In order to help output parsers, it is sometimes useful to prefix log output with the key of each item.

Example 1:

%X{user} 

would be easier to parse if it were written as

user=%X{user} 

Example 2:

%X{user} %{sessionId} %level %logger %msg

would be easier to parse if written as

user=%X{user} sessionId=%{sessionId} level=%level logger=%logger msg=%msg

The prefix composite converter does the prefixing for you. Thus, the following two patterns generate equivalent output

%prefix(%X{user} %{sessionId} %level %logger %msg)

and

user=%X{user} sessionId=%{sessionId} level=%level logger=$logger msg=%msg}