appender.0.type = File
appender.0.name = FILE
appender.0.fileName = app.log
appender.0.ignoreExceptions = false

appender.1.type = Console
appender.1.name = CONSOLE

appender.2.type = Failover
appender.2.name = FAILOVER
appender.2.primary = FILE
appender.2.fail.type = Failovers
appender.2.fail.0.type = AppenderRef
appender.2.fail.0.ref = CONSOLE

https://logging.apache.org/log4j/2.x/manual/appenders/delegating.html