Configure the Data Migration Tool

class Integrity extends \Migration\App\Step\AbstractIntegrity
{
    ...
    /**
     * Integrity constructor.
     * @param ProgressBar\LogLevelProcessor $progress
     * @param Logger $logger
     * @param Config $config
     * @param ResourceModel\Source $source
     * @param ResourceModel\Destination $destination
     * @param MapFactory $mapFactory
     * @param string $mapConfigOption
     */
    public function __construct(
        ProgressBar\LogLevelProcessor $progress,
        Logger $logger,
        Config $config,
        ResourceModel\Source $source,
        ResourceModel\Destination $destination,
        MapFactory $mapFactory,
        $mapConfigOption = 'greatblog_map_file'
    ) {
        parent::__construct($progress, $logger, $config, $source, $destination, $mapFactory, $mapConfigOption);
    }

    /**
     * @inheritDoc
     */
    public function perform()
    {
        $this->progress->start($this->getIterationsCount());
        $this->check(['greatblog_post'], MapInterface::TYPE_SOURCE);
        $this->check(['greatblog_post', 'greatblog_post_tags'], MapInterface::TYPE_DEST);
        $this->progress->finish();
        return $this->checkForErrors();
    }
    ...
}

class Data implements \Migration\App\Step\StageInterface
{
    ...
    /**
     * Data constructor.
     *
     * @param ProgressBar\LogLevelProcessor $progress
     * @param ResourceModel\Source $source
     * @param ResourceModel\Destination $destination
     * @param ResourceModel\RecordFactory $recordFactory
     * @param RecordTransformerFactory $recordTransformerFactory
     * @param MapFactory $mapFactory
     */
    public function __construct(
        ProgressBar\LogLevelProcessor $progress,
        ResourceModel\Source $source,
        ResourceModel\Destination $destination,
        ResourceModel\RecordFactory $recordFactory,
        RecordTransformerFactory $recordTransformerFactory,
        MapFactory $mapFactory
    ) {
        $this->progress = $progress;
        $this->destination = $destination;
        $this->recordFactory = $recordFactory;
        $this->source = $source;
        $this->recordTransformerFactory = $recordTransformerFactory;
        $this->map = $mapFactory->create('greatblog_map_file');
    }

    /**
     * @inheritDoc
     */
    public function perform()
    {
        $sourceDocName = 'greatblog_post';
        $sourceDocument = $this->source->getDocument($sourceDocName);
        $destinationDocName = 'greatblog_post';
        $destinationDocument = $this->destination->getDocument($destinationDocName);
        /** @var \Migration\RecordTransformer $recordTransformer */
        $recordTransformer = $this->recordTransformerFactory->create(
            [
                'sourceDocument' => $sourceDocument,
                'destDocument'   => $destinationDocument,
                'mapReader'      => $this->map
            ]
        );
        $recordTransformer->init();

        $this->progress->start($this->source->getRecordsCount($sourceDocName));
        $pageNumber = 0;
        while (!empty($items = $this->source->getRecords($sourceDocName, $pageNumber))) {
            $pageNumber++;
            $recordsToSave = $destinationDocument->getRecords();
            foreach ($items as $item) {
                $sourceRecord = $this->recordFactory->create(
                    ['document' => $sourceDocument, 'data' => $item]
                );
                $destinationRecord = $this->recordFactory->create(['document' => $destinationDocument]);
                $recordTransformer->transform($sourceRecord, $destinationRecord);
                $recordsToSave->addRecord($destinationRecord);
            }
            $this->destination->saveRecords($destinationDocName, $recordsToSave);

            $tags = $this->getTags($items);
            $this->destination->saveRecords('greatblog_post_tags', $tags);
            $this->progress->advance();
        }

        $this->progress->finish();
        return true;
    }
    ...
}

class Volume extends \Migration\App\Step\AbstractVolume
{
    ...
    /**
     * @inheritdoc
     */
    public function perform()
    {
        $documentName = 'greatblog_post';
        $sourceCount = $this->source->getRecordsCount($documentName);
        $destinationCount = $this->destination->getRecordsCount($documentName);
        if ($sourceCount != $destinationCount) {
            $this->errors[] = sprintf(
                'Mismatch of entities in the document: %s Source: %s Destination: %s',
                $documentName,
                $sourceCount,
                $destinationCount
            );
        }

        return $this->checkForErrors(Logger::ERROR);
    }
    ...
}

class Delta extends \Migration\App\Step\AbstractDelta
{
    /**
     * @var string
     */
    protected $mapConfigOption = 'greatblog_map_file';

    /**
     * @var string
     */
    protected $groupName = 'delta_greatblog';

    /**
     * @inheritDoc
     */
    public function perform()
    {
        $sourceDocumentName = 'greatblog_post';
        $idKeys = ['post_id'];
        $page = 0;
        while (!empty($items = $this->source->getChangedRecords($sourceDocumentName, $idKeys, $page++))) {
            $this->destination->deleteRecords(
                'greatblog_post_tags',
                $idKeys,
                $items
            );

            $tags = $this->getTags($items);
            $this->destination->saveRecords('greatblog_post_tags', $tags);
        }

        //parent class takes care of greatblog_post records automatically
        return parent::perform();
    }
}