Support LazyConnectionDataSourceProxy

[ttddyy]

Hi,

I would like to suggest a support for LazyConnectionDataSourceProxy in spring-boot.

Here is my opinion:

Transaction management in spring:
When a transactional method(@Transactional) is called, the configured impl of PlatformTransactionManager kicks in and starts/prepare transaction logic(call AbstractPlatformTransactionManager#doBegin).

In the implementation of starting transaction(e.g: JpaTransactionManager, HibernateTransactionManager), when @Transactional definition has non default isolation level or readonly flag is set to true, it acquires a physical connection and set isolation level and/or readonly flag to the connection in its preparation phase. (call to DataSourceUtils#prepareConnectionForTransaction)
Since this happens before invoking the actual target method, this consumes a connection from pool(underlying DataSource) regardless of the method really needs a connection or not.

It is problematic especially on high traffic application with:

• Many readonly transactional methods (or any non-default transactional methods)
  • This holds connection longer than it needs to do
• High hits for hibernate 2nd level cache
  • The method may not even need a connection

It is better to delay getting the physical connection as much as possible.

LazyConnectionDataSourceProxy in spring solves this problem. When isolation-level, read-only flag, etc methods are called, this proxy internally keeps those values. Physical connection acquisition is delayed until connection is really required.

Currently, spring-boot doesn't have any support for LazyConnectionDataSourceProxy.
My suggestion is to add a configuration of LazyConnectionDataSourceProxy in spring-boot.
Probably, application property with spring.datasource.lazy-connection=true would wrap the datasource with LazyConnectionDataSourceProxy. I also think it is beneficial to turn this on by default.
LazyConnectionDataSourceProxy should be recommended more, in general, IMO.

Thanks,