Create 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, in normal circumstances. You can use a read replica to offload read requests or analytics traffic from the primary instance.

Additionally, for disaster recovery, you can perform a regional migration. If a replica is a cross-region replica, you can perform a failover to another region; specifically, you can promote a replica to a standalone instance (in which case, existing replicas would not consider that instance as 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.

Create a read replica

The steps for creating a read replica are below.

Console

  1. In the Google Cloud console, go to the Cloud SQL Instances page.

    Go to Cloud SQL Instances

  2. Find the instance for which you want to create a replica, and then open the more actions menu next to the listing.
  3. Select Create read replica.

    If you don't see that choice, then the instance is a replica; you can't create a replica of a replica.

  4. If the instance had backups and binary logging enabled, continue with the next step. 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 Customize your instance section of the page, update settings for your replica. Begin by clicking Show configuration options to display the groups of settings. Then, expand the desired groups to review and customize the settings. A Summary of all the options you select appears on the right. Customizing these settings is optional. Defaults are assigned in every case where no customizations are made.

    For more details about each setting, see the About instance settings page.

    For example, to allow other Google Cloud services, such as BigQuery, to access data in Cloud SQL and make queries against this data over an internal connection, expand the Connections group, and then deselect the Public IP check box.

  6. Click Create replica.

    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 PRIMARY_INSTANCE_NAME \
--enable-bin-log
    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. If you're creating a replica from a primary instance for MySQL 8.4 and later, and the Cloud SQL edition for the instance is either Enterprise or is Enterprise Plus, then you don't have to specify a value for this parameter. The replica inherits the machine type from the primary instance.

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

    You can add more parameters for other instance settings. To learn more, see gcloud sql instances create.

    If the primary instance has an internal IP address only, and you want to allow other Google Cloud services, such as BigQuery, to access data in Cloud SQL and make queries against this data over an internal connection, then add the --enable-google-private-path parameter to the command.

    You must create the replica in the same VPC network as the primary instance. You can also specify an allocated-ip-range-name in that VPC network. If no range is specified, the replica is created in a random range.

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

    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.

Terraform

To create a read replica, use Terraform resource. 

resource "google_sql_database_instance" "read_replica" {
  name                 = "mysql-replica-instance-name"
  master_instance_name = google_sql_database_instance.primary.name
  region               = "europe-west4"
  database_version     = "MYSQL_8_0"

  replica_configuration {
    failover_target = false
  }

  settings {
    tier              = "db-n1-standard-2"
    availability_type = "ZONAL"
    disk_size         = "100"
  }
  # set `deletion_protection` to true, will ensure that one cannot accidentally delete this instance by
  # use of Terraform whereas `deletion_protection_enabled` flag protects this instance at the GCP level.
  deletion_protection = false
}

Apply the changes

To apply your Terraform configuration in a Google Cloud project, complete the steps in the following sections.

Prepare Cloud Shell

  1. Launch Cloud Shell.

  2. Set the default Google Cloud project where you want to apply your Terraform configurations.

    You only need to run this command once per project, and you can run it in any directory.

    export GOOGLE_CLOUD_PROJECT=PROJECT_ID

    Environment variables are overridden if you set explicit values in the Terraform configuration file.

Prepare the directory

Each Terraform configuration file must have its own directory (also called a root module).

  1. In Cloud Shell, create a directory and a new file within that directory. The filename must have the .tf extension—for example main.tf. In this tutorial, the file is referred to as main.tf. 
    mkdir DIRECTORY && cd DIRECTORY && touch main.tf

  2. If you are following a tutorial, you can copy the sample code in each section or step.

    Copy the sample code into the newly created main.tf.

    Optionally, copy the code from GitHub. This is recommended when the Terraform snippet is part of an end-to-end solution.

  3. Review and modify the sample parameters to apply to your environment.
  4. Save your changes.
  5. Initialize Terraform. You only need to do this once per directory. 
    terraform init

    Optionally, to use the latest Google provider version, include the -upgrade option: 

    terraform init -upgrade

Apply the changes

  1. Review the configuration and verify that the resources that Terraform is going to create or update match your expectations: 
    terraform plan

    Make corrections to the configuration as necessary.

  2. Apply the Terraform configuration by running the following command and entering yes at the prompt: 
    terraform apply

    Wait until Terraform displays the "Apply complete!" message.

  3. Open your Google Cloud project to view the results. In the Google Cloud console, navigate to your resources in the UI to make sure that Terraform has created or updated them.

Delete the changes

To delete your changes, do the following:

  1. To disable deletion protection, in your Terraform configuration file set the deletion_protection argument to false. 
    deletion_protection =  "false"
  2. Apply the updated Terraform configuration by running the following command and entering yes at the prompt: 
    terraform apply

  1. Remove resources previously applied with your Terraform configuration by running the following command and entering yes at the prompt:

    terraform destroy

REST v1

  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, make the following replacements:

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

    HTTP method and URL: 

    GET https://sqladmin.googleapis.com/v1/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 pointInTimeEnabled are false, use the patch method of the instances resource to enable them both. In the request, specify any properties of the backup configuration that you wish 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 pointInTimeEnabled to true.

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

    • PROJECT_ID: the ID or project number of the Google Cloud project that contains the instance
    • INSTANCE_NAME: the name of the primary or read replica instance that you're configuring for high availability
    • START_TIME: the time (in hours and minutes)

    HTTP method and URL: 

    PATCH https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances/INSTANCE_NAME

    Request JSON body: 

    {
  "settings":
  {
    "backupConfiguration":
    {
      "startTime": "START_TIME",
      "enabled": true,
      "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. If the primary instance uses an internal IP address, you can specify an allocatedIpRange the same way as when you create a primary instance. If no range is specified, the replica is created in a random range. For a cross-region read replica, specify a region other than the primary instance's region.

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

    • project-id: The project ID
    • database-version: Enum version string (for example, MYSQL_8_0)
    • 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 type. For example: "db-custom-1-3840"
    • private-network: The authorized network that you're adding or selecting to create a private connection.

    HTTP method and URL: 

    POST https://sqladmin.googleapis.com/v1/projects/project-id/instances

    Request JSON body: 

    {
  "masterInstanceName": "primary-instance-name",
  "project": "project-id",
  "databaseVersion": "database-version",
  "name": "replica-name",
  "region": "replica-region",
  "settings":
  {
    "tier": "machine-type",
    "settingsVersion": 0,
    "ipConfiguration": {
    object (IpConfiguration)
  },
  {
  "ipv4Enabled": false,
  "privateNetwork": private-network,
  "requireSsl": boolean,
  "authorizedNetworks": [
    {
      object (AclEntry)
    }
  ],
  "allocatedIpRange": string
    }
  }
}

    To send your request, expand one of these options:

    You should receive a JSON response similar to the following:

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, make the following replacements:

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

    HTTP method and URL: 

    GET https://sqladmin.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 later 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, make the following replacements:

    • PROJECT_ID: the ID or project number of the Google Cloud project that contains the instance
    • INSTANCE_NAME: the name of the primary or read replica instance that you're configuring for high availability
    • START_TIME: the time (in hours and minutes)

    HTTP method and URL: 

    PATCH https://sqladmin.googleapis.com/v1beta4/projects/PROJECT_ID/instances/INSTANCE_NAME

    Request JSON body: 

    {
  "settings":
  {
    "backupConfiguration":
    {
      "startTime": "START_TIME",
      "enabled": true,
      "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. If the primary instance uses an internal IP address, you can specify an allocatedIpRange the same way as when you create a primary instance. For a cross-region read replica, specify a region other than the primary instance's region.

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

    • project-id: The project ID
    • database-version: Enum version string (for example, MYSQL_8_0)
    • 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 type. For example: "db-custom-1-3840"
    • private-network: The authorized network that you're adding or selecting to create a private connection.

    HTTP method and URL: 

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

    Request JSON body: 

    {
  "masterInstanceName": "primary-instance-name",
  "project": "project-id",
  "databaseVersion": "database-version",
  "name": "replica-name",
  "region": "replica-region",
  "settings":
  {
    "tier": "machine-type",
    "settingsVersion": 0,
    
    "ipConfiguration": {
    object (IpConfiguration)
  },
  {
  "ipv4Enabled": false,
  "privateNetwork": private-network,
  "requireSsl": boolean,
  "authorizedNetworks": [
    {
      object (AclEntry)
    }
  ],
  "allocatedIpRange": string
    }
    
  }
}

    To send your request, expand one of these options:

    You should receive a JSON response similar to the following:

Create a read replica of an instance with Private Service Connect enabled

To create a read replica of an instance with Private Service Connect enabled, use gcloud CLI or the API. You can create this replica in either the same region or in a different region from the primary instance (cross-region read replica).

The read replica can't replicate from an instance with a different connectivity type. For example, an instance with Private Service Connect enabled can replicate only from another Private Service Connect instance. It also can't replicate from an instance that supports external IP connections or from an instance that's configured with private services access.

gcloud

To create a read replica of an instance, use the gcloud sql instances create command:

gcloud sql instances create REPLICA_INSTANCE_NAME \
--master-instance-name=PRIMARY_INSTANCE_NAME \
--project=PROJECT_ID \
--region=REGION_NAME \
--enable-private-service-connect \
--allowed-psc-projects=ALLOWED_PROJECTS \
--availability-type=AVAILABILITY_TYPE \
--no-assign-ip

Make the following replacements:

  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID or project number of the Google Cloud project that contains the instance.
  • REGION_NAME: the region name for the replica instance.

  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create a Private Service Connect endpoint. If you're using the Cloud SQL Auth Proxy or Cloud SQL Language Connectors, then create a DNS zone and DNS record for the replicas.

  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, see Configure an existing instance for high availability and Deactivate high availability for an instance.

REST v1

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

  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID or project number of the Google Cloud project that contains the instance.
  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • REGION_NAME: the region name for the replica instance.
  • MACHINE_TYPE: the machine type for the instance.
  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, see Configure an existing instance for high availability and Deactivate high availability for an instance.

  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create a Private Service Connect endpoint. If you're using the Cloud SQL Auth proxy or Cloud SQL Language Connectors, then you must create a DNS zone and DNS record for the replicas.

HTTP method and URL: 

POST https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances

Request JSON body: 

{
  "masterInstanceName": "PRIMARY_INSTANCE_NAME",
  "project": "PROJECT_ID",
  "databaseVersion": "MYSQL_8_0",
  "name": "REPLICA_INSTANCE_NAME",
  "region": "REGION_NAME",
  "kind": "sql#instance",
  "settings":
  {
    "tier": "MACHINE_TYPE",
    "availabilityType": "AVAILABILITY_TYPE",
    "settingsVersion": 0,
    "ipConfiguration": {
      "ipv4Enabled": false,
      "pscConfig": {
        "allowedConsumerProjects": [ALLOWED_PROJECTS],
        "pscEnabled": true
      }
    },
    "kind": "sql#settings",
    "pricingPlan": "PER_USE",
    "replicationType": "ASYNCHRONOUS",
    "tier": "MACHINE_TYPE"
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

{
  "kind": "sql#operation",
  "targetLink": "https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances/REPLICA_INSTANCE_NAME",
  "status": "PENDING",
  "user": "user@example.com",
  "insertTime": "2020-01-16T02:32:12.281Z",
  "operationType": "CREATE_REPLICA",
  "name": "OPERATION_ID",
  "targetId": "REPLICA_INSTANCE_NAME",
  "selfLink": "https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/operations/OPERATION_ID",
  "targetProject": "PROJECT_ID"
}

REST v1beta4

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

  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID or project number of the Google Cloud project that contains the instance.
  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • REGION_NAME: the region name for the replica instance.
  • MACHINE_TYPE: the machine type for the instance.
  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, see Configure an existing instance for high availability and Deactivate high availability for an instance.

  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create a Private Service Connect endpoint. If you're using the Cloud SQL Auth proxy or Cloud SQL Language Connectors, then you must create a DNS zone and DNS record for the replicas.

HTTP method and URL: 

PATCH https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances

Request JSON body: 

{
  "masterInstanceName": "PRIMARY_INSTANCE_NAME",
  "project": "PROJECT_ID",
  "databaseVersion": "MYSQL_8_0",
  "name": "REPLICA_INSTANCE_NAME",
  "region": "REGION_NAME",
  "kind": "sql#instance",
  "settings":
  {
    "tier": "MACHINE_TYPE",
    "availabilityType": "AVAILABILITY_TYPE",
    "settingsVersion": 0,
    "ipConfiguration": {
      "ipv4Enabled": false,
      "pscConfig": {
        "allowedConsumerProjects": [ALLOWED_PROJECTS],  
        "pscEnabled": true
      }
    },
    "kind": "sql#settings",
    "pricingPlan": "PER_USE",
    "replicationType": "ASYNCHRONOUS",
    "tier": "MACHINE_TYPE"
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

{
  "kind": "sql#operation",
  "targetLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances/REPLICA_INSTANCE_NAME",
  "status": "PENDING",
  "user": "user@example.com",
  "insertTime": "2020-01-16T02:32:12.281Z",
  "operationType": "CREATE_REPLICA",
  "name": "OPERATION_ID",
  "targetId": "REPLICA_INSTANCE_NAME",
  "selfLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/operations/OPERATION_ID",
  "targetProject": "PROJECT_ID"
}

Configure read replicas for IAM database authentication

Read replicas do not have the cloudsql_iam_authentication flag enabled automatically when it is enabled on the primary instance.

To configure a read replica for IAM database authentication:

  1. In the Google Cloud console, go to the Cloud SQL Instances page.

    Go to Cloud SQL Instances

  2. To open the Overview page of an instance, click the instance name.
  3. In the Configuration tile, look for the cloudsql_iam_authentication flag. If the flag isn't on the list, enabling the flag in the read replica is unnecessary. If the flag is on the list, you must enable the flag on the read replica. If you need to enable the flag on the read replica, continue with the next step..
  4. Select Replicas from the SQL navigation menu.
  5. Click the name of the replica you want to edit.
  6. Click Edit.
  7. In the Configuration options section, expand Flags.
  8. Select + Add item.
  9. Enter cloudsql_iam_authentication for the flag name. Make sure that On is selected for this flag.
  10. Click Save.

Create cascading replicas

This section describes how to create and manage cascading replicas.

For information about how cascading replicas work, see Cascading replicas.

Steps to create a cascading replica

Console

  1. In the Google Cloud console, go to the Cloud SQL Instances page.

    Go to Cloud SQL Instances

  2. For MySQL 5.7 or later, enable replication.
  3. Click the Replicas tab for the replica that will act as a parent for the replica that you want to create.
  4. Click Create replica.
  5. On the Create read replica page, update the instance ID, and any other configuration options, including the name, region, and zone.
  6. Click Create.

    Cloud SQL creates a replica. You're returned to the instance page for the parent replica.

  7. Follow steps 4-6 for each new cascading replica that you want to create.

gcloud

  1. If you're using MySQL version 5.7 or later, then enable binlogs for the primary of the new replica:
    gcloud sql instances patch --enable-bin-log PARENT_REPLICA_NAME
    Replace PARENT_REPLICA_NAME with the name of the parent replica.
  2. Create the new replica by specifying your primary replica as the primary instance using the --master-instance-name flag:
    3. gcloud sql instances create REPLICA_NAME \
      --master-instance-name=PARENT_REPLICA_NAME \
    Replace the following:
    • REPLICA_NAME: the unique ID for the replica that you are creating
    • PARENT_REPLICA_NAME: the name of the parent replica
  3. After you create the cascading replica, you can see that the changes made to the primary instance are replicated through all the replicas in the cascading replicas chain.

curl

  1. If you are using MySQL version 5.7 or later, enable binary logging:

    To enable binary logging, save the following JSON in a file named request.JSON, then invoke the curl command to enable binary logging. 
    {
  "settings":
  {
    "backupConfiguration":
    {
      "enabled": false,
      "binaryLogEnabled": true
    }
  }
}
  2. To create a replica under the parent replica, edit the following JSON code sample, and save it to a file called request.json: 
    {
  "masterInstanceName": "PARENT_REPLICA_NAME",
  "project": "PROJECT_ID",
  "name": "REPLICA_NAME",
  "region": "REPLICA_REGION",
  "settings":
    {
      "tier": "MACHINE_TYPE",
    }
}
  3. Run the following command: 
    curl -X POST
-H "Authorization: Bearer "$(gcloud auth print-access-token)
-H "Content-Type: application/json; charset=utf-8"
-d @request.json
"https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances"

Troubleshoot

Issue Troubleshooting
Read replica didn't start replicating on creation. There's probably a more specific error in the log files. Inspect the logs in Cloud Logging to find the actual error.
Unable to create read replica - invalidFlagValue error. One of the flags in the request is invalid. It could be a flag you provided explicitly or one that was set to a default value.

First, check that the value of the max_connections flag is greater than or equal to the value on the primary.

If the max_connections flag is set appropriately, inspect the logs in Cloud Logging to find the actual error.
Unable to create read replica - unknown error. There's probably a more specific error in the log files. 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. The primary instance disk size can become full during replica creation. Edit the primary instance to upgrade it to a larger disk size.
The replica instance is using too much memory. The replica uses temporary memory to cache often-requested read operations, which can lead it to use more memory than the primary instance.

Restart the replica instance to reclaim the temporary memory space.
Replication stopped. The maximum storage limit was reached and automatic storage increase isn't enabled.

Edit the instance to enable automatic storage increase.
Replication lag is consistently high. 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.

Some possible solutions include:
Replication lag suddenly spikes. This is caused by long-running transaction(s). When a transaction (single statement or multi-statements) commits on the source instance, the start time of the transaction is recorded in the binary log. When the replica receives this binlog event, it compares that timestamp with the current timestamp to calculate replication lag. Hence, a long-running transaction on the source would result in an immediate large replication lag on the replica. If the amount of row changes in the transaction is large, the replica would also spend a long time to execute it. During the time, replication lag is increasing. Once the replica finishes this transaction, the catch up period would depend on the write workload on the source and the replica's processing speed.

To avoid a long transaction, some possible solutions include:

  • Break the transaction into multiple small transactions
  • Chunk a single large write query into smaller batches
  • Try to separate long SELECT queries from a transaction mixed with DMLs
Changing parallel replication flags results in an error. An incorrect value is set for one of or more of these flags.

On the primary instance that's displaying the error message, set the parallel replication flags:

  1. Modify the binlog_transaction_dependency_tracking and transaction_write_set_extractionflags:
    • binlog_transaction_dependency_tracking=COMMIT_ORDER
    • transaction_write_set_extraction=OFF
  2. Add the slave_pending_jobs_size_max flag:

    slave_pending_jobs_size_max=33554432

  3. Modify the transaction_write_set_extraction flag:

    transaction_write_set_extraction=XXHASH64

  4. Modify the binlog_transaction_dependency_tracking flag:

    binlog_transaction_dependency_tracking=WRITESET
Replica creation fails with timeout. Long-running uncommitted transactions on the primary instance can cause read replica creation to fail.

Recreate the replica after stopping all running queries.

What's next