Details: Last Updated: 14 March 2021

What is Apache Solr?

Apache Solr is an open-source search server platform written in Java language by Apache software foundation. It is highly scalable and ready to deploy search engine to handle a large volume of text-centric data. The purpose of using Apache Solr is to index and search large amount of web content and give relevant content based on search query.

Apache Solr is a REST-API based HTTP wrapper around the full-text search engine called Apache Lucene. An inverted index is a list of words where each word-entry links to the documents it is stored in. That way getting all documents for the search query "guru99" with simple 'get' operation.

In this Apache Solr tutorial for beginners, you will learn-

What is Apache Solr?
History of Apache Solr
Features of Apache Solr
Key Terms used in Apache Solr
Apache Solr Architecture
Apache Solr Applications
How to install Apache Solr
Elasticsearch Vs. Apache Solr
Advantages of Apache Solr
DisAdvantages of Apache Solr

History of Apache Solr

1999: Doug Cutting published Lucene
2004: Solr was developed at CNET by Yonik Seeley as a part company in-house project
2006: CNET publish the source code by donating it to the Apache Software Foundation
2008: Solr 1.3 was released with enhanced search capabilities and performance enhancements
2010: Merger of Lucene and Solr
2012: Solr version 4.0 was released, with new Solr Cloud feature
2016: Solr 6.0, was released which offers support for the execution of parallel SQL queries

Features of Apache Solr

Here, are Important features of Apache Solr:

Automatic Load Balancing
Standards-Based Open Interfaces – XML, JSON, and HTTP
Recommendations & Spell Suggestions are supported
Support for Auto-Completion and Geo-Spatial Search
Built-in Security for Authentication and Authorization
Allows you to perform a multilingual Keyword search
Autocomplete/Type-ahead Prediction
Batch and Streaming processing
Building machine-learning models are easy
Specially optimized for high volume web Traffic
Comprehensive HTML Admiration Interfaces
Supports both Schema and Schemaless configuration
Faceted Search and Filtering
Central Configuration For Entire Cluster

Key Terms used in Apache Solr

Now in this Solr search engine tutorial, we will learn about key terms used in Apache Solr:

Key Term	Description
Solr Core	Solr Core can be defined as an index of texts and fields derived from all the documents. One Solr Instance may have single or multiple Solr Cores. Core = an instance of Lucene Index + Solr configuration
Solr Instance	Solr Instance is an instance a Solr running in the Java Virtual Machine (JVM). In Standalone mode, it only offers one instance whereas in cloud mode you can have one or more instances.
Indexing	Indexing is a method for adding document's content to Solr Index. Apache Solr uses Apache Lucene Inverted Index technique.
Document	It is a group of fields and their values. A document is a basic unit of data stored in Apache Core. One Apache core may contain one or more Documents.
Field	The field is a key-value pair that stores the actual data in a Document. Key specifies the field name and value contains that Field data. A document may have a one or multiple fields. It is used by Apache Solr to index the document content.
Restful APIs	To communicate with Solr, it is not necessary to have used Java programming. Instead, Apache Solr provides restful services to communicate with it. You can send documents and receive results in various file formats like JSON, XML, and CSV.
Full-text search	Solr offers features for full-text search such as tokens, phrases, spell checking, auto-complete, wildcard, etc.
Admin Interface	Solr offers an easy-to-use, user-friendly, feature powered, user interface. Using the interface you can perform tasks like managing logs, add, delete, update and search documents.
Text-Centric and Sorted by Relevance	Apache Solr is used to search text documents, and the results are delivered according to the user's query.
Node	In Solr cloud, every single instance is known as a node.
Cluster	A cluster is a collection of nodes.
Collection	A cluster has a logical index which is also called a collection.
Shard	It is a small area of the collection which offers single or multiple replicas of the index.
Replica	A replica is a copy of shard which runs in a node.
Leader	It is a replica of the shard, which sends the requests of the Solr Cloud for the rest of replicas.

Apache Solr Architecture

Now in this Solr search tutorial, let's learn about Apache Solr Architecture:

Apache Solr compromises following components:

Query:

The query parser parses the queries which you need to pass to Solr. It verifies your query to check syntactical errors. After parsing the queries, it translates into a format which is known by Lucene.

Request Handler:

The request sends to Apache Solr are processed by the request handler. The request can be a query request or index update requests. You need to select the request handler according to your requirement. To pass a request to Solr, you need to map the handler to a specific URL end-point.

Response Writer

A response writer will generate formatted outputs for input queries. It supports various formats like XML, JSON, CSV.etc. You may have different response writers for different type of requests.

Update Handler:

When you send an update request to Apache Solr, it is run through a set of plugins, signature, logging, indexing. This process is known as update request processor. Update handler also responsible for modifications like adding or dropping filed, etc.

Apache Solr Applications

Application	Usage
Intranet Portal	Easy access to search Application launching News and event notification Single sign-on authentication
Federated Client	Simplified presentation Search across all content Authorized access only Document viewing
Instrument Datasets	Optimized for scientists Data dependent menus Specialized grid filters
Regulatory Documents	Designed for researchers Rich meta-data access Spreadsheet exports View document accelerator
Embedded in PLM Application	Offers better search experience than an RDBMS could provide Late-binding security model Document actions exposed on the toolbar

How to install Apache Solr

Step 1) Go to this link,

Click "Continue to Subscribe."

Step 2) On the next page,

Click on Accept Terms.

Step 3) Wait for some time and then,

Request is accepted after some time.

Step 4) Refresh the page and,

Click "Continue to Configuration"

Step 5) Keep the settings default and,

Click "Continue to Launch."

Step 6) On the next page,

Keep the settings default

Ensure you have the pem file of the key
Click "Launch"

You will see this success message

Step 7) In EC2 console,

Note the public DNS of your instance

Step 8) To access Solr,

Simply use the URL

http://publicdns:8983

in our case it becomes

http://ec2-18-221-175-53.us-east-2.compute.amazonaws.com:8983

Note: If you have trouble accessing the instance, change inbound and outbound rules in your instance to allow all traffic as shown in the below Solr query example:

Elasticsearch Vs. Apache Solr

Parameters	Apache Solr	Elastic Search
Nature	It is an open source project.	Not an open-sourced project.
Static status	Static in shema.xml	Static in elasticsearch.yml
Format	XML, CSV, JSON	Only JSON
Index	Can be reloaded during runtime with collection/core reload	Defined during index/type creation with a REST call
Documentation	It is well documented.	It is badly documented.
Splitting Shards	Possible	Not possible

Advantages of Apache Solr

Helps you to reduce the amount of time taken to locate Information
It is fast, simple, powerful and flexible search engine
Helps you to make your products and services more accessible
Increase customer spend on a web application
Helps you to improve user experience on the web application to increase revenue and profit
Comprehensive HTML based Administration Interface
Flexible and Adaptable with XML configuration
Extensible Plugin Architecture
Highly Scalable, robust, fault-tolerant search engine
Supports Distributed, Shading, Replication, Clustering and Multi-Node Architecture

DisAdvantages of Apache Solr

It is not an ACID compliant Data Store
It is not useful as a primary data store. Only useful as Secondary Data Store
Not offers support for transactions and distributed transactions
Not support Joins and Complex Queries
Not optimal for Normalized Data

Summary

Apache Solr is an Open-source REST-API based search server
Apache Solr was developed at CNET by Yonik Seeley as a part company in-house project
Apache Solr offers features like Auto-Completion and Geo-Spatial Search
Solr Core, Solr Instance, Indexing, Document, Restful APIs, Full-text search, Admin Interface,etc. are some key terms used in Apache Solr
Query, Request handler, Request writer and update handlers are some of the important components of Apache Solr
Intranet Portal, Federated Client, Instrument Datasets, Regulatory Documents, Embedded in PLM Application are useful Apache Solr Applications
Apache Solr is well documented compares to elastic search which is badly documented
The biggest advantage of Apache Solr is that it helps you to reduce the amount of time taken to locate Information
The main drawback of this application It is not an ACID compliant Data Store