Flush

Flushing Explained

Flushing in Lucene is the process of writing In-Memory Buffer changes (newly indexed documents and deletions) to the disk. This creates a new Segment, which is an immutable data structure. Once a segment is written to disk, it becomes searchable, although changes only become visible to readers once a commit has occurred.

When Flushing Occurs

Automatic Flushing: Lucene's IndexWriter automatically flushes its in-memory buffer to disk when the buffer grows large enough to reach a threshold set by memory settings or the number of documents. This behavior is controlled by configurations such as setMaxBufferedDocs and setRAMBufferSizeMB on IndexWriterConfig. Automatic flushing helps in managing memory usage and keeping the index responsive.
Manual Flushing: You can manually trigger a flush by calling the flush() method on IndexWriter. This is useful if you have specific needs regarding when data should be written to disk (for instance, before a potentially risky operation or before a program section that should not be delayed by automatic flushing).
Committing: When you call commit() on an IndexWriter, Lucene flushes all the buffered documents to disk and then writes a new commit point. This commit point includes information about all segments in the index, which makes the changes visible to new readers. A commit is more than just a flush because it also ensures that the changes are durable and visible to new IndexReader instances.

IndexWriter writer = new IndexWriter(directory, config);
// Add or delete documents
writer.addDocument(doc);
// Flushes changes to disk and writes a new commit point
writer.commit();

Commit vs Flush

Flush: Writes the current in-memory buffer to a new segment on disk without marking these changes as committed. These changes are not visible to IndexReader until a commit occurs.
Commit: Includes a flush (if there are unflushed changes) and then marks the state of the index (including all segments) as committed, which makes the changes visible to readers.

Best Practices

Regular Commits: Regularly committing changes is crucial in a production environment to ensure data durability and consistency. However, too frequent commits can degrade performance due to the overhead associated with writing commit points and managing multiple Segments.
Handling Flushes: Typically, you don’t need to manually manage flushing because Lucene handles it efficiently based on the configured thresholds. Manually flushing can be useful in low-memory environments or in situations where precise control over disk writes is necessary.

Understanding the role of flushing and committing in Lucene helps in optimizing index performance and ensuring that updates are managed according to your application’s requirements for durability and responsiveness.

Q

There is no automatic time based flushing?
To delete the document do we delete entire index or mark the document within the index?
How often does auto merge happen? Will index auto merge? How expensive is merge? Is searching available when merge happens? Are there duplicate results during the merge?

Backlinks

Segment Merge Merging