{"record":{"id":"05ea1e5909f44c46","repo":"apache/beam","slug":"may-only-have-a-single-beam-eventtime-parameter-and-it-must","errorCode":null,"errorMessage":"may only have a single beam.EventTime parameter and it must precede the main input parameter","messagePattern":"may only have a single beam\\.EventTime parameter and it must precede the main input parameter","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"sdks/go/pkg/beam/core/funcx/fn.go","lineNumber":532,"sourceCode":"\t\tif paramState, err = nextParamState(paramState, p.Kind); err != nil {\n\t\t\treturn errors.WithContextf(err, \"validating parameter %d for %s\", i, u.Fn.Name())\n\t\t}\n\t}\n\t// Validate the return value ordering.\n\tretState := rsStart\n\tfor i, r := range u.Ret {\n\t\tif retState, err = nextRetState(retState, r.Kind); err != nil {\n\t\t\treturn errors.WithContextf(err, \"validating return value %d for %s\", i, u.Fn.Name())\n\t\t}\n\t}\n\treturn nil\n}\n\nvar (\n\terrContextParam                      = errors.New(\"may only have a single context.Context parameter and it must be the first parameter\")\n\terrPaneParamPrecedence               = errors.New(\"may only have a single PaneInfo parameter and it must precede the WindowParam, EventTime and main input parameter\")\n\terrWindowParamPrecedence             = errors.New(\"may only have a single Window parameter and it must precede the EventTime and main input parameter\")\n\terrEventTimeParamPrecedence          = errors.New(\"may only have a single beam.EventTime parameter and it must precede the main input parameter\")\n\terrWatermarkEstimatorParamPrecedence = errors.New(\"may only have a single sdf.WatermarkEstimator parameter and it must precede the main input parameter\")\n\terrReflectTypePrecedence             = errors.New(\"may only have a single reflect.Type parameter and it must precede the main input parameter\")\n\terrRTrackerPrecedence                = errors.New(\"may only have a single sdf.RTracker parameter and it must precede the main input parameter\")\n\terrBundleFinalizationPrecedence      = errors.New(\"may only have a single BundleFinalization parameter and it must precede the main input parameter\")\n\terrStateProviderPrecedence           = errors.New(\"may only have a single state.Provider parameter and it must precede the main input parameter\")\n\terrTimerProviderPrecedence           = errors.New(\"may only have a single timer.Provider parameter and it must precede the main input parameter\")\n\terrInputPrecedence                   = errors.New(\"inputs parameters must precede emit function parameters\")\n)\n\ntype paramState int\n\nconst (\n\tpsStart paramState = iota\n\tpsContext\n\tpsPane\n\tpsWindow\n\tpsEventTime\n\tpsWatermarkEstimator","sourceCodeStart":514,"sourceCodeEnd":550,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/core/funcx/fn.go#L514-L550","documentation":"funcx requires a single beam.EventTime (typex.EventTime) parameter and that it precede the main input. nextParamState returns errEventTimeParamPrecedence when an EventTime parameter appears after the main input or more than once.","triggerScenarios":"Signatures like func(int, typex.EventTime) — EventTime after the main input — or two EventTime params, registered through beam.ParDo/TestNew.","commonSituations":"Adding event-timestamp access to an existing DoFn by appending the parameter instead of inserting it before the main input.","solutions":["Move typex.EventTime so it precedes the main input parameter.","Remove duplicate EventTime parameters.","Verify only special (non-input) parameters precede the main input, in documented order."],"exampleFix":"// before\nfunc(v int, et typex.EventTime) { ... }\n// after\nfunc(et typex.EventTime, v int) { ... }","handlingStrategy":"validation","validationCode":"for i, p := range paramTypes(fn) {\n\tif p == eventTimeType && i >= mainInputIndex(paramTypes(fn)) {\n\t\treturn errors.New(\"EventTime must precede the main input\")\n\t}\n}","typeGuard":null,"tryCatchPattern":"if _, err := funcx.New(reflect.ValueOf(fn)); err != nil {\n\treturn fmt.Errorf(\"invalid DoFn signature: %w\", err)\n}","preventionTips":["Insert typex.EventTime before the main input parameter, not after.","Only one EventTime parameter per function.","Test each DoFn signature with funcx.New during development."],"tags":["go","apache-beam","function-signature","reflection"],"backgroundTag":"invalid-argument-value","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T16:17:12.679Z"}