How to log MDC with Spring Sleuth?

Question

I have a Spring boot + sleuth based application. All works as expected. I have for now logs like this:

2017-05-04 17:55:52.226  INFO [alert,692d0eeca479e216,c3c8b680dc29ad02,false] 17292 --- [cTaskExecutor-1] c.k.a.b.s.alert.impl.AlertServiceImpl    : Alert state to process: xxx

Now, I want to add custom MDC to my log like the contract reference for example. I want to have logs like this:

2017-05-04 17:55:52.226  INFO [alert,692d0eeca479e216,c3c8b680dc29ad02,false] [CONTRACT_REF] 17292 --- [cTaskExecutor-1] c.k.a.b.s.alert.impl.AlertServiceImpl    : Alert state to process: xxx

I tried various things with no success:

1. Use the Spring Sleuth Tracer to add a tag;
2. Add logging.pattern.level=%5p %mdc to my application.properties file with MDC.put(xxx, xxx)

How can I add custom MDC/tags to my log?

Answer 1

You have to create your own implementation of a SpanLogger. The easiest way will be to extend te Slf4jSpanLogger and provide your own code to add / update and remove the entries from MDC context. Then you can change your logging pattern and that way your logs will contain what they need.

Answer 2

I was able to add data to the MDC fairly easily by doing MDC.put("yourCoolKey", "your cool value") (see MDC.put JavaDoc).

Once you put the value into the MDC, you can use the sequence %X{yourCoolKey} in your logging pattern (in my case, the value of logging.pattern.console) to print the string "your cool value" as part of each log statement.

Optionally, you can specify a default value in the pattern string by adding :-<defaultValue> after the key, such as %X{yourCoolKey:-N/A}, which will print the string "N/A" whenever the MDC does not have an entry for "yourCoolKey". The default, if not specified, is a blank string ("")