Global Variables

Concept

Sometimes you need Data to persist between the call of a payload decoder. This is normally done by using global environment variables that store the data between function calls.

Access Database Fields

On Datacake this can be achieved by creating a dedicated field in the Database Section of your Datacake Device and by using this as the storage for this data.

function decoder() {
// get current value from field in Database
var numberOfCalls = measurements.NUMBER_OF_CALLS.value;
// do some processing
numberOfCalls = numberOfCalls + 1;
// return that field in order to write new value into database
return [
{ "field": "NUMBER_OF_CALLS", "value": numberOfCalls }
]
}

Time between changes

Storing data between payload decoder function calls can also be used to measure the time elapsed between a change of a value. This can be done by accessing not only the current value of a Database-Field but also the timestamp.

function decoder() {
// pseudo code here - removed the parts before
// get number of changes happened before from Database
var lastNumberOfChanges = measurements.NUMBER_OF_CHANGES.value;
// get state stored in Database
var lastState = measurements.RO1_STATUS.value;
// when new state is not equal state in database -> change happened
if (lastState != current_RO1_STATUS) {
// increase number of changes
lastNumberOfChanges += 1;
// store number of changes in database
decoded.push({ "field":"NUMBER_OF_CHANGES","value":lastNumberOfChanges });
// calculate the time between changes
var currentTimestamp = Math.floor(Date.now() / 1000);
var lastTimestamp = measurements.RO1_STATUS.timestamp;
var timeDelta = (currentTimestamp - lastTimestamp) / 60;
var timeBetweenChanges = Math.abs(Math.round(timeDelta));
// store new time between last changes in database
decoded.push({ "field":"TIME_BETWEEN_CHANGES","value":timeBetweenChanges });
}
​
return decoded;
}