ConnectionPool: pool is empty - increase either maxPoolSize or borrowConnectionTimeout

I was facing this issue for my springboot application that connects to a DB and MQ, and uses Atomikos Transaction manager.

com.atomikos.jms.AtomikosJMSException|Connection pool exhausted - try increasing 'maxPoolSize' and/or 'borrowConnectionTimeout' on the AtomikosConnectionFactoryBean.
com.atomikos.datasource.pool.PoolExhaustedException: ConnectionPool: pool is empty - increase either maxPoolSize or borrowConnectionTimeout
    at com.atomikos.datasource.pool.ConnectionPool.waitForAtLeastOneAvailableConnection(ConnectionPool.java:326)
    at com.atomikos.datasource.pool.ConnectionPool.findOrWaitForAnAvailableConnection(ConnectionPool.java:144)
    at com.atomikos.datasource.pool.ConnectionPool.borrowConnection(ConnectionPool.java:132)
    at com.atomikos.datasource.pool.ConnectionPoolWithSynchronizedValidation.borrowConnection(ConnectionPoolWithSynchronizedValidation.java:23)
    at com.atomikos.jms.AtomikosConnectionFactoryBean.createConnection(AtomikosConnectionFactoryBean.java:601)
    at org.springframework.jms.support.JmsAccessor.createConnection(JmsAccessor.java:196)
    at org.springframework.jms.listener.AbstractPollingMessageListenerContainer.access$100(AbstractPollingMessageListenerContainer.java:77)
    at org.springframework.jms.listener.AbstractPollingMessageListenerContainer$MessageListenerContainerResourceFactory.createConnection(AbstractPollingMessageListenerContainer.java:490)
    at org.springframework.jms.connection.ConnectionFactoryUtils.doGetTransactionalSession(ConnectionFactoryUtils.java:325)
    at org.springframework.jms.listener.AbstractPollingMessageListenerContainer.doReceiveAndExecute(AbstractPollingMessageListenerContainer.java:281)
    at org.springframework.jms.listener.AbstractPollingMessageListenerContainer.receiveAndExecute(AbstractPollingMessageListenerContainer.java:245)
    at org.springframework.jms.listener.DefaultMessageListenerContainer$AsyncMessageListenerInvoker.invokeListener(DefaultMessageListenerContainer.java:1189)
    at org.springframework.jms.listener.DefaultMessageListenerContainer$AsyncMessageListenerInvoker.executeOngoingLoop(DefaultMessageListenerContainer.java:1179)
    at org.springframework.jms.listener.DefaultMessageListenerContainer$AsyncMessageListenerInvoker.run(DefaultMessageListenerContainer.java:1076)
    at java.lang.Thread.run(Thread.java:748)

I tried printing the maxPoolSize and found that it is 1. This page came across in between (https://www.atomikos.com/Documentation/ConfiguringJms) and I found the line where they increased the MaxPoolSize to 5. I just tried setting it to 2 and it worked.

AtomikosConnectionFactoryBean xaConnectionFactory = new AtomikosConnectionFactoryBean();
        xaConnectionFactory.setXaConnectionFactory(ibmMQXAConnectionFactory);
        xaConnectionFactory.setMaxPoolSize(2);

Can someone help me to understand what should be the ideal poolsize. what it is for etc?

In order to process messages Atomikos uses DB and JMS connections (in your case). These connections are taken from the pools of available connections. To get the idea why connection pools are needed, please follow this link as a starting point - Connection_pool

To put it simple - in order to process one message at a time Atomikos needs one DB and one JMS connection/session. So if you plan to process 10 messages in parallel, each connection pool size must be at least 10 (10 for DB and 10 for JMS connection pools respectively).