Transactional Lucene

Many users don't appreciate the transactional semantics of Lucene's APIs and how this can be useful in search applications. For starters, Lucene implements ACID properties:

• Atomicity: when you make changes (adding, removing documents) in an IndexWriter session, and then commit, either all (if the commit succeeds) or none (if the commit fails) of your changes will be visible, never something in-between. Some methods have their own atomic behavior: if you call updateDocument, which is implemented as a delete followed by an add, you'll never see the delete without the add, even if you open a near-real-time (NRT) reader or commit from a separate thread. Similarly if you add a block of documents, using the relatively new addDocuments method, you'll see either none or all of the documents in any reader you obtain.
  
  
• Consistency: if the computer or OS crashes, or the JVM crashes or is killed, or power is lost, your index will remain intact (ie, not corrupt). Note that other problems, such as bad RAM, a bit-flipping CPU or file system corruption, can still easily corrupt the index!
  
  
• Isolation: while IndexWriter is making changes, nothing is visible to any IndexReader searching the index, until you commit or open a new NRT reader. Only one IndexWriter instance at a time can change the index.
  
  
• Durability: once commit returns, all changes have been written to durable storage (assuming your I/O system correctly implements fsync). If the computer or OS crashes, or the JVM crashes or is killed, or power is lost to the computer, all changes will still be present in the index.

Lucene provides a two-phased commit API: call the prepareCommit method to do all of the hard work (applying buffered deletes, writing buffered documents, fsyncing files). If something is going to go wrong (e.g., disk fills up) it'll almost certainly happen during this first phase. Then, call commit to complete the transaction.

When you close the IndexWriter, it calls commit under the hood. If, instead, you want to discard all changes since the last commit, call the rollback method instead, which also closes the writer. You can even rollback a CREATE: if you have an existing index, and you open an IndexWriter on it with OpenMode.CREATE, and then rollback, the index will be unchanged. Likewise, if you call deleteAll and then rollback.

Note that merely opening an IndexWriter on a new directory does not create an empty commit; ie, you cannot open an IndexReader on the directory until you've called commit yourself.

Lucene does not implement a transaction log itself, but it's easy to build that layer out on top. For example, popular search servers such as Solr and ElasticSearch, do so.

Multiple commits in one index

A single Lucene index is free to contain more than one commit; this is a powerful yet often overlooked feature. Each commit holds a point-in-time view of the index as it existed when the commit was created.

This is similar to the snapshots and writable clones available in modern filesystems like ZFS and the up-and-coming Btrfs. In fact, Lucene is able to efficiently expose multiple commits for the very same underlying reason: all index segments and files are write-once, just like the file blocks in ZFS and Btrfs.

To save multiple commits in your index, just implement your own IndexDeletionPolicy and pass it to IndexWriter. This is the class Lucene uses to know which commits should be deleted: IndexWriter invokes it on opening an index and whenever a commit succeeds. The default policy, KeepOnlyLastCommitDeletionPolicy, deletes all but the last commit. If you use NoDeletionPolicy then every commit is retained!

You can pass a userData (Map<String,String>) to commit, to record custom information (opaque to Lucene) about that commit, and then use IndexReader.listCommits to find all commits in the index. Once you've found a commit, you can open an IndexReader on it to search the index as of that commit.

You can also open an IndexWriter on a prior commit, to effectively roll back all changes after it: this is just like the rollback method, except it enables you to rollback across commits and not just the changes made in the current IndexWriter session.

Old commits are still kept even when you open an index with OpenMode.CREATE. It's also fine to pass OpenMode.CREATE when IndexReaders are still searching the old commits. This enables fun use cases, such as fully re-indexing your content between each commit without affecting any open readers.

Combining all of these fun transactional features, you can do some cool things:

• Hot backups, using SnapshotDeletionPolicy or PersistentSnapshotDeletionPolicy: these deletion policies make it trivial to take a "live" backup of the index without blocking ongoing changes with IndexWriter. The backup can easily be incremental (just copy the new files, remove the deleted ones), and you can freely throttle the IO to minimize any interference with searching.
  
  
• Searching different catalog versions: perhaps you run an e-commerce site, and but you ship multiple versions of your catalog. In this case you can keep older commits around, each searching a specific version of your catalog, enabling users to choose which catalog to search.
  
  
• Repeatable indexing tests from the same initial index: maybe you want to run a bunch of performance tests, perhaps trying different RAM buffer sizes or merge factors, starting from a large initial index. To do this, simply run each test, but in the end, instead of closing the IndexWriter, use the rollback method to quickly return the index to its initial state, ready for the next test.
  
  
• Force all index segments to be merged down to a single segment, but also keep the prior multi-segment commit. Then you can do tests to compare multi-segment vs single-segment performance.
  
  
• Indexing and searching over the NFS file system: because NFS does not protect still-open files from deletion, you must use an IndexDeletionPolicy to keep each commit around until all open readers have finished with the commit (ie, reopened to a newer commit). The simple approach is time-based, for example: don't delete the commit until it is 15 minutes old, and then always reopen your readers every 5 minutes. Without this you'll hit all sorts of scary exceptions when searching over NFS.
  
  
• Distributed commit: if you have other resources that must commit atomically along with the changes to your Lucene index, you can use the two-phased commit API. This is simple, but vulnerable to failures during the 2nd phaes; to also recover from such cases, for example if Lucene completed its 2nd phase commit but the database's 2nd phase hit some error or crash or power loss, you can easily rollback Lucene's commit by opening an IndexWriter on the prior commit.
  
  
• Experimental index changes: maybe you want to try re-indexing some subset of your index in a new way, but you're not sure it'll work out. In this case, just keep the old commit around, and then rollback if it didn't work out, or delete the old commit if it did.
  
  
• Time-based snapshots: maybe you'd like the freedom to roll back to your index as it existed 1 day ago, 1 week ago, 1 month ago, etc., so you preserve commits based on their age.

Remember that keeping more than one commit alive will necessarily consume additional disk space, however, the overhead is often small since the multiple commits will usually share common segments, especially the larger, older ones.