Read-Write and Read-Only Transaction Routing With Spring

A step-by-step guide to implementing read-write and read-only transaction routing in Spring using AbstractRoutingDataSource. The approach uses the @Transactional(readOnly) flag combined with TransactionSynchronizationManager to automatically direct write operations to a primary database and read queries to a replica. Key implementation details include wrapping the routing DataSource in LazyConnectionDataSourceProxy to ensure the read-only flag is set before a connection is acquired, manually configuring EntityManagerFactory and JpaTransactionManager, and verifying routing behavior with integration tests using unsynchronized H2 databases. Practical considerations cover replication lag, nested transaction propagation, and scaling to multiple read replicas.