apache/pulsar · error · IllegalArgumentException

Window length must be positive [${windowLengthCount}]

Error message

Window length must be positive [${windowLengthCount}]

What it means

Window-function config validation in WindowConfigUtils.validate: a count-based window length was supplied (windowLengthCount set alone) but its value is zero or negative, which cannot define a valid tumbling window; the windowLengthCount value is the faulty input.

Source

Thrown at pulsar-functions/utils/src/main/java/org/apache/pulsar/functions/utils/WindowConfigUtils.java:40

public class WindowConfigUtils {

    public static final long DEFAULT_MAX_LAG_MS = 0; // no lag
    public static final long DEFAULT_WATERMARK_EVENT_INTERVAL_MS = 1000; // 1s

    public static void validate(WindowConfig windowConfig) {
        if (windowConfig.getWindowLengthDurationMs() == null && windowConfig.getWindowLengthCount() == null) {
            throw new IllegalArgumentException("Window length is not specified");
        }

        if (windowConfig.getWindowLengthDurationMs() != null && windowConfig.getWindowLengthCount() != null) {
            throw new IllegalArgumentException(
                    "Window length for time and count are set! Please set one or the other.");
        }

        if (windowConfig.getWindowLengthCount() != null) {
            if (windowConfig.getWindowLengthCount() <= 0) {
                throw new IllegalArgumentException(
                        "Window length must be positive [" + windowConfig.getWindowLengthCount() + "]");
            }
        }

        if (windowConfig.getWindowLengthDurationMs() != null) {
            if (windowConfig.getWindowLengthDurationMs() <= 0) {
                throw new IllegalArgumentException(
                        "Window length must be positive [" + windowConfig.getWindowLengthDurationMs() + "]");
            }
        }

        if (windowConfig.getSlidingIntervalCount() != null) {
            if (windowConfig.getSlidingIntervalCount() <= 0) {
                throw new IllegalArgumentException(
                        "Sliding interval must be positive [" + windowConfig.getSlidingIntervalCount() + "]");
            }
        }

View on GitHub (pinned to 820761864e)

Solutions

Set windowLengthCount to a positive integer

Example fix

// before
config.setWindowLengthCount(0);
// after
config.setWindowLengthCount(100);

Defensive patterns

Strategy: validation

Validate before calling

static void requirePositiveWindowLengthCount(WindowConfig wc) {
    if (wc.getWindowLengthCount() != null && wc.getWindowLengthCount() <= 0) {
        throw new IllegalArgumentException("windowLengthCount must be >= 1, got " + wc.getWindowLengthCount());
    }
}

Type guard

static boolean isValidWindowLengthCount(Integer count) {
    return count != null && count > 0;
}

Try / catch

try {
    WindowConfigUtils.validate(windowConfig);
} catch (IllegalArgumentException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Window length must be positive")) {
        throw new IllegalStateException("Fix window length settings: count must be a positive integer", e);
    }
    throw e;
}

Prevention

Clamp computed counts with Math.max(1, count) before setting them.
Reject zero/empty numeric strings when parsing window config from properties.
Never leave 0 as a placeholder value in window configs.

When it happens

Trigger: Calling WindowConfigUtils.validate(windowConfig) where getWindowLengthCount() != null and getWindowLengthCount() <= 0 — e.g. setWindowLengthCount(0), a negative value from a computed expression, or a zero default from a template.

Common situations: Building the count from an unvalidated property (parseInt of empty/0 string); math that computes the count as floor of something that can be 0; placeholder values left in config intending to be replaced later.

Understand the failure class

Background: "Invalid configuration value" and "Unsupported/Unknown setting value" errors: why libraries reject your config strings, numbers, and types — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apache/pulsar@820761864e (2026-09-06). Data as JSON: /api/errors/13213c94ae151c5a. Report an issue: GitHub.