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Mastering Elasticsearch 5.x
Mastering Elasticsearch 5.x

Mastering Elasticsearch 5.x: Master the intricacies of Elasticsearch 5 and use it to create flexible and scalable search solutions , Third Edition

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Mastering Elasticsearch 5.x

Chapter 2. The Improved Query DSL

In the previous chapter, we looked at the overview of Lucene, how it works, how analysis processing is done, and how to use the Apache Lucene query language. In addition to that, we discussed the basic concepts of Elasticsearch. We also introduced Elasticsearch 5.x and covered the latest features introduced in version 2.x as well as 5.x. We also talked about the most important changes/removals of features Elasticsearch has gone through during the transition from 1.x to 5.x. In this chapter, we will dive deep into Elasticsearch, focusing on the Query DSL. We will first go through Lucene similarity algorithm formulas before turning to advanced queries. By the end of this chapter, we will have covered the following topics:

  • The changed default text scoring in Lucene: BM25
  • Understanding precision and recall
  • How BM25 differs from TF-IDF
  • Elasticsearch Query DSL
  • Understanding bool query syntax
  • Which query you should use for your particular use case
  • Important...

The changed default text scoring in Lucene - BM25

Scoring is the most important part of Apache Lucene. It is the process of calculating the score property of a document in a scope of a given query. A score is a factor that describes how well the document matches the query. For score calculation, Lucene supports many algorithms, but since the beginning of Lucene, TF-IDF (term frequency-inverse document frequency) has been the default scoring algorithm. With the release of Apache Lucene 6.0, one of the major changes in Lucene is the changed default scoring algorithm. The default algorithm is now BM25 (Best Matching). In this section, we will also cover two fundamental concepts of search relevancy: precision and recall, and after that, we'll look at the new default Apache Lucene scoring mechanism and how it differs from TF-IDF.

Precision versus recall

After executing a search query, an obvious question comes to mind: Have I found the most relevant documents or am I missing important...

Re-factored Query DSL

The release of Elasticsearch 2.0.0 saw a major refactoring in Elasticsearch Query DSL, the interface provided by Elasticsearch to write queries in the JSON format. Covering complete Query DSL is out of the scope of this book. But for a person who doesn't have much prior experience with a full text search engine, the number of queries exposed by Elasticsearch can be overwhelming and very confusing. So, we have decided to cover the most frequently used queries, along with making readers aware about how to use queries and filters with revised Query DSL syntax. Thus, this book can serve the purposes of both types of readers: those who are aware of old versions as well as those who are starting with version 5.0.

Choosing the right query for the job

In this section, we will start with different categories of queries available in Elasticsearch, looking at which query to use in which situation.

Query categorization

Of course, categorizing queries is a hard task and the following list of categories is not the only correct one. If you were to ask other Elasticsearch users, they would provide their own categories, or say that each query can be assigned to more than a single category, and they would be right. We also think that there is no single way of categorizing the queries; however, in our opinion, each Elasticsearch query can be assigned to one (or more) of the following categories:

  • Basic queries: A category that groups queries allowing searching for a part of the index, either in an analyzed or a non-analyzed manner. The key point in this category is that you can nest queries inside a basic query. An example of a basic query is the term query.
  • Compound queries: A category grouping queries that allows...

Query rewrite explained

We have already talked about scoring, which is valuable knowledge, especially when trying to improve the relevance of our queries. We also think that when debugging your queries, it is valuable to know how all the queries are executed; therefore, it is because of this that we decided to include this section on how query rewrite works in Elasticsearch, why it is used, and how to control it.

If you have ever used queries, such as the prefix query and the wildcard query, basically any query that is said to be multiterm, you've probably heard about query rewriting. Elasticsearch does that because of performance reasons. The rewrite process is about changing the original, expensive query to a set of queries that are far less expensive from Lucene's point of view, and thus speed up the query execution. The rewrite process is not visible to the client, but it is good to know that we can alter the rewrite process behavior. For example, let's look at what Elasticsearch...

The changed default text scoring in Lucene - BM25


Scoring is the most important part of Apache Lucene. It is the process of calculating the score property of a document in a scope of a given query. A score is a factor that describes how well the document matches the query. For score calculation, Lucene supports many algorithms, but since the beginning of Lucene, TF-IDF (term frequency-inverse document frequency) has been the default scoring algorithm. With the release of Apache Lucene 6.0, one of the major changes in Lucene is the changed default scoring algorithm. The default algorithm is now BM25 (Best Matching). In this section, we will also cover two fundamental concepts of search relevancy: precision and recall, and after that, we'll look at the new default Apache Lucene scoring mechanism and how it differs from TF-IDF.

Precision versus recall

After executing a search query, an obvious question comes to mind: Have I found the most relevant documents or am I missing important documents...

Re-factored Query DSL


The release of Elasticsearch 2.0.0 saw a major refactoring in Elasticsearch Query DSL, the interface provided by Elasticsearch to write queries in the JSON format. Covering complete Query DSL is out of the scope of this book. But for a person who doesn't have much prior experience with a full text search engine, the number of queries exposed by Elasticsearch can be overwhelming and very confusing. So, we have decided to cover the most frequently used queries, along with making readers aware about how to use queries and filters with revised Query DSL syntax. Thus, this book can serve the purposes of both types of readers: those who are aware of old versions as well as those who are starting with version 5.0.

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Key benefits

  • Master the searching, indexing, and aggregation features in ElasticSearch
  • Improve users’ search experience with Elasticsearch’s functionalities and develop your own Elasticsearch plugins
  • A comprehensive, step-by-step guide to master the intricacies of ElasticSearch with ease

Description

Elasticsearch is a modern, fast, distributed, scalable, fault tolerant, and open source search and analytics engine. Elasticsearch leverages the capabilities of Apache Lucene, and provides a new level of control over how you can index and search even huge sets of data. This book will give you a brief recap of the basics and also introduce you to the new features of Elasticsearch 5. We will guide you through the intermediate and advanced functionalities of Elasticsearch, such as querying, indexing, searching, and modifying data. We’ll also explore advanced concepts, including aggregation, index control, sharding, replication, and clustering. We’ll show you the modules of monitoring and administration available in Elasticsearch, and will also cover backup and recovery. You will get an understanding of how you can scale your Elasticsearch cluster to contextualize it and improve its performance. We’ll also show you how you can create your own analysis plugin in Elasticsearch. By the end of the book, you will have all the knowledge necessary to master Elasticsearch and put it to efficient use.

Who is this book for?

If you have some prior working experience with Elasticsearch and want to take your knowledge to the next level, this book will be the perfect resource for you.If you are a developer who wants to implement scalable search solutions with Elasticsearch, this book will also help you. Some basic knowledge of the query DSL and data indexing is required to make the best use of this book.

What you will learn

  • Understand Apache Lucene and Elasticsearch 5's design and architecture
  • Use and configure the new and improved default text scoring mechanism in Apache Lucene 6
  • Know how to overcome the pitfalls while handling relational data in Elasticsearch
  • Learn about choosing the right queries according to the use cases and master the scripting module including new default scripting language, painlessly
  • Explore the right way of scaling production clusters to improve the performance of Elasticsearch
  • Master the searching, indexing, and aggregation features in Elasticsearch
  • Develop your own Elasticsearch plugins to extend the functionalities of Elasticsearch
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Publication date : Feb 21, 2017
Length: 428 pages
Edition : 3rd
Language : English
ISBN-13 : 9781786460189
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ISBN-13 : 9781786460189
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Table of Contents

12 Chapters
1. Revisiting Elasticsearch and the Changes Chevron down icon Chevron up icon
2. The Improved Query DSL Chevron down icon Chevron up icon
3. Beyond Full Text Search Chevron down icon Chevron up icon
4. Data Modeling and Analytics Chevron down icon Chevron up icon
5. Improving the User Search Experience Chevron down icon Chevron up icon
6. The Index Distribution Architecture Chevron down icon Chevron up icon
7. Low-Level Index Control Chevron down icon Chevron up icon
8. Elasticsearch Administration Chevron down icon Chevron up icon
9. Data Transformation and Federated Search Chevron down icon Chevron up icon
10. Improving Performance Chevron down icon Chevron up icon
11. Developing Elasticsearch Plugins Chevron down icon Chevron up icon
12. Introducing Elastic Stack 5.0 Chevron down icon Chevron up icon

Customer reviews

Rating distribution
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1 star 100%
Michael Bell Jun 03, 2017
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Poorly written. Basically it's a regurgitation of the online docs with little attempt to explain the implications of your query and indexing optionsVery disappointing
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