Enhancing RAG Pipelines in Haystack: Introducing DiversityRanker and LostInTheMiddleRanker | by Vladimir Blagojevic | Aug, 2023

How the most recent rankers optimize LLM context window utilization in Retrieval-Augmented Technology (RAG) pipelines

The latest enhancements in Pure Language Processing (NLP) and Lengthy-Kind Query Answering (LFQA) would have, just some years in the past, seemed like one thing from the area of science fiction. Who may have thought that these days we’d have programs that may reply advanced questions with the precision of an skilled, all whereas synthesizing these solutions on the fly from an unlimited pool of sources? LFQA is a sort of Retrieval-Augmented Technology (RAG) which has lately made vital strides, using the most effective retrieval and technology capabilities of Massive Language Fashions (LLMs).

However what if we may refine this setup even additional? What if we may optimize how RAG selects and makes use of data to boost its efficiency? This text introduces two progressive elements aiming to enhance RAG with concrete examples drawn from LFQA, based mostly on the most recent analysis and our expertise — the DiversityRanker and the LostInTheMiddleRanker.

Think about the LLM’s context window as a connoisseur meal, the place every paragraph is a singular, flavorful ingredient. Simply as a culinary masterpiece requires various, high-quality substances, LFQA question-answering calls for a context window crammed with high-quality, diversified, related, and non-repetitive paragraphs.

Within the intricate world of LFQA and RAG, profiting from the LLM’s context window is paramount. Any wasted area or repetitive content material limits the depth and breadth of the solutions we are able to extract and generate. It’s a fragile balancing act to put out the content material of the context window appropriately. This text presents new approaches to mastering this balancing act, which is able to improve RAG’s capability for delivering exact, complete responses.

Let’s discover these thrilling developments and the way they enhance LFQA and RAG.


Haystack is an open-source framework offering end-to-end options for sensible NLP builders. It helps a variety of use instances, from question-answering and semantic doc search all the best way to LLM brokers. Its modular design permits the combination of state-of-the-art NLP fashions, doc shops, and numerous different elements required in right this moment’s NLP toolbox.

One of many key ideas in Haystack is the thought of a pipeline. A pipeline represents a sequence of processing steps {that a} particular element executes. These elements can carry out numerous kinds of textual content processing, permitting customers to simply create highly effective and customizable programs by defining how knowledge flows by way of the pipeline and the order of nodes that carry out their processing steps.

The pipeline performs an important function in web-based long-form query answering. It begins with a WebRetriever element, which searches and retrieves query-relevant paperwork from the online, routinely stripping HTML content material into uncooked textual content. However as soon as we fetch query-relevant paperwork, how will we benefit from them? How will we fill the LLM’s context window to maximise the standard of the solutions? And what if these paperwork, though extremely related, are repetitive and quite a few, generally overflowing the LLM context window?

That is the place the elements we’ll introduce right this moment come into play — the DiversityRanker and the LostInTheMiddleRanker. Their purpose is to deal with these challenges and enhance the solutions generated by the LFQA/RAG pipelines.

The DiversityRanker enhances the variety of the paragraphs chosen for the context window. LostInTheMiddleRanker, normally positioned after DiversityRanker within the pipeline, helps to mitigate the LLM efficiency degradation noticed when fashions should entry related data in the course of an extended context window. The next sections will delve deeper into these two elements and show their effectiveness in a sensible use case.


The DiversityRanker is a novel element designed to boost the variety of the paragraphs chosen for the context window within the RAG pipeline. It operates on the precept {that a} various set of paperwork can enhance the LLM’s capacity to generate solutions with extra breadth and depth.

Determine 1: An inventive interpretation of the DiversityRanker algorithm’s doc ordering course of, courtesy of MidJourney. Please notice that this visualization is extra illustrative than exact.

The DiversityRanker makes use of sentence transformers to calculate the similarity between paperwork. The sentence transformers library provides highly effective embedding fashions for creating significant representations of sentences, paragraphs, and even complete paperwork. These representations, or embeddings, seize the semantic content material of the textual content, permitting us to measure how comparable two items of textual content are.

DiversityRanker processes the paperwork utilizing the next algorithm:

1. It begins by calculating the embeddings for every doc and the question utilizing a sentence-transformer mannequin.

2. It then selects the doc semantically closest to the question as the primary chosen doc.

3. For every remaining doc, it calculates the common similarity to the already chosen paperwork.

4. It then selects the doc that’s, on common, least just like the already chosen paperwork.

5. This choice course of continues till all paperwork are chosen, leading to a listing of paperwork ordered from the doc contributing essentially the most to the general variety to the doc that contributes the least.

A technical notice to remember: the DiversityRanker makes use of a grasping native method to pick out the following doc so as, which could not discover essentially the most optimum general order for the paperwork. DiversityRanker focuses on variety greater than relevance, so it needs to be positioned within the pipeline after one other element like TopPSampler or one other similarity ranker that focuses extra on relevance. Through the use of it after a element that selects essentially the most related paperwork, we be certain that we choose various paperwork from a pool of already related paperwork.


The LostInTheMiddleRanker optimizes the structure of the chosen paperwork within the LLM’s context window. This element is a option to work round an issue recognized in latest analysis [1] that implies LLMs battle to concentrate on related passages in the course of an extended context. The LostInTheMiddleRanker alternates inserting the most effective paperwork initially and finish of the context window, making it straightforward for the LLM’s consideration mechanism to entry and use them. To know how LostInTheMiddleRanker orders the given paperwork, think about a easy instance the place paperwork include a single digit from 1 to 10 in ascending order. LostInTheMiddleRanker will order these ten paperwork within the following order: [1 3 5 7 9 10 8 6 4 2].

Though the authors of this analysis targeted on a question-answering process — extracting the related spans of the reply from the textual content — we’re speculating that the LLM’s consideration mechanism will even have a neater time specializing in the paragraphs at first and the tip of the context window when producing solutions.

Determine 2. LLMs battle to extract solutions from the center of the context, tailored from Liu et al. (2023)[1]

LostInTheMiddleRanker is finest positioned because the final ranker within the RAG pipeline because the given paperwork are already chosen based mostly on similarity (relevance) and ordered by variety.

Utilizing the brand new rankers in pipelines

On this part, we’ll look into the sensible use case of the LFQA/RAG pipeline, specializing in easy methods to combine the DiversityRanker and LostInTheMiddleRanker. We’ll additionally focus on how these elements work together with one another and the opposite elements within the pipeline.

The primary element within the pipeline is a WebRetriever which retrieves question related paperwork from the online utilizing a programmatic search engine API (SerperDev, Google, Bing and so forth). The retrieved paperwork are first stripped of HTML tags, transformed to uncooked textual content, and optionally preprocessed into shorter paragraphs. They’re then, in flip handed to a TopPSampler element, which selects essentially the most related paragraphs based mostly on their similarity to the question.

After TopPSampler selects the set of related paragraphs, they’re handed to the DiversityRanker. DiversityRanker, in flip, orders the paragraphs based mostly on their variety, lowering the repetitiveness of the TopPSampler-ordered paperwork.

The chosen paperwork are then handed to the LostInTheMiddleRanker. As we beforehand talked about, LostInTheMiddleRanker locations essentially the most related paragraphs initially and the tip of the context window, whereas pushing the worst-ranked paperwork to the center.

Lastly, the merged paragraphs are handed to a PromptNode, which circumstances an LLM to reply the query based mostly on these chosen paragraphs.

Determine 3. LFQA/RAG pipeline — Picture by creator

The brand new rankers are already merged into Haystack’s most important department and shall be accessible within the upcoming 1.20 launch slated for the tip of August 2023. We included a brand new LFQA/RAG pipeline demo within the venture’s examples folder.

The demo exhibits how DiversityRanker and LostInTheMiddleRanker may be simply built-in right into a RAG pipeline to enhance the standard of the generated solutions.

Case research

To show the effectiveness of the LFQA/RAG pipelines that embrace the 2 new rankers, we’ll use a small pattern of half a dozen questions requiring detailed solutions. The questions embrace: “What are the primary causes for long-standing animosities between Russia and Poland?”, “What are the first causes of local weather change on each international and native scales?”, and extra. To reply these questions effectively, LLMs require a variety of historic, political, scientific, and cultural sources, making them supreme for our use case.

Evaluating the generated solutions of the RAG pipeline with two new rankers (optimized pipeline) and a pipeline with out them (non-optimized) would require advanced analysis involving human skilled judgment. To simplify analysis and to guage the impact of the DiversityRanker primarily, we calculated the common pairwise cosine distance of the context paperwork injected into the LLM context as a substitute. We restricted the context window dimension in each pipelines to 1024 phrases. By operating these pattern Python scripts [2], we’ve discovered that the optimized pipeline has a median 20–30% enhance in pairwise cosine distance [3] for the paperwork injected into the LLM context. This enhance within the pairwise cosine distance basically implies that the paperwork used are extra various (and fewer repetitive), thus giving the LLM a wider and richer vary of paragraphs to attract upon for its solutions. We’ll depart the analysis of LostInTheMiddleRanker and its impact on generated solutions for one in all our upcoming articles.


We’ve explored how Haystack customers can improve their RAG pipelines through the use of two progressive rankers: DiversityRanker and LostInTheMiddleRanker.

DiversityRanker ensures that the LLM’s context window is crammed with various, non-repetitive paperwork, offering a broader vary of paragraphs for the LLM to synthesize the reply from. On the identical time, the LostInTheMiddleRanker optimizes the location of essentially the most related paragraphs within the context window, making it simpler for the mannequin to entry and make the most of the best-supporting paperwork.

Our small case research confirmed the effectiveness of the DiversityRanker by calculating the common pairwise cosine distance of the paperwork injected into the LLM’s context window within the optimized RAG pipeline (with two new rankers) and the non-optimized pipeline (no rankers used). The outcomes confirmed that an optimized RAG pipeline elevated the common pairwise cosine distance by roughly 20–30%.

We’ve got demonstrated how these new rankers can doubtlessly improve Lengthy-Kind Query-Answering and different RAG pipelines. By persevering with to spend money on and develop on these and comparable concepts, we are able to additional enhance the capabilities of Haystack’s RAG pipelines, bringing us nearer to crafting NLP options that appear extra like magic than actuality.


[1] “Misplaced within the Center: How Language Fashions Use Lengthy Contexts” at

[2] Script:

[3] Script output (solutions):

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