Creating read replicas

This page describes how to create a read replica for a Cloud SQL instance.

A read replica is a copy of the primary instance that reflects changes to the primary in almost real time. You create a replica to offload read requests or analytics traffic from the primary.

For more information about how replication works, see Replication in Cloud SQL.

Before you begin

If you are creating the first replica for this instance, ensure that the instance meets the requirements for primary instances. Learn more.

Creating a read replica

The steps for creating a read replica are below.

Console

  1. Go to the Cloud SQL Instances page in the Google Cloud Console.

    Go to the Cloud SQL Instances page

  2. Find the instance you want to create a replica for, and open its more actions menu at the far right of the listing.
  3. Select Create read replica.

    If you do not see that choice, the instance is a replica; you cannot create a replica of a replica.

  4. In the Create read replica page, update the instance ID, if needed, and any other configuration options required, including name, region, and zone.
  5. Click Create.

    Cloud SQL creates a backup, if needed, and creates the replica. You are returned to the instance page for the primary.

gcloud

  1. Check the status of the primary instance:
    gcloud sql instances describe [PRIMARY_INSTANCE_NAME]

    If the databaseReplicationEnabled property is true, the instance is a replica; you cannot create a replica of a replica.

  2. If the enabled property under backupConfiguration is false, enable backups for the primary instance now:
    gcloud sql instances patch [PRIMARY_INSTANCE_NAME] --backup-start-time [HH:MM]
    The backup-start-time parameter is specified in 24-hour time, in the UTC±00 time zone, and specifies the start of a 4-hour backup window. Backups can start any time during the backup window.
  3. Create the replica:
    gcloud sql instances create [REPLICA_NAME] --master-instance-name=[PRIMARY_INSTANCE_NAME]
    

    You can specify a different tier size using the --tier parameter, if needed.

    You can specify a different region using the --region parameter.

    If the primary instance has a private IP address only, add the --no-assign-ip parameter to the command.

REST

  1. Get the current backup configuration

    Use the get method of the instances resource to return the database version and current backup configuration for the primary.

    Before using any of the request data below, make the following replacements:

    • project-id: The project ID
    • primary-instance-name: The name of the primary instance

    HTTP method and URL:

    GET https://www.googleapis.com/sql/v1beta4/projects/project-id/instances/primary-instance-name

    To send your request, expand one of these options:

    You should receive a JSON response similar to the following:

  2. Create the read replica

    Use the insert method of the instances resource to create the read replica. The databaseVersion property must be the same as the primary. For a cross-region read replica, specify a region other than the primary instance's region.

    Before using any of the request data below, make the following replacements:

    • project-id: The project ID
    • primary-instance-name: The name of the primary instance
    • primary-instance-region: The region of the primary instance
    • replica-region: The region of the replica instance
    • replica-name: The name of the replica instance
    • machine-type: Enum string of the machine (tier) type. For example: "db-n1-standard-4"

    HTTP method and URL:

    POST https://www.googleapis.com/sql/v1beta4/projects/project-id/instances

    Request JSON body:

    {
      "masterInstanceName": "primary-instance-name",
      "project": "project-id",
      "name": "replica-name",
      "region": "replica-region",
      "settings":
      {
        "tier": "machine-type",
        "settingsVersion": 0,
      }
    }
    

    To send your request, expand one of these options:

    You should receive a JSON response similar to the following:

Troubleshooting

Click the links in the table for details:

For this problem... The issue might be... Try this...
Read replica did not start replicating on creation. At least one backup must have been created after binary logging was enabled. Wait until at least one backup has been created after enabling binary logs.
Unable to create read replica - unknown error. There could be many root causes. Check the logs to find more information.
Disk is full. The primary instance disk size can become full during replica creation. Upgrade the primary instance to a larger disk size.
Replica instance is using too much memory. Replicas can cache often-requested read operations. Restart the replica instance to reclaim the temporary memory space.
Replication stopped. Max storage space was reached and automatic storage increase is not enabled. Enable automatic storage increase.
Replication lag is consistently high. There can be many different root causes. Here are a few things to try.

Read replica did not start replicating on creation

Read replica did not start replicating on creation.

The issue might be

The primary instance must have at least a week's worth of binlogs or else replicas cannot start replicating.

Things to try

Wait until there are enough binlogs.


Unable to create read replica - unknown error

Unable to create read replica - unknown error.

The issue might be

There is probably a more specific error in the log files.

Things to try

Inspect the logs in Cloud Logging to find the actual error. If the error is: set Service Networking service account as servicenetworking.serviceAgent role on consumer project, then disable and re-enable the Service Networking API. This action creates the service account necessary to continue with the process.


Disk is full

error: disk is full

The issue might be

The primary instance disk size can become full during replica creation.

Things to try

Edit the primary instance to upgrade it to a larger disk size.


Replica instance is using too much memory

The replica instance is using too much memory.

The issue might be

The replica uses temporary memory to cache often-requested read operations, which can lead it to use more memory than the primary instance.

Things to try

Restart the replica instance to reclaim the temporary memory space.


Replication stopped

Replication stopped.

The issue might be

The maximum storage limit was reached and >automatic storage increase is disabled.

Things to try

Edit the instance to enable automatic storage increase.


Replication lag is consistently high

Replication lag is consistently high.

The issue might be

The write load is too high for the replica to handle. Replication lag takes place when the SQL thread on a replica is unable to keep up with the IO thread. Some kinds of queries or workloads can cause temporary or permanent high replication lag for a given schema. Some of the typical causes of replication lag are:

  • Slow queries on the replica. These are discoverable by enabling log_slow_slave_statements and fixing them.
  • All tables must have a unique/primary key. Every update on such a table without a unique/primary key causes full table scans on th replica.
  • Queries like DELETE ... WHERE field < 50000000 cause replication lag with row-based replication since a huge number of updates are piled up on the replica.

Things to try

Some possible solutions:

  • Edit the instance to increase the size of the replica.
  • Reduce the load on the database.
  • Index the tables.
  • Identify and fix slow queries.
  • Recreate the replica.

What's next