Multi-Region Capabilities Overview

This page provides an overview of CockroachDB multi-region capabilities.

Overview

CockroachDB multi-region capabilities make it easier to run global applications. To use these capabilities effectively, you should understand the following concepts:

  • Cluster region is a geographic region that you specify at node start time.
  • Database region is a geographic region in which a database operates. You must choose a database region from the list of available cluster regions.
  • Survival goal dictates how many simultaneous failure(s) a database can survive.
  • Table locality determines how CockroachDB optimizes access to a table's data.

At a high level, the simplest process for running a multi-region cluster is:

  1. Set region information for each node in the cluster at startup using node startup locality options.
  2. Add one or more regions to a database, making it a "multi-region" database. One of these regions must be the primary region.
  3. (Optional) Change table localities (global, regional by table, regional by row). This step is optional because by default the tables in a database will be homed in the database's primary region (as set during Step 1).
  4. (Optional) Change the database's survival goals (zone or region). This step is optional because by default multi-region databases will be configured to survive zone failures.

These steps describe the simplest case, where you accept all of the default settings. The latter two steps are optional, but table locality and survival goals have a significant impact on performance. Therefore Cockroach Labs recommends that you give these aspects some consideration when you choose a multi-region configuration.

Tip:

For new clusters using the multi-region SQL abstractions, Cockroach Labs recommends lowering the --max-offset setting to 250ms. This setting is especially helpful for lowering the write latency of global tables. For existing clusters, changing the setting will require restarting all of the nodes in your cluster at the same time; it cannot be done with a rolling restart.

Cluster regions

You define a cluster region at the node level using the region key and the zone using the zone key in the node startup locality options.

For example, the following command adds us-east-1 to the list of cluster regions and us-east-1b to the list of zones:

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cockroach start --locality=region=us-east-1,zone=us-east-1b # ... other required flags go here

To show all of a cluster's regions, execute the following SQL statement:

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SHOW REGIONS FROM CLUSTER;

Database regions

A database region is a high-level abstraction for a geographic region. Each region is broken into multiple zones. These terms are meant to correspond directly to the region and zone terminology used by cloud providers.

The regions added during node startup become database regions when you add them to a database. To add the first region, use the ALTER DATABASE ... PRIMARY REGION statement.

While the database has only one region assigned to it, it is considered a "multi-region database." This means that all data in that database is stored within its assigned regions, and CockroachDB optimizes access to the database's data from the primary region. If the default survival goals and table localities meet your needs, there is nothing else you need to do once you have set a database's primary region.

To add another database region, use the ALTER DATABASE ... ADD REGION statement.

To show all of a database's regions, execute the SHOW REGIONS FROM DATABASE statement.

Note:

If the default survival goals and table localities meet your needs, there is nothing else you need to do once you have set a database's primary region.

Super regions

Warning:

This is an experimental feature. The interface and output are subject to change.

New in v22.1: Super regions allow you to define a set of database regions such that the following schema objects will have all of their replicas stored only in regions that are members of the super region:

The primary use case for super regions is data domiciling. As mentioned above, data from regional and regional by row tables will be stored only in regions that are members of the super region. Further, if the super region contains 3 or more regions and if you use REGION survival goals, the data domiciled in the super region will remain available if you lose a region.

To use super regions, you must keep the following considerations in mind:

For more information about how to enable and use super regions, see:

Note that super regions take a different approach to data domiciling than ALTER DATABASE ... PLACEMENT RESTRICTED. Specifically, super regions make it so that all replicas (both voting and non-voting) are placed within the super region, whereas PLACEMENT RESTRICTED makes it so that there are no non-voting replicas.

For more information about data domiciling using PLACEMENT RESTRICTED, see Data Domiciling with CockroachDB.

Note:

If you want to do data domiciling for databases with region survival goals using the higher-level multi-region abstractions, you must use super regions. Using ALTER DATABASE ... PLACEMENT RESTRICTED will not work for databases that are set up with region survival goals.

Note:

Super regions rely on the underlying replication zone system, which was historically built for performance, not for domiciling. The replication system's top priority is to prevent the loss of data and it may override the zone configurations if necessary to ensure data durability. For more information, see Configure Replication Zones.

Survival goals

A survival goal dictates how many simultaneous failure(s) a database can survive. All tables within the same database operate with the same survival goal. Each database can have its own survival goal setting.

The following survival goals are available:

  • Zone failure
  • Region failure

The zone failure survival goal is the default. You can configure a database to survive region failures at the cost of slower write performance (due to network hops) using the following statement:

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ALTER DATABASE <db> SURVIVE REGION FAILURE;

For more information about the survival goals supported by CockroachDB, see the following sections:

Survive zone failures

With the zone level survival goal, the database will remain fully available for reads and writes, even if a zone becomes unavailable. However, the database may not remain fully available if multiple zones in the same region fail. This is the default setting for multi-region databases.

You can configure a database to survive zone failures using the ALTER DATABASE ... SURVIVE ZONE FAILURE statement.

If your application has performance or availability needs that are different than what the default settings provide, you can explore the other customization options described on this page.

Survive region failures

The region level survival goal has the property that the database will remain fully available for reads and writes, even if an entire region becomes unavailable. This added survival comes at a cost: write latency will be increased by at least as much as the round-trip time to the nearest region. Read performance will be unaffected. In other words, you are adding network hops and making writes slower in exchange for robustness.

You can upgrade a database to survive region failures using the ALTER DATABASE ... SURVIVE REGION FAILURE statement.

Setting this goal on a database in a cluster with 3 cluster regions will automatically increase the replication factor of the ranges underlying the database from 3 (the default) to 5. This ensures that there will be 5 replicas of each range spread across the 3 regions (2+2+1=5). This is how CockroachDB is able to provide region level resiliency while maintaining good read performance in the leaseholder's region. For writes, CockroachDB will need to coordinate across 2 of the 3 regions, so you will pay additional write latency in exchange for the increased resiliency.

Note:

To survive region failures, you must add at least 3 database regions.

Table localities

Table locality determines how CockroachDB optimizes access to the table's data from that locality. Every table in a multi-region database has a "table locality setting". By default, all tables in a multi-region database are regional tables -- that is, CockroachDB optimizes access to the table's data from a single region (by default, the database's primary region).

For information about the table localities CockroachDB supports, see the sections:

  • Regional tables provide low-latency reads and writes for an entire table from a single region.
  • Regional by row tables provide low-latency reads and writes for one or more rows of a table from a single region. Different rows in the table can be optimized for access from different regions.
  • Global tables are optimized for low-latency reads from all regions.
Note:

Table locality settings are used for optimizing latency under different read and write patterns. If you are optimizing for read and write access to all of your tables from a single region (the primary region), there is nothing else you need to do once you set your database's primary region.

Regional tables

In a regional table, access to the table will be fast in the table's "home region" and slower in other regions. In other words, CockroachDB optimizes access to data in a regional table from a single region. By default, a regional table's home region is the database's primary region, but that can be changed to use any region in the database. Regional tables work well when your application requires low-latency reads and writes for an entire table from a single region.

For instructions showing how to set a table's locality to REGIONAL BY TABLE, see ALTER TABLE ... SET LOCALITY.

By default, all tables in a multi-region database are regional tables that use the database's primary region. Unless you know your application needs different performance characteristics than regional tables provide, there is no need to change this setting.

Regional by row tables

In a regional by row table, individual rows are optimized for access from different regions. This setting automatically divides a table and all of its indexes into partitions, with each partition optimized for access from a different region. Like regional tables, regional by row tables are optimized for access from a single region. However, that region is specified at the row level instead of applying to the whole table.

Use regional by row tables when your application requires low-latency reads and writes at a row level where individual rows are primarily accessed from a single region. For example, a users table in a global application may need to keep some users' data in specific regions for better performance.

For an example of a table that can benefit from the regional by row setting in a multi-region deployment, see the users table from the MovR application.

For instructions showing how to set a table's locality to REGIONAL BY ROW, see ALTER TABLE ... SET LOCALITY.

Global tables

A global table is optimized for low-latency reads from every region in the database. The tradeoff is that writes will incur higher latencies from any given region, since writes have to be replicated across every region to make the global low-latency reads possible. Use global tables when your application has a "read-mostly" table of reference data that is rarely updated, and needs to be available to all regions.

For an example of a table that can benefit from the global table locality setting in a multi-region deployment, see the promo_codes table from the MovR application.

For instructions showing how to set a table's locality to GLOBAL, see ALTER TABLE ... SET LOCALITY.

Additional features

The features listed in this section make working with multi-region clusters easier.

Indexes on REGIONAL BY ROW tables

In multi-region deployments, most users should use REGIONAL BY ROW tables instead of explicit index partitioning. When you add an index to a REGIONAL BY ROW table, it is automatically partitioned on the crdb_region column. Explicit index partitioning is not required.

While CockroachDB process an ADD REGION or DROP REGION statement on a particular database, creating or modifying an index will throw an error. Similarly, all ADD REGION and DROP REGION statements will be blocked while an index is being modified on a REGIONAL BY ROW table within the same database.

This behavior also applies to GIN indexes.

For an example that uses unique indexes but applies to all indexes on REGIONAL BY ROW tables, see Add a unique index to a REGIONAL BY ROW table.

Regional by row tables can take advantage of hash-sharded indexes provided the crdb_region column is not part of the columns in the hash-sharded index.

Schema changes in multi-region clusters

To reduce latency while making online schema changes, we recommend specifying a lease_preference zone configuration on the system database to a single region and running all subsequent schema changes from a node within that region. For example, if the majority of online schema changes come from machines that are geographically close to us-east1, run the following:

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ALTER DATABASE system CONFIGURE ZONE USING constraints = '{"+region=us-east1": 1}', lease_preferences = '[[+region=us-east1]]';

Run all subsequent schema changes from a node in the specified region.

Next steps

See also


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