Goal
Apache Solr's query performance is critical. Solr's preferLocalShards feature is a powerful tool that can significantly boost performance in distributed setups. Let's dive into the basics of this feature, how it works, and why it's crucial for Solr users.
Environment
Solr 6, 7, 8
Guide
Sharding is the process of breaking up a big Solr index into more manageable, smaller sections known as shards. In essence, each shard is a distinct Solr core that is in charge of storing a fraction of the dataset. Sharding has advantages including increased query parallelism, fault tolerance, and scalability.
preferLocalShards is a query-time parameter in Solr that optimizes query execution by favoring shards hosted on the same server as the client making the request. This feature is particularly useful in distributed Solr architectures with multiple nodes. Without it, queries may be sent to remote shards, introducing network latency and slowing down response times.
When a query with preferLocalShards=true is sent to Solr, the system's query coordinator identifies the shards residing on the same node as the client and prioritizes them for query execution. This approach minimizes cross-node communication, reducing network overhead and improving query response times.
Use Cases and Benefits
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Multi-Node Clusters: preferLocalShards is ideal for multi-node Solr clusters, as it ensures that most query processing occurs locally, even in geographically dispersed setups.
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Geo-Distributed Applications: Applications serving a global audience can benefit from routing queries to local shards, improving search performance for users in specific regions.
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Resource Efficiency: By minimizing network communication,
preferLocalShardsconserves network resources and balances the load on individual nodes.
Enabling preferLocalShards
To use preferLocalShards, include it as a query parameter in your Solr requests. For example:
q=my_query&preferLocalShards=true
In this query, preferLocalShards is set to true to activate the preference for local shards.
Note: The preferLocalShards parameter in Solr was deprecated in version 8.4, which was released on March 12, 2020.
The preferred alternative parameter is shards.preference=replica.location:local. This parameter has the same effect as preferLocalShards, but it is more explicit and easier to understand.
It is important to note that using preferLocalShards or shards.preference=replica.location:local can adversely affect performance if there are many shards in a collection and few local replicas. This is because the query controller will have to direct the query to non-local replicas for most of the shards.
Example:
http://localhost:8983/solr/my_collection/select?q=*:*&shards.preference=replica.location:local
Therefore, it is recommended to only use these parameters if you have a small number of shards and many replicas, and if you are sure that the performance benefits outweigh the potential drawbacks.
References:
https://solr.apache.org/guide/6_6/distributed-requests.html#DistributedRequests-PreferLocalShards
https://solr.apache.org/guide/7_4/distributed-requests.html#shards-preference-parameter
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