Writer configuration

IndexWriterConfig controls buffering, merging, compression, and analysis.

Common setup

var config = new IndexWriterConfig
{
    DefaultAnalyser = new StandardAnalyser(),
    RamBufferSizeMB = 512.0,
    MaxBufferedDocs = 10_000,
    MaxQueuedDocs   = 20_000,
    MergeThreshold  = 10,
    DurableCommits  = true,
    CompressionPolicy = FieldCompressionPolicy.Deflate,
    StoredFieldBlockSize = 16,
};

Defaults

Setting	Default	What it does
`RamBufferSizeMB`	`512.0`	Memory buffer before flush
`MaxBufferedDocs`	`10_000`	Doc count before flush
`MaxQueuedDocs`	`20_000`	Backpressure cap; `AddDocument` blocks past this
`DefaultAnalyser`	`StandardAnalyser`	Analyser for fields without a mapping
`Similarity`	`Bm25Similarity.Instance`	Scoring model
`DeletionPolicy`	`KeepLatestCommitPolicy`	Which old commits survive
`DurableCommits`	`true`	`fsync` before declaring commit successful
`CompressionPolicy`	`Deflate`	Stored field compression
`StoredFieldBlockSize`	`16`	Docs per compression block
`PostingsSkipInterval`	`128`	Postings skip-list frequency
`MergeThreshold`	`10`	Segment count that triggers a merge
`BKDMaxLeafSize`	`512`	BKD tree leaf capacity
`MaxTokensPerDocument`	`0` (unlimited)	Token cap per document
`TokenBudgetPolicy`	`Truncate`	What happens when the cap is hit
`StoreTermVectors`	`false`	Whether to persist term vectors
`Metrics`	`NullMetricsCollector.Instance`	Metrics backend

Backpressure and merge throttling

MaxQueuedDocs (default 20,000) caps the number of documents waiting to be flushed. When the queue is full, AddDocument blocks until a flush frees space. Prevents out-of-memory conditions under sustained write load.

MergeThrottleSegments (default 0, disabled) blocks writes when the segment count exceeds the threshold. Set it to force merges to catch up before more documents are accepted, keeping the segment count bounded when indexing faster than merges can consolidate:

var config = new IndexWriterConfig
{
    MergeThrottleSegments = 100,
};

Field boosts and index sort

var document = new LeanDocument();
document.Add(new TextField(
    "title",
    "A compact corpus",
    stored: true,
    boost: 3.0f));

var config = new IndexWriterConfig
{
    IndexSort = new IndexSort(
        SortField.Int64(
            "publishedAt",
            descending: true)),
};

Field boosts belong to field values and are persisted in segment norms. IndexSort controls physical document order for newly written and merged segments.

Schema validation