Github user srdo commented on a diff in the pull request:

    https://github.com/apache/storm/pull/2538#discussion_r165859312
  
    --- Diff: 
external/storm-kafka-client/src/main/java/org/apache/storm/kafka/spout/internal/CommitMetadataManager.java
 ---
    @@ -0,0 +1,90 @@
    +/*
    + * Copyright 2018 The Apache Software Foundation.
    + *
    + * Licensed under the Apache License, Version 2.0 (the "License");
    + * you may not use this file except in compliance with the License.
    + * You may obtain a copy of the License at
    + *
    + *      http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.storm.kafka.spout.internal;
    +
    +import com.fasterxml.jackson.core.JsonProcessingException;
    +import com.fasterxml.jackson.databind.ObjectMapper;
    +import java.io.IOException;
    +import java.util.Map;
    +import org.apache.kafka.clients.consumer.OffsetAndMetadata;
    +import org.apache.kafka.common.TopicPartition;
    +import org.apache.storm.kafka.spout.KafkaSpout;
    +import org.apache.storm.kafka.spout.KafkaSpoutConfig.ProcessingGuarantee;
    +import org.apache.storm.task.TopologyContext;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +/**
    + * Generates and reads commit metadata.
    + */
    +public class CommitMetadataManager {
    +
    +    private static final ObjectMapper JSON_MAPPER = new ObjectMapper();
    +    private static final Logger LOG = 
LoggerFactory.getLogger(CommitMetadata.class);
    +    // Metadata information to commit to Kafka. It is unique per spout 
instance.
    +    private final String commitMetadata;
    +    private final ProcessingGuarantee processingGuarantee;
    +    private final TopologyContext context;
    +
    +    /**
    +     * Create a manager with the given context.
    +     */
    +    public CommitMetadataManager(TopologyContext context, 
ProcessingGuarantee processingGuarantee) {
    +        this.context = context;
    +        try {
    +            commitMetadata = JSON_MAPPER.writeValueAsString(new 
CommitMetadata(
    --- End diff --
    
    I'm happy to declare the class final, hopefully that'll discourage people 
from using the class directly if the internal package hint doesn't do the trick.
    
    Regarding creating objects in the constructor, I agree with you in the 
general case (DI is great), but for pure data objects like CommitMetadata I'm 
not sure I understand the harm? 


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