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What Is Sharding?

Sharding is the practice of optimizing database management systems by separating the rows or columns of a larger database table into multiple smaller tables. The new tables are called “shards” (or partitions), and each new table either has the same schema but unique rows (as is the case for “horizontal sharding”) or has a schema that is a proper subset of the original table’s schema (as is the case for “vertical sharding”).

Why Is Sharding Used?

Sharding is a common concept in scalable database architectures. By sharding a larger table, you can store the new chunks of data, called logical shards, across multiple nodes to achieve horizontal scalability and improved performance. Once the logical shard is stored on another node, it is referred to as a physical shard.

When running a database on a single machine, you will eventually reach the limit of the amount of computing resources you can apply to any queries, and you will obviously reach a maximum amount of data with which you can efficiently work. By horizontally scaling out, you can enable a flexible database design that increases performance in two key ways:

  • With massively parallel processing, you can take advantage of all the compute resources across your cluster for every query.
  • Because the individual shards are smaller than the logical table as a whole, each machine has to scan fewer rows when responding to a query.

Horizontal sharding is effective when queries tend to return a subset of rows that are often grouped together. For example, queries that filter data based on short date ranges are ideal for horizontal sharding since the date range will necessarily limit querying to only a subset of the servers.

Vertical sharding is effective when queries tend to return only a subset of columns of the data. For example, if some queries request only names, and others request only addresses, then the names and addresses can be sharded onto separate servers.

Also, sharded databases can offer higher levels of availability. In the event of an outage on an unsharded database, the entire application is unusable. With a sharded database, only the portions of the application that relied on the missing chunks of data are unusable. In practice, sharded databases often further mitigate the impact of such outages by replicating backup shards on additional nodes.

What Is the Difference between Sharding and Partitioning?

Sharding and partitioning are both about breaking up a large data set into smaller subsets. The difference is that sharding implies the data is spread across multiple computers while partitioning does not. Partitioning is about grouping subsets of data within a single database instance. In many cases, the terms sharding and partitioning are even used synonymously, especially when preceded by the terms “horizontal” and “vertical.” Thus, “horizontal sharding” and “horizontal partitioning” can mean the same thing.

Related Topics

In-Memory Computation

Edge Computing

Grid Computing

Further Reading

Hazelcast IMDG 3.12 Introduces CP Subsystem