Snapshots and deletion policies

A snapshot pins a specific commit so its segment files survive future deletion policy passes. Use it to take consistent backups while indexing continues.

Take a snapshot

var snapshot = writer.CreateSnapshot();
try
{
    BackupSegmentFiles(snapshot.SegmentIds, snapshot.Generation);
}
finally
{
    writer.ReleaseSnapshot(snapshot);
}

The snapshot exposes the commit Generation and the list of segment IDs the commit references.

Deletion policies

IndexWriterConfig.DeletionPolicy controls which old commits and segments survive after each commit:

Policy	Behaviour
KeepLatestCommitPolicy (default)	Retains only the newest commit.
KeepLastNCommitsPolicy(n)	Retains the last n generations.

var config = new IndexWriterConfig
{
    DeletionPolicy = new KeepLastNCommitsPolicy(maxCommits: 5),
};

Active snapshots always protect their segments, regardless of policy.

See also

• CreateSnapshot
• KeepLastNCommitsPolicy
• KeepLatestCommitPolicy