If you start your database server and application server at the same time, Liferay DXP might try connecting to the data source before the database is ready. By default, Liferay DXP doesn’t retry connecting to the database; it just fails. But there is a way to avoid this situation: database connection retries.
portal-ext.propertiesfile in your Liferay Home folder.
Set the property
retry.jdbc.on.startup.max.retriesequal to the number of times to retry connecting to the data source.
retry.jdbc.on.startup.delayequal to the number of seconds to wait before retrying connection.
If at first the connection doesn’t succeed, Liferay DXP uses the retry settings to try again.