How to Set Number of Replicas to 0 in Elasticsearch

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Elasticsearch is a powerful open-source search and analytics engine that allows for the storage, search, and analysis of big volumes of data quickly and in near real-time. Elasticsearch uses sharding to distribute data across multiple nodes within a cluster to enhance performance and capacity. Along with sharding, Elasticsearch also uses replicas to provide redundancy and increase search capacity. However, there might be scenarios where you need to set the number of replicas to 0, especially in development environments or to optimize resource usage in certain production scenarios. This article will guide you on how to set the number of replicas to 0 in Elasticsearch.

Table of Contents

Understanding Elasticsearch Replicas

Before we dive into the process, it’s essential to understand what replicas are and why they’re used. In Elasticsearch, an index can be divided into multiple shards to distribute the data. For each shard, there can be zero or more replicas. Replicas are essentially copies of the shard data, serving two main purposes: increasing fault tolerance (by providing data redundancy) and improving query throughput (by allowing queries to be executed on replica shards).

Setting the number of replicas to 0 means that no additional copies of the shard data will be maintained. While this reduces data redundancy and potential query throughput, it can be beneficial for conserving resources in environments where data redundancy is not a concern or during initial data loading to improve indexing speed.

Setting Number of Replicas to 0

There are a few different ways to set the number of replicas to 0 in Elasticsearch, including at index creation time or by updating an existing index.

At Index Creation

When creating a new index, you can specify the number of replicas in the settings. Here’s how you can do it using a curl command:

curl -X PUT "localhost:9200/my_index" -H "Content-Type: application/json" -d'
{
  "settings": {
    "index": {
      "number_of_replicas": 0
    }
  }
}'

This command creates an index named my_index with the number of replicas set to 0.

Updating an Existing Index

To update the number of replicas for an existing index, you can use the Index Update Settings API. Here’s an example command to set the number of replicas to 0 for an existing index:

curl -X PUT "localhost:9200/my_index/_settings" -H "Content-Type: application/json" -d'
{
  "index": {
    "number_of_replicas": 0
  }
}'

This command updates the my_index index, setting the number of replicas to 0.

Template Setting for New Indices

If you want to ensure that all new indices created in your Elasticsearch cluster have the number of replicas set to 0 by default, you can specify this setting in an index template:

curl -X PUT "localhost:9200/_template/default_template" -H "Content-Type: application/json" -d'
{
  "template": "*",
  "settings": {
    "number_of_replicas": 0
  }
}'

This command creates a template named default_template that applies to all new indices (as indicated by the "template": "*" setting) and sets the number of replicas to 0.

Considerations

While setting the number of replicas to 0 can save resources, it’s essential to understand the implications:

Data Safety: Without replicas, if a node holding a primary shard fails and cannot be recovered, the data on that shard is lost.
Query Throughput: Having replicas can improve read throughput by distributing search queries across multiple copies of the data.

It’s generally recommended to use replicas in production environments to ensure data redundancy and high availability.

Conclusion

Setting the number of replicas to 0 in Elasticsearch can be useful in certain scenarios, especially during development or when optimizing resources. However, it’s crucial to weigh the benefits against the potential risks related to data safety and query performance. Always ensure that your data is backed up and consider increasing the number of replicas before moving to a production environment to safeguard against data loss and improve system resilience.

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Anastasios Antoniadis

Anastasios Antoniadis is the founder and editor-in-chief of BORDERPOLAR... He is a software engineer, blogger, and avid gamer covering tech, gaming, and coding guides for over 4 years. He is a 2014 graduate of the Department of Informatics and Telecommunications of the University of Athens, an M.Sc. holder in Computer Science, and a Ph.D. student in Program Analysis.