Automatic context propagation with MDC using Project Reactor

build.gradle

// ...

dependencies {
    implementation 'io.micrometer:context-propagation:1.0.6'
    // ...
}

logback.xml

<configuration>

    <appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>
                %d{HH:mm:ss.SSS} [%thread] [key=%X{key}] %-5level %logger{36} - %msg%n
            </pattern>
        </encoder>
    </appender>
    <logger name="reactor" level="info"/>

    <root level="info">
        <appender-ref ref="stdout"/>
    </root>

</configuration>

TestMdc.java

    @Test
    void testMDC() {
        Logger log = LoggerFactory.getLogger("test");

        Hooks.enableAutomaticContextPropagation();

        // To deal with the entire MDC (if we ensured no third-party code modifies it):
        // ContextRegistry.getInstance().registerThreadLocalAccessor(new MdcAccessor());

        // To deal with an individual key in the MDC:
        ContextRegistry.getInstance().registerThreadLocalAccessor(
            "key",
            () -> MDC.get("key"),
            value -> MDC.put("key", value),
            () -> MDC.remove("key"));

        MDC.put("key", "Hello");

        Mono.just("Delayed")
            .delayElement(Duration.ofMillis(10))
            .doOnNext(log::info)
            // It is vital to capture MDC contents into Reactor's context
            // as it is the source of truth for propagating ThreadLocals
            //.contextCapture() // Can be skipped since reactor-core:3.5.7
            .block();
    }

    // This implementation modifies the entire MDC.
    // If any other code (especially third-party libraries) interacts with the MDC
    // it would collide and the results will be corrupted.
    // The best strategy is to pick specific keys instead of working on the entire MDC map.
    static class MdcAccessor implements ThreadLocalAccessor<Map<String, String>> {

        static final String KEY = "mdc";

        @Override
        public Object key() {
            return KEY;
        }

        @Override
        public Map<String, String> getValue() {
            return MDC.getCopyOfContextMap();
        }

        @Override
        public void setValue(Map<String, String> value) {
            MDC.setContextMap(value);
        }

        @Override
        public void reset() {
            MDC.clear();
        }
    }

I'll do! 👍 Btw. I've started wondering if Hooks.enableAutomaticContextPropagation(); may have impact on reactive transaction propagation...

be more specific please :)

@leakin185
https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Hooks.html#enableAutomaticContextPropagation--

Please make sure you're using the latest version :) reactor-core is now 3.5.10, and this feature was made available since 3.5.3. If it's missing, probably you're using an older version of the library.

I updated the gist to reflect the best practices.