Function 'GetDynamodb' failed to execute for rule. Not able to retrieve and/or parse the requested item from dynamo.

Question

We have a rule where the condition is based on data from dynamodb. We wanted the filtering based on if given key had an item in the database. The query looks like:

```SELECT * FROM '$aws/events/presence/connected/+' WHERE NOT isNull(get_dynamodb(MY_TABLE, 'HashKey', MY_HASHKEY, 'RangeKey', MY_RANGEKEY,   MY_ROLE))```

When the data is present the rule works as expected, but when it is not we get these errors logged in CloudWatch:

```
details: Not able to retrieve and/or parse the requested item from dynamo. ErrorMessage:
eventType: FunctionExecution
logLevel: ERROR
```

```
details: Function 'GetDynamodb' failed to execute for rule MY_RULE. Not able to retrieve and/or parse the requested item from dynamo. ErrorMessage:
eventType: RuleExecution
logLevel: ERROR
reason: ExternFunctionException
```

So the questions are: 
1. Can i use the get_dynamodb function to check if a item exists in the database, or can I only use this function if I know that the data for a given key is present?
2. Should I expect my error action to trigger in the case described above (testing indicates that my error action is not triggered)?

Accepted Answer

Hi Magne.

1. You can use it when an item is not present, but it will cause the `FunctionExecution` error you're seeing. So it's probably not the behaviour you're wanting.
2. No. [The error action is only triggered if a rule action suffers an error](https://docs.aws.amazon.com/iot/latest/developerguide/rule-error-handling.html). Your rule query failed to produce a match (because the error occurred in the WHERE clause) and hence no action was triggered.

Function 'GetDynamodb' failed to execute for rule. Not able to retrieve and/or parse the requested item from dynamo.

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