Managing read replicas

This page describes how you can disable and enable replication for a read replica, as well as how to promote a replica to a stand-alone instance or delete it. For information about working with read replicas, see Replication in Cloud SQL.

Disabling replication

By default, a replica starts with replication enabled. However, you can disable replication, for example, to debug or analyze the state of an instance. When you are ready, you explicitly re-enable replication. Disabling or re-enabling replication restarts the replica.

Disabling replication does not stop the replica instance; it becomes a read-only instance that is no longer replicating from its primary instance. You continue to be charged for the instance. You can re-enable replication on the disabled replica, delete the replica, or promote the replica to a stand-alone instance. You cannot stop the replica.

To disable replication:

Console

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

    Go to the Cloud SQL Instances page

  2. Open a replica instance by clicking its name.
  3. Click Disable replication in the button bar.
  4. Click OK.

gcloud

gcloud sql instances patch [REPLICA_NAME] --no-enable-database-replication

REST v1beta4

To execute this cURL command at a command line prompt, you acquire an access token by using the gcloud auth print-access-token command. You can also use the APIs Explorer on the Instances:patch page to send the REST API request.

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

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

HTTP method and URL:

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

Request JSON body:

{
  "settings":
  {
    "databaseReplicationEnabled": "False"
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

Enabling replication

If a replica has not been replicating for a long time, it will take longer for it to catch up to the primary instance. In this case, delete the replica and create a new one.

To enable replication:

Console

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

    Go to the Cloud SQL Instances page

  2. Select a replica instance by clicking its name.
  3. Click Enable replication in the button bar.
  4. Click OK.

gcloud

gcloud sql instances patch [REPLICA_NAME] --enable-database-replication

REST v1beta4

To execute this cURL command at a command line prompt, you acquire an access token by using the gcloud auth print-access-token command. You can also use the APIs Explorer on the Instances:patch page to send the REST API request.

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

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

HTTP method and URL:

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

Request JSON body:

{
  "settings":
  {
    "databaseReplicationEnabled": "True"
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

Promoting a replica

Promoting a read replica converts it to a standalone Cloud SQL primary instance with read and write capabilities. Once promoted, the replica instance stops replicating from the primary instance. This is irreversible and the newly promoted instance cannot be converted back to a read replica.

You can promote a read replica as part of disaster recovery, if the original primary instance becomes unavailable for an extended period of time.

Before promoting a read replica, you should stop all writes to the primary instance, if it is still available, and check the replication status of the replica (follow the instructions in the psql Client tab). You should verify that the replica is replicating, if the primary is available, and then wait until the replication lag reported by the replay_lag metric is 0. Otherwise, the newly promoted instance may be missing some of the transactions that were committed to the primary instance.

To promote a replica to a standalone instance:

Console

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

    Go to the Cloud SQL Instances page

  2. Select a replica instance by clicking its name.
  3. Click Promote replica in the button bar.
  4. Click OK.

gcloud

gcloud sql instances promote-replica [REPLICA_NAME]
  

REST v1beta4

To execute this cURL command at a command line prompt, you acquire an access token by using the gcloud auth print-access-token command. You can also use the APIs Explorer on the Instances:promoteReplica page to send the REST API request.

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

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

HTTP method and URL:

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

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

Confirm that the promoted instance is configured correctly. In particular, enable automated backups and consider configuring the instance for high availability if needed.

Checking replication status

When you log into an individual replica instance using an administration client, you get details about replication, including status and metrics. When you use the Google Cloud Console or the gcloud command-line tool, you get a brief summary of replication.

To check replication status:

Console

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

    Go to the Cloud SQL Instances page

  2. Select a replica instance by clicking its name.
  3. The replication status is provided in a banner at the top of the page.

gcloud

For a replica instance, check the replication status with:

gcloud sql instances describe [REPLICA_NAME]

In the output, look for the properties databaseReplicationEnabled and masterInstanceName.

For a primary instance, check if there are replicas with:

gcloud sql instances describe [PRIMARY_INSTANCE_NAME]

In the output, look for the property replicaNames.

psql Client

  1. Connect to the replica with a PostgreSQL client.

    For information, see Connection options for external applications.

  2. Check the replica's status, on the primary:
    select * from pg_stat_replication;
  3. Look for the following metrics in the output of the command:
    • client_addr: The IP address of the replica instance.
    • state: Indicates whether the SQL thread for executing events in the relay log is running. The value is streaming when replication is started.
    • replay_lag: The number of bytes that the replica SQL thread is behind the primary instance. The value is O or a small number of bytes.
  4. Check the replica's status, on the replica:
    select * from pg_stat_wal_receiver;
  5. Look for the following metrics in the output of the command:
    • sender_host: The IP address of the primary instance.
    • status: Indicates whether the SQL thread for executing events in the relay log is running. The value is streaming when replication is started.
    • last_msg_send_time and last_msg_receipt_time: The difference between these two timestamps is the lag time.

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