*
 * <p>This interface extends {@link java.io.Closeable}, so implementations
 * should also handle resource cleanup when the {@link #close()} method is called.</p>
 */
public interface DocumentReader extends Closeable {
    /**
     * Reads a document and returns its contents as a map.
     *
     * @return a map containing the document's data, or null if there are no more documents to read.
     */

            try (final DocumentReader documentReader = reader.get()) {
                Map<String, Object> doc = documentReader.read();
                while (doc != null) {
                    if (Thread.currentThread().isInterrupted()) {
                        break;
                    }
                    docs.add(doc);
                    doc = documentReader.read();

        ensureOperations();
        return contentOps.indexFromDocument(createContext(), documents);
    }

    /**
     * Indexes documents from a DocumentReader asynchronously.
     * @param reader The supplier for DocumentReader.
     * @param docPerReq The number of documents to process per request.
     * @param waitController The runnable to control waiting between requests.

import org.opensearch.search.sort.SortBuilder;
import org.opensearch.transport.client.Client;

/**
 * <p>
 * {@link ESSourceReader} reads documents from Elasticsearch using the scroll API.
 * It implements the {@link DocumentReader} interface to provide a way to iterate over documents
 * in a large index without loading all of them into memory at once.
 * </p>
 *
 * <p>

## Advanced Usage

### Index from Existing Documents

```java
import org.codelibs.fess.suggest.index.contents.document.ESSourceReader;

// Index suggestions from existing Elasticsearch documents
DocumentReader reader = new ESSourceReader(
    client,
    suggester.settings(),
    "content-index",        // source index
    "document"             // document type
);

suggester.indexer()