为什么说 elastic search 是近实时的？

偶尔发呆 / 2023-08-14 / 原文

我们都知道 elastic search 是近实时的搜索系统，这里面的原因究竟是什么呢？es 是近实时，是因为 lucene 是近实时的。我们看一段 lucene 的示例代码：

使用的 lucene 版本如下：

<dependency>
    <groupId>org.apache.lucene</groupId>
    <artifactId>lucene-core</artifactId>
    <version>8.9.0</version>
</dependency>

 1 Analyzer analyzer = new StandardAnalyzer();
 2 
 3 Path indexPath = new File("tmp_index").toPath();
 4 IOUtils.rm(indexPath);
 5 Directory directory = FSDirectory.open(indexPath);
 6 IndexWriterConfig config = new IndexWriterConfig(analyzer);
 7 // 使用 compound file
 8 config.setUseCompoundFile(true);
 9 // config.setInfoStream(System.out);
10 IndexWriter iw = new IndexWriter(directory, config);
11 // Now search the index:
12 StandardDirectoryReader oldReader = (StandardDirectoryReader) DirectoryReader.open(iw);
13 
14 // 添加文档 1
15 Document doc = new Document();
16 doc.add(new Field("name", "Pride and Prejudice", TextField.TYPE_STORED));
17 doc.add(new Field("line", "It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.", TextField.TYPE_STORED));
18 doc.add(new Field("chapters", "1", TextField.TYPE_STORED));
19 iw.addDocument(doc);
20 
21 IndexSearcher searcher = new IndexSearcher(oldReader);
22 // Parse a simple query that searches for "text":
23 QueryParser parser = new QueryParser("name", analyzer);
24 Query query = parser.parse("prejudice");
25 ScoreDoc[] hits = searcher.search(query, 10).scoreDocs;
26 // Iterate through the results:
27 System.out.println("第一次查询：");
28 System.out.printf("version=%d, segmentInfos=%s\n", oldReader.getVersion(),
29         oldReader.getSegmentInfos());
30 for (int i = 0; i < hits.length; i++) {
31     System.out.println(hits[i]);
32 }
33 
34 StandardDirectoryReader newReader = (StandardDirectoryReader) DirectoryReader.openIfChanged(oldReader);
35 oldReader.close();
36 
37 searcher = new IndexSearcher(newReader);
38 hits = searcher.search(query, 10).scoreDocs;
39 System.out.println("第二次查询：");
40 System.out.printf("version=%d, segmentInfos=%s\n", newReader.getVersion(),
41         newReader.getSegmentInfos());
42 // Iterate through the results:
43 for (int i = 0; i < hits.length; i++) {
44     System.out.println(hits[i]);
45 }
46 
47 iw.commit();
48 newReader.close();
49 directory.close();