To configure logback for the JobScheduler engine put a logback configuration into a directory of you choice and reference it by setting the java system property logback.configurationFile, e.g. -ea -Dlogback.configurationFile=etc/logback.xml.
A sample configuration:
<configuration>
<appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d\{dd HH:mm:ss.SSS\} [%thread] %-5level %logger - %msg%n</pattern>
</encoder>
</appender>
<root level="INFO">
<appender-ref ref="STDERR" />
</root>
<logger name="JobScheduler" level="TRACE"/>
</configuration>
The logger JobScheduler controls the output of the JobScheduler engine.
The setting of the property log_level in the factory.ini filters the messages that will be available for slf4j. That means, that the level you set in the factory.ini should never be higher than the level you have specified in the logger JobScheduler.