Elasticsearch Understanding Shards in Elasticsearch

By Opster Team

Updated: Jun 6, 2023

| 2 min read


What are shards in Elasticsearch?

Shards are the fundamental building blocks of Elasticsearch’s distributed architecture. They enable horizontal scaling, improve search performance, and ensure high availability.

In this article, we will delve into the concept of shards, their types, and how they contribute to the overall performance and reliability of Elasticsearch.

Types of Shards

There are two types of shards in Elasticsearch:

1. Primary Shards: These shards hold the original data and are responsible for indexing and searching operations. The number of primary shards is determined at the time of index creation and cannot be changed afterward.

2. Replica Shards: These are the copies of primary shards, created to provide redundancy and improve search performance. Replica shards can be added or removed dynamically, and Elasticsearch automatically balances them across the nodes in the cluster.

Benefits of Sharding

1. Horizontal Scaling: Shards allow Elasticsearch to distribute data across multiple nodes, enabling the cluster to grow horizontally as the data volume increases. This distribution ensures that no single node becomes a bottleneck, thus maintaining optimal performance.

2. Improved Search Performance: By splitting the data into smaller units, Elasticsearch can execute search queries on multiple shards concurrently, resulting in faster response times.

3. High Availability: Replica shards ensure that the data remains available even if a node fails. Elasticsearch automatically routes search and indexing operations to the available shards, maintaining the cluster’s overall health.

Best Practices for Sharding

1. Choose the Right Number of Primary Shards: Selecting the appropriate number of primary shards is crucial for optimal performance. It is essential to consider factors such as data volume, query load, and hardware resources when determining the number of primary shards.

2. Use a Suitable Number of Replica Shards: Having an adequate number of replica shards ensures high availability and improved search performance. However, too many replicas can consume additional resources and affect indexing performance. It is recommended to have at least one replica shard for each primary shard.

3. Monitor Shard Allocation: Regularly monitoring shard allocation helps identify imbalances and potential issues in the cluster. Elasticsearch provides APIs, such as the Cluster Health API and the Shard Allocation API, to monitor and manage shard allocation.

4. Rebalance Shards When Necessary: In case of an imbalance, Elasticsearch provides the Cluster Reroute API to manually move shards between nodes. However, use this API with caution, as it can impact cluster performance if not used correctly.


In conclusion, understanding shards and their role in Elasticsearch’s architecture is crucial for optimizing performance and ensuring high availability. By following best practices and monitoring shard allocation, you can maintain a healthy and efficient Elasticsearch cluster.

If you want to learn more about the concept of shards in Elasticsearch and see examples, check out this guide.

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