Sharding

Table of contents

Introduction

Every table partition is split into a configured number of shards. Shards are then distributed across the cluster. As nodes are added to the cluster, CrateDB will move shards around to achieve maximum possible distribution.

Tip

Non-partitioned tables function as a single partition, so non-partitioned tables are still split into the configured number of shards.

Shards are transparent at the table-level. You do not need to know about or think about shards when querying a table.

Read requests are broken down and executed in parallel across multiple shards on multiple nodes, massively improving read performance.

Number of shards

The number of shards can be defined by using the CLUSTERED INTO <number> SHARDS statement upon the table creation.

Example:

cr> create table my_table5 (
...   first_column integer
... ) clustered into 10 shards;
CREATE OK, 1 row affected (... sec)

If the number of shards is not defined explicitly, the sensible default value is applied.

Note

The number of shards can be changed after table creation, providing the value is a multiple of number_of_routing_shards (set at table-creation time). Altering the number of shards will put the table into a read-only state until the operation has completed.

Caution

Well tuned shard allocation is vital. Read the Sharding Guide to make sure you’re getting the best performance out ot CrateDB.

Routing

Given a fixed number of primary shards, individual rows can be routed to a fixed shard number with a simple formula:

shard number = hash(routing column) % total primary shards

When hash values are distributed evenly (which will be approximately true in most cases), rows will be distributed evenly amongst the fixed amount of available shards.

The routing column can be specified with the CLUSTERED clause when creating the table. All rows that have the same routing column row value are stored in the same shard. If a primary key has been defined, it will be used as the default routing column, otherwise the internal document ID is used.

Example:

cr> create table my_table6 (
...   first_column integer,
...   second_column text
... ) clustered by (first_column);
CREATE OK, 1 row affected (... sec)

If primary key constraints are defined, the routing column definition can be omitted as primary key columns are always used for routing by default.

If the routing column is defined explicitly, it must match a primary key column:

cr> create table my_table8 (
...   first_column integer primary key,
...   second_column text primary key,
...   third_column text
... ) clustered by (first_column);
CREATE OK, 1 row affected (... sec)

Example for combining custom routing and shard definition:

cr> create table my_table9 (
...   first_column integer primary key,
...   second_column text primary key,
...   third_column text
... ) clustered by (first_column) into 10 shards;
CREATE OK, 1 row affected (... sec)

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