PolarDB for MySQL supports transaction splitting. Transaction splitting ensures that data is consistent in a session and allows PolarDB to send read requests to read-only nodes to reduce the loads on the primary node.
Background information
If you use a PolarDB for PostgreSQL cluster endpoint in read/write mode, the proxy forwards read and write requests to the primary node and read-only nodes. To ensure read/write consistency of transactions in a session, the proxy sends all transaction requests in the session to the primary node.
For example, some database client drivers, such as the Java Database Connectivity (JDBC) driver, encapsulate all requests in transactions by default. In this case, all requests from applications are sent to the primary node. This results in heavy loads on the primary node and low loads on read-only nodes, as shown in the following figure.
To fix the preceding issue, PolarDB provides the transaction splitting feature that can be used to ensure read/write consistency. This feature reduces the loads on the primary node by sending read requests in transactions to read-only nodes.
Description
Basic transaction splitting
To reduce the loads on the primary node, PolarProxy sends read requests that are received before the first write request in a transaction is sent to read-only nodes. Uncommitted data in transactions cannot be queried from read-only nodes. To ensure data consistency in transactions, all read and write requests that are received after the first write request are still forwarded to the primary node. For more information about how to enable basic transaction splitting, see Create and modify a custom cluster endpoint.
Benefits
This feature allows you to transfer the read loads from the primary node to read-only nodes without modifying application code or configurations. This makes the primary node more stable.
Note
- Only transactions that use the Read Committed isolation level can be split.
- If you enable basic transaction splitting and the consistency level is not set to eventual consistency, the proxy sends the read requests that are received before the first write request in a transaction to read-only nodes only after read-only nodes synchronize all data from the primary node. Otherwise, the proxy still sends read requests to the primary node. For more information about consistency levels, see .
Related API operations
| API | Description |
| ModifyDBClusterEndpoint | Modifies the attributes of a PolarDB for MySQL cluster endpoint. For example, you can modify the following attributes: read/write mode, consistency level, transaction splitting, and offload read requests from the primary node. You can also specify whether to associate the specified cluster endpoint with newly added nodes. |