Java Database Connectivity (JDBC) API is still blocking and synchronous. There are proposals to make it fully non-blocking and asynchronous, but that work is still in progress. This means, as of writing this book, there is no way to do reactive programming if you persist to a database using JDBC. 

But there is a small caveat that can be used to mimic reactive behavior, you can use a pool of threads to execute synchronous tasks asynchronously. 

The following DbConfig class is used to do just that:

@Configuration
public class DbConfig {

    private Integer connectionPoolSize;

    public DbConfig(@Value("${spring.datasource.maximum-pool-size:10}") 
    Integer connectionPoolSize) {
        this.connectionPoolSize = connectionPoolSize;
    }

    @Bean
    public Scheduler dbScheduler() {
        return Schedulers.fromExecutor(Executors.newFixedThreadPool
        (connectionPoolSize));
    }
}

The dbScheduler() method creates a Scheduler bean, which encapsulates a standard Java fixed-size thread pool for executive tasks. This bean will be used in the service layer, as explained in the next section.