an excerpt from the log of the Alfen Eve Double Station, connected via ocpp1.6j shows that the current TransactionId has been lost at the station for some reason. Instead of the valid Transaction Id, 9223372036854775807 is sent that is not according to the specification (Transaction Id can be omitted, if it is set, it must be valid).

2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:-> [2,"42179","MeterValues",{"connectorId":1,"transactionId":9223372036854775807
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:,"meterValue":[{"timestamp":"2023-04-15T02:25:11Z","sampledValue":[{"value":"120
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:701.000","context":"Sample.Periodic","measurand":"Energy.Active.Import.Register"
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:,"location":"Outlet","unit":"Wh"},{"value":"0.000","context":"Sample.Periodic","
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:measurand":"Current.Import","location":"Outlet","unit":"A","phase":"L1"},{"value
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:":"0.000","context":"Sample.Periodic","measurand":"Current.Import","location":"O
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:utlet","unit":"A","phase":"L2"},{"value":"5.561","context":"Sample.Periodic","me
2023-04-19T09:52:16.062Z:COM:ocpp_rpc.c:681:asurand":"Current.Import","location":"Outlet","unit":"A","phase":"L3"}]}]}]
2023-04-19T09:52:16.167Z:COM:ocpp_rpc.c:1127:<- [4,"42179","FormationViolation","json: cannot unmarshal number 92233720368547
2023-04-19T09:52:16.167Z:COM:ocpp_rpc.c:1127:76000 into Go struct field MeterValuesRequest.transactionId of type int",null]

As a consequence, a conversion of the large number (9223372036854775807 -> 9.223372036854776e+18 -> 9223372036854776000) leads to a rounding error and ultimately to a parsing error (json: cannot unmarshal number 9223372036854776000), since the value range for 64Bit Integer is exceeded.

This is done by the double unmarchalling

1. in ocppj/server.go::ocppMessageHandler - ParseRawJsonMessage(data))
   data ist correct, parsedJson contains transactionId:9.223372036854776e+18 (9223372036854776000)
2. ParseMessage(parsedJson, pending) -> FormationViolation - json: cannot unmarshal number 9223372036854776000 into Go struct field MeterValuesRequest.transactionId of type int

Certainly, Alfen has to make sure in a newer firmware (currently 6.2.0) that a valid TransactionId is not lost and as a consequence a non-valid TransactionId is not sent as MaxInt 64Bit but omitted, but basically MaxInt 64Bit (9223372036854775807) would be a valid TransactionId according to ocpp1.6j.

The following code works (if type INT is known):

type MeterValuesRequest struct {
	ConnectorId   int  `json:"connectorId" validate:"gte=0"`
	TransactionId *int `json:"transactionId,omitempty"`
}
var m MeterValuesRequest = MeterValuesRequest{}
err = json.Unmarshal(jsonMsg, &m)
if err != nil {
log.Fatal(err)
}
logrus.Infof("msg TransactionId: %d", *m.TransactionId)

The following ultimatively leads to the above-mentioned 'error'

var arr []interface{}
err := json.Unmarshal(dataJson, &arr)
if err != nil {
return nil, err
}

In principle, it would be desirable that valid values according to the spec do not lead to a parsing error. Furthermore, it is somehow unsightly that the error message with implementation details is ultimately sent back to the client (and ends up in the Alfen log).

Due to the error when sending the MeterValues, the StopTransaction is not sent in the Alfen firmware in the further course, so the station is basically unusable. This transaction with an invalid Id can no longer be cancelled from the outside (via OCPP).

regards