Flow of scan results for DynamoDb

package com.vertigo.user;

import java.util.Collections;
import java.util.Map;
import java.util.concurrent.atomic.AtomicReference;
import com.amazonaws.auth.AWSStaticCredentialsProvider;
import com.amazonaws.auth.BasicAWSCredentials;
import com.amazonaws.regions.Regions;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDB;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDBClientBuilder;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;
import com.amazonaws.services.dynamodbv2.model.ScanRequest;
import com.amazonaws.services.dynamodbv2.model.ScanResult;
import io.reactivex.BackpressureStrategy;
import io.reactivex.Flowable;
import lombok.val;
import org.apache.commons.lang3.tuple.Pair;

public class DynaRun {

    public static void main(String[] args) {
        val awsCredentials = new BasicAWSCredentials(
            "access-key",
            "secret-key"
        );
        val client = AmazonDynamoDBClientBuilder
            .standard()
            .withCredentials(new AWSStaticCredentialsProvider(awsCredentials))
            .withRegion(Regions.US_WEST_2)
            .build();

        val disposable = Flowable.range(0, Integer.MAX_VALUE)
            .zipWith(flowableScan(client, "load-post", 1000), Pair::of)
            // .map(ScanResult::getItems)
            .subscribe(pair ->
                System.out.println(
                    "page: " + pair.getLeft() +
                    ", items: " + pair.getRight().getCount()
                )
            );
        disposable.dispose(); // not needed in this case, but in general we might want to do it

        // other example: the flow of batches of keys
        // Flowable<Set<String>> itemsFlow =
        //     flowableScan(client, "staging-post", 1000)
        //     .flatMapIterable(ScanResult::getItems)
        //     .map(Map::keySet);

    }

    public static Flowable<ScanResult> flowableScan(
        AmazonDynamoDB client, String table, int batchSize
    ) {
        val ref = new AtomicReference<Map<String, AttributeValue>>();
        return Flowable.create(e -> {
            if (e.isCancelled()) {
                return;
            }
            try {
                do {
                    ScanRequest scanRequest = new ScanRequest()
                        .withTableName(table)
                        .withAttributesToGet("postid", "body")
                        .withLimit(batchSize)
                        .withExclusiveStartKey(ref.get());

                    val scanResult = client.scan(scanRequest);
                    // NOTE: the last page returns getLastEvaluatedKey == null
                    ref.set(scanResult.getLastEvaluatedKey());
                    e.onNext(scanResult);
                } while (ref.get() != null && !ref.get().isEmpty());
            } catch (Exception ex) {
                e.onError(ex);
            }
        }, BackpressureStrategy.BUFFER);
    }


}

// pom file
    <dependency>
      <groupId>com.amazonaws</groupId>
      <artifactId>aws-java-sdk-core</artifactId>
      <version>1.11.421</version>
    </dependency>
    <dependency>
      <groupId>com.amazonaws</groupId>
      <artifactId>aws-java-sdk-dynamodb</artifactId>
      <version>1.11.421</version>
    </dependency>
    <dependency>
      <groupId>io.reactivex.rxjava2</groupId>
      <artifactId>rxjava</artifactId>
      <version>2.2.2</version>
    </dependency>