Enterprise Java

DataSource Routing with Spring @Transactional

I was inspired by Carl Papa’s use of aspects with the Spring Framework to determine the DataSource to use (either read-write or read-only). So, I’m writing this post.

I must admit that I have long been familiar with Spring’s AbstractRoutingDataSource. But I did not have a good idea where it can be used. Thanks to Carl and team, and one of their projects. Now, I know a good use case.


With Spring, read-only transactions are typically marked with annotations.

public class ... {
    public void ...() {...}

    @Transactional // read-write
    public void ...() {...}

To take advantage of this, we use Spring’s TransactionSynchronizationManager to determine if the current transaction is read-only or not.


Here, we use Spring’s AbstractRoutingDataSource to route to the read-only replica if the current transaction is read-only. Otherwise, it routes to the default which is the master.

public class ... extends AbstractRoutingDataSource {
    protected Object determineCurrentLookupKey() {
        if (TransactionSynchronizationManager
                .isCurrentTransactionReadOnly() ...) {
            // return key to a replica
        return null; // use default

Upon using the above approach, we found out that the TransactionSynchronizationManager is one step behind because Spring will have already called DataSource.getConnection() before a synchronization is established. Thus, a LazyConnectionDataSourceProxy needs to be configured as well.

As we were discussing this, we figured if there was another way to determine if the current transaction is read-only or not (without resorting to LazyConnectionDataSourceProxy). So, we came up with an experimental approach where an aspect captures the TransactionDefinition (from the @Transactional annotation, if any) as a thread-local variable, and an AbstractRoutingDataSource that routes based on the captured information.

The relevant source code can be found on GitHub. Thanks again, Carl! BTW, Carl is also an award-winning movie director. Wow, talent definitely knows no boundaries.

Published on Java Code Geeks with permission by Lorenzo Dee, partner at our JCG program. See the original article here: DataSource Routing with Spring @Transactional

Opinions expressed by Java Code Geeks contributors are their own.

Lorenzo Dee

Lorenzo is a software engineer, trainer, manager, and entrepreneur, who loves developing software systems that make people and organizations productive, profitable, and happy. He is a co-founder of the now dormant Haybol.ph, a Philippine real estate search site. He loves drinking coffee, root beer, and milk shakes.
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