Multi-region Capabilities Overview

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Overview

New in v21.1: CockroachDB has improved multi-region capabilities that make it easier to run global applications.

To take advantage of these improved capabilities, you will need to understand the following concepts:

• Cluster Regions are geographic regions that a user specifies at node start time.
• Database Regions are geographic regions that a given database operates within. A database region must be chosen from the list of available cluster regions.
• Survival Goals, which dictate how many simultaneous failure(s) that a database can survive.
• Table Localities, which tell CockroachDB how to optimize 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 2).
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.

The steps above describe the simplest case, where you accept all of the default settings. If your application has performance or availability needs that are different than what the default settings provide, there are customization options to explore.

For more information about CockroachDB's multi-region capabilities and the customization options that are available, see below.

Cluster Regions

Regions and zones are defined at the node level using the following node startup locality options:

• Regions are specified using the region key.
• Zones through the use of the zone key.

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

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:

SHOW REGIONS FROM CLUSTER;

Database Regions

Database regions are 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 they are added 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.

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.

Survival goals

All tables within the same database operate with the same survival goal. Each database is allowed to have its own survival goal setting.

The following survival goals are available:

• Zone failure
• Region failure

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

ALTER DATABASE <db> SURVIVE REGION FAILURE;

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

• Surviving zone failures
• Surviving region failures

Surviving zone failures

With the zone level survival goal, the database will remain fully available for reads and writes, even if a zone goes down. However, the database may not remain fully available if multiple zones fail in the same region.

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

Surviving 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 goes down. 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.

Table locality

Every table in a multi-region database has a "table locality setting" applied to it. CockroachDB uses the table locality setting to determine how to optimize access to the table's data from that locality.

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 more information about the table localities CockroachDB supports, see the following 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.

Regional tables

Regional tables work well when your application requires low-latency reads and writes for an entire table from a single region.

For regional tables, 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 regional tables 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.

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

Regional by row tables

In regional by row tables, 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

Global tables are 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 are designed to make working with multi-region clusters easier.

Indexes on REGIONAL BY ROW tables

New in v21.1: 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.

Note that the behavior described above 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.

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:

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

• Choosing a multi-region configuration

See also

• When to use ZONE vs. REGION survival goals
• When to use REGIONAL vs. GLOBAL tables
• Topology Patterns
• Disaster Recovery
• Develop and Deploy a Global Application
• Multi-region SQL performance
• Migrate to Multi-region SQL

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