[FIXED] Deep understanding of Spring boot with HikariCP

Issue

I have a spring boot app which uses spring data and hikaricp for db connection pooling. I noticed the following behaviour that looks strange to me:

I have one method which is not transactional and in that method several db queries are executed using spring data repositories

public void testMethod(final Long firstRepositoryId, final Long secondRepositoryId) {

    final DomainObject result = firstRepository.findById(firstRepositoryId);
    // here there's some code that is processing the result without db queries
    secondRepository.findById(secondRepositoryId);
    // some more logic here without additional db queries
  }

So as expected when there's no transaction on the method then the spring data methods opens a transaction for executing the query and complete it after the methods returns. I have enabled transaction logging so there's the following log output:

2021-06-03 15:34:30.961 TRACE c681f76a-5d7e-41d5-9e50-fb6f96169681 --- [tp659271212-291] o.s.t.i.TransactionInterceptor : Getting transaction for [com.test.FirstRepository.findById] 2021-06-03 15:34:30.966 TRACE c681f76a-5d7e-41d5-9e50-fb6f96169681 --- [tp659271212-291] o.s.t.i.TransactionInterceptor : Completing transaction for [com.test.FirstRepository.findById] 2021-06-03 15:34:30.967 TRACE c681f76a-5d7e-41d5-9e50-fb6f96169681 --- [tp659271212-291] o.s.t.i.TransactionInterceptor : Getting transaction for [com.test.SecondRepository.findById] 2021-06-03 15:34:30.972 TRACE c681f76a-5d7e-41d5-9e50-fb6f96169681 --- [tp659271212-291] o.s.t.i.TransactionInterceptor : Completing transaction for [com.test.SecondRepository.findById]

Everything seems to be exactly how I expects to be. The thing I can't understand is the hikari behaviour. This method is invoked within a http request. A connection is taken from hikari cp right after the execution of the firstRepository.findById but this connection is returned in the pool only after the http controller returns response. What I expect is that a connection is taken after a transaction is opened and returned back after the transaction is completed. Is there something that I miss or maybe I have some wrong configuration?

P.S. I'm monitoring the active hikari connections through the spring boot actuator prometheus data. And to be able to reproduce the behavior I explained above I'm suspending the connection thread with several debug breakpoints.

Solution

I found out what causes this behaviour- it's Spring functionality to maintain the hibernate session in view in order to be able to retrieve lazy loaded data in the view. In order to disable this you need the following property:

spring.jpa.open-in-view=false

Here's another SO post where is explained what it does:

What is this spring.jpa.open-in-view=true property in Spring Boot?

Answered By - Hristo Angelov