When you have full control over your logging format we recommend human readable key-value pairs whenever possible. JSON and XML are also acceptable, though XML is harder to read and parse than JSON. 

Whatever you use follow these rules:

For example, from our own production logs, we use the following:

2012-08-16 13:39:36,979 [metrics] INFO com.sumologic.util.Reporter - com.sumologic.util.scala.Aggregator.Buffer[/usr/sumo/stream-19.5-3/logs/stream.log]-Size-Meter (bytes/s) count=78847, fifteenMinuteRate=0.26, fiveMinuteRate=0.11, oneMinuteRate=0.00, meanRate=1.06

This is a key-value pair log, which is easy to parse and easy to read in Sumo.