Replication with Amazon Aurora MySQL - Amazon Aurora
Aurora ReplicasReplication optionsReplication performanceZero-downtime restart (ZDR)Configuring replication filtersMonitoring replication

Replication with Amazon Aurora MySQL

The Aurora MySQL replication features are key to the high availability and performance of your cluster. Aurora makes it easy to create or resize clusters with up to 15 Aurora Replicas.

All the replicas work from the same underlying data. If some database instances go offline, others remain available to continue processing queries or to take over as the writer if needed. Aurora automatically spreads your read-only connections across multiple database instances, helping an Aurora cluster to support query-intensive workloads.

In the following topics, you can find information about how Aurora MySQL replication works and how to fine-tune replication settings for best availability and performance.

Topics

Using Aurora Replicas

Aurora Replicas are independent endpoints in an Aurora DB cluster, best used for scaling read operations and increasing availability. Up to 15 Aurora Replicas can be distributed across the Availability Zones that a DB cluster spans within an AWS Region. Although the DB cluster volume is made up of multiple copies of the data for the DB cluster, the data in the cluster volume is represented as a single, logical volume to the primary instance and to Aurora Replicas in the DB cluster. For more information about Aurora Replicas, see Aurora Replicas.

Aurora Replicas work well for read scaling because they are fully dedicated to read operations on your cluster volume. Write operations are managed by the primary instance. Because the cluster volume is shared among all instances in your Aurora MySQL DB cluster, no additional work is required to replicate a copy of the data for each Aurora Replica. In contrast, MySQL read replicas must replay, on a single thread, all write operations from the source DB instance to their local data store. This limitation can affect the ability of MySQL read replicas to support large volumes of read traffic.

With Aurora MySQL, when an Aurora Replica is deleted, its instance endpoint is removed immediately, and the Aurora Replica is removed from the reader endpoint. If there are statements running on the Aurora Replica that is being deleted, there is a three minute grace period. Existing statements can finish gracefully during the grace period. When the grace period ends, the Aurora Replica is shut down and deleted.

Important

Aurora Replicas for Aurora MySQL always use the REPEATABLE READ default transaction isolation level for operations on InnoDB tables. You can use the SET TRANSACTION ISOLATION LEVEL command to change the transaction level only for the primary instance of an Aurora MySQL DB cluster. This restriction avoids user-level locks on Aurora Replicas, and allows Aurora Replicas to scale to support thousands of active user connections while still keeping replica lag to a minimum.

Note

DDL statements that run on the primary instance might interrupt database connections on the associated Aurora Replicas. If an Aurora Replica connection is actively using a database object, such as a table, and that object is modified on the primary instance using a DDL statement, the Aurora Replica connection is interrupted.

Note

The China (Ningxia) Region does not support cross-Region read replicas.

Replication options for Amazon Aurora MySQL

You can set up replication between any of the following options:

Note

Rebooting the primary instance of an Amazon Aurora DB cluster also automatically reboots the Aurora Replicas for that DB cluster, to re-establish an entry point that guarantees read/write consistency across the DB cluster.

Performance considerations for Amazon Aurora MySQL replication

The following features help you to fine-tune the performance of Aurora MySQL replication.

The replica log compression feature automatically reduces network bandwidth for replication messages. Because each message is transmitted to all Aurora Replicas, the benefits are greater for larger clusters. This feature involves some CPU overhead on the writer node to perform the compression. Thus, the feature is only available on the 8xlarge and 16xlarge instance classes, which have high CPU capacity. It is enabled by default on these instance classes. You can control this feature by turning off the aurora_enable_replica_log_compression parameter. For example, you might turn off replica log compression for larger instance classes if your writer node is near its maximum CPU capacity.

The binlog filtering feature automatically reduces network bandwidth for replication messages. Because the Aurora Replicas don't use the binlog information that is included in the replication messages, that data is omitted from the messages sent to those nodes. You control this feature by changing the aurora_enable_repl_bin_log_filtering parameter. This parameter is on by default. Because this optimization is intended to be transparent, you might turn off this setting only during diagnosis or troubleshooting for issues related to replication. For example, you can do so to match the behavior of an older Aurora MySQL cluster where this feature was not available.

Zero-downtime restart (ZDR) for Amazon Aurora MySQL

The zero-downtime restart (ZDR) feature can preserve some or all of the active connections to DB instances during certain kinds of restarts. ZDR applies to restarts that Aurora performs automatically to resolve error conditions, for example when a replica begins to lag too far behind the source.

Important

The ZDR mechanism operates on a best-effort basis. The Aurora MySQL versions, instance classes, error conditions, compatible SQL operations, and other factors that determine where ZDR applies are subject to change at any time.

In Aurora MySQL 1.* versions where ZDR is available, you turn on this feature by turning on the aurora_enable_zdr parameter in the cluster parameter group. ZDR for Aurora MySQL 2.* requires version 2.10 and higher. ZDR is available in all minor versions of Aurora MySQL 3.*. In Aurora MySQL version 2 and 3, the ZDR mechanism is turned on by default and Aurora doesn't use the aurora_enable_zdr parameter.

Aurora reports on the Events page activities related to zero-downtime restart. Aurora records an event when it attempts a restart using the ZDR mechanism. This event states why Aurora performs the restart. Then Aurora records another event when the restart finishes. This final event reports how long the process took, and how many connections were preserved or dropped during the restart. You can consult the database error log to see more details about what happened during the restart.

Although connections remain intact following a successful ZDR operation, some variables and features are reinitialized. The following kinds of information aren't preserved through a restart caused by zero-downtime restart:

  • Global variables. Aurora restores session variables, but it doesn't restore global variables after the restart.

  • Status variables. In particular, the uptime value reported by the engine status is reset.

  • LAST_INSERT_ID.

  • In-memory auto_increment state for tables. The in-memory auto-increment state is reinitialized. For more information about auto-increment values, see MySQL Reference Manual.

  • Diagnostic information from INFORMATION_SCHEMA and PERFORMANCE_SCHEMA tables. This diagnostic information also appears in the output of commands such as SHOW PROFILE and SHOW PROFILES.

The following table shows the versions, instance roles, instance classes, and other circumstances that determine whether Aurora can use the ZDR mechanism when restarting DB instances in your cluster.

Aurora MySQL version Does ZDR apply to the writer? Does ZDR apply to readers? Notes

Aurora MySQL version 2.*, before 2.10.0

No

No

ZDR isn't available for these versions. The aurora_enable_zdr parameter isn't available in the default cluster parameter group for Aurora MySQL version 2.

Aurora MySQL version 2.*, 2.10.0 and higher

Yes

Yes

The ZDR mechanism is always enabled.

In these Aurora MySQL versions, the following conditions apply to the ZDR mechanism:

  • Aurora rolls back any transactions that are in progress on active connections. Your application must retry the transactions.

  • Aurora cancels any connections that use TLS/SSL, temporary tables, table locks, or user locks.

Aurora MySQL version 3.*

Yes

Yes

The ZDR mechanism is always enabled.

The same conditions apply as in Aurora MySQL version 2.10. ZDR applies to all instance classes.

Configuring replication filters with Aurora MySQL

You can use replication filters to specify which databases and tables are replicated with a read replica. Replication filters can include databases and tables in replication or exclude them from replication.

The following are some use cases for replication filters:

  • To reduce the size of a read replica. With replication filtering, you can exclude the databases and tables that aren't needed on the read replica.

  • To exclude databases and tables from read replicas for security reasons.

  • To replicate different databases and tables for specific use cases at different read replicas. For example, you might use specific read replicas for analytics or sharding.

  • For a DB cluster that has read replicas in different AWS Regions, to replicate different databases or tables in different AWS Regions.

  • To specify which databases and tables are replicated with an Aurora MySQL DB cluster that is configured as a replica in an inbound replication topology. For more information about this configuration, see Replication between Aurora and MySQL or between Aurora and another Aurora DB cluster (binary log replication).

Setting replication filtering parameters for Aurora MySQL

To configure replication filters, set the following replication filtering parameters on the Aurora read replica:

  • binlog-do-db – Replicate changes to the specified binary logs. When you set this parameter for a read replica, only the binary logs specified in the parameter are replicated.

  • binlog-ignore-db – Don't replicate changes to the specified binary logs. When the binlog-do-db parameter is set for a read replica, this parameter isn't evaluated.

  • replicate-do-db – Replicate changes to the specified databases. When you set this parameter for a read replica, only the databases specified in the parameter are replicated.

  • replicate-ignore-db – Don't replicate changes to the specified databases. When the replicate-do-db parameter is set for a read replica, this parameter isn't evaluated.

  • replicate-do-table – Replicate changes to the specified tables. When you set this parameter for a read replica, only the tables specified in the parameter are replicated. Also, when the replicate-do-db or replicate-ignore-db parameter is set, make sure to include the database that includes the specified tables in replication with the read replica.

  • replicate-ignore-table – Don't replicate changes to the specified tables. When the replicate-do-table parameter is set for a read replica, this parameter isn't evaluated.

  • replicate-wild-do-table – Replicate tables based on the specified database and table name patterns. The % and _ wildcard characters are supported. When the replicate-do-db or replicate-ignore-db parameter is set, make sure to include the database that includes the specified tables in replication with the read replica.

  • replicate-wild-ignore-table – Don't replicate tables based on the specified database and table name patterns. The % and _ wildcard characters are supported. When the replicate-do-table or replicate-wild-do-table parameter is set for a read replica, this parameter isn't evaluated.

The parameters are evaluated in the order that they are listed. For more information about how these parameters work, see the MySQL documentation:

By default, each of these parameters has an empty value. On each read replica, you can use these parameters to set, change, and delete replication filters. When you set one of these parameters, separate each filter from others with a comma.

You can use the % and _ wildcard characters in the replicate-wild-do-table and replicate-wild-ignore-table parameters. The % wildcard matches any number of characters, and the _ wildcard matches only one character.

The binary logging format of the source DB instance is important for replication because it determines the record of data changes. The setting of the binlog_format parameter determines whether the replication is row-based or statement-based. For more information, see Configuring Aurora MySQL binary logging.

Note

All data definition language (DDL) statements are replicated as statements, regardless of the binlog_format setting on the source DB instance.

Replication filtering limitations for Aurora MySQL

The following limitations apply to replication filtering for Aurora MySQL:

  • Replication filters are supported only for Aurora MySQL version 3.

  • Each replication filtering parameter has a 2,000-character limit.

  • Commas aren't supported in replication filters.

  • Replication filtering doesn't support XA transactions.

    For more information, see Restrictions on XA Transactions in the MySQL documentation.

Replication filtering examples for Aurora MySQL

To configure replication filtering for a read replica, modify the replication filtering parameters in the DB cluster parameter group associated with the read replica.

Note

You can't modify a default DB cluster parameter group. If the read replica is using a default parameter group, create a new parameter group and associate it with the read replica. For more information on DB cluster parameter groups, see Working with parameter groups.

You can set parameters in a DB cluster parameter group using the AWS Management Console, AWS CLI, or RDS API. For information about setting parameters, see Modifying parameters in a DB parameter group. When you set parameters in a DB cluster parameter group, all of the DB clusters associated with the parameter group use the parameter settings. If you set the replication filtering parameters in a DB cluster parameter group, make sure that the parameter group is associated only with read replica clusters. Leave the replication filtering parameters empty for source DB instances.

The following examples set the parameters using the AWS CLI. These examples set ApplyMethod to immediate so that the parameter changes occur immediately after the CLI command completes. If you want a pending change to be applied after the read replica is rebooted, set ApplyMethod to pending-reboot.

The following examples set replication filters:

Example Including databases in replication

The following example includes the mydb1 and mydb2 databases in replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-do-db,ParameterValue='mydb1,mydb2',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-do-db,ParameterValue='mydb1,mydb2',ApplyMethod=immediate"
Example Including tables in replication

The following example includes the table1 and table2 tables in database mydb1 in replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-do-table,ParameterValue='mydb1.table1,mydb1.table2',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-do-table,ParameterValue='mydb1.table1,mydb1.table2',ApplyMethod=immediate"
Example Including tables in replication using wildcard characters

The following example includes tables with names that begin with order and return in database mydb in replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-wild-do-table,ParameterValue='mydb.order%,mydb.return%',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-wild-do-table,ParameterValue='mydb.order%,mydb.return%',ApplyMethod=immediate"
Example Excluding databases from replication

The following example excludes the mydb5 and mydb6 databases from replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-ignore-db,ParameterValue='mydb5,mydb6',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-ignore-db,ParameterValue='mydb5,mydb6,ApplyMethod=immediate"
Example Excluding tables from replication

The following example excludes tables table1 in database mydb5 and table2 in database mydb6 from replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-ignore-table,ParameterValue='mydb5.table1,mydb6.table2',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-ignore-table,ParameterValue='mydb5.table1,mydb6.table2',ApplyMethod=immediate"
Example Excluding tables from replication using wildcard characters

The following example excludes tables with names that begin with order and return in database mydb7 from replication.

For Linux, macOS, or Unix:

aws rds modify-db-cluster-parameter-group \
  --db-cluster-parameter-group-name myparametergroup \
  --parameters "ParameterName=replicate-wild-ignore-table,ParameterValue='mydb7.order%,mydb7.return%',ApplyMethod=immediate"

For Windows:

aws rds modify-db-cluster-parameter-group ^
  --db-cluster-parameter-group-name myparametergroup ^
  --parameters "ParameterName=replicate-wild-ignore-table,ParameterValue='mydb7.order%,mydb7.return%',ApplyMethod=immediate"

Viewing the replication filters for a read replica

You can view the replication filters for a read replica in the following ways:

  • Check the settings of the replication filtering parameters in the parameter group associated with the read replica.

    For instructions, see Viewing parameter values for a DB parameter group.

  • In a MySQL client, connect to the read replica and run the SHOW REPLICA STATUS statement.

    In the output, the following fields show the replication filters for the read replica:

    • Binlog_Do_DB

    • Binlog_Ignore_DB

    • Replicate_Do_DB

    • Replicate_Ignore_DB

    • Replicate_Do_Table

    • Replicate_Ignore_Table

    • Replicate_Wild_Do_Table

    • Replicate_Wild_Ignore_Table

    For more information about these fields, see Checking Replication Status in the MySQL documentation.

Monitoring Amazon Aurora MySQL replication

Read scaling and high availability depend on minimal lag time. You can monitor how far an Aurora Replica is lagging behind the primary instance of your Aurora MySQL DB cluster by monitoring the Amazon CloudWatch AuroraReplicaLag metric. The AuroraReplicaLag metric is recorded in each Aurora Replica.

The primary DB instance also records the AuroraReplicaLagMaximum and AuroraReplicaLag Amazon CloudWatch metrics. The AuroraReplicaLagMaximum metric records the maximum amount of lag between the primary DB instance and each Aurora Replica in the DB cluster. The AuroraReplicaLag metric records the minimum amount of lag between the primary DB instance and each Aurora Replica in the DB cluster.

If you need the most current value for Aurora Replica lag, you can check the AuroraReplicaLag metric in Amazon CloudWatch. The Aurora Replica lag is also recorded on each Aurora Replica of your Aurora MySQL DB cluster in the information_schema.replica_host_status table. For more information on this table, see information_schema.replica_host_status.

For more information on monitoring RDS instances and CloudWatch metrics, see Monitoring metrics in an Amazon Aurora cluster.