What causes "Can't find metrics, {}" in apache/skywalking?

Check the supported function list in the OAL docs (docs/en/concepts-and-designs/oal.md) and correct the function name in the .oal file

apache/skywalking · error · IllegalArgumentException

Can't find metrics, {}

Error message

Can't find metrics, {}

What it means

Thrown by MetricsHolder.find(functionName) during OAL script analysis when the aggregation function used in an OAL rule is not registered. MetricsHolder lazily scans and registers all classes implementing the Metrics aggregator interface (discovered via SPI/service files) into a REGISTER map keyed by function name. A null lookup means no registered aggregator answers to that function name.

Source

Thrown at oap-server/oal-rt/src/main/java/org/apache/skywalking/oal/v2/metadata/MetricsHolder.java:60

                MetricsFunction metricsFunction = aClass.getAnnotation(MetricsFunction.class);
                REGISTER.put(
                    metricsFunction.functionName(),
                    (Class<? extends Metrics>) aClass
                );
            }
        }
    }

    @SneakyThrows
    public static Class<? extends Metrics> find(String functionName) {
        if (!INITIALIZED) {
            init();
            INITIALIZED = true;
        }

        Class<? extends Metrics> metricsClass = REGISTER.get(functionName);
        if (metricsClass == null) {
            throw new IllegalArgumentException("Can't find metrics, " + functionName);
        }
        return metricsClass;
    }
}

View on GitHub (pinned to 102af09b4a)

Solutions

Check the supported function list in the OAL docs (docs/en/concepts-and-designs/oal.md) and correct the function name in the .oal file
If you intended a custom function, implement the Metrics class and register it via META-INF/services so MetricsHolder.init() picks it up, then rebuild
Verify the exact spelling/case of the registered key by inspecting the registered Metrics implementations' name() methods

Example fix

// before (OAL)
service_resp_time = from ServiceSpan.sum(duration) --> no; wrong usage
endpoint_avg = from Endpoint.latency.longAvg() --> endpointAvg
// after: use a registered function name
endpoint_avg = from Endpoint.latency.avg() --> endpointAvg

Defensive patterns

Strategy: validation

Validate before calling

// Lint OAL function names against the registry before deploy:
Set<String> valid = Arrays.stream(metricsImplementations).map(m -> m.name()).collect(toSet());
if (!valid.contains(oalFunctionName)) fail("Unknown OAL function: " + oalFunctionName);

Try / catch

Catch IllegalArgumentException around OALScriptParserV2 / MetricsHolder usage in tooling (CLI lint, dsl-debugging install) and surface the message; in OAP startup, let it fail fast — an OAL rule with an unknown function cannot work.

Prevention

Validate new OAL rules in a staging OAP or via the dsl-debugging API before applying them through dynamic configuration
For custom aggregators, add an SPI registration test that fails the build if the function name is not resolvable via MetricsHolder.find()

When it happens

Trigger: An OAL script uses an aggregation function that does not exist, e.g. 'percentile90(...)' or 'median(...)' when only registered names like 'longAvg', 'avg', 'sum', 'percentile' are available; or a custom Metrics implementation was written but its service-file entry in META-INF/services is missing; or the function name case does not match the registered key.

Common situations: Writing new OAL rules with a guessed function name; upgrading SkyWalking where function names changed; adding a custom aggregator without SPI registration; typo in the OAL rule (e.g. 'longAVG' vs 'longAvg').

Related errors

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/757d98da14c67c00. Report an issue: GitHub.