How to manage log file switching through custom user event in Spring?

Suppose we use Logback for logging.

It’s required to change the path to the log file every time a certain event (i.e. function call) occurs.

For example, somewhere we call a function.

startNewLogSegment("A")

After this event, the logger is expected to start writing to the logs/mySegment_A.log file. Then, again a call performed:

startNewLogSegment("B")

After this event, the logger is expected to finish writing to the previous file and start writing to the logs/mySegment_B.log file.

Let's assume that a state changed by startNewLogSegment should be visible in the whole application (all threads).

I tried to apply the approach with MDC:

logback.xml

...
<appender name="SIFTING_BY_ID" class="ch.qos.logback.classic.sift.SiftingAppender">
    <discriminator>
        <key>id</key>
        <defaultValue>initial</defaultValue>
    </discriminator>

    <sift>
        <appender name="FULL-${id}" class="ch.qos.logback.core.FileAppender">
            <file>logs/mySegment_${id}.log</file>
            <append>false</append>
            <encoder >
                <pattern>%d{dd-MM-yyyy HH:mm:ss.SSS} [%thread] [%-5level] %logger{36}.%M - %msg%n</pattern>
            </encoder>
        </appender>
    </sift>
</appender>
...

and calling MDC.put("id", "A") when a custom event appears.

But it works a different way than I need.

It’s known that the MDC manages contextual information on a per thread basis, so at least we need a control over threads creation to accomplish the goal described above.

I wonder if this approach could be used with Spring, and in particular with async operations performed by Spring Reactor. I’ve found no information about using a custom thread pool for internal Spring activities.

Possibly, I hope, there’s a more simple way to tune logging that way without abusing Spring internals.

Solution

I ended up with a cusom implementation of discriminator AbstractDiscriminator<ILoggingEvent> allowing uasge of globally visible values.

GVC.java

/**
 * Global values context.
 * Allows to sift log files globally independent from a thread calling log operation.
 * <p>
 * Used API analogous to standard {@link org.slf4j.MDC}.
 */
public final class GVC {

    private static Map<String, String> STORED = new HashMap<>();

    private GVC() {
    }

    public static synchronized void put(String key, String value) {
        STORED.put(key, value);
    }

    public static synchronized String get(String key) {
        return STORED.get(key);
    }
}

GVCBasedDiscriminator.java

/**
 * Customized analogue of MDCBasedDiscriminator.
 * <p>
 * GVCBasedDiscriminator essentially returns the value mapped to an GVC key.
 * If the value is null, then a default value is returned.
 * <p>
 * Both Key and the DefaultValue are user specified properties.
 */
public class GVCBasedDiscriminator extends AbstractDiscriminator<ILoggingEvent> {

    private String key;
    private String defaultValue;

    public String getDiscriminatingValue(ILoggingEvent event) {
        String value = GVC.get(key);

        if (value == null) {
            return defaultValue;
        } else {
            return value;
        }
    }

    @Override
    public String getKey() {
        return key;
    }

    @Override
    public void start() {
        int errors = 0;
        if (OptionHelper.isEmpty(key)) {
            errors++;
            addError("The \"Key\" property must be set");
        }
        if (OptionHelper.isEmpty(defaultValue)) {
            errors++;
            addError("The \"DefaultValue\" property must be set");
        }
        if (errors == 0) {
            started = true;
        }
    }

    /**
     * Key for this discriminator instance
     *
     * @param key
     */
    public void setKey(String key) {
        this.key = key;
    }

    /**
     * The default GVC value in case the GVC is not set for
     * {@link #setKey(String) mdcKey}.
     * <p/>
     * <p> For example, if {@link #setKey(String) Key} is set to the value
     * "someKey", and the MDC is not set for "someKey", then this appender will
     * use the default value, which you can set with the help of this method.
     *
     * @param defaultValue
     */
    public void setDefaultValue(String defaultValue) {
        this.defaultValue = defaultValue;
    }
}

logback.xml

<appender name="TRACES_PER_SESSION_FILE" class="ch.qos.logback.classic.sift.SiftingAppender">
  <!-- here the custom discriminator implementation is applied -->
  <discriminator class="internal.paxport.misc.logging.GVCBasedDiscriminator">             
    <key>id</key>
    <defaultValue>initial</defaultValue>
  </discriminator>

  <sift>
    <appender name="FULL-${id}" class="ch.qos.logback.core.FileAppender">
      <file>logs/mySegment_${id}.log</file>
      <append>false</append>
      <encoder>
        <pattern>%d{dd-MM-yyyy HH:mm:ss.SSS} [%thread] [%-5level] %logger{36}.%M - %msg%n</pattern>
      </encoder>
    </appender>
  </sift>
</appender>