Introduction
High availability in Elasticsearch refers to the system’s ability to remain accessible and operational over time, even in the event of component failures. This article delves into the strategies and best practices to ensure high availability in Elasticsearch.
Strategies and Best Practices to Ensure High Availability in Elasticsearch
1. Shard Allocation and Replication
Shard allocation and replication are key strategies to ensure high availability in Elasticsearch. Elasticsearch automatically divides the data in an index into multiple shards, each of which can be hosted on any node in the cluster.
To ensure high availability, it’s crucial to have a sufficient number of replicas for each shard. If a node fails, Elasticsearch can use these replicas to ensure that no data is lost and that the system remains available.
For example, if you have an index with five shards and one replica (the default setting), your data will be distributed across ten shards in total. If a node fails, the replicas on the other nodes will ensure that your data is still available.
2. Node Types and Roles
Elasticsearch has different types of nodes, each with a specific role. Understanding these roles and configuring your nodes correctly is crucial for high availability.
- Master-eligible nodes: These nodes can be elected as the master node, which controls the cluster.
- Data nodes: These nodes hold the data and perform data-related operations such as CRUD, search, and aggregations.
- Ingest nodes: These nodes are used to pre-process documents before indexing.
- Coordinating (client) nodes: These nodes route search and bulk requests to the appropriate nodes.
By default, every node is implicitly a master-eligible node, a data node, and a coordinating node. However, you can change these settings to suit your needs. For example, you can increase the number of master-eligible nodes to ensure that there’s always a node available to take over if the current master node fails. You should always have an odd number of master-eligible nodes to prevent split-brain situations. The ideal number of master-eligible nodes is three, as there is rarely the need to have more master-eligible nodes to control a cluster.
3. Cluster Health and Monitoring
Monitoring your Elasticsearch cluster’s health is crucial for maintaining high availability. Elasticsearch provides several APIs that you can use to check the status of your cluster and its components.
The `_cluster/health` API, for example, provides a high-level overview of the cluster’s health. It shows the status of the cluster (green, yellow, or red), the number of nodes and data nodes, the number of active shards, and other useful information.
The `_cat/nodes` API provides information about the nodes in the cluster, such as heap usage, CPU usage, load average, and more.
Regularly monitoring these metrics and setting proper alerts can help you detect potential issues before they affect the availability of your system.
4. Index Lifecycle Management
Index Lifecycle Management (ILM) is a feature in Elasticsearch that allows you to automate the management of your indices as they pass through different stages of their lifecycle: hot, warm, cold, frozen and delete.
By automating index management with ILM, you can ensure that your data is stored on the appropriate hardware at each stage of its lifecycle, which can help improve the availability and performance of your system.
For example, you might want to store hot data (data that’s being actively updated and queried) on high-performance hardware, while moving cold data (data that’s rarely accessed) to less expensive, slower hardware.
5. Backup and Restore
Regular backups are a crucial part of any high availability strategy. Elasticsearch’s snapshot and restore feature allows you to create backups of your indices and clusters. These backups, called snapshots, are stored in a repository, which can be a shared file system, Amazon S3, Azure Storage, Google Cloud Storage, or Hadoop Distributed File System (HDFS).
In the event of a failure, you can use these snapshots to restore your data. The restore process is flexible: you can restore the entire cluster, or specific indices.
6. Cross-Cluster Replication
Cross-cluster replication (CCR) is a very useful feature that can help you replicate your data into a remote cluster located in another data center that can be used for disaster recovery but also for high-availability.
It is also worth noting that the CCR feature requires at least a Platinum license. However, you can still try this feature by converting your basic license to a trial one, which gives you access to all features for 30 days.
Conclusion
In conclusion, high availability in Elasticsearch is achieved through a combination of strategies, including shard allocation and replication, node configuration, cluster health monitoring, index lifecycle management, cross-cluster replication, and regular backups. By understanding and correctly implementing these strategies, you can ensure that your Elasticsearch system remains available and resilient, even in the face of component failures.
