如何生成 _id 像 elasticsearch 但对于 apache lucene？

Question

I want to generate _id of Elasticsearch document the same way in apache Lucene, to have _id like Elasticsearch but in Apache Lucene. How can I do? Where Can I find algorithm that generate Elasticsearch _id?

Answer 1

The algorithm is based on Flake IDs and can be found here: https://github.com/elastic/elasticsearch/blob/be7c7415627377a1b795400fb8dfcc6cbdf0e322/server/src/main/java/org/elasticsearch/common/TimeBasedUUIDGenerator.java#L49

Answer 2

Apache Lucene doesn't have a direct equivalent to the "_id" field in Elasticsearch, but you can simulate this behavior by using a unique identifier as a field in your Lucene document.

One way to generate a unique identifier is by using a UUID. You can use a library like Java's java.util.UUID to generate a unique identifier for each document and store it as a field in the Lucene document.

Another way is to use a hash value of your document as the identifier. You can use a hashing algorithm like SHA-256 to hash the contents of your document and store the resulting hash value as the identifier field.

It's important to note that the Elasticsearch _id is not only unique but also deterministic. If you want to generate an _id that is deterministic in Apache Lucene, you need to use a specific, deterministic algorithm to generate the identifier.

Here is an example in Java for generating a unique identifier for each document using java.util.UUID :

import java.util.UUID;
import org.apache.lucene.document.Document;

public class DocumentIDGenerator {
    public static String generateID(Document document) {
        return UUID.randomUUID().toString();
    }
}

In this example, the generateID method takes a Lucene Document object as input and returns a newly generated UUID as a string. To use this method, simply call it for each document before adding it to the index and store the returned identifier as a field in the document.

It's important to note that UUID is not deterministic, so if you need to generate a deterministic identifier, this method is not suitable. In that case, you may want to consider using a hash value as described in the next example.

Here is an example in Java for generating a deterministic identifier for each document using the SHA-256 hash value:

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import org.apache.lucene.document.Document;

public class DocumentIDGenerator {
    public static String generateID(Document document) {
        StringBuilder sb = new StringBuilder();
        for (IndexableField field : document.getFields()) {
            sb.append(field.stringValue());
        }
        String contents = sb.toString();
        try {
            MessageDigest digest = MessageDigest.getInstance("SHA-256");
            byte[] hash = digest.digest(contents.getBytes(StandardCharsets.UTF_8));
            StringBuilder hexString = new StringBuilder();
            for (byte b : hash) {
                hexString.append(String.format("%02X", b));
            }
            return hexString.toString();
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
    }
}

In this example, the generateID method takes a Lucene Document object as input and returns a hexadecimal string representation of the SHA-256 hash of the contents of the document. To use this method, simply call it for each document before adding it to the index and store the returned identifier as a field in the document.

It's important to note that this is just one example, and there are many other ways to generate a deterministic identifier. You should choose the method that is most appropriate for your use case and requirements.

Hope this help.