Spring Boot Interview Question: HikariCP Connection Timeout

Connection pool, transaction boundary, timeouts and more...

Scenario

You have a Spring Boot application using HikariCP with the following configuration:

spring.datasource.hikari:
  maximum-pool-size: 10
  connection-timeout: 30000

The application receives low QPS (5–10 requests/sec), but you occasionally see this error in the logs:

HikariPool-1 - Connection is not available, request timed out after 30000ms.

What does this HikariCP error actually mean?

All connections in the pool are checked out and none became available within connectionTimeout(30s).

Common misconception: It’s not necessarily due to high traffic; it often means connections are held too long or never returned.

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Why it happens at low QPS

1. Connections are not being released (most common)

   Typical causes:

   • Missing transaction boundaries

     @Transactional
     public void methodA() {
         methodB(); // starts another transaction?
     }

   • Nested / mis-scoped transactions

   • Long-lived transactions

   • Transactions around non-DB logic

   
   Symptom

   • QPS is low

   • Active connections slowly climb

   • Pool never recovers

   Fix

   • Keep transactions short

   • Wrap only DB work

   • Avoid @Transactional on controllers