In order to understand replicas in Elasticsearch, you need to have a thorough understanding of shards and their use in Elasticsearch. While each shard contains a single copy of the data, an index can contain multiple copies of the shard. There are thus two types of shards, the primary shard and a replica, or copy. Each replica is located on a different node, which ensures access to your data in the event of a node failure. In addition to providing redundancy and their role in preventing data loss and downtime, replicas can also help boost search performance by allowing queries to be processed in parallel with the primary shard, and therefore faster.
Notes and good things to know
- There are some important differences in how primary and replica shards behave. While both are capable of processing queries, indexing requests must first go through primary shards before they can be replicated to the replica shards. As noted above, if a primary shard becomes unavailable—for example, due to a node disconnection or hardware failure—a replica is promoted to take over its role.
- While replicas can help in the case of a node failure, replicas use up memory and disk space, as do primary shards. They also use compute powers when indexing, so it is also important not to have too many. Another difference between the primary shards and replicas is that while the number of primary shards cannot be changed after the index has been created, the number of replicas can be altered at any time.
- Another factor to consider with replicas is the number of nodes available. Replicas are always placed on different nodes from the primary shard, since two copies of the same data on the same node would add no protection if the node were to fail. As a result, for a system to support n replicas, there need to be at least n + 1 nodes in the cluster. For instance, if there are two nodes in a system and an index is configured with six replicas, only one replica will be allocated. On the other hand, a system with seven nodes is perfectly capable of handling one primary shard and six replicas.
- Even after an index with the right balance of shards and replicas has been created, these need to be monitored, as the dynamics around an index change over time. For instance, when dealing with time series data, indices with recent data are generally more active than older ones. Without tuning these indices, they would all consume the same amount of resources, despite their very different requirements.
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