDeleteFn = getIsDeleteFn();
      checkState(
          (backendVersion != null || connectionConfiguration != null),
          "withBackendVersion() or withConnectionConfiguration() is required");
      checkArgument(
          isDeleteFn == null || idFn != null,
          "Id needs to be specified by withIdFn for delete operation");

      return docs.apply(ParDo.of(new DocToBulkFn(this)));
    }

    
    private static class DocumentMetadata implements Serializable {
      final String index;
      final String type;
      final String id;
      final Integer retryOnConflict;
      final String routing;
      final Integer backendVersion;
      final String version;
      final String versionType;

      DocumentMetadata(
          String index,
          String type,
          String id,
          Integer retryOnConflict,
          String routing,
          Integer backendVersion,
          String version,
          String versionType) {
        this.index = index;
        this.id = id;
        this.type = type;
        this.retryOnConflict = retryOnConflict;
        this.routing = routing;
        this.backendVersion = backendVersion;
        this.version = version;
        this.versionType = versionType;
      }
    }

    private static class DocumentMetadataSerializer extends StdSerializer<DocumentMetadata> {
      private DocumentMetadataSerializer() {
        super(DocumentMetadata.class);
      }

      @Override
      public void serialize(DocumentMetadata value, JsonGenerator gen, SerializerProvider provider)
          throws IOException {
        gen.writeStartObject();
        if (value.index != null) {
          gen.writeStringField("_index", value.index);
        }
        if (value.type != null) {
          gen.writeStringField("_type", value.type);
        }
        if (value.id != null) {
          gen.writeStringField("_id", value.id);
        }
        if (value.routing != null) {
          gen.writeStringField("routing", value.routing);
        }
        if (value.retryOnConflict != null && value.backendVersion <= 6) {
          gen.writeNumberField("_retry_on_conflict", value.retryOnConflict);
        }
        if (value.retryOnConflict != null && value.backendVersion >= 7) {
          gen.writeNumberField("retry_on_conflict", value.retryOnConflict);
        }
        if (value.version != null) {
          gen.writeStringField("version", value.version);
        }
        if (value.versionType != null) {
          gen.writeStringField("version_type", value.versionType);
        }
        gen.writeEndObject();
      }
    }

    @VisibleForTesting
    static String createBulkApiEntity(DocToBulk spec, String document, int backendVersion)
        throws IOException {
      String documentMetadata = "{}";
      boolean isDelete = false;
      if (spec.getIndexFn() != null
          || spec.getTypeFn() != null
          || spec.getIdFn() != null
          || spec.getRoutingFn() != null) {
        
        JsonNode parsedDocument = OBJECT_MAPPER.readTree(document);
        documentMetadata = getDocumentMetadata(spec, parsedDocument, backendVersion);
        if (spec.getIsDeleteFn() != null) {
          isDelete = spec.getIsDeleteFn().apply(parsedDocument);
        }
      }

      if (isDelete) {
        
        return String.format("{ \"delete\" : %s }%n", documentMetadata);
      } else {
        
        
        if (spec.getUsePartialUpdate()) {
          return String.format(
              "{ \"update\" : %s }%n{ \"doc\" : %s, " + "\"doc_as_upsert\" : true }%n",
              documentMetadata, document);
        } else if (spec.getUpsertScript() != null) {
          return String.format(
              "{ \"update\" : %s }%n{ \"script\" : {\"source\": \"%s\", "
                  + "\"params\": %s}, \"upsert\" : %s, \"scripted_upsert\": true}%n",
              documentMetadata, spec.getUpsertScript(), document, document);
        } else {
          return String.format("{ \"index\" : %s }%n%s%n", documentMetadata, document);
        }
      }
    }

    private static String lowerCaseOrNull(String input) {
      return input == null ? null : input.toLowerCase();
    }

    /**
     * Extracts the components that comprise the document address from the document using the {@link
     * Write.FieldValueExtractFn} configured. This allows any or all of the index, type and document
     * id to be controlled on a per document basis. If none are provided then an empty default of
     * {@code {}} is returned. Sanitization of the index is performed, automatically lower-casing
     * the value as required by Elasticsearch.
     *
     * @param parsedDocument the json from which the index, type and id may be extracted
     * @return the document address as JSON or the default
     * @throws IOException if the document cannot be parsed as JSON
     */
    private static String getDocumentMetadata(
        DocToBulk spec, JsonNode parsedDocument, int backendVersion) throws IOException {
      DocumentMetadata metadata =
          new DocumentMetadata(
              spec.getIndexFn() != null
                  ? lowerCaseOrNull(spec.getIndexFn().apply(parsedDocument))
                  : null,
              spec.getTypeFn() != null ? spec.getTypeFn().apply(parsedDocument) : null,
              spec.getIdFn() != null ? spec.getIdFn().apply(parsedDocument) : null,
              (spec.getUsePartialUpdate()
                      || (spec.getUpsertScript() != null && !spec.getUpsertScript().isEmpty()))
                  ? DEFAULT_RETRY_ON_CONFLICT
                  : null,
              spec.getRoutingFn() != null ? spec.getRoutingFn().apply(parsedDocument) : null,
              backendVersion,
              spec.getDocVersionFn() != null ? spec.getDocVersionFn().apply(parsedDocument) : null,
              spec.getDocVersionType());
      return OBJECT_MAPPER.writeValueAsString(metadata);
    }

    /** {@link DoFn} to for the {@link DocToBulk} transform. */
    @VisibleForTesting
    static class DocToBulkFn extends DoFn<String, String> {
      private final DocToBulk spec;
      private int backendVersion;

      public DocToBulkFn(DocToBulk spec) {
        this.spec = spec;
      }

      @Setup
      public void setup() throws IOException {
        if (spec.getBackendVersion() != null) {
          backendVersion = spec.getBackendVersion();
        } else {
          backendVersion = ElasticsearchIO.getBackendVersion(spec.getConnectionConfiguration());
        }
      }

      @ProcessElement
      public void processElement(ProcessContext c) throws IOException {
        c.output(createBulkApiEntity(spec, c.element(), backendVersion));
      }
    }
  }

  /**
   * A {@link PTransform} writing data to Elasticsearch.
   *
   * <p>This {@link PTransform} acts as a convenience wrapper for doing both document to bulk API
   * serialization as well as batching those Bulk API entities and writing them to an Elasticsearch
   * cluster. This class is effectively a thin proxy for DocToBulk->BulkIO all-in-one for
   * convenience and backward compatibility.
   */
  public static class Write extends PTransform<PCollection<String>, PDone> {
    public interface FieldValueExtractFn extends SerializableFunction<JsonNode, String> {}

    public interface BooleanFieldValueExtractFn extends SerializableFunction<JsonNode, Boolean> {}

    private DocToBulk docToBulk =
        new AutoValue_ElasticsearchIO_DocToBulk.Builder()
            .setUsePartialUpdate(false) 
            .build();

    private BulkIO bulkIO =
        new AutoValue_ElasticsearchIO_BulkIO.Builder()
            
            .setMaxBatchSize(1000L)
            
            .setMaxBatchSizeBytes(5L * 1024L * 1024L)
            .setUseStatefulBatches(false)
            .setMaxParallelRequestsPerWindow(1)
            .build();

    public DocToBulk getDocToBulk() {
      return docToBulk;
    }

    public BulkIO getBulkIO() {
      return bulkIO;
    }

    
    /** Refer to {@link DocToBulk
    public Write withIdFn(FieldValueExtractFn idFn) {
      docToBulk = docToBulk.withIdFn(idFn);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withIndexFn(FieldValueExtractFn indexFn) {
      docToBulk = docToBulk.withIndexFn(indexFn);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withRoutingFn(FieldValueExtractFn routingFn) {
      docToBulk = docToBulk.withRoutingFn(routingFn);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withTypeFn(FieldValueExtractFn typeFn) {
      docToBulk = docToBulk.withTypeFn(typeFn);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withDocVersionFn(FieldValueExtractFn docVersionFn) {
      docToBulk = docToBulk.withDocVersionFn(docVersionFn);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withDocVersionType(String docVersionType) {
      docToBulk = docToBulk.withDocVersionType(docVersionType);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withUsePartialUpdate(boolean usePartialUpdate) {
      docToBulk = docToBulk.withUsePartialUpdate(usePartialUpdate);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withUpsertScript(String source) {
      docToBulk = docToBulk.withUpsertScript(source);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withBackendVersion(int backendVersion) {
      docToBulk = docToBulk.withBackendVersion(backendVersion);
      return this;
    }

    /** Refer to {@link DocToBulk
    public Write withIsDeleteFn(Write.BooleanFieldValueExtractFn isDeleteFn) {
      docToBulk = docToBulk.withIsDeleteFn(isDeleteFn);
      return this;
    }
    

    /** Refer to {@link BulkIO
    public Write withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");
      docToBulk = docToBulk.withConnectionConfiguration(connectionConfiguration);
      bulkIO = bulkIO.withConnectionConfiguration(connectionConfiguration);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withMaxBatchSize(long batchSize) {
      bulkIO = bulkIO.withMaxBatchSize(batchSize);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withMaxBatchSizeBytes(long batchSizeBytes) {
      bulkIO = bulkIO.withMaxBatchSizeBytes(batchSizeBytes);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withRetryConfiguration(RetryConfiguration retryConfiguration) {
      bulkIO = bulkIO.withRetryConfiguration(retryConfiguration);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withIgnoreVersionConflicts(boolean ignoreVersionConflicts) {
      bulkIO = bulkIO.withIgnoreVersionConflicts(ignoreVersionConflicts);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withUseStatefulBatches(boolean useStatefulBatches) {
      bulkIO = bulkIO.withUseStatefulBatches(useStatefulBatches);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withMaxBufferingDuration(Duration maxBufferingDuration) {
      bulkIO = bulkIO.withMaxBufferingDuration(maxBufferingDuration);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withMaxParallelRequestsPerWindow(int maxParallelRequestsPerWindow) {
      bulkIO = bulkIO.withMaxParallelRequestsPerWindow(maxParallelRequestsPerWindow);
      return this;
    }

    /** Refer to {@link BulkIO
    public Write withAllowableResponseErrors(@Nullable Set<String> allowableResponseErrors) {
      if (allowableResponseErrors == null) {
        allowableResponseErrors = new HashSet<>();
      }

      bulkIO = bulkIO.withAllowableResponseErrors(allowableResponseErrors);
      return this;
    }

    @Override
    public PDone expand(PCollection<String> input) {
      return input.apply(docToBulk).apply(bulkIO);
    }
  }

  /**
   * A {@link PTransform} writing Bulk API entities created by {@link ElasticsearchIO.DocToBulk} to
   * an Elasticsearch cluster. Typically, using {@link ElasticsearchIO.Write} is preferred, whereas
   * using {@link ElasticsearchIO.DocToBulk} and BulkIO separately is for advanced use cases such as
   * mirroring data to multiple clusters or data lakes without recomputation.
   */
  @AutoValue
  public abstract static class BulkIO extends PTransform<PCollection<String>, PDone> {
    @VisibleForTesting
    static final String RETRY_ATTEMPT_LOG = "Error writing to Elasticsearch. Retry attempt[%d]";

    @VisibleForTesting
    static final String RETRY_FAILED_LOG =
        "Error writing to ES after %d attempt(s). No more attempts allowed";

    abstract @Nullable ConnectionConfiguration getConnectionConfiguration();

    abstract long getMaxBatchSize();

    abstract long getMaxBatchSizeBytes();

    abstract @Nullable Duration getMaxBufferingDuration();

    abstract boolean getUseStatefulBatches();

    abstract int getMaxParallelRequestsPerWindow();

    abstract @Nullable RetryConfiguration getRetryConfiguration();

    abstract @Nullable Set<String> getAllowedResponseErrors();

    abstract Builder builder();

    @AutoValue.Builder
    abstract static class Builder {
      abstract Builder setConnectionConfiguration(ConnectionConfiguration connectionConfiguration);

      abstract Builder setMaxBatchSize(long maxBatchSize);

      abstract Builder setMaxBatchSizeBytes(long maxBatchSizeBytes);

      abstract Builder setRetryConfiguration(RetryConfiguration retryConfiguration);

      abstract Builder setAllowedResponseErrors(Set<String> allowedResponseErrors);

      abstract Builder setMaxBufferingDuration(Duration maxBufferingDuration);

      abstract Builder setUseStatefulBatches(boolean useStatefulBatches);

      abstract Builder setMaxParallelRequestsPerWindow(int maxParallelRequestsPerWindow);

      abstract BulkIO build();
    }

    /**
     * Provide the Elasticsearch connection configuration object.
     *
     * @param connectionConfiguration the Elasticsearch {@link ConnectionConfiguration} object
     * @return the {@link BulkIO} with connection configuration set
     */
    public BulkIO withConnectionConfiguration(ConnectionConfiguration connectionConfiguration) {
      checkArgument(connectionConfiguration != null, "connectionConfiguration can not be null");

      return builder().setConnectionConfiguration(connectionConfiguration).build();
    }

    /**
     * Provide a maximum size in number of documents for the batch see bulk API
     * (https:
     * docs (like Elasticsearch bulk size advice). See
     * https:
     * execution engine, size of bundles may vary, this sets the maximum size. Change this if you
     * need to have smaller ElasticSearch bulks.
     *
     * @param batchSize maximum batch size in number of documents
     * @return the {@link BulkIO} with connection batch size set
     */
    public BulkIO withMaxBatchSize(long batchSize) {
      checkArgument(batchSize > 0, "batchSize must be > 0, but was %s", batchSize);
      return builder().setMaxBatchSize(batchSize).build();
    }

    /**
     * Provide a maximum size in bytes for the batch see bulk API
     * (https:
     * (like Elasticsearch bulk size advice). See
     * https:
     * execution engine, size of bundles may vary, this sets the maximum size. Change this if you
     * need to have smaller ElasticSearch bulks.
     *
     * @param batchSizeBytes maximum batch size in bytes
     * @return the {@link BulkIO} with connection batch size in bytes set
     */
    public BulkIO withMaxBatchSizeBytes(long batchSizeBytes) {
      checkArgument(batchSizeBytes > 0, "batchSizeBytes must be > 0, but was %s", batchSizeBytes);
      return builder().setMaxBatchSizeBytes(batchSizeBytes).build();
    }

    /**
     * Provides configuration to retry a failed batch call to Elasticsearch. A batch is considered
     * as failed if the underlying {@link RestClient} surfaces 429 HTTP status code as error for one
     * or more of the items in the {@link Response}. Users should consider that retrying might
     * compound the underlying problem which caused the initial failure. Users should also be aware
     * that once retrying is exhausted the error is surfaced to the runner which <em>may</em> then
     * opt to retry the current bundle in entirety or abort if the max number of retries of the
     * runner is completed. Retrying uses an exponential backoff algorithm, with minimum backoff of
     * 5 seconds and then surfacing the error once the maximum number of retries or maximum
     * configuration duration is exceeded.
     *
     * <p>Example use:
     *
     * <pre>{@code
     * ElasticsearchIO.write()
     *   .withRetryConfiguration(ElasticsearchIO.RetryConfiguration.create(10, Duration.standardMinutes(3))
     *   ...
     * }</pre>
     *
     * @param retryConfiguration the rules which govern the retry behavior
     * @return the {@link BulkIO} with retrying configured
     */
    public BulkIO withRetryConfiguration(RetryConfiguration retryConfiguration) {
      checkArgument(retryConfiguration != null, "retryConfiguration is required");
      return builder().setRetryConfiguration(retryConfiguration).build();
    }

    /**
     * Whether or not to suppress version conflict errors in a Bulk API response. This can be useful
     * if your use case involves using external version types.
     *
     * @param ignoreVersionConflicts true to suppress version conflicts, false to surface version
     *     conflict errors.
     * @return the {@link BulkIO} with version conflict handling configured
     */
    public BulkIO withIgnoreVersionConflicts(boolean ignoreVersionConflicts) {
      Set<String> allowedResponseErrors = getAllowedResponseErrors();
      if (allowedResponseErrors == null) {
        allowedResponseErrors = new HashSet<>();
      }
      if (ignoreVersionConflicts) {
        allowedResponseErrors.add(VERSION_CONFLICT_ERROR);
      }

      return builder().setAllowedResponseErrors(allowedResponseErrors).build();
    }

    /**
     * Provide a set of textual error types which can be contained in Bulk API response
     * items[].error.type field. Any element in @param allowableResponseErrorTypes will suppress
     * errors of the same type in Bulk responses.
     *
     * <p>See also
     * https:
     *
     * @param allowableResponseErrorTypes
     * @return the {@link BulkIO} with allowable response errors set
     */
    public BulkIO withAllowableResponseErrors(@Nullable Set<String> allowableResponseErrorTypes) {
      if (allowableResponseErrorTypes == null) {
        allowableResponseErrorTypes = new HashSet<>();
      }

      return builder().setAllowedResponseErrors(allowableResponseErrorTypes).build();
    }

    /**
     * If using {@link BulkIO
     * time before buffered elements are emitted to Elasticsearch as a Bulk API request. If this
     * config is not set, Bulk requests will not be issued until {@link BulkIO
     * number of documents have been buffered. This may result in higher latency in particular if
     * your max batch size is set to a large value and your pipeline input is low volume.
     *
     * @param maxBufferingDuration the maximum duration to wait before sending any buffered
     *     documents to Elasticsearch, regardless of maxBatchSize.
     * @return the {@link BulkIO} with maximum buffering duration set
     */
    public BulkIO withMaxBufferingDuration(Duration maxBufferingDuration) {
      LOG.warn(
          "Use of withMaxBufferingDuration requires withUseStatefulBatches(true). "
              + "Setting that automatically.");
      return builder()
          .setUseStatefulBatches(true)
          .setMaxBufferingDuration(maxBufferingDuration)
          .build();
    }

    /**
     * Whether or not to use Stateful Processing to ensure bulk requests have the desired number of
     * entities i.e. as close to the maxBatchSize as possible. By default without this feature
     * enabled, Bulk requests will not contain more than maxBatchSize entities, but the lower bound
     * of batch size is determined by Beam Runner bundle sizes, which may be as few as 1.
     *
     * @param useStatefulBatches true enables the use of Stateful Processing to ensure that batches
     *     are as close to the maxBatchSize as possible.
     * @return the {@link BulkIO} with Stateful Processing enabled or disabled
     */
    public BulkIO withUseStatefulBatches(boolean useStatefulBatches) {
      return builder().setUseStatefulBatches(useStatefulBatches).build();
    }

    /**
     * When using {@link BulkIO
     * batches are maintained per-key-per-window. BE AWARE that low values for @param
     * maxParallelRequestsPerWindow, in particular if the input data has a finite number of windows,
     * can reduce parallelism greatly. If data is globally windowed and @param
     * maxParallelRequestsPerWindow is set to 1,there will only ever be 1 request in flight. Having
     * only a single request in flight can be beneficial for ensuring an Elasticsearch cluster is
     * not overwhelmed by parallel requests,but may not work for all use cases. If this number is
     * less than the number of maximum workers in your pipeline, the IO work will result in a
     * sub-distribution of the last write step with most of the runners.
     *
     * @param maxParallelRequestsPerWindow the maximum number of parallel bulk requests for a window
     *     of data
     * @return the {@link BulkIO} with maximum parallel bulk requests per window set
     */
    public BulkIO withMaxParallelRequestsPerWindow(int maxParallelRequestsPerWindow) {
      checkArgument(
          maxParallelRequestsPerWindow > 0, "parameter value must be positive " + "a integer");
      return builder().setMaxParallelRequestsPerWindow(maxParallelRequestsPerWindow).build();
    }

    /**
     * Creates batches of documents using Stateful Processing based on user configurable settings of
     * withMaxBufferingDuration and withMaxParallelRequestsPerWindow.
     *
     * <p>Mostly exists for testability of withMaxParallelRequestsPerWindow.
     */
    @VisibleForTesting
    static class StatefulBatching
        extends PTransform<PCollection<String>, PCollection<KV<Integer, Iterable<String>>>> {
      final BulkIO spec;

      private StatefulBatching(BulkIO bulkSpec) {
        spec = bulkSpec;
      }

      public static StatefulBatching fromSpec(BulkIO spec) {
        return new StatefulBatching(spec);
      }

      @Override
      public PCollection<KV<Integer, Iterable<String>>> expand(PCollection<String> input) {
        GroupIntoBatches<Integer, String> groupIntoBatches =
            GroupIntoBatches.ofSize(spec.getMaxBatchSize());

        if (spec.getMaxBufferingDuration() != null) {
          groupIntoBatches =
              groupIntoBatches.withMaxBufferingDuration(spec.getMaxBufferingDuration());
        }

        return input
            .apply(ParDo.of(new Reshuffle.AssignShardFn<>(spec.getMaxParallelRequestsPerWindow())))
            .apply(groupIntoBatches);
      }
    }

    @Override
    public PDone expand(PCollection<String> input) {
      ConnectionConfiguration connectionConfiguration = getConnectionConfiguration();

      checkState(connectionConfiguration != null, "withConnectionConfiguration() is required");

      if (getUseStatefulBatches()) {
        input.apply(StatefulBatching.fromSpec(this)).apply(ParDo.of(new BulkIOStatefulFn(this)));
      } else {
        input.apply(ParDo.of(new BulkIOBundleFn(this)));
      }
      return PDone.in(input.getPipeline());
    }

    static class BulkIOBundleFn extends BulkIOBaseFn<String> {
      @VisibleForTesting
      BulkIOBundleFn(BulkIO bulkSpec) {
        super(bulkSpec);
      }

      @ProcessElement
      public void processElement(ProcessContext context) throws Exception {
        String bulkApiEntity = context.element();
        addAndMaybeFlush(bulkApiEntity);
      }
    }

    /*
    Intended for use in conjunction with {@link GroupIntoBatches}
     */
    static class BulkIOStatefulFn extends BulkIOBaseFn<KV<Integer, Iterable<String>>> {
      @VisibleForTesting
      BulkIOStatefulFn(BulkIO bulkSpec) {
        super(bulkSpec);
      }

      @ProcessElement
      public void processElement(ProcessContext context) throws Exception {
        Iterable<String> bulkApiEntities = context.element().getValue();
        for (String bulkApiEntity : bulkApiEntities) {
          addAndMaybeFlush(bulkApiEntity);
        }
      }
    }

    /** {@link DoFn} to for the {@link BulkIO} transform. */
    @VisibleForTesting
    private abstract static class BulkIOBaseFn<T> extends DoFn<T, Void> {
      private static final Duration RETRY_INITIAL_BACKOFF = Duration.standardSeconds(5);

      private transient FluentBackoff retryBackoff;

      private BulkIO spec;
      private transient RestClient restClient;
      private ArrayList<String> batch;
      long currentBatchSizeBytes;

      protected BulkIOBaseFn(BulkIO bulkSpec) {
        this.spec = bulkSpec;
      }

      @Setup
      public void setup() throws IOException {
        ConnectionConfiguration connectionConfiguration = spec.getConnectionConfiguration();

        restClient = connectionConfiguration.createClient();

        retryBackoff =
            FluentBackoff.DEFAULT.withMaxRetries(0).withInitialBackoff(RETRY_INITIAL_BACKOFF);

        if (spec.getRetryConfiguration() != null) {
          retryBackoff =
              FluentBackoff.DEFAULT
                  .withInitialBackoff(RETRY_INITIAL_BACKOFF)
                  .withMaxRetries(spec.getRetryConfiguration().getMaxAttempts() - 1)
                  .withMaxCumulativeBackoff(spec.getRetryConfiguration().getMaxDuration());
        }
      }

      @StartBundle
      public void startBundle(StartBundleContext context) {
        batch = new ArrayList<>();
        currentBatchSizeBytes = 0;
      }

      @FinishBundle
      public void finishBundle(FinishBundleContext context)
          throws IOException, InterruptedException {
        flushBatch();
      }

      protected void addAndMaybeFlush(String bulkApiEntity)
          throws IOException, InterruptedException {
        batch.add(bulkApiEntity);
        currentBatchSizeBytes += bulkApiEntity.getBytes(StandardCharsets.UTF_8).length;

        if (batch.size() >= spec.getMaxBatchSize()
            || currentBatchSizeBytes >= spec.getMaxBatchSizeBytes()) {
          flushBatch();
        }
      }

      private boolean isRetryableClientException(Throwable t) {
        
        
        
        return t.getCause() instanceof ConnectTimeoutException
            || t.getCause() instanceof SocketTimeoutException
            || t.getCause() instanceof ConnectionClosedException
            || t.getCause() instanceof ConnectException;
      }

      private void flushBatch() throws IOException, InterruptedException {
        if (batch.isEmpty()) {
          return;
        }

        LOG.info(
            "ElasticsearchIO batch size: {}, batch size bytes: {}",
            batch.size(),
            currentBatchSizeBytes);

        StringBuilder bulkRequest = new StringBuilder();
        for (String json : batch) {
          bulkRequest.append(json);
        }

        batch.clear();
        currentBatchSizeBytes = 0L;

        Response response = null;
        HttpEntity responseEntity = null;

        
        
        
        
        String endPoint = spec.getConnectionConfiguration().getBulkEndPoint();

        HttpEntity requestBody =
            new NStringEntity(bulkRequest.toString(), ContentType.APPLICATION_JSON);
        try {
          Request request = new Request("POST", endPoint);
          request.addParameters(Collections.emptyMap());
          request.setEntity(requestBody);
          response = restClient.performRequest(request);
          responseEntity = new BufferedHttpEntity(response.getEntity());
        } catch (java.io.IOException ex) {
          if (spec.getRetryConfiguration() == null || !isRetryableClientException(ex)) {
            throw ex;
          }
          LOG.error("Caught ES timeout, retrying", ex);
        }

        if (spec.getRetryConfiguration() != null
            && (response == null
                || responseEntity == null
                || spec.getRetryConfiguration().getRetryPredicate().test(responseEntity))) {
          if (responseEntity != null
              && spec.getRetryConfiguration().getRetryPredicate().test(responseEntity)) {
            LOG.warn("ES Cluster is responding with HTP 429 - TOO_MANY_REQUESTS.");
          }
          responseEntity = handleRetry("POST", endPoint, Collections.emptyMap(), requestBody);
        }
        checkForErrors(responseEntity, spec.getAllowedResponseErrors());
      }

      /** retry request based on retry configuration policy. */
      private HttpEntity handleRetry(
          String method, String endpoint, Map<String, String> params, HttpEntity requestBody)
          throws IOException, InterruptedException {
        Response response;
        HttpEntity responseEntity = null;
        Sleeper sleeper = Sleeper.DEFAULT;
        BackOff backoff = retryBackoff.backoff();
        int attempt = 0;
        
        while (BackOffUtils.next(sleeper, backoff)) {
          LOG.warn(String.format(RETRY_ATTEMPT_LOG, ++attempt));
          try {
            Request request = new Request(method, endpoint);
            request.addParameters(params);
            request.setEntity(requestBody);
            response = restClient.performRequest(request);
            responseEntity = new BufferedHttpEntity(response.getEntity());
          } catch (java.io.IOException ex) {
            if (isRetryableClientException(ex)) {
              LOG.error("Caught ES timeout, retrying", ex);
              continue;
            }
          }
          
          if (!Objects.requireNonNull(spec.getRetryConfiguration())
              .getRetryPredicate()
              .test(responseEntity)) {
            return responseEntity;
          } else {
            LOG.warn("ES Cluster is responding with HTP 429 - TOO_MANY_REQUESTS.");
          }
        }
        throw new IOException(String.format(RETRY_FAILED_LOG, attempt));
      }

      @Teardown
      public void closeClient() throws IOException {
        if (restClient != null) {
          restClient.close();
        }
      }
    }
  }

  static int getBackendVersion(ConnectionConfiguration connectionConfiguration) {
    try (RestClient restClient = connectionConfiguration.createClient()) {
      Request request = new Request("GET", "");
      Response response = restClient.performRequest(request);
      JsonNode jsonNode = parseResponse(response.getEntity());
      int backendVersion =
          Integer.parseInt(jsonNode.path("version").path("number").asText().substring(0, 1));
      checkArgument(
          (VALID_CLUSTER_VERSIONS.contains(backendVersion)),
          "The Elasticsearch version to connect to is %s.x. "
              + "This version of the ElasticsearchIO is only compatible with "
              + "Elasticsearch v7.x, v6.x, v5.x and v2.x",
          backendVersion);
      return backendVersion;

    } catch (IOException e) {
      throw new IllegalArgumentException("Cannot get Elasticsearch version", e);
    }
  }
}