| Keyword | Description | Default |

|-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------|

| `max_elements` | Maximum number of vector that can be stored | 1024 |

| `space` | Vector space (distance metric) the index operates in. Supports 'l2', 'ip', and 'cosine'. <br/><span style="color:black">**Note:** In contrast to the other backends, for HnswDocumentIndex `'cosine'` refers to **cosine distance**, not cosine similarity. To transform one to the other, you can use: `cos_sim = 1 - cos_dist`. For more details see [here](https://en.wikipedia.org/wiki/Cosine_similarity#Cosine_Distance)</span>. | 'l2' |

| `index` | Whether or not an index should be built for this field. | True |

| `ef_construction` | defines a construction time/accuracy trade-off | 200 |

| `ef` | parameter controlling query time/accuracy trade-off | 10 |

| `M` | parameter that defines the maximum number of outgoing connections in the graph | 16 |

| `allow_replace_deleted` | enables replacing of deleted elements with new added ones | True |

| `num_threads` | sets the number of cpu threads to use | 1 |

!!! note

In HnswLibDocIndex `space='cosine'` refers to cosine distance, not to cosine similarity, as it does for the other backends.