LOGBACK-1337 LogbackMDCAdapter should store mdc in LinkedHashMap to predict order of MDC in %X

[motlin]

No description provided.

[ceki]

Hi Craig,
So the idea is to iterate over the keys in the order of insertion. Correct?

[motlin]

That's right! I noticed that MDC iteration order is pseudorandom when working with logstash/logstash-logback-encoder. This surprised me a bit so I searched to see if anyone had written about it and I found LOGBACK-1337.

[ceki]

@motlin Thank you for this PR.

Operations such as get() and put() for LinkedHashMap are known to be slower than that for HashMap. Given that MDC is a performance critical component, the change you propose needs more investigation.

However, the need for ordered iteration is duly noted.

[motlin]

I was hoping to revisit this and see if it could be included.

As an example of why it's desirable, I include MDC in my console output...

    <appender name="Console" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%highlight(%-5level) %cyan(%date{HH:mm:ss.SSS, ${LOGGING_TIMEZONE}}) %gray(\(%file:%line\)) [%white(%thread)] %blue(%marker) {%magenta(%mdc)} %green(%logger): %message%n%rootException</pattern>
        </encoder>
    </appender>

And I add context to my MDC. It's not usually in one method, but spread across a few methods in the call stack. Usually the context has some sort of order. The pieces of context added later are more specific than the pieces added earlier.

MDC.put("first", "1");
MDC.put("second", "2");
MDC.put("third", "3");
LOGGER.info("Hello, World!");

With the current implementation, MDC gets printed to the console in pseudo-random order.

INFO  19:02:29.201 (Example.java:76) [main]  {third=3, first=1, second=2} com.example.Example: Hello, World!

[ceki]

Would output in alphabetical order suffice?

[motlin]

I think in some cases I could construct MDC key names with numbers inside to make sure they are sortable later.

In other cases I think the keys will be unpredictable in advance.

I suspect the way I use MDC is pretty common. I attach context about a service request, then about data reads/writes, then about the response. Some of the keys only last for a single log statement, others last for the duration of request.