Implementation:Deepset ai Haystack DocumentWriter

Knowledge Sources	Haystack Haystack Docs
Domains	Data_Storage, ETL
Last Updated	2026-02-11 00:00 GMT

Overview

Concrete tool for writing documents to a document store provided by the Haystack framework.

Description

The DocumentWriter component persists a list of Document objects into any Haystack-compatible DocumentStore. It delegates the actual write operation to the store's write_documents method, passing along a DuplicatePolicy that controls how existing documents with matching IDs are handled. It supports both synchronous and asynchronous execution.

Usage

Import this class as the final component in indexing pipelines, after embedding or preprocessing steps. Connect the output of an embedder or splitter to the writer's documents input.

Code Reference

Source Location

• Repository: haystack
• File: haystack/components/writers/document_writer.py
• Lines: L12-128

Signature

class DocumentWriter:
    def __init__(
        self,
        document_store: DocumentStore,
        policy: DuplicatePolicy = DuplicatePolicy.NONE,
    ):
        """
        Args:
            document_store: The target document store instance.
            policy: Duplicate handling policy (NONE, SKIP, OVERWRITE, FAIL).
        """

    def run(
        self,
        documents: list[Document],
        policy: DuplicatePolicy | None = None,
    ) -> dict[str, int]:
        """
        Args:
            documents: Documents to write.
            policy: Optional runtime override for duplicate policy.
        Returns:
            {"documents_written": int}
        """

Import

from haystack.components.writers import DocumentWriter
from haystack.document_stores.types import DuplicatePolicy

I/O Contract

Inputs

Name	Type	Required	Description
documents	list[Document]	Yes	Documents to persist to the store
policy	DuplicatePolicy or None	No	Runtime override for duplicate handling

Outputs

Name	Type	Description
documents_written	int	Number of documents successfully written

Usage Examples

Basic Document Writing

from haystack import Document
from haystack.components.writers import DocumentWriter
from haystack.document_stores.in_memory import InMemoryDocumentStore

doc_store = InMemoryDocumentStore()
writer = DocumentWriter(document_store=doc_store)

docs = [
    Document(content="Python is a popular programming language"),
    Document(content="Java is used in enterprise applications"),
]

result = writer.run(documents=docs)
print(f"Documents written: {result['documents_written']}")

With Skip Duplicate Policy

from haystack.components.writers import DocumentWriter
from haystack.document_stores.types import DuplicatePolicy
from haystack.document_stores.in_memory import InMemoryDocumentStore

doc_store = InMemoryDocumentStore()
writer = DocumentWriter(document_store=doc_store, policy=DuplicatePolicy.SKIP)

# Second write of same documents will skip duplicates
writer.run(documents=docs)
result = writer.run(documents=docs)
print(f"Documents written on second run: {result['documents_written']}")

Related Pages

Implements Principle

• Principle:Deepset_ai_Haystack_Document_Writing