What is number_of_replicas in Elasticsearch: Explanation and Usage

number_of_replicas in Elasticsearch is a setting that defines how many copies of each shard are kept as replicas. These replicas provide data redundancy and improve search performance by distributing queries across multiple copies.

How It Works

Imagine you have important documents stored in several folders. To keep them safe, you make copies and store them in different places. In Elasticsearch, data is split into parts called shards. The number_of_replicas setting tells Elasticsearch how many extra copies of each shard it should keep.

These replica shards act like backups. If one copy is lost or a server goes down, Elasticsearch can still access the data from the replicas. Also, when you search, Elasticsearch can use these replicas to answer faster by sharing the work.

Example

This example shows how to set number_of_replicas to 2 for an index named my_index. This means Elasticsearch will keep two copies of each shard besides the original.

json

PUT /my_index/_settings
{
  "index" : {
    "number_of_replicas" : 2
  }
}

Output

{
  "acknowledged" : true
}

When to Use

Use number_of_replicas to improve data safety and search speed. For example, in a live website where uptime is critical, having replicas ensures the site keeps working even if a server fails.

If you expect many users searching at the same time, replicas help by allowing Elasticsearch to handle more queries in parallel. However, more replicas mean more storage space is needed.

Key Points

• Replicas are copies of shards for safety and speed.
• Setting number_of_replicas controls how many copies exist.
• Replicas help keep data available if a server fails.
• More replicas improve search performance but use more storage.

Key Takeaways

number_of_replicas sets how many copies of each shard Elasticsearch keeps.

Replicas increase data safety and allow Elasticsearch to handle more search requests.

More replicas require more disk space but improve availability and performance.

You can change number_of_replicas anytime without reindexing data.