Elastic Search interview questions for freshers/Elastic search Interview Questions and Answers for Freshers & Experienced

What are frozen indices in Elasticsearch?

Frozen indices are those indices which are rarely accessed. So, the users freeze these indices. Such indices are called frozen indices. Basically, we freeze those indices which are not in use to free up the memory.

Frozen indices become read-only and its resources are no longer kept active. So, these indices are searchable, but to write again, we have to unfreeze them. Elasticsearch offers an ignore_throttled parameter, which is used to include the frozen indices in your search. Thus, we don't need to re-open them to make available for search.

Does Elasticsearch provide an interactive graphical user interface to its users?

No, Elasticsearch does not offer its own graphical user interface (GUI). We run it through a batch file (i.e., elasticsearch.bat) that provides a Command Line Interface (CLI).

To interact with Elasticsearch, we have to install a plugin or a data visualization tool. There are several plugins available, such as elasticsearch-head, icu-analyzer, etc. Despite this, you can install Kibana for data visualization, which is an essential component of ELK Stack.

Please explain the working of aggregation in Elasticsearch?

Aggregations help in the collection of data from the query used in the search. Different types of aggregations are Metrics, Average, Minimum, Maximum, Sum and stats, based on different purposes.

What are Aggregations?

The aggregations framework helps you to provide aggregated data based on a search query. It is based on simple building blocks known as aggregations. It can be composed to build complex summaries of the data.

Can you compare between Term-based queries and Full-text queries?

Domain Specific Language (DSL) Elasticsearch query which is known as Full-text queries utilizes the HTTP request body, offers the advantage of clear and detailed in their intent, over time it is simpler to tune these queries.

Term based queries utilize the inverted index, a hash map-like data structure that helps to locate text or string from the body of email, keyword or numbers or dates, etc. used in analysis purposes.

What do you mean by ingest node?

Ingest node is used to pre-process the documents before the actual document indexing is done. It intercepts bulk and index requests and applies transformations to pass the documents back to the bulk API and index.

How can you create an Index in Elasticsearch?

For example:

PUT /client?pretty

GET /_cat/indices?v

What is Single document APIs in Elasticsearch?

>> Get API
>> Index API
>> Delete API
>> Update API

What are the various possible ways in which we can perform a search in Elasticsearch?

Mentioned below are the various possible ways in which we can perform a search in Elasticsearch:

<> Applying search API across multiple types and multiple indexes: Search API, we can search an entity across multiple types and indices.

<> Search request using a Uniform Resource Identifier: We can search requests using parameters along with URI i.e. Uniform Resource Identifier.

<> Search using Query DSL i.e. (Domain Specific Language) within the body: DSL i.e. Domain Specific Language is utilized for JSON request body.

How do you check the version of Elasticsearch you are working with?

To find out which version of Elasticsearch you are running locally execute the following curl command from your command line: curl -XGET 'http://localhost:9200'

What are the various ways of using X-Pack Commands?

Here, are X-Pack commands that help you configure security:

Certgen
migrate
syskeygen
certutil
saml-metadata
setup-passwords
users

What is the port number to access the Elasticsearch on the web? Can we change it?

Elasticsearch is accessed by using HTTP protocol on the web, which needs a port number along with localhost address. The default port number of Elasticsearch is 9200.

In case the Elasticsearch port 9200 is already in use or used by any other tool, you can change the port number. Elasticsearch port number can be changed in the elasticsearch.yml file. This file exists inside the config folder.

What is Apache Lucene?

Apache Lucene is an open-source information retrieval software library written in Java language.

Please explain relevancy and scoring in Elasticsearch?

When you search on the internet about say, Apple. It could either display the search results about fruit or company with name as an Apple. You may want to buy fruit online, check the recipe from the fruit or health benefits of eating fruit, apple.

In contrast, you may want to check Apple.com to find the latest product range offered by the company, check Apple Inc.’s stock prices and how a company is performing in NASDAQ in the last 6 months, 1 or 5 years.

Similarly, when we search for a document (a record) from Elasticsearch, you are interested in getting the relevant information that you are looking for. Based on the relevance, the probability of getting the relevant information is calculated by the Lucene scoring algorithm.

The Lucene technology helps to search a particular record i.e. document which is indexed based on the frequency of the term in search appearing in the document, how often its appearance across an index and query which is designed using various parameters.

What is the syntax or code to retrieve a document by ID in Elasticsearch?

GET API retrieves the specified JSON document from an index.

Syntax:

GET <index_name>/_doc/<_id>

Where is Elastic search stored?

Elastic search results are stored in a distributed document in different directories. Also, a user can retrieve complex data structures that are serialized as JSON documents.

What is cat API in Elasticsearch?

These commands accept a query string parameter. This helps to see all info and headers and info they provide and the /_cat command, which allows you to lists all the available commands.

Where do you configure settings for X-Pack?

You can configure settings for X-Pack. It has features in the elasticsearch, logstash, and kibana.yml (ELK stack ) configuration files.

What is NRT in Elasticsearch?

NRT is a full form of (Near Real-Time Search) platform. It is a near real-time search platform. It means there is a slight latency (mostly one second) from when you index a document until it becomes very searchable.

Here, are important configuration management tool supported by Elasticsearch:

* Puppet – puppet-elasticsearch
* Chef – cookbook-elasticsearch
* Ansible – ansible-elastic search

What is Apache Lucene?

Apache Lucene is an open-source information retrieval software library. It is originally written in Java language.

What is the latest version of Elasticsearch?

Lastest version of Elastic Search on Jan 2020, which is the latest and stable version of Elasticsearch.

What are the Disadvantages of Elasticsearch?

* Elasticsearch does not support multiple languages while handling request and response data in JSON.

* In rare cases, it has a problem with Split-Brain situations.

What is Elasticsearch REST API and use of it?

Elasticsearch provides a very comprehensive and powerful REST API that you can use to interact with your cluster. Among the few things that can be done with the API are as follows:

Check your cluster, node, and index health, status, and statistics
Administer your cluster, node, and index data and metadata
Perform CRUD (Create, Read, Update, and Delete) and search operations against your indexes
Execute advanced search operations viz. aggregations, filtering, paging, scripting, sorting, among many others

What is Token filters in Elasticsearch Analyzer?

A token filter obtains the token stream, later on add, delete, or alter the tokens. For instance, a lowercase token filter modifies all tokens into lowercase, a stop token filter deletes stop words, and a synonym token filter includes synonyms into the token stream.

Token filters will be unable to change the position or character offsets of any certain token.

What are the various ways of searching in Elasticsearch?

Following are the way of search in Elasticsearch:

Multi-index, Multitype search: You can search APIs that can be applied across all multiple indices by using the multi-index support system.

In Elastic search, we can create certain tags across all indices across all indices and all types.

>> URI search: A search request is executed using a URI by providing requested parameters.
>> Request body search: A search request need to be executed by a search DSL. It includes the query DSL within the body.

Explain the method to add a mapping in an Index

Elasticsearch allows you to create the mapping according to the data provided by the user in the request body. Its bulk feature can be used to add more than one JSON object in the index.

For example, POST website /_bulk.

How you can delete an index in Elastic search?

To delete an index in Elasticsearch, You need to write command:

DELETE /index name.
For example, DELETE /website.

What is a replica in ElasticSearch?

Each shard in ElasticSearch has 2 copy, which is called replicas. They help you for high-availability and fault-tolerance.

Explain Tokenizer in ElasticSearch

A Tokenizer breakdown fields which values of a document into a stream. Inverted indexes are created and updated by using these values. After that, these stream of values are stored in the document.

What are the various ways of searching in Elasticsearch?

We have different ways of searching in Elasticsearch:

>> Multi-index, Multitype search: A user can search APIs that can be applied across several indices through a multi-index support system.

>> URI (uniform resource identifier) search: A user can execute a search request using a URI by providing the requested parameters.

>> Request body search: A search request needs to be executed by a search DSL.

Can you define REPLICA and what is the advantage of creating a replica?

A replica is an exact copy of the Shard, used to increase query throughput or achieve high availability during extreme load conditions. These replicas help to efficiently manage requests.

How can you delete an index in Elasticsearch?

To delete an index in Elasticsearch, use the below command.

DELETE/index name

For eg. DELETE/website

Can you please define Mapping in an Elasticsearch?

Mapping is the outline of the documents stored in an index. The mapping defines how a document is indexed, how its fields are indexed and stored by Lucene.

What are the primary operations performed in a Document?

Here, are important operation performed on documents:

* Indexing a document
* Fetching documents
* Updating documents
* Deleting documents

What is a Type in an Elastic search?

Type, here is a table in the relational database. These types (tables) hold multiple Documents (rows), and each document has Properties (columns).

What is an index in an Elasticsearch cluster?

An Elasticsearch cluster can contain multiple indices, which are database as compared with a relational database, these indices contain multiple types (tables). The types (tables) contain multiple Documents (records/rows) and these documents contain Properties (columns).

Name 10 companies that have an Elasticsearch as their search engine and database for their application?

Following are the list of some companies that use Elasticsearch along with Logstash and Kibana:

* Uber
* Instacart
* Slack
* Shopify
* Stack Overflow
* DigitalOcean
* Udemy
* 9GAG
* Wikipedia
* Netflix
* Accenture
* Fujitsu

What is the ELK stack?

The ELK Stack is a collection of three open-source products — Elasticsearch, Logstash, and Kibana. They are all developed, managed, and maintained by the company Elastic.

>> E stands for ElasticSearch: It is used for storing logs.
>> L stands for LogStash: It is used for both shipping as well as the processing and storing logs.
>> K stands for Kibana: It is a visualization tool (a web interface) that is hosted through Nginx or Apache.

What are the important advantages of Elastic Search?

Here are the important advantages of Elasticsearch:

* Store schema-less data and also creates a schema for your data.
* Manipulate your data record by record with the help of Multi-document APIs
* Perform filtering and querying your data for insights
* Based on Apache Lucene and provides RESTful API
* It provides horizontal scalability, reliability, and multitenant capability for real-time use of indexing.
* Helps you to scale vertically and horizontally

Define the Term Shard

Every index can be split into several shards to be able to distribute data. The shard is the atomic part of an index, which can be distributed over the cluster if you want to add more nodes.

What is a document in Elastic Search?

In an Elastic search, a document is a basic unit of information that can be indexed. It is expressed in JSON (key: value) pair. ‘{“user”: “nullcon”}’. Every single Document is associated with a type and a unique id.

Explain Index

A node is an elastic search Instance. It is created when an elasticsearch instance begins.

What are the important features of Elasticsearch?

Here are important features of Elasticsearch:

* An open-source search server written using Java.
* Used to index any kind of heterogeneous data
* Has REST API web-interface with JSON output
* Full-Text Search
* Near Real-Time (NRT) search
* Sharded, replicated searchable, JSON document store.
* Schema-free, REST & JSON based distributed document store
* Multi-language & Geolocation support

What is the ELK stack?

In Elasticsearch, ELK Stack is a collection of three open-source products — Elasticsearch, Logstash, and Kibana.

E stands for ElasticSearch: used for storing logs.

L means LogStash: used for both shipping, processing, and storing logs.

K stands for Kibana: a visualization tool.

What is a cluster?

A cluster is a group of nodes with the same cluster. name attribute which together holds data and provides joined indexing and search capabilities.

To install Elasticsearch, what software is required as a prerequisite?

Latest JDK 8 or Java version 1.8.0 is recommended as the software required for running Elasticsearch on your device.

Can you state the stable Elasticsearch version currently available for download?

The latest stable version of Elasticsearch is 7.5.0.

What is Elasticsearch?

Elasticsearch is a NoSQL database. It is based on the Lucene search engine, and it is built with RESTful APIS. It offers simple deployment, maximum reliability, and easy management. It also provides advanced queries to perform detailed analysis and stores all the data centrally. It helps execute a quick search of the documents.

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