Elasticsearch employs multiple mechanisms to ensure data reliability. The following are key measures:
1. Replicas and Shards
Elasticsearch ensures high availability and data security through data replication across multiple nodes. Each index can be divided into multiple shards, each of which can have one or more replicas. Primary shards handle write operations and a portion of read operations, while replica shards handle read operations and can take over write operations if the primary shard fails.
Example: Suppose an index has 5 primary shards and 3 replicas per primary shard. Even if up to 3 nodes fail, the data remains available with no data loss.
2. Write Acknowledgment
Elasticsearch uses a 'quorum-based' write acknowledgment mechanism for data writes. By default, an operation is considered successful only after data has been written to the primary shard and a majority of replica shards.
Example: If an index has three replicas, a write operation only returns success after successfully writing to the primary shard and two replica shards, ensuring data consistency and reliability.
3. Persistent Storage
Although Elasticsearch is a distributed search engine, it persists data to disk to ensure data is not lost after system restarts.
Example: Whenever data is written to Elasticsearch, it is stored in memory and asynchronously written to disk. This ensures data can be recovered from disk even during system crashes.
4. Snapshots and Backups
Elasticsearch supports creating periodic full index snapshots. These snapshots can be stored in external storage systems like Amazon S3 or HDFS for recovery in case of data loss or corruption.
Example: Users can configure a scheduled task, such as taking an index snapshot daily at midnight, and storing it in a secure external storage system. In the event of a catastrophic failure, these snapshots enable data restoration.
5. Failover
Elasticsearch automatically performs failover when a node or primary shard fails. This involves selecting an active replica shard to promote as the new primary shard, maintaining service continuity.
Example: If a node suddenly fails, Elasticsearch selects an active replica shard to replace the failed node's primary shard, allowing data write and query operations to continue seamlessly.
Through these mechanisms, Elasticsearch ensures data remains secure and reliable even during hardware failures, network issues, or other unexpected events.