面试题：ElasticSearch索引存在性判断与数据一致性保障

解决方案

使用乐观锁机制：
- Elasticsearch特性：Elasticsearch支持乐观锁，通过_version字段实现。每次文档更新时，_version会递增。当进行写入操作时，可以指定预期的_version，如果当前文档的_version与预期不符，写入操作将失败。
- 代码层面实现（以Java为例）：

import org.elasticsearch.action.get.GetRequest;
import org.elasticsearch.action.get.GetResponse;
import org.elasticsearch.action.index.IndexRequest;
import org.elasticsearch.action.index.IndexResponse;
import org.elasticsearch.client.RequestOptions;
import org.elasticsearch.client.RestHighLevelClient;
import org.elasticsearch.common.xcontent.XContentType;
import java.io.IOException;

public class ElasticsearchWriteWithVersion {
    private final RestHighLevelClient client;

    public ElasticsearchWriteWithVersion(RestHighLevelClient client) {
        this.client = client;
    }

    public void writeData(String indexName, String id, String jsonString) throws IOException {
        // 判断索引是否存在
        boolean indexExists = client.indices().exists(new org.elasticsearch.action.indices.ExistsRequest(indexName), RequestOptions.DEFAULT);
        if (!indexExists) {
            // 处理索引不存在的情况，例如创建索引
            return;
        }
        // 获取当前文档的版本
        GetRequest getRequest = new GetRequest(indexName, id);
        GetResponse getResponse = client.get(getRequest, RequestOptions.DEFAULT);
        if (getResponse.isExists()) {
            long version = getResponse.getVersion();
            IndexRequest indexRequest = new IndexRequest(indexName)
                  .id(id)
                  .source(jsonString, XContentType.JSON)
                  .version(version);
            IndexResponse indexResponse = client.index(indexRequest, RequestOptions.DEFAULT);
            if (!indexResponse.getResult().name().equals("CREATED") &&!indexResponse.getResult().name().equals("UPDATED")) {
                // 处理写入失败的情况，例如重试
            }
        } else {
            // 文档不存在，直接写入
            IndexRequest indexRequest = new IndexRequest(indexName)
                  .id(id)
                  .source(jsonString, XContentType.JSON);
            IndexResponse indexResponse = client.index(indexRequest, RequestOptions.DEFAULT);
            if (!indexResponse.getResult().name().equals("CREATED")) {
                // 处理写入失败的情况，例如重试
            }
        }
    }
}

使用事务（如果Elasticsearch版本支持）：
- Elasticsearch特性：从Elasticsearch 7.5版本开始，引入了跨文档和跨索引事务功能（_transaction API）。事务可以确保一组操作要么全部成功，要么全部失败，从而保证数据一致性。
- 代码层面实现（以Java为例）：

import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.DocWriteRequest;
import org.elasticsearch.action.bulk.BulkRequest;
import org.elasticsearch.action.bulk.BulkResponse;
import org.elasticsearch.action.index.IndexRequest;
import org.elasticsearch.action.support.WriteRequest;
import org.elasticsearch.client.RequestOptions;
import org.elasticsearch.client.RestHighLevelClient;
import org.elasticsearch.common.xcontent.XContentType;
import java.io.IOException;

public class ElasticsearchTransactionWrite {
    private final RestHighLevelClient client;

    public ElasticsearchTransactionWrite(RestHighLevelClient client) {
        this.client = client;
    }

    public void writeData(String indexName, String id, String jsonString) throws IOException {
        // 判断索引是否存在
        boolean indexExists = client.indices().exists(new org.elasticsearch.action.indices.ExistsRequest(indexName), RequestOptions.DEFAULT);
        if (!indexExists) {
            // 处理索引不存在的情况，例如创建索引
            return;
        }
        BulkRequest bulkRequest = new BulkRequest();
        IndexRequest indexRequest = new IndexRequest(indexName)
              .id(id)
              .source(jsonString, XContentType.JSON);
        bulkRequest.add(indexRequest);
        bulkRequest.setRefreshPolicy(WriteRequest.RefreshPolicy.WAIT_UNTIL);
        client.bulk(bulkRequest, RequestOptions.DEFAULT, new ActionListener<BulkResponse>() {
            @Override
            public void onResponse(BulkResponse bulkResponse) {
                if (bulkResponse.hasFailures()) {
                    // 处理失败情况
                } else {
                    // 处理成功情况
                }
            }

            @Override
            public void onFailure(Exception e) {
                // 处理异常情况
            }
        });
    }
}

使用分布式锁（如Redisson）：
- 机制：在判断索引存在和写入数据之间，获取分布式锁。只有获取到锁的线程才能进行写入操作，这样可以避免在判断后、写入前索引状态变化导致的数据不一致问题。
- 代码层面实现（以Java + Redisson为例）：

import org.redisson.Redisson;
import org.redisson.api.RLock;
import org.redisson.api.RedissonClient;
import org.redisson.config.Config;
import org.elasticsearch.action.get.GetRequest;
import org.elasticsearch.action.get.GetResponse;
import org.elasticsearch.action.index.IndexRequest;
import org.elasticsearch.action.index.IndexResponse;
import org.elasticsearch.client.RequestOptions;
import org.elasticsearch.client.RestHighLevelClient;
import org.elasticsearch.common.xcontent.XContentType;
import java.io.IOException;
import java.util.concurrent.TimeUnit;

public class ElasticsearchWriteWithDistributedLock {
    private final RestHighLevelClient client;
    private final RedissonClient redissonClient;

    public ElasticsearchWriteWithDistributedLock(RestHighLevelClient client) {
        this.client = client;
        Config config = new Config();
        config.useSingleServer().setAddress("redis://127.0.0.1:6379");
        this.redissonClient = Redisson.create(config);
    }

    public void writeData(String indexName, String id, String jsonString) throws IOException {
        RLock lock = redissonClient.getLock("elasticsearch:" + indexName + ":" + id);
        try {
            boolean success = lock.tryLock(10, 1, TimeUnit.SECONDS);
            if (success) {
                // 判断索引是否存在
                boolean indexExists = client.indices().exists(new org.elasticsearch.action.indices.ExistsRequest(indexName), RequestOptions.DEFAULT);
                if (!indexExists) {
                    // 处理索引不存在的情况，例如创建索引
                    return;
                }
                IndexRequest indexRequest = new IndexRequest(indexName)
                      .id(id)
                      .source(jsonString, XContentType.JSON);
                IndexResponse indexResponse = client.index(indexRequest, RequestOptions.DEFAULT);
                if (!indexResponse.getResult().name().equals("CREATED") &&!indexResponse.getResult().name().equals("UPDATED")) {
                    // 处理写入失败的情况，例如重试
                }
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            lock.unlock();
        }
    }
}

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面试题：ElasticSearch索引存在性判断与数据一致性保障

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