Read-only Instance Learner in the 3-node Enterprise Edition of RDS for MySQL


Read/write splitting is a common way of using databases. Middleware, like a MySQL proxy, sends write and update traffic to a primary node and forwards query traffic to a read-only node. This frees up CPU and I/O resources on the primary node and improves overall database availability.

Features of the Learner

The Three-node Enterprise Edition implements read-only instances by adding a learner. The learner receives committed logs from the leader and stores the logs in the consensus log. The secondary thread reads and distributes the logs to worker threads. Finally, the logs are concurrently played back to the state machine. In this case, the learner node is read-only for external clients.

Incubation of the Learner

The Three-node Enterprise Edition uses a special version of XtraBackup to back up and recover data for instances. We have improved XtraBackup based on the snapshot interface provided by X-Paxos. It now supports the creation of physical backup snapshots with consistent points in time and quickly incubates and adds a new learner node to the cluster to extend its read capability. In the forthcoming Three-node Edition of RDS 8.0, we will also integrate the new clone plug-in feature released in the Official Edition of MySQL 8.0, to provide snapshots with consistent points in time-based on the clone plug-in. This will make the incubation feature of the learner node faster and easier to maintain.

Custom Data Sources

The read-only node of the Three-node Enterprise Edition easily implements a flexible replication topology by using the LearnerSource feature of X-Paxos and custom data sources. The three-node replication topology is configured through the control SQL statements related to membership change on the leader. Centralized configuration management ensures data consistency at the cluster level. The advantage of custom data sources is that, when a large number of read-only nodes are involved, the custom data sources divert loads of log transfers on the leader and scatter the volumes of data transmitted on the network, to reduce the latency of log synchronization.

Session-wide Read Consistency

The read-only node receives and plays backlogs and receives external query requests. However, there is a challenge. Log synchronization and playback are asynchronous on the learner. Although the latency is less than 5 seconds in most scenarios, this does not guarantee that the data returned for each query is the latest. In particular, when a data definition language (DDL) statement for large tables or a large transaction is executed on the primary database, significant latency may occur on the read-only nodes. To solve this problem, the Three-node Enterprise Edition introduces MaxScale as a proxy for read/write splitting and implements session-wide read consistency in MaxScale. This ensures that subsequent read operations within a single session can read the data previously written in the same session, but does not ensure that the latest version of the data in other sessions can be read.


By using the learner role from X-Paxos, the Three-node Enterprise Edition allows creating read-only instances to elastically scale the read capability and share the load of the primary database. Use read-only instances to process massive volumes of read requests sent to the database to increase the throughput of applications. Currently, the Alibaba Cloud official website allows users to create and use read-only instances in the Three-node Enterprise Edition of RDS 5.7. We welcome you to try it out.

Original Source:

Follow me to keep abreast with the latest technology news, industry insights, and developer trends.