Dynamically set DEBUG or INFO logging level per request based on request header in reactive Spring Boot with Log4j2

I'm working on a Reactive Spring Boot application that uses Log4j2 as the logging framework. By default, my application logs at the INFO level, and I set the logging level in the application.yml

I would like to be able to log at the DEBUG level for only one specific request without changing the logging level globally or affecting other concurrent requests. For example, if I pass a custom header (e.g., X-Debug-Logging: true) for a request, I want that request to log at the DEBUG level, but the other parallel requests should continue logging at the default INFO level.

What I've Tried: I have implemented a WebFilter to check for the X-Debug-Logging header and adjust the logging level, but I'm concerned that since Reactive Spring handles requests asynchronously, changing the log level could affect other requests that are processed concurrently.

Question: How can I dynamically log at the DEBUG level for a single request in a Reactive Spring application, using Log4j2, while ensuring that other concurrent requests continue logging at the default INFO level?

Use cases:

If request A has X-Debug-Logging: true, it should log at DEBUG.
If request B doesn't have this header, it should log at INFO (default).
Requests should remain isolated, and the logging level change should only affect the specific request, not others.

Solution

What you are looking for is the DynamicThresholdFilter or ContextMapFilter, which allows you to filter messages based on the current context data.

When filters are used as global filters (i.e. direct children of <Configuration>):

a result of ACCEPT will accept the message unconditionally,
a result of NEUTRAL will also check the configured logger level.

(see Logger stage for details).

So all you need to do is:

At each request add a value to the Log4j API Thread Context (e.g. sampled) and propagate it to all the threads, were the request will be handled.
Configure a Log4j filter to return ACCEPT if sampled is true and NEUTRAL otherwise.

Setting and propagating the Thread Context

As you noticed, propagating the value is probably the hardest task. For that I suggest you to read Dariusz blog series on context propagation. In this particular case you need to:

Add io.micrometer:context-propagation to your application:

<dependency>
  <groupId>io.micrometer</groupId>
  <artifactId>context-propagation</artifactId>
</dependency>

Enable propagation in the main method of your application:
```
Hooks.enableAutomaticContextPropagation();
```
Add a ThreadLocalAccessor for the Log4j ThreadContext in your main method. There is no direct accessor, but the SLF4J MDC is bridged to ThreadContext:
```
ContextRegistry.getInstance().registerThreadLocalAccessor(new Slf4jThreadLocalAccessor());
```

Add a WebFilter that sets and removes the sampled key from the thread context map:

@Component
public class TracingFilter implements WebFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        boolean sampled =
                Boolean.parseBoolean(exchange.getRequest().getHeaders().getFirst("X-Debug-Logging"));
        return chain.filter(exchange).contextWrite(c -> {
            // Get a copy of the MDC map from the Reactor context
            Map<String, Object> mdc =
                    new HashMap<>(c.getOrDefault(Slf4jThreadLocalAccessor.KEY, Collections.emptyMap()));
            // Add or remove an entry
            mdc.put("sampled", sampled ? "true" : null);
            // Return a modified context
            return c.put(Slf4jThreadLocalAccessor.KEY, Collections.unmodifiableMap(mdc));
        });
    }
}

Configuring the Log4j filter

You can use DynamicThresholdFilter, which:

Checks the message against DEBUG if sampled is true,
Checks the message against defaultThreshold if sampled has any other non-null value,
Returns onMismatch otherwise.

<Configuration xmlns="https://logging.apache.org/xml/ns"
               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xsi:schemaLocation="
                   https://logging.apache.org/xml/ns
                   https://logging.apache.org/xml/ns/log4j-config-2.xsd">
  <DynamicThresholdFilter key="sampled"
                          onMatch="ACCEPT"
                          onMismatch="NEUTRAL">
    <KeyValuePair key="true" value="DEBUG"/>
  </DynamicThresholdFilter>
  <!-- Your `Appenders` and `Loggers` go here: -->
</Configuration>