haystack/docs/v0.5.0/_src/tutorials/tutorials/6.md

<!---
title: "Tutorial 6"
metaTitle: "Better retrieval via Dense Passage Retrieval"
metaDescription: ""
slug: "/docs/tutorial6"
date: "2020-09-03"
id: "tutorial6md"
--->

# Better Retrieval via "Dense Passage Retrieval"

EXECUTABLE VERSION: [colab](https://colab.research.google.com/github/deepset-ai/haystack/blob/master/tutorials/Tutorial6_Better_Retrieval_via_DPR.ipynb)

### Importance of Retrievers

The Retriever has a huge impact on the performance of our overall search pipeline.


### Different types of Retrievers
#### Sparse
Family of algorithms based on counting the occurrences of words (bag-of-words) resulting in very sparse vectors with length = vocab size.

**Examples**: BM25, TF-IDF

**Pros**: Simple, fast, well explainable

**Cons**: Relies on exact keyword matches between query and text
 

#### Dense
These retrievers use neural network models to create "dense" embedding vectors. Within this family there are two different approaches: 

a) Single encoder: Use a **single model** to embed both query and passage.  
b) Dual-encoder: Use **two models**, one to embed the query and one to embed the passage

Recent work suggests that dual encoders work better, likely because they can deal better with the different nature of query and passage (length, style, syntax ...). 

**Examples**: REALM, DPR, Sentence-Transformers

**Pros**: Captures semantinc similarity instead of "word matches" (e.g. synonyms, related topics ...)

**Cons**: Computationally more heavy, initial training of model


### "Dense Passage Retrieval"

In this Tutorial, we want to highlight one "Dense Dual-Encoder" called Dense Passage Retriever. 
It was introdoced by Karpukhin et al. (2020, https://arxiv.org/abs/2004.04906. 

Original Abstract: 

_"Open-domain question answering relies on efficient passage retrieval to select candidate contexts, where traditional sparse vector space models, such as TF-IDF or BM25, are the de facto method. In this work, we show that retrieval can be practically implemented using dense representations alone, where embeddings are learned from a small number of questions and passages by a simple dual-encoder framework. When evaluated on a wide range of open-domain QA datasets, our dense retriever outperforms a strong Lucene-BM25 system largely by 9%-19% absolute in terms of top-20 passage retrieval accuracy, and helps our end-to-end QA system establish new state-of-the-art on multiple open-domain QA benchmarks."_

Paper: https://arxiv.org/abs/2004.04906  
Original Code: https://fburl.com/qa-dpr 


*Use this* [link](https://colab.research.google.com/github/deepset-ai/haystack/blob/master/tutorials/Tutorial6_Better_Retrieval_via_DPR.ipynb) *to open the notebook in Google Colab.*


### Prepare environment

#### Colab: Enable the GPU runtime
Make sure you enable the GPU runtime to experience decent speed in this tutorial.  
**Runtime -> Change Runtime type -> Hardware accelerator -> GPU**

<img src="https://raw.githubusercontent.com/deepset-ai/haystack/master/docs/img/colab_gpu_runtime.jpg">


```python
# Make sure you have a GPU running
!nvidia-smi
```


```python
# Install the latest release of Haystack in your own environment 
#! pip install farm-haystack

# Install the latest master of Haystack and install the version of torch that works with the colab GPUs
!pip install git+https://github.com/deepset-ai/haystack.git
!pip install torch==1.6.0+cu101 -f https://download.pytorch.org/whl/torch_stable.html
```


```python
from haystack import Finder
from haystack.preprocessor.cleaning import clean_wiki_text
from haystack.preprocessor.utils import convert_files_to_dicts, fetch_archive_from_http
from haystack.reader.farm import FARMReader
from haystack.reader.transformers import TransformersReader
from haystack.utils import print_answers
```

### Document Store

FAISS is a library for efficient similarity search on a cluster of dense vectors.
The `FAISSDocumentStore` uses a SQL(SQLite in-memory be default) database under-the-hood
to store the document text and other meta data. The vector embeddings of the text are
indexed on a FAISS Index that later is queried for searching answers.


```python
from haystack.document_store.faiss import FAISSDocumentStore

document_store = FAISSDocumentStore()
```

### Cleaning & indexing documents

Similarly to the previous tutorials, we download, convert and index some Game of Thrones articles to our DocumentStore


```python
# Let's first get some files that we want to use
doc_dir = "data/article_txt_got"
s3_url = "https://s3.eu-central-1.amazonaws.com/deepset.ai-farm-qa/datasets/documents/wiki_gameofthrones_txt.zip"
fetch_archive_from_http(url=s3_url, output_dir=doc_dir)

# Convert files to dicts
dicts = convert_files_to_dicts(dir_path=doc_dir, clean_func=clean_wiki_text, split_paragraphs=True)

# Now, let's write the dicts containing documents to our DB.
document_store.write_documents(dicts)
```

### Initalize Retriever, Reader,  & Finder

#### Retriever

**Here:** We use a `DensePassageRetriever`

**Alternatives:**

- The `ElasticsearchRetriever`with custom queries (e.g. boosting) and filters
- Use `EmbeddingRetriever` to find candidate documents based on the similarity of embeddings (e.g. created via Sentence-BERT)
- Use `TfidfRetriever` in combination with a SQL or InMemory Document store for simple prototyping and debugging


```python
from haystack.retriever.dense import DensePassageRetriever
retriever = DensePassageRetriever(document_store=document_store,
                                  query_embedding_model="facebook/dpr-question_encoder-single-nq-base",
                                  passage_embedding_model="facebook/dpr-ctx_encoder-single-nq-base",
                                  use_gpu=True,
                                  embed_title=True,
                                  max_seq_len=256,
                                  batch_size=16,
                                  remove_sep_tok_from_untitled_passages=True)
# Important: 
# Now that after we have the DPR initialized, we need to call update_embeddings() to iterate over all
# previously indexed documents and update their embedding representation. 
# While this can be a time consuming operation (depending on corpus size), it only needs to be done once. 
# At query time, we only need to embed the query and compare it the existing doc embeddings which is very fast.
document_store.update_embeddings(retriever)
```

#### Reader

Similar to previous Tutorials we now initalize our reader.

Here we use a FARMReader with the *deepset/roberta-base-squad2* model (see: https://huggingface.co/deepset/roberta-base-squad2)



##### FARMReader


```python
# Load a  local model or any of the QA models on
# Hugging Face's model hub (https://huggingface.co/models)

reader = FARMReader(model_name_or_path="deepset/roberta-base-squad2", use_gpu=True)
```

#### Finder

The Finder sticks together reader and retriever in a pipeline to answer our actual questions. 


```python
finder = Finder(reader, retriever)
```

### Voilà! Ask a question!


```python
# You can configure how many candidates the reader and retriever shall return
# The higher top_k_retriever, the better (but also the slower) your answers. 
prediction = finder.get_answers(question="Who created the Dothraki vocabulary?", top_k_retriever=10, top_k_reader=5)

#prediction = finder.get_answers(question="Who is the father of Arya Stark?", top_k_retriever=10, top_k_reader=5)
#prediction = finder.get_answers(question="Who is the sister of Sansa?", top_k_retriever=10, top_k_reader=5)
```


```python
print_answers(prediction, details="minimal")
```


```python

```