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Amazon Relational Database Service
User Guide (API Version 2014-10-31)

AWS Documentation » Amazon Relational Database Service (RDS) » User Guide » Amazon RDS DB Instance Lifecycle » Working with PostgreSQL, MySQL, and MariaDB Read Replicas

Working with PostgreSQL, MySQL, and MariaDB Read Replicas

Amazon RDS uses the MySQL, MariaDB, and PostgreSQL (version 9.3.5 and later) DB engines' built-in replication functionality to create a special type of DB instance called a Read Replica from a source DB instance. Updates made to the source DB instance are asynchronously copied to the Read Replica. You can reduce the load on your source DB instance by routing read queries from your applications to the Read Replica. Using Read Replicas, you can elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads.

Note

The information following applies to creating Amazon RDS Read Replicas either in the same region as the source DB instance, or in a separate region. The information following doesn't apply to setting up replication with an instance that is running on an Amazon EC2 instance or that is on-premises.

When you create a Read Replica, you first specify an existing DB instance as the source. Then, Amazon RDS takes a snapshot of the source instance and creates a read-only instance from the snapshot. Amazon RDS then uses the asynchronous replication method for the DB engine to update the Read Replica whenever there is a change to the source DB instance. The Read Replica operates as a DB instance that allows only read-only connections. Applications connect to a Read Replica the same way they do to any DB instance. Amazon RDS replicates all databases in the source DB instance.

Amazon RDS sets up a secure communications channel between the source DB instance and a Read Replica if that Read Replica is in a different AWS Region from the DB instance. Amazon RDS establishes any AWS security configurations needed to enable the secure channel, such as adding security group entries. PostgreSQL DB instances use a secure connection that you can encrypt by setting the ssl parameter to 1 for both the source and the replica instances.

Topics

Amazon RDS Read Replica Overview

Deploying one or more Read Replicas for a given source DB instance might make sense in a variety of scenarios, including the following:

  • Scaling beyond the compute or I/O capacity of a single DB instance for read-heavy database workloads. You can direct this excess read traffic to one or more Read Replicas.

  • Serving read traffic while the source DB instance is unavailable. If your source DB instance cannot take I/O requests (for example, due to I/O suspension for backups or scheduled maintenance), you can direct read traffic to your Read Replicas. For this use case, keep in mind that the data on the Read Replica might be "stale" because the source DB instance is unavailable.

  • Business reporting or data warehousing scenarios where you might want business reporting queries to run against a Read Replica, rather than your primary, production DB instance.

By default, a Read Replica is created with the same storage type as the source DB instance. However, you can create a Read Replica that has a different storage type from the source DB instance based on the options listed in the following table.

Source DB Instance Storage Type Source DB Instance Storage Allocation Read Replica Storage Type Options
PIOPS 100 GB - 3 TB PIOPS | GP2 | Standard
GP2 100 GB - 3 TB PIOPS | GP2 | Standard
GP2 Less than 100 GB GP2 | Standard
Standard 100 GB - 3 TB PIOPS | GP2 | Standard
Standard Less than 100 GB GP2 | Standard

Amazon RDS doesn't support circular replication. You cannot configure a DB instance to serve as a replication source for an existing DB instance; you can only create a new Read Replica from an existing DB instance. For example, if MyDBInstance replicates to ReadReplica1, you cannot configure ReadReplica1 to replicate back to MyDBInstance. From ReadReplica1, you can only create a new Read Replica, such as ReadReplica2.

For MySQL, MariaDB, and PostgreSQL Read Replicas, you can monitor replication lag in Amazon CloudWatch by viewing the Amazon RDS ReplicaLag metric. For MySQL and MariaDB, the ReplicaLag metric reports the value of the Seconds_Behind_Master field of the SHOW SLAVE STATUS command. For PostgreSQL, the ReplicaLag metric reports the value of SELECT extract(epoch from now() - pg_last_xact_replay_timestamp()) AS slave_lag.

Common causes for replication lag for MySQL and MariaDB are the following:

  • A network outage.

  • Writing to tables with indexes on a Read Replica. If the read_only parameter is not set to 0 on the Read Replica, it can break replication.

  • Using a non-transactional storage engine such as MyISAM. Replication is only supported for the InnoDB storage engine on MySQL and the XtraDB storage engine on MariaDB.

When the ReplicaLag metric reaches 0, the replica has caught up to the source DB instance. If the ReplicaLag metric returns -1, then replication is currently not active. ReplicaLag = -1 is equivalent to Seconds_Behind_Master = NULL.

Differences Between PostgreSQL and MySQL or MariaDB Read Replicas

Because the PostgreSQL DB engine implements replication differently than the MySQL and MariaDB DB engines, there are several significant differences you should know about, as shown in the following table.

Feature/Behavior PostgreSQL MySQL and MariaDB

What is the replication method?

Physical replication. 

Logical replication.

How are transaction logs purged?

PostgreSQL has a parameter, wal_keep_segments, that dictates how many write ahead log (WAL) files are kept to provide data to the Read Replicas. The parameter value specifies the number of logs to keep.

Amazon RDS won't delete any binary logs that have not been applied.

Can a replica be made writable?

No. A PostgreSQL Read Replica is a physical copy and PostgreSQL doesn't allow for a Read Replica to be made writeable.

Yes. You can enable the MySQL or MariaDB Read Replica to be writable.

Can backups be performed on the replica?

Yes, you can create a snapshot of a PostgreSQL Read Replica, but you cannot enable automatic backups.

Yes. You can enable automatic backups on a MySQL or MariaDB Read Replica.

Can you use parallel replication?

No.  PostgreSQL has a single process handling replication.

Yes. MySQL version 5.6 and later and all supported MariaDB versions allow for parallel replication threads.

PostgreSQL Read Replicas (Version 9.3.5 and Later)

Amazon RDS PostgreSQL 9.3.5 and later uses PostgreSQL native streaming replication to create a read-only copy of a source (a "master" in PostgreSQL terms) DB instance. This Read Replica (a "standby" in PostgreSQL terms) DB instance is an asynchronously created physical replication of the master DB instance. It is created by a special connection that transmits write ahead log (WAL) data between the source DB instance and the Read Replica where PostgreSQL asynchronously streams database changes as they are made.

PostgreSQL uses a "replication" role to perform streaming replication. The role is privileged, but cannot be used to modify any data. PostgreSQL uses a single process for handling replication.

Creating a PostgreSQL Read Replica doesn't require an outage for the master DB instance. Amazon RDS sets the necessary parameters and permissions for the source DB instance and the Read Replica without any service interruption. A snapshot is taken of the source DB instance, and this snapshot becomes the Read Replica. No outage occurs when you delete a Read Replica.

You can create up to five Read Replicas from one source DB instance. For replication to operate effectively, each Read Replica should have the same amount of compute and storage resources as the source DB instance. If you scale the source DB instance, you should also scale the Read Replicas.

Amazon RDS overrides any incompatible parameters on a Read Replica if it prevents the Read Replica from starting. For example, if the max_connections parameter value is higher on the source DB instance than on the Read Replica, Amazon RDS updates the parameter on the Read Replica to be the same value as that on the source DB instance.

Here are some important facts about PostgreSQL Read Replicas:

  • PostgreSQL Read Replicas are read-only and cannot be made writeable.

  • You cannot create a Read Replica from another Read Replica (that is, you cannot create cascading Read Replicas).

  • You can promote a PostgreSQL Read Replica to be a new source DB instance. However, the Read Replica doesn't become the new source DB instance automatically. The Read Replica, when promoted, stops receiving WAL communications and is no longer a read-only instance. You must set up any replication you intend to have going forward because the promoted Read Replica is now a new source DB instance.

  • A PostgreSQL Read Replica reports a replication lag of up to 5 minutes if there are no user transactions occurring on the source DB instance.

  • Before a DB instance can serve as a source DB instance, you must enable automatic backups on the source DB instance by setting the backup retention period to a value other than 0.

Situations That Break PostgreSQL Replication

In several situations, a PostgreSQL source DB instance can unintentionally break replication with a Read Replica. These situations include the following:

  • The max_wal_senders parameter is set too low to provide enough data to the number of Read Replicas. This situation causes replication to stop.

  • The PostgreSQL parameter, wal_keep_segments, dictates how many WAL files are kept to provide data to the Read Replicas. The parameter value specifies the number of logs to keep. If you set the parameter value too low, you can cause a Read Replica to fall so far behind that streaming replication stops. In this case, Amazon RDS reports a replication error and begins recovery on the Read Replica by replaying the source DB instance's archived WAL logs. This recovery process continues until the Read Replica has caught up enough to continue streaming replication. For more information on this process and how to determine the appropriate parameter setting, see Troubleshooting a PostgreSQL Read Replica Problem.

  • A PostgreSQL Read Replica requires a reboot if the source DB instance endpoint changes.

When the WAL stream that provides data to a Read Replica is broken, PostgreSQL switches into recovery mode to restore the Read Replica by using archived WAL files. When this process is compete, PostgreSQL will attempt to re-establish streaming replication.

MySQL and MariaDB Read Replicas

Before a MySQL or MariaDB DB instance can serve as a replication source, you must enable automatic backups on the source DB instance by setting the backup retention period to a value other than 0. This requirement also applies to a Read Replica that is the source DB instance for another Read Replica. Automatic backups are supported only for Read Replicas running any version of MariaDB or MySQL 5.6 and later.

You can configure replication based on binary log coordinates for both MySQL and MariaDB instance. For MariaDB instances, you can also configure replication based on global transaction IDs (GTIDs), which provides better crash safety. For more information about configuring replication using GTIDs on a MariaDB DB instance, see Configuring GTID-Based Replication into an Amazon RDS MariaDB DB instance.

You can create up to five Read Replicas from one DB instance. In order for replication to operate effectively, each Read Replica should have as much compute and storage resources as the source DB instance. If you scale the source DB instance, you should also scale the Read Replicas.

If a Read Replica is running any version of MariaDB or MySQL 5.6 and later, you can specify it as the source DB instance for another Read Replica. For example, you can create ReadReplica1 from MyDBInstance, and then create ReadReplica2 from ReadReplica1. Updates made to MyDBInstance are replicated to ReadReplica1 and then replicated from ReadReplica1 to ReadReplica2. You cannot have more than four instances involved in a replication chain. For example, you can create ReadReplica1 from MySourceDBInstance, and then create ReadReplica2 from ReadReplica1, and then create ReadReplica3 from ReadReplica2, but you cannot create a ReadReplica4 from ReadReplica3.

To enable automatic backups on a Read Replica for Amazon RDS MariaDB or MySQL version 5.6 and later, first create the Read Replica, then modify the Read Replica to enable automatic backups.

Read Replicas are designed to support read queries, but you might need occasional updates. For example, you might need to add an index to speed the specific types of queries accessing the replica. You can enable updates by setting the read_only parameter to 0 in the DB parameter group for the Read Replica.

You can run multiple concurrent Read Replica create or delete actions that reference the same source DB instance, as long as you stay within the limit of five Read Replicas for the source instance.

You can create a Read Replica from either single-AZ or Multi-AZ DB instance deployments. You use a Multi-AZ deployment to improve the durability and availability of a critical system, but you cannot use the Multi-AZ secondary to serve read-only queries. You must create Read Replicas from a high-traffic, Multi-AZ DB instance to offload read queries from the source DB instance. If the source instance of a Multi-AZ deployment fails over to the secondary, any associated Read Replicas are switched to use the secondary as their replication source.

For MySQL and MariaDB DB instances, in some cases Read Replicas cannot be switched to the secondary if some binlog events are not flushed during the failure. In these cases, you must manually delete and recreate the Read Replicas. You can reduce the chance of this happening in MySQL 5.5 by setting the sync_binlog=1 and innodb_support_xa=1 dynamic variables. These settings might reduce performance, so test their impact before implementing the changes to a production environment. These problems are less likely to occur if you use MySQL 5.6 and later or MariaDB. For instances running MySQL 5.6 and later or MariaDB, the parameters are set by default to sync_binlog=1 and innodb_support_xa=1.

You usually configure replication between Amazon RDS DB instances, but you can configure replication to import databases from instances of MySQL or MariaDB running outside of Amazon RDS, or to export databases to such instances. For more information, see Importing Data to an Amazon RDS MySQL or MariaDB DB Instance with Reduced Downtime and Using Replication to Export MySQL Data.

You can stop and restart the replication process on an Amazon RDS DB instance by calling the system stored procedures mysql.rds_stop_replication and mysql.rds_start_replication. You can do this when replicating between two Amazon RDS instances for long-running operations such as creating large indexes. You also need to stop and start replication when importing or exporting databases. For more information, see Importing Data to an Amazon RDS MySQL or MariaDB DB Instance with Reduced Downtime and Using Replication to Export MySQL Data.

You must explicitly delete Read Replicas, using the same mechanisms for deleting a DB instance. If you delete the source DB instance without deleting the replicas, each replica is promoted to a stand-alone, single-AZ DB instance.

If you promote a MySQL or MariaDB Read Replica that is in turn replicating to other Read Replicas, those Read Replicas remain active. Consider an example where MyDBInstance1 replicates to MyDBInstance2, and MyDBInstance2 replicates to MyDBInstance3. If you promote MyDBInstance2, replication from MyDBInstance1 to MyDBInstance2 no longer occurs, but MyDBInstance2 still replicates to MyDBInstance3.

If replication is stopped for more than 30 consecutive days, either manually or due to a replication error, Amazon RDS terminates replication between the master DB instance and all Read Replicas to prevent increased storage requirements on the master DB instance and long failover times. The Read Replica DB instance is still available, but replication cannot be resumed because the binary logs required by the Read Replica are deleted from the master DB instance after replication is terminated. You can create a new Read Replica for the master DB instance to reestablish replication

Creating a Read Replica

You can create a Read Replica from an existing MySQL, MariaDB, or PostgreSQL DB instance using the AWS Management Console, AWS CLI, or AWS API. You create a Read Replica by specifying the SourceDBInstanceIdentifier, which is the DB instance identifier of the source DB instance from which you wish to replicate.

When you initiate the creation of a Read Replica, Amazon RDS takes a DB snapshot of your source DB instance and begins replication. As a result, you experience a brief I/O suspension on your source DB instance as the DB snapshot occurs. The I/O suspension typically lasts about one minute and can be avoided if the source DB instance is a Multi-AZ deployment (in the case of Multi-AZ deployments, DB snapshots are taken from the standby). An active, long-running transaction can slow the process of creating the Read Replica, so wait for long-running transactions to complete before creating a Read Replica. If you create multiple Read Replicas in parallel from the same source DB instance, Amazon RDS takes only one snapshot at the start of the first create action.

When creating a Read Replica, there are a few things to consider. First, you must enable automatic backups on the source DB instance by setting the backup retention period to a value other than 0. This requirement also applies to a Read Replica that is the source DB instance for another Read Replica. For MySQL DB instances, automatic backups are supported only for Read Replicas running MySQL 5.6 and later, but not for MySQL versions 5.5. To enable automatic backups on an Amazon RDS MySQL version 5.6 and later Read Replica, first create the Read Replica, then modify the Read Replica to enable automatic backups.

Preparing MySQL DB Instances That Use MyISAM

If your MySQL DB instance uses a non-transactional engine such as MyISAM, you need to perform the following steps to successfully set up your Read Replica. These steps are required to ensure that the Read Replica has a consistent copy of your data. These steps are not required if all of your tables use a transactional engine such as InnoDB.

  1. Stop all data manipulation language (DML) and data definition language (DDL) operations on non-transactional tables in the source DB instance and wait for them to complete. SELECT statements can continue running.

  2. Flush and lock the tables in the source DB instance.

  3. Create the Read Replica using one of the methods in the following sections.

  4. Check the progress of the Read Replica creation using, for example, the DescribeDBInstances API operation. Once the Read Replica is available, unlock the tables of the source DB instance and resume normal database operations.

AWS Management Console

To create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance

  1. Sign in to the AWS Management Console and open the Amazon RDS console at https://console.aws.amazon.com/rds/.

  2. In the navigation pane, choose DB Instances.

  3. In the Instances pane, choose the MySQL, MariaDB, or PostgreSQL DB instance that you want to use as the source for a Read Replica and choose Create Read Replica from Instance Actions.

  4. Choose the instance specifications you want to use. It is a best practice to use the same DB instance class and storage type for the Read Replica.

  5. Choose the settings you want to use. For DB Instance Identifier, type a name for the Read Replica. Adjust other settings as needed.

  6. Choose the network, security, database, and maintenance settings you want to use.

  7. Choose Create Read Replica.

CLI

To create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance, use the AWS CLI command create-db-instance-read-replica.

Example

For Linux, OS X, or Unix:

Copy
aws rds create-db-instance-read-replica \
    --db-instance-identifier myreadreplica \
    --source-db-instance-identifier mydbinstance

For Windows:

Copy
aws rds create-db-instance-read-replica ^
    --db-instance-identifier myreadreplica ^
    --source-db-instance-identifier mydbinstance

API

To create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance, call the Amazon RDS API function CreateDBInstanceReadReplica. 

Copy
https://rds.amazonaws.com/
	?Action=CreateDBInstanceReadReplica
	&DBInstanceIdentifier=myreadreplica
	&SourceDBInstanceIdentifier=mydbinstance
	&Version=2012-01-15						
	&SignatureVersion=2
	&SignatureMethod=HmacSHA256
	&Timestamp=2012-01-20T22%3A06%3A23.624Z
	&AWSAccessKeyId=<AWS Access Key ID>
	&Signature=<Signature>

Promoting a Read Replica to Be a DB Instance

You can promote a MySQL, MariaDB, or PostgreSQL Read Replica into a stand-alone, single-AZ DB instance. When you promote a Read Replica, the DB instance is rebooted before it becomes available.

There are several reasons you might want to convert a Read Replica into a single-AZ DB instance:

  • Performing DDL operations (MySQL and MariaDB only) – DDL operations, such as creating or rebuilding indexes, can take time and impose a significant performance penalty on your DB instance. You can perform these operations on a MySQL or MariaDB Read Replica once the Read Replica is in sync with its source DB instance. Then you can promote the Read Replica and direct your applications to use the promoted instance.

  • Sharding – Sharding embodies the "share-nothing" architecture and essentially involves breaking a large database into several smaller databases. Common ways to split a database include splitting tables that are not joined in the same query onto different hosts or duplicating a table across multiple hosts and then using a hashing algorithm to determine which host receives a given update. You can create Read Replicas corresponding to each of your shards (smaller databases) and promote them when you decide to convert them into standalone shards. You can then carve out the key space (if you are splitting rows) or distribution of tables for each of the shards depending on your requirements.

  • Implementing failure recovery – You can use Read Replica promotion as a data recovery scheme if the source DB instance fails; however, if your use case requires synchronous replication, automatic failure detection, and failover, we recommend that you run your DB instance as a Multi-AZ deployment instead. If you are aware of the ramifications and limitations of asynchronous replication and you still want to use Read Replica promotion for data recovery, you first create a Read Replica and then monitor the source DB instance for failures. In the event of a failure, do the following:

    1. Promote the Read Replica.

    2. Direct database traffic to the promoted DB instance.

    3. Create a replacement Read Replica with the promoted DB instance as its source.

    You can perform all of these operations using the Amazon Relational Database Service API Reference, and you can automate the process by using the Amazon Simple Workflow Service Developer Guide.

The new DB instance that is created when you promote a Read Replica retains the backup retention period, backup window period, and parameter group of the former Read Replica source. The promotion process can take several minutes or longer to complete, depending on the size of the Read Replica. Once you promote the Read Replica into a single-AZ DB instance, it is just like any other single-AZ DB instance. For example, you can convert the new DB instance into a Multi-AZ DB instance, and you can create Read Replicas from it. You can also take DB snapshots and perform Point-In-Time Restore operations. Because the promoted DB instance is no longer a Read Replica, you cannot use it as a replication target. If a source DB instance has several Read Replicas, promoting one of the Read Replicas to a DB instance has no effect on the other replicas.

We recommend that you disable automated backups on your Read Replica before promoting the Read Replica. This approach ensures that no backup is performed during the promotion process. Once the instance is promoted to a primary instance, backups are performed based on your backup settings.

The following steps show the general process for promoting a Read Replica to a single-AZ DB instance:

  1. Stop any transactions from being written to the Read Replica source DB instance, and then wait for all updates to be made to the Read Replica. Database updates occur on the Read Replica after they have occurred on the source DB instance, and this replication lag can vary significantly. Use the Replica Lag metric to determine when all updates have been made to the Read Replica.

  2. For MySQL and MariaDB only: If you need to make changes to the MySQL or MariaDB Read Replica, you must the set the read_only parameter to 0 in the DB parameter group for the Read Replica. You can then perform all needed DDL operations, such as creating indexes, on the Read Replica. Actions taken on the Read Replica don't affect the performance of the source DB instance.

  3. Promote the Read Replica by using the Promote Read Replica option on the Amazon RDS console, the AWS CLI command promote-read-replica, or the PromoteReadReplica Amazon RDS API operation.

    Note

    The promotion process takes a few minutes to complete. When you promote a Read Replica, replication is stopped and the Read Replica is rebooted. When the reboot is complete, the Read Replica is available as a single-AZ DB instance.

AWS Management Console

To promote a Read Replica to a DB instance

  1. Sign in to the AWS Management Console and open the Amazon RDS console at https://console.aws.amazon.com/rds/.

  2. In the Amazon RDS console, choose Read Replicas.

  3. In the Read Replicas pane, select the check box beside the Read Replica that you want to promote.

  4. Choose Promote Read Replica.

  5. In the Promote Read Replica dialog box, enter the backup retention period and the backup window for the new promoted DB instance.

  6. When the settings are as you want them, choose Continue.

  7. On the acknowledgment page, choose Yes, Promote.

CLI

To promote a Read Replica to a DB instance, use the AWS CLI promote-read-replica command.

Example

For Linux, OS X, or Unix:

Copy
aws rds promote-read-replica \
    --db-instance-identifier myreadreplica

For Windows:

Copy
aws rds promote-read-replica ^
    --db-instance-identifier myreadreplica

API

To promote a Read Replica to a DB instance, call PromoteReadReplica. 

Copy
https://rds.amazonaws.com/
	?Action=PromoteReadReplica
	&DBInstanceIdentifier=myreadreplica
	&Version=2012-01-15						
	&SignatureVersion=2
	&SignatureMethod=HmacSHA256
	&Timestamp=2012-01-20T22%3A06%3A23.624Z
	&AWSAccessKeyId=<AWS Access Key ID>
	&Signature=<Signature>

Replicating a Read Replica Across Regions

With Amazon Relational Database Service (Amazon RDS), you can create a MySQL, PostgreSQL, or MariaDB Read Replica in a different AWS Region than the source DB instance. You create a Read Replica to do the following:

  • Improve your disaster recovery capabilities.

  • Scale read operations into a region closer to your users.

  • Make it easier to migrate from a data center in one region to a data center in another region.

Creating a MySQL, PostgreSQL, or MariaDB Read Replica in a different region than the source instance is very similar to creating a replica in the same region. To create a Read Replica across regions, you can use the AWS Management Console, run the create-db-instance-read-replica command, or call the CreateDBInstanceReadReplica API action.

To create an encrypted Read Replica in a different AWS Region than the source DB instance, the source DB instance must be encrypted.

Note

You can also create a replica of an Amazon Aurora DB cluster in a different region. For more information, see Replicating Amazon Aurora DB Clusters Across AWS Regions.

Following, you can find information on how to create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance in a different AWS Region.

AWS Management Console

You can create a Read Replica across regions using the AWS Management Console.

To create a Read Replica across regions with the console

  1. Sign in to the AWS Management Console and open the Amazon RDS console at https://console.aws.amazon.com/rds/.

  2. In the navigation pane, choose DB Instances.

  3. In the Instances window, choose the MySQL, MariaDB, or PostgreSQL DB instance that you want to use as the source for a Read Replica, and then choose Create Read Replica from Instance Actions. To create an encrypted Read Replica, the source DB instance must be encrypted. To learn more about encrypting the source DB instance, see Encrypting Amazon RDS Resources.

  4. Choose the instance specifications you want to use. We recommend that you use the same DB instance class and storage type for the Read Replica.

  5. Choose the other settings you want to use:

    • For DB Instance Identifier, type a name for the Read Replica.

    • In the Network & Security section, choose a value for Designation Region and Designation DB Subnet Group.

    • To create an encrypted Read Replica in another region, choose Enable Encryption, and then choose Master Key. For Master Key, choose the KMS key identifier of the destination region.

    • Choose the remaining network, security, database, and maintenance settings you want to use.

  6. Choose Create Read Replica.

AWS CLI

To create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance in a different AWS Region, you can use the create-db-instance-read-replica command. In this case, you use create-db-instance-read-replica from the AWS Region where you want the Read Replica and specify the Amazon Resource Name (ARN) for the source DB instance. An ARN uniquely identifies a resource created in Amazon Web Services.

For example, if your source DB instance is in the US East (N. Virginia) region, the ARN looks similar to the following.

arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance

For information about ARNs, see Working with Amazon Resource Names (ARNs) in Amazon RDS.

To create an encrypted Read Replica in a different AWS Region than the source DB instance, you can use the AWS CLI create-db-instance-read-replica command from the destination region. The following parameters are used to create an encrypted Read Replica in another AWS Region:

  • --source-region — The AWS Region that the encrypted Read Replica is created in. If source-region is not specified, you must specify a pre-signed-url. A pre-signed-url is a URL that contains a Signature Version 4 signed request for the CreateDBInstanceReadReplica action that is called in the source region where the Read Replica is created from. To learn more about the pre-signed-url, see CreateDBInstanceReadReplica.

  • --source-db-instance-identifier — The DB instance identifier for the encrypted Read Replica that is created. This identifier must be in the ARN format for the source region. The region specified in source-db-instance-identifier must match the region specified as the source-region.

  • --db-instance-identifier — The identifier for the encrypted Read Replica in the destination region.

  • --kms-key-id — The AWS KMS key identifier for the key to use to encrypt the Read Replica in the destination region.

The following code creates a Read Replica in the us-west-2 region.

Example

For Linux, OS X, or Unix:

Copy
aws rds create-db-instance-read-replica \
    --db-instance-identifier DBInstanceIdentifier \
    --region us-west-2 \
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance

For Windows:

Copy
aws rds create-db-instance-read-replica ^
    --db-instance-identifier DBInstanceIdentifier ^
    --region us-west-2 ^
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance

The following code creates a Read Replica in a different AWS Region than the source DB instance. The region where you call the create-db-instance-read-replica command is the destination region for the encrypted Read Replica.

Example

For Linux, OS X, or Unix:

Copy
aws rds create-db-instance-read-replica \
    --db-instance-identifier DBInstanceIdentifier \
    --region us-west-2 \
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance \
    --source-region us-east-1 \
    --kms-key-id my-us-east-1-key

For Windows:

Copy
aws rds create-db-instance-read-replica ^
    --db-instance-identifier DBInstanceIdentifier ^
    --region us-west-2 ^
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance ^
    --source-region us-east-1 ^	
    --kms-key-id my-us-east-1-key

API

To create a Read Replica from a source MySQL, MariaDB, or PostgreSQL DB instance in a different AWS Region, you can call the Amazon RDS API function CreateDBInstanceReadReplica. In this case, you call CreateDBInstanceReadReplica from the AWS Region where you want the Read Replica and specify the Amazon Resource Name (ARN) for the source DB instance. An ARN uniquely identifies a resource created in Amazon Web Services.

To create an encrypted Read Replica in a different AWS Region than the source DB instance, you can use the Amazon RDS API CreateDBInstanceReadReplica action from the destination region. To create an encrypted Read Replica in another region, you must specify a value for PreSignedURL. PreSignedURL should contain a request for the CreateDBInstanceReadReplica action to call in the source region where the Read Replica is created in. To learn more about PreSignedUrl, see CreateDBInstanceReadReplica.

For example, if your source DB instance is in the US East (N. Virginia) region, the ARN looks similar to the following.

arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance

For information about ARNs, see Working with Amazon Resource Names (ARNs) in Amazon RDS.

Example 

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https://us-west-2.rds.amazonaws.com/
    ?Action=CreateDBInstanceReadReplica
    &KmsKeyId=my-us-east-1-key
    &PreSignedUrl=https%253A%252F%252Frds.us-west-2.amazonaws.com%252F
         %253FAction%253D CreateDBInstanceReadReplica
         %2526DestinationRegion%253Dus-east-1
         %2526KmsKeyId%253Dmy-us-east-1-key
         %2526SourceDBInstanceIdentifier%253Darn%25253Aaws%25253Ards%25253Aus-west-2%1234567890
12%25253Adb%25253Amy-mysql-instance
         %2526SignatureMethod%253DHmacSHA256
         %2526SignatureVersion%253D4%2526SourceDBInstanceIdentifier%253Darn%25253Aaws%25253Ards%25253Aus-west-2%25253A123456789012%25253Ainstance%25253Amysql-instance1-instance-20161115
         %2526Version%253D2014-10-31
         %2526X-Amz-Algorithm%253DAWS4-HMAC-SHA256
         %2526X-Amz-Credential%253DAKIADQKE4SARGYLE%252F20161117%252Fus-west-2%252Frds%252Faws4_request
         %2526X-Amz-Date%253D20161117T215409Z
         %2526X-Amz-Expires%253D3600
         %2526X-Amz-SignedHeaders%253Dcontent-type%253Bhost%253Buser-agent%253Bx-amz-content-sha256%253Bx-amz-date
         %2526X-Amz-Signature%253D255a0f17b4e717d3b67fad163c3ec26573b882c03a65523522cf890a67fca613
    &DBInstanceIdentifier=myreadreplica
    &SourceDBInstanceIdentifier=arn:aws:rds:us-east-1:123456789012:db:my-mysql-instance
    &Version=2012-01-15						
    &SignatureVersion=2
    &SignatureMethod=HmacSHA256
    &Timestamp=2012-01-20T22%3A06%3A23.624Z
    &AWSAccessKeyId=<AWS Access Key ID>
    &Signature=<Signature>

Cross-Region Replication Considerations

All of the considerations for performing replication within a region apply to cross-region replication. The following extra considerations apply when replicating between regions:

  • You can only replicate between regions when using Amazon RDS DB instances of MariaDB, PostgreSQL (versions 9.4.7 and 9.5.2 and later), or MySQL 5.6 and later.

  • A source DB instance can have cross-region Read Replicas in multiple regions.

  • You can only create a cross-region Amazon RDS Read Replica from a source Amazon RDS DB instance that is not a Read Replica of another Amazon RDS DB instance.

  • You cannot set up a replication channel into or out of the AWS GovCloud (US) region.

  • You can expect to see a higher level of lag time for any Read Replica that is in a different region than the source instance, due to the longer network channels between regional data centers.

  • Within a region, all cross-region Read Replicas created from the same source DB instance must either be in the same Amazon VPC or be outside of a VPC. For cross-region Read Replicas, any of the create Read Replica commands that specify the --db-subnet-group-name parameter must specify a DB subnet group from the same VPC.

  • You can create a cross-region Read Replica in a VPC from a source DB instance that is not in an VPC. You can also create a cross-region Read Replica that is not in an VPC from a source DB instance that is in a VPC.

  • Due to the limit on the number of access control list (ACL) entries for a VPC, we cannot guarantee more than 5 cross-region Read Replica instances.

Cross-Region Replication Costs

The data transferred for cross-region replication incurs Amazon RDS data transfer charges. These cross-region replication actions generate charges for the data transferred out of the source region:

  • When you create a Read Replica, Amazon RDS takes a snapshot of the source instance and transfers the snapshot to the Read Replica region.

  • For each data modification made in the source databases, Amazon RDS transfers data from the source region to the Read Replica region.

For more information about Amazon RDS data transfer pricing, see Amazon Relational Database Service Pricing.

For MySQL and MariaDB instances, you can reduce your data transfer costs by reducing the number of cross-region Read Replicas that you create. For example, if you have a source DB instance in one region and want to have three Read Replicas in another region, only create one of the Read Replicas from the source DB instance, and then create the other two replicas from the first Read Replica instead of the source DB instance. For example, if you have source-instance-1 in one region, you can do the following:

  • Create read-replica-1 in the new region, specifying source-instance-1 as the source.

  • Create read-replica-2 from read-replica-1.

  • Create read-replica-3 from read-replica-1.

In this example, you are only charged for the data transferred from source-instance-1 to read-replica-1. You are not charged for the data transferred from read-replica-1 to the other two replicas because they are all in the same region. If you create all three replicas directly from source-instance-1, you are charged for the data transfers to all three replicas.

How Amazon RDS Does Cross-Region Replication

Amazon RDS uses the following process to create a cross-region Read Replica. Depending on the regions involved and the amount of data in the databases, this process can take hours to complete. You can use this information to determine how far the process has proceeded when you create a cross-region Read Replica:

  1. Amazon RDS begins configuring the source DB instance as a replication source and sets the status to modifying.

  2. Amazon RDS begins setting up the specified Read Replica in the destination region and sets the status to creating.

  3. Amazon RDS creates an automated DB snapshot of the source DB instance in the source region. The format of the DB snapshot name is rds:<InstanceID>-<timestamp>, where <InstanceID> is the identifier of the source instance, and <timestamp> is the date and time the copy started. For example, rds:mysourceinstance-2013-11-14-09-24 was created from the instance mysourceinstance at 2013-11-14-09-24. During the creation of an automated DB snapshot, the source DB instance status remains modifying, the Read Replica status remains creating, and the DB snapshot status is creating. The progress column of the DB snapshot page in the console reports how far the DB snapshot creation has progressed. When the DB snapshot is complete, the status of both the DB snapshot and source DB instance are set to available.

  4. Amazon RDS begins a cross-region snapshot copy for the initial data transfer. The snapshot copy is listed as an automated snapshot in the destination region with a status of creating. It has the same name as the source DB snapshot. The progress column of the DB snapshot display indicates how far the copy has progressed. When the copy is complete, the status of the DB snapshot copy is set to available.

  5. Amazon RDS then uses the copied DB snapshot for the initial data load on the Read Replica. During this phase, the Read Replica is in the list of DB instances in the destination, with a status of creating. When the load is complete, the Read Replica status is set to available, and the DB snapshot copy is deleted.

  6. When the Read Replica reaches the available status, Amazon RDS starts by replicating the changes made to the source instance since the start of the create Read Replica operation. During this phase, the replication lag time for the Read Replica will be greater than 0.

    For MySQL, MariaDB, and PostgreSQL Read Replicas, you can monitor replication lag in Amazon CloudWatch by viewing the Amazon RDS ReplicaLag metric. For MySQL and MariaDB, the ReplicaLag metric reports the value of the Seconds_Behind_Master field of the SHOW SLAVE STATUS command. For PostgreSQL, the ReplicaLag metric reports the value of SELECT extract(epoch from now() - pg_last_xact_replay_timestamp()) AS slave_lag.

    Common causes for replication lag for MySQL and MariaDB are the following:

    • A network outage.

    • Writing to tables with indexes on a Read Replica. If the read_only parameter is not set to 0 on the Read Replica, it can break replication.

    • Using a non-transactional storage engine such as MyISAM. Replication is only supported for the InnoDB storage engine on MySQL and the XtraDB storage engine on MariaDB.

    When the ReplicaLag metric reaches 0, the replica has caught up to the source DB instance. If the ReplicaLag metric returns -1, then replication is currently not active. ReplicaLag = -1 is equivalent to Seconds_Behind_Master = NULL.

    PostgreSQL (versions 9.4.7 and 9.5.2 exclusively) uses physical replication slots to manage Write Ahead Log (WAL) retention on the source instance. For each cross-region Read Replica instance, Amazon RDS creates a physical replication slot and associates it with the instance. Two Amazon CloudWatch metrics, Oldest Replication Slot Lag and Transaction Logs Disk Usage, show how far behind the most lagging replica is in terms of WAL data received and how much storage is being used for WAL data. The Transaction Logs Disk Usage value can substantially increase when a cross-region Read Replica is lagging significantly.

Cross-Region Replication Examples

Example Create a Cross-Region Read Replica Outside of any VPC

The following example creates a Read Replica in us-west-2 from a source DB instance in us-east-1. The Read Replica is created outside of a VPC:

For Linux, OS X, or Unix:

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aws rds create-db-instance-read-replica \
    --db-instance-identifier SimCoProd01Replica01 \
    --region us-west-2
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:SimcoProd01

For Windows:

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aws rds create-db-instance-read-replica ^
    --db-instance-identifier SimCoProd01Replica01 ^
    --region us-west-2
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:SimcoProd01

Example Create Cross-Region Read Replica in a VPC

This example creates a Read Replica in us-west-2 from a source DB instance in us-east-1. The Read Replica is created in the VPC associated with the specified DB subnet group:

For Linux, OS X, or Unix:

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aws rds create-db-instance-read-replica \
    --db-instance-identifier SimCoProd01Replica01 \
    --region us-west-2
    --db-subnet-group-name my-us-west-2-subnet 
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:SimcoProd01

For Windows:

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aws rds create-db-instance-read-replica ^
    --db-instance-identifier SimCoProd01Replica01 ^
    --region us-west-2
    --db-subnet-group-name my-us-west-2-subnet 
    --source-db-instance-identifier arn:aws:rds:us-east-1:123456789012:db:SimcoProd01

Monitoring Read Replication

You can monitor the status of a Read Replica in several ways. The Amazon RDS console shows the status of a Read Replica; you can also see the status of a Read Replica using the AWS CLI describe-db-instances command or the Amazon RDS API DescribeDBInstances action.

Read Replica status

The status of a Read Replica can be one of the following:

  • Replicating—The Read Replica is replicating successfully.

  • Error—An error has occurred with the replication. Check the Replication Error field in the Amazon RDS console or the event log to determine the exact error. For more information about troubleshooting a replication error, see Troubleshooting a MySQL or MariaDB Read Replica Problem.

  • Stopped—(MySQL or MariaDB only) Replication has stopped because of a customer initiated request.

  • Terminated—Replication is terminated. This occurs if replication is stopped for more than thirty consecutive days, either manually or due to a replication error. In this case, Amazon RDS terminates replication between the master DB instance and all Read Replicas in order to prevent increased storage requirements on the master DB instance and long failover times.

    Broken replication can affect storage because the logs can grow in size and number due to the high volume of errors messages being written to the log. Broken replication can also affect failure recovery due to the time Amazon RDS requires to maintain and process the large number of logs during recovery.

You can monitor how far a MySQL or MariaDB Read Replica is lagging the source DB instance by viewing the Seconds_Behind_Master data returned by the MySQL or MariaDB Show Slave Status command, or the CloudWatch Replica Lag statistic. If a replica lags too far behind for your environment, consider deleting and recreating the Read Replica. Also consider increasing the scale of the Read Replica to speed replication.

Troubleshooting a MySQL or MariaDB Read Replica Problem

MySQL and MariaDB's replication technologies are asynchronous. Because they are asynchronous, occasional BinLogDiskUsage increases on the source DB instance and ReplicaLag on the Read Replica are to be expected. For example, a high volume of write operations to the source DB instance can occur in parallel, while write operations to the Read Replica are serialized using a single I/O thread, can lead to a lag between the source instance and Read Replica. For more information about read-only replicas in the MySQL documentation, see Replication Implementation Details. For more information about read-only replicas in the MariaDB documentation, go to Replication Overview.

You can do several things to reduce the lag between updates to a source DB instance and the subsequent updates to the Read Replica, such as the following:

  • Sizing a Read Replica to have a storage size and DB instance class comparable to the source DB instance.

  • Ensuring that parameter settings in the DB parameter groups used by the source DB instance and the Read Replica are compatible. For more information and an example, see the discussion of the max_allowed_packet parameter later in this section.

Amazon RDS monitors the replication status of your Read Replicas and updates the Replication State field of the Read Replica instance to Error if replication stops for any reason, such as DML queries being run on your Read Replica that conflict with the updates made on the source DB instance. You can review the details of the associated error thrown by the MySQL or MariaDB engines by viewing the Replication Error field. Events that indicate the status of the Read Replica are also generated, including RDS-EVENT-0045, RDS-EVENT-0046, and RDS-EVENT-0047. For more information about events and subscribing to events, see Using Amazon RDS Event Notification. If a MySQL error message is returned, review the error number in the MySQL error message documentation. If a MariaDB error message is returned, review the error in the MariaDB error message documentation.

One common issue that can cause replication errors is when the value for the max_allowed_packet parameter for a Read Replica is less than the max_allowed_packet parameter for the source DB instance. The max_allowed_packet parameter is a custom parameter that you can set in a DB parameter group that is used to specify the maximum size of DML that can be executed on the database. If the max_allowed_packet parameter value in the DB parameter group associated with a source DB instance is smaller than the max_allowed_packet parameter value in the DB parameter group associated with the source's Read Replica, the replication process can throw an error (Packet bigger than 'max_allowed_packet' bytes) and stop replication. You can fix the error by having the source and Read Replica use DB parameter groups with the same max_allowed_packet parameter values.

Other common situations that can cause replication errors include the following:

  • Writing to tables on a Read Replica. If you are creating indexes on a Read Replica, you need to have the read_only parameter set to 0 to create the indexes. If you are writing to tables on the Read Replica, it might break replication.

  • Using a non-transactional storage engine such as MyISAM. Read replicas require a transactional storage engine. Replication is only supported for the InnoDB storage engine on MySQL and the XtraDB storage engine on MariaDB.

  • Using unsafe nondeterministic queries such as SYSDATE(). For more information, see Determination of Safe and Unsafe Statements in Binary Logging.

If you decide that you can safely skip an error, you can follow the steps described in the section Skipping the Current Replication Error. Otherwise, you can delete the Read Replica and create a instance using the same DB instance identifier so that the endpoint remains the same as that of your old Read Replica. If a replication error is fixed, the Replication State changes to replicating.

Troubleshooting a PostgreSQL Read Replica Problem

PostgreSQL uses replication slots for cross-region replication, so the process for troubleshooting same region and cross region replication problems is different.

Troubleshooting PostgreSQL Read Replica Problems Within a Region

The PostgreSQL parameter, wal_keep_segments, dictates how many Write Ahead Log (WAL) files are kept to provide data to the Read Replicas. The parameter value specifies the number of logs to keep. If you set the parameter value too low, you can cause a Read Replica to fall so far behind that streaming replication stops. In this case, Amazon RDS will report a replication error and begin recovery on the Read Replica by replaying the source DB instance's archived WAL logs. This recovery process continues until the Read Replica has caught up enough to continue streaming replication.

The PostgreSQL log on the Read Replica will show when Amazon RDS is recovering a Read Replica that is this state by replaying archived WAL files. 

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  2014-11-07 19:01:10 UTC::@:[23180]:DEBUG:  switched WAL source from archive to stream after
      failure 2014-11-07 19:01:10 UTC::@:[11575]:LOG:  started streaming WAL from primary at
      1A/D3000000 on timeline 1 2014-11-07 19:01:10 UTC::@:[11575]:FATAL:  could not receive
      data from WAL stream: ERROR:  requested WAL segment 000000010000001A000000D3 has already been
      removed 2014-11-07 19:01:10 UTC::@:[23180]:DEBUG:  could not restore file
      "00000002.history" from archive: return code 0 2014-11-07 19:01:15
      UTC::@:[23180]:DEBUG:  switched WAL source from stream to archive after failure
      recovering 000000010000001A000000D3 2014-11-07 19:01:16 UTC::@:[23180]:LOG:  restored log file "000000010000001A000000D3"
        from archive

Once Amazon RDS has replayed enough archived WAL files on the replica to catch up and allow the Read Replica to begin streaming again, PostgreSQL will resume streaming and write a similar line to the following to the log file: 

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  2014-11-07 19:41:36 UTC::@:[24714]:LOG:  started streaming WAL from primary at 1B/B6000000
      on timeline 1

You can determine how many WAL files you should keep by looking at the checkpoint information in the log. The PostgreSQL log shows the following information at each checkpoint. By looking at the "# recycled" transaction log files of these log statements, a user can understand how many transaction files will be recycled during a time range and use this information to tune the wal_keep_segments parameter. 

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  2014-11-07 19:59:35 UTC::@:[26820]:LOG:  checkpoint complete: wrote 376 buffers (0.2%); 0
      transaction log file(s) added, 0 removed, 1 recycled; write=35.681 s, sync=0.013 s,
      total=35.703 s; sync files=10, longest=0.013 s, average=0.001 s

For example, if the PostgreSQL log shows that 35 files are recycled from the "checkpoint completed" log statements within a 5 minute time frame, we know that with this usage pattern a Read Replica relies on 35 transaction files in 5 minutes and could not survive 5 minutes in a non-streaming state if the source DB instance is set to the default wal_keep_segments parameter value of 32.

Troubleshooting PostgreSQL Read Replica Problems Across Regions

PostgreSQL (versions 9.4.7 and 9.5.2 exclusively) uses physical replication slots to manage Write Ahead Log (WAL) retention on the source DB instance. For each cross-region Read Replica instance, Amazon RDS creates and associates a physical replication slot. You can use two Amazon CloudWatch metrics, Oldest Replication Slot Lag and Transaction Logs Disk Usage, to see how far behind the most lagging replica is in terms of WAL data received and to see how much storage is being used for WAL data. The Transaction Logs Disk Usage value can substantially increase when a cross-region Read Replica is lagging significantly.

If the workload on your DB instance generates a large amount of WAL data, you might need to change the DB instance class of your source DB instance and Read Replica to one with High / 10Gb network performance for the replica to keep up. The Amazon CloudWatch metric Transaction Logs Generation can help you understand the rate at which your workload is generating WAL data.

To determine the status of a cross-region Read Replica, you can query pg_replication_slots on the source instance, as in the following example: 

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postgres=# select * from pg_replication_slots;

                  slot_name                  | plugin | slot_type | datoid | database | active | active_pid | xmin | catalog_xmin | restart_lsn
_______________________________________________________________________________________________________________________________________________
 rds_us_east_1_db_uzwlholddgpblksce6hgw4nkte |        | physical  |        |          | t      |      12598 |      |              | 4E/95000060
(1 row)