Read from Standby - CockroachDB

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In addition to providing capabilities for , allows you to direct read-only queries to your standby cluster. This process offloads ad-hoc analytics query traffic from the primary cluster. Use this page to understand how the read from standby feature works and how to utilize it.

How the read from standby feature works

PCR utilizes to separate a cluster’s control plane from its data plane. A cluster always has one control plane, called a system virtual cluster (system VC), and at least one data plane, called an application virtual cluster (app VC). The standby cluster’s system VC manages the PCR job and other cluster metadata, and is not used for application queries. All data tables, system tables, and cluster settings in the standby cluster’s app VC are identical to the primary cluster’s app VC. The standby cluster’s app VC itself remains offline during replication. When using read from standby, applications can read from the standby cluster, but they do not connect directly to the standby cluster’s app VC. Instead, PCR introduces a reader virtual cluster (reader VC). The reader VC ensures a clean, isolated environment specifically for serving read queries without interfering with replication or system metadata. It reads continuously from the standby cluster’s app VC using internal pointers, providing access to the replicated data while keeping the app VC offline. The reader VC itself only stores a small amount of metadata and no user data, so it is not expected to take up additional storage space. The standby cluster’s reader VC has its own system tables and . The reader VC replicates a subset of system tables, including Users and Roles, from the app VC, so that existing primary users can authenticate using the same as on the primary cluster’s app VC. Other system tables and cluster settings are set to defaults in the reader VC. For more information, consult . In the event of failover, the reader VC is destroyed.

Use the read from standby feature

The read from standby feature allows you to increase PCR standby cluster hardware utilization during replication. However, PCR’s primary use case is disaster recovery, not workload isolation. If you need a workload isolation solution but do not need disaster recovery, deploy a single cluster and use instead.

Before you begin

Prior to setting up read from standby, ensure that:

You have already configured PCR between a primary cluster and a standby cluster. For information on configuring PCR, refer to .
Your CockroachDB version is v24.3 or later. The read from standby option is not supported in earlier versions.

Start a PCR stream with read from standby

To start a PCR stream that allows read access to the standby cluster, use the statement with the READ VIRTUAL CLUSTER option:

CREATE VIRTUAL CLUSTER main FROM REPLICATION OF main ON 'postgresql://{connection string to primary}' WITH READ VIRTUAL CLUSTER;

Add read from standby to a PCR stream

To add read from standby capabilities to an existing PCR stream, use the statement:

ALTER VIRTUAL CLUSTER main SET REPLICATION READ VIRTUAL CLUSTER;

The standby cluster’s app VC must have a status of replicating before you can create your reader VC. Use the command to check the status of the app VC.

Check the status of your reader virtual cluster

To confirm that your reader virtual cluster is active:

SHOW VIRTUAL CLUSTERS;

The output shows a standby-readonly virtual cluster in addition to the system VC and app VC:

  id |       name       | data_state  | service_mode
-----+------------------+-------------+---------------
   1 | system           | ready       | shared
   3 | standby          | replicating | none
   4 | standby-readonly | ready       | shared

The reader VC cannot serve reads until after the PCR initial scan is complete. After completing the initial scan, wait until the reader VC’s service_mode is shared, then wait about one minute before connecting to the reader VC.

Run read-only queries on the standby cluster

Once you have created a reader virtual cluster on the standby cluster, you can connect to it and run . For example:

SELECT COUNT(*) FROM customers;
SELECT region, SUM(amount) FROM orders GROUP BY region;

The results of queries on the standby cluster reflect the state of the primary cluster as of a historical time that approaches the . Write operations are not permitted on the standby cluster.

​How the read from standby feature works

​Use the read from standby feature

​Before you begin

​Start a PCR stream with read from standby

​Add read from standby to a PCR stream

​Check the status of your reader virtual cluster

​Run read-only queries on the standby cluster

​See also

How the read from standby feature works

Use the read from standby feature

Before you begin

Start a PCR stream with read from standby

Add read from standby to a PCR stream

Check the status of your reader virtual cluster

Run read-only queries on the standby cluster

See also