Configure external replicas

This page describes how to configure a Cloud SQL instance that replicates to one or more replicas external to Cloud SQL, and how to demote the primary instance in an external replica configuration to reverse the configuration.

For more information about the external replica configuration, see About external replication.

Set up the external replica configuration

Before you begin

Before you start this task, you must have a Cloud SQL instance and an external MySQL instance that meets the requirements for external replicas.

Configure the primary instance

  1. Go to the Cloud SQL Instances page in the Google Cloud console.
  2. Enable access on the primary instance for the IP address of the external replica.

    For information about enabling IP access, see Configuring access for IP connections.

  3. Record the public IP address and the public outgoing IP address of the primary instance for later use. You can find these values on the instance's Overview page.
  4. Click the Cloud Shell icon in the upper right corner.
  5. At the Cloud Shell prompt, use the built-in MySQL client to connect to your primary instance:
    gcloud sql connect PRIMARY_INSTANCE_NAME \
  6. Enter your root password. You should then see the mysql prompt.
  7. Create a special user for replication and grant replication privileges:
  8. If you are starting with a new database, create the same database and tables on both the primary and replica instances. For example:
    USE test;
    CREATE TABLE replica_test (id SERIAL PRIMARY KEY, data text);
    INSERT INTO replica_test (data) VALUES ('apple'), ('banana'), ('cherry');
  9. If you already have a database on the primary instance, you must create the same on the replica. To do this, export the database from the primary instance to a Cloud Storage bucket and import it into the replica. Learn more about Exporting data from Cloud SQL to a SQL dump file in Cloud Storage.

Configure the external replica

Warning: This procedure overwrites any data hosted in a MySQL database on the replica, including users and passwords, with the settings and data from the primary instance.
  1. On the machine hosting the replica, seed your new external MySQL instance with the export file you created from the primary instance.

    For example, the following command loads an exported file named mydump.sql:

    mysql --user=root --password < mydump.sql
  2. Determine the server ID for this replica-primary pair.

    The server ID is a numeric value (for example, "3") that must be unique across the external replica configuration (each replica must have a unique server ID).

  3. Add the following options to the replica's my.cnf option file:

    For more information about MySQL replication options, see Replication and Binary Logging Options.

  4. Restart the mysqld process to cause the configuration file to be read.
  5. In a mysql client on the replica, enter the following command:
  6. Start replication on the replica:
  7. Confirm replication status:


    If you see "Waiting for master to send event", replication is working.

Demote the primary instance of an external replica

When you have a Cloud SQL instance with an external replica, you can reverse the configuration, which causes these changes to happen:

  • The external replica becomes the new primary instance.
  • The Cloud SQL instance becomes a read replica, replicating from the server that was previously the external replica (now called the source database server).

To reverse the external replica configuration:

  1. Create a source representation instance.

    This instance will represent the source database server to the Cloud SQL replica after the demotion operation completes.

    gcloud auth login
    ACCESS_TOKEN="$(gcloud auth print-access-token)"
    curl --header "Authorization: Bearer ${ACCESS_TOKEN}" \
         --header 'Content-Type: application/json' \
         --data '{
             "name": "SOURCE_REPRESENTATION_NAME",
             "region": "REGION",
             "databaseVersion": "EXTERNAL_SERVER_DATABASE_VERSION",
             "onPremisesConfiguration": {
                 "hostPort": "EXTERNAL_SERVER_IP:EXTERNAL_SERVER_PORT"
         }' \
         -X POST \

    Use the region where you want your Cloud SQL replica to reside.

  2. Start the demotion process.

    Because this API call requires you to provide sensitive information, you should use a JSON file to provide your data to cURL, rather than providing it on the command line.

    Create the data file:

        "demoteMasterContext": {
            "replicaConfiguration": {
                "mysqlReplicaConfiguration": {
                    "username": "REPLICATION_USERNAME",
                    "password": "PASSWORD",
                    "caCertificate": "EXTERNAL_SERVER_CA",
                    "clientCertificate": "CLIENT_CERT",
                    "clientKey": "PRIVATE_KEY"
            "masterInstanceName": "SOURCE_REPRESENTATION_NAME",

    Then, call the API.

    curl --header "Authorization: Bearer ${ACCESS_TOKEN}" \
         --header 'Content-Type: application/json' \
         --data @PATH_TO_DATA_FILE \

    For more information about your options for SSL/TLS, see SSL/TLS options. For more information about the properties used by the replicaConfiguration object, see Replicating from an External Server.

  3. Wait for the external replica to complete all pending transactions from the primary instance.

    When the replica is caught up, the SHOW SLAVE STATUS command will show Seconds Behind Master as 0, and the Executed_Gtid_Set value will be identical between the external replica and the Cloud SQL primary.

  4. Use the mysql client to stop replication on the external replica:

  5. Wait for the Cloud SQL instance to start replicating from the external server, which is now the source database server.

    Running the SHOW SLAVE STATUS command on the Cloud SQL instance provides replication status.

  6. When the Cloud SQL instance is successfully replicating from the source database server, set the read_only flag on the source database server to off and update your applications to point to the source database server.


Issue Troubleshooting
Error message: The slave is connecting ... master has purged binary logs containing GTIDs that the slave requires. The primary Cloud SQL instance has automatic backups and binary logs and point-in-time recovery is enabled, so it should have enough logs for the replica to be able to catch up. However, in this case although the binary logs exist, the replica doesn't know which row to start reading from.

Create a new dump file using the correct flag settings, and configure the external replica using that file

  1. Connect to your mysql client through a Compute Engine instance.
  2. Run mysqldump and use the --master-data=1 and --flush-privileges flags.

    Important: Do not include the --set-gtid-purged=OFF flag.

    Learn more.

  3. Ensure that the dump file just created contains the SET @@GLOBAL.GTID_PURGED='...' line.
  4. Upload the dump file to a Cloud Storage bucket and configure the replica using the dump file.

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