apache/seatunnel · error · IllegalArgumentException

Already an MDCFunction

Error message

Already an MDCFunction

What it means

MDCTracer.tracing(context, Function) rejects a delegate already wrapped as MDCFunction with IllegalArgumentException. This prevents nested MDC capture/restore around a single function application, which would restore stale state.

Source

Thrown at seatunnel-api/src/main/java/org/apache/seatunnel/api/tracing/MDCTracer.java:158

    public static <T> MDCConsumer<T> tracing(MDCContext context, Consumer<T> delegate) {
        if (delegate instanceof MDCConsumer) {
            throw new IllegalArgumentException("Already an MDCConsumer");
        }
        return new MDCConsumer<>(context, delegate);
    }

    public static <T, R> MDCFunction<T, R> tracing(Function<T, R> delegate) {
        return tracing(MDCContext.current(), delegate);
    }

    public static <T, R> MDCFunction<T, R> tracing(Long jobId, Function<T, R> delegate) {
        return tracing(MDCContext.of(jobId), delegate);
    }

    public static <T, R> MDCFunction<T, R> tracing(MDCContext context, Function<T, R> delegate) {
        if (delegate instanceof MDCFunction) {
            throw new IllegalArgumentException("Already an MDCFunction");
        }
        return new MDCFunction<>(context, delegate);
    }

    public static <T> MDCPredicate<T> tracing(Predicate<T> delegate) {
        return tracing(MDCContext.current(), delegate);
    }

    public static <T> MDCPredicate<T> tracing(Long jobId, Predicate<T> delegate) {
        return tracing(MDCContext.of(jobId), delegate);
    }

    public static <T> MDCPredicate<T> tracing(MDCContext context, Predicate<T> delegate) {
        if (delegate instanceof MDCPredicate) {
            throw new IllegalArgumentException("Already an MDCPredicate");
        }
        return new MDCPredicate<>(context, delegate);
    }

View on GitHub (pinned to cf67b549a7)

Solutions

1. Trace the Function once at the outermost composition point
2. Pass through already-traced instances: `if (f instanceof MDCFunction) return f;`
3. Unwrap the duplicated tracing call in one of the layers

Example fix

// before
MDCFunction<T,R> f = MDCTracer.tracing(jobId, MDCTracer.tracing(jobId, fn));
// after
MDCFunction<T,R> f = MDCTracer.tracing(jobId, fn);

MDCFunction<T,R> safeWrap(Long jobId, Function<T,R> f) {
    return f instanceof MDCFunction ? (MDCFunction<T,R>) f : MDCTracer.tracing(jobId, f);
}

boolean isWrapped(Function<?,?> f) {
    return f instanceof MDCFunction;
}

try {
    return MDCTracer.tracing(jobId, function);
} catch (IllegalArgumentException e) {
    return (MDCFunction<T,R>) function;
}

When it happens

Trigger: Calling tracing(jobId, function) or tracing(context, function) where function is already an MDCFunction from a previous tracing() call, e.g. mapping functions wrapped by both a transform utility and the caller.

Common situations: Transform chains where map() helpers and the engine both trace the same Function; caching traced functions and re-tracing.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Already an MDCRunnable
• Already an MDCCallable
• Already an MDCExecutor
• Already an MDCConsumer
• Already an MDCPredicate

AI-assisted analysis of apache/seatunnel@cf67b549a7 (2026-09-10). Data as JSON: /api/errors/89a181a7152ae364. Report an issue: GitHub.