Poor query performance and latency in sharded clusters often result from one or more of the following factors:
Inefficient Shard Key: A poorly selected shard key can lead to data imbalances and uneven query loads across shards.
Suboptimal Query Patterns: Queries that do not use the shard key cause scatter-gather operations, where all shards must be queried, leading to increased latency.
Unbalanced Data Distribution and Chunk Migrations: Uneven chunk distribution among shards, often due to inefficient shard keys or limited chunk migrations, can lead to high traffic on certain shards. Write-intensive workloads in sharded clusters may increase chunk migrations, adding overhead and impacting performance.
Hardware Limitations: Insufficient resources (CPU, memory, disk I/O) on sharded servers can degrade performance under heavy load, which may require scaling up.
When clusters are sharded, one or more collections within the cluster may be partitioned across shards. To optimize the benefits of sharding, special thought has to be put into your most critical queries and how you model your data accordingly. For example:
MongoDB's golden rule—data that is accessed together should be stored together—helps minimize cross-shard queries and latency issues.
A thoughtfully chosen shard key ensures balanced data distribution across shards, efficient query routing, and minimized chunk migrations during high-write workloads. Queries that include the shard key can avoid scatter-gather operations, reducing latency and resource contention. Utilize the Shard Key Analyzer to find your shard key.
Embedding related data in a single document is ideal for frequently accessed data; this strategy works best when combined with a shard key that clusters related data on the same shard.
Referencing is better suited for independently accessed data or cases that would lead to bloated documents or unbounded arrays.
Schema design patterns like the subset pattern, where frequently accessed subsets of data are embedded into parent documents, simplify queries and improve performance by reducing the need for cross-shard retrieval.
To learn more about data modeling in sharded clusters, consider taking MongoDB’s Schema Design Optimization Skill Badge and Sharding Strategies Skill Badge.