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 can use a read replica to do the following:

  • Offload read requests or analytics traffic from the primary
  • Perform a regional migration or fail over to another region for disaster recovery purposes (if the replica is a cross-region replica, that is, a replica created in a region different from that of 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. If the instance had backups and binary logging enabled, continue with Step 6. Otherwise, select Automate backups and Enable binary logging, click Continue, and then click Save and restart to restart the instance.

    Enabling binary logging causes the instance to be restarted.

  5. In the Create read replica page, update the instance ID, if needed, and any other configuration options required, including name, region, and zone.
  6. 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. If the binaryLogEnabled property is false, enable binary logs on the primary instance:
    gcloud sql instances patch --enable-bin-log PRIMARY_INSTANCE_NAME
    Enabling binary logs causes the instance to be restarted.
  4. 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.

  • Binary logging is supported on read replica instances (MySQL 5.7 and 8.0 only. Not supported on legacy HA failover replicas). Enable binary logging on a replica with the same gcloud command, using the replica's instance name instead of the primary's instance name.
    gcloud sql instances patch --enable-bin-log REPLICA_INSTANCE_NAME
        

    Binary logging durability on the replica (but not on the primary) instance can be set with the sync_binlog flag, which controls how often the MySQL server synchronizes the binary log to disk.

    Backups cannot be enabled on replica instances, but binary logging can be enabled on a replica even when backups are disabled, unlike the primary.

    The binlog retention period on replica instances is automatically set to one day, unlike seven days on primary instances.

REST v1beta4

  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 master.

    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. Verify that the replication fields are set

    If either enabled or binaryLogEnabled are false on the primary instance, use the patch method of the instances resource to enable them both. In the request, specify any properties of the backup configuration that you want to change.

    To enable backups, set enabled to true and the startTime to a time of day in HH:MM format. The startTime 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.

    To enable point-in-time recovery, set binaryLogEnabled to true on the primary instance.

    Binary logging is supported on read replica instances (MySQL 5.7 and 8.0 only). Enable binary logging on a replica with the same API, using the replica's instance ID instead of the primary's instance ID.

    Binary logging durability on the replica (but not on the primary) instance can be set with the sync_binlog flag, which controls how often the MySQL server synchronizes the binary log to disk.

    Backups cannot be enabled on replica instances, but binary logging can be enabled on a replica even when backups are disabled, unlike the primary.

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

    • project-id: The project ID
    • instance-id: The instance ID (primary or replica)
    • start-time The time in the format "HH:MM"
    • enabled: Set to true for a primary instance. Set to false for a replica instance

    HTTP method and URL:

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

    Request JSON body:

    {
      "settings":
      {
        "backupConfiguration":
        {
          "startTime": "start-time",
          "enabled": enabled,
          "binaryLogEnabled": true
        }
      }
    }
    

    To send your request, expand one of these options:

    You should receive a JSON response similar to the following:

  3. 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

UPDATE_DISK_SIZE or mysqld: disk is full error.

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. Find and fix 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