The mongo logs have a lot of varied information and, as a result, lack a consistent structure. This makes writing accurate and efficient parsers difficult because your parser has to understand how the line will be printed in each context. For instance, queries are logged very differently from updates:
Outside of the realm of op queries, the lines get even more diverse:
This type of data seems like a good candidate for JSON formatting. It would be helpful to have a config option for having all log lines printed in JSON. This would make parsing straightforward and consistent regardless of the presence or absence of various fields.