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Preprocessing

Haystack includes a suite of tools to:

  • extract text from different file types,
  • normalize white space
  • split text into smaller pieces to optimize retrieval

These data preprocessing steps can have a big impact on the systems performance and effective handling of data is key to getting the most out of Haystack.

The Document Store expects its inputs to come in the following format. The sections below will show you all the tools you'll need to ready your data for storing.

docs = [
{
'text': DOCUMENT_TEXT_HERE,
'meta': {'name': DOCUMENT_NAME, ...}
}, ...
]

File Conversion

There are a range of different file converters in Haystack that can extract text from files and cast them into the unified dictionary format shown above. Haystack features support for txt, pdf and docx files and there is even a converter that leverages Apache Tika. Please refer to the API docs to see which converter best suits you.

PreProcessor

While each of the above conversion methods produce documents that are already in the format expected by the Document Store, it is recommended that they are further processed in order to ensure optimal Retriever and Reader performance. The PreProcessor takes one of the documents created by the converter as input, performs various cleaning steps and splits them into multiple smaller documents.

doc = converter.convert(file_path=file, meta=None)
processor = PreProcessor(clean_empty_lines=True,
clean_whitespace=True,
clean_header_footer=True,
split_by="word",
split_length=200,
split_respect_sentence_boundary=True)
docs = processor.process(d)
  • clean_empty_lines will normalize 3 or more consecutive empty lines to be just a two empty lines
  • clean_whitespace will remove any whitespace at the beginning or end of each line in the text
  • clean_header_footer will remove any long header or footer texts that are repeated on each page
  • split_by determines what unit the document is split by: 'word', 'sentence' or 'passage'
  • split_length sets a maximum number of 'word', 'sentence' or 'passage' units per output document
  • split_respect_sentence_boundary ensures that document boundaries do not fall in the middle of sentences

Impact of Document Splitting

The File Converters will treat each file as a single document regardless of length. This is not always ideal as long documents can have negative impacts on both speed and accuracy.

Document length has a very direct impact on the speed of the Reader. If you halve the length of your documents, you can expect that the Reader will double in speed.

It is generally not a good idea to let document boundaries fall in the middle of sentences. Doing so means that each document will contain incomplete sentence fragments which maybe be hard for both retriever and reader to interpret.

For sparse retrievers, very long documents pose a challenge since the signal of the relevant section of text can get washed out by the rest of the document. We would recommend making sure that documents are no longer than 10,000 words.

Dense retrievers are limited in the length of text that they can read in one pass. As such, it is important that documents are not longer than the dense retriever's maximum input length. By default, Haystack's DensePassageRetriever model has a maximum length of 256 tokens. As such, we recommend that documents contain significantly less words. We have found decent performance with documents around 100 words long.