Elasticsearch Elasticsearch Index Number_of_Replicas

By Opster Team

Updated: Aug 29, 2023

| 2 min read

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Elasticsearch is designed to be a distributed system that can handle a large amount of data and provide high availability. One of the key features that enable this is the concept of index replication, which is controlled by the `number_of_replicas` setting. This article will delve into the details of this setting, its implications, and how to properly configure it.

The role of replicas in Elasticsearch

In Elasticsearch, an index is a collection of documents that are partitioned across multiple primary shards. Each primary shard is a self-contained Apache Lucene index, and the documents within an index are distributed among all primary shards. To ensure high availability and data redundancy, Elasticsearch allows each shard to have one or more copies, known as replicas.

The `number_of_replicas` setting controls the number of replica shards (copies) that Elasticsearch creates for each primary shard in an index. By default, Elasticsearch creates one replica for each primary shard, but this can be changed according to the requirements of your system.

Configuring the number_of_replicas

The `number_of_replicas` setting can be configured at the time of index creation or updated later. Here’s how you can set it during index creation:

PUT /my_index
  "settings": {
    "number_of_replicas": 2

In this example, Elasticsearch will create two replicas for each primary shard in the `my_index` index.

To update the `number_of_replicas` setting for an existing index, you can use the `_settings` API:

PUT /my_index/_settings
  "number_of_replicas": 3

This command will update the `my_index` index to have three replicas for each primary shard.

Implications of the number_of_replicas setting

The `number_of_replicas` setting has a significant impact on the performance and resilience of your Elasticsearch cluster. Here are some key points to consider:

1. Data Redundancy and Availability: Increasing the `number_of_replicas` enhances the availability of your data by creating more copies of each shard. If a node fails, Elasticsearch can still serve data from the replica shards on the remaining nodes.

2. Search Performance: Replica shards can serve read requests, so having more replicas can improve search performance by distributing the load across more shards.

3. Write Performance: However, each write operation must be performed on every copy of a shard. Therefore, a higher `number_of_replicas` can slow down indexing performance as it increases the number of operations that must be performed for each write.

4. Storage Requirements: More replicas mean more storage space. You should ensure that your cluster has enough capacity to store the additional replicas.

5. Resilience to Node Failure: The `number_of_replicas` should be set considering the number of nodes in your cluster. If the `number_of_replicas` is equal to or greater than the number of nodes, your cluster can tolerate the failure of multiple nodes without data loss.

Best practices for setting number_of_replicas

The optimal `number_of_replicas` setting depends on the specific requirements of your system. However, here are some general best practices:

– For a single-node cluster, `number_of_replicas` should be set to 0, as there are no other nodes to hold replicas.

– For a multi-node cluster, `number_of_replicas` should be set to at least 1 to ensure data redundancy and high availability.

– If search performance is a priority, consider increasing the `number_of_replicas`. However, keep in mind the trade-off with write performance and storage requirements.

– Always ensure that your cluster has enough capacity to store the additional replicas.