What's the best practice for unmarshalling data returned from a dynamo operation in aws step functions? - Amazon web services

BeeTaylor
October 5, 2022
237 views
0 votes
2 Answers

I am running a state machine running a dynamodb query (called using CallAwsService). The format returned looks like this:

{
  Items: [
    { 
      "string" : { 
         "B": blob,
         "BOOL": boolean,
         "BS": [ blob ],
         "L": [ 
            "AttributeValue"
         ],
         "M": { 
            "string" : "AttributeValue"
         },
         "N": "string",
         "NS": [ "string" ],
         "NULL": boolean,
         "S": "string",
         "SS": [ "string" ]
      }
   }
  ]
}

I would like to unmarshall this data efficiently and would like to avoid using a lambda call for this

The CDK code we’re currently using for the query is below

interface FindItemsStepFunctionProps {
  table: Table
  id: string
}

export const FindItemsStepFunction = (scope: Construct, props: FindItemStepFunctionProps): StateMachine => {
  const { table, id } = props

  const definition = new CallAwsService(scope, 'Query', {
    service: 'dynamoDb',
    action: 'query',
    parameters: {
      TableName: table.tableName,
      IndexName: 'exampleIndexName',
      KeyConditionExpression: 'id = :id',
      ExpressionAttributeValues: {
        ':id': {
          'S.$': '$.path.id',
        },
      },
    },
    iamResources: ['*'],
  })

  return new StateMachine(scope, id, {
    logs: {
      destination: new LogGroup(scope, `${id}LogGroup`, {
        logGroupName: `${id}LogGroup`,
        removalPolicy: RemovalPolicy.DESTROY,
        retention: RetentionDays.ONE_WEEK,
      }),
      level: LogLevel.ALL,
    },
    definition,
    stateMachineType: StateMachineType.EXPRESS,
    stateMachineName: id,
    timeout: Duration.minutes(5),
  })
}

Answers

- LeeHannigan
- October 5, 2022 at 7:25 pm
- 0 votes
0
Can you unmarshall the data downstream? I’m not too well versed on StepFunctions, do you have the ability to import utilities?

Unmarshalling DDB JSON is as simple as calling the unmarshall function from DynamoDB utility:

https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/modules/_aws_sdk_util_dynamodb.html

You may need to do so downstream as StepFunctions seems to implement the low level client.

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- Guy
- October 6, 2022 at 1:19 am
- 0 votes
0
Step functions still don’t make it easy enough to call DynamoDB directly from a step in a state machine without using a Lambda function. The main missing parts are the handling of the different cases of finding zero, one or more records in a query, and the unmarshaling of the slightly complicated format of DynamoDB records. Sadly the $utils library is still not supported in step functions.

You will need to implement these two in specific steps in the graph.

Here is a diagram of the steps that we use as DynamoDB query template:

The first step is used to provide parameters to the query. This step can be omitted and define the parameters in the query step:
```
"Set Query Parameters": {
      "Type": "Pass",
      "Next": "DynamoDB Query ...",
      "Result": {
        "tableName": "<TABLE_NAME>",
        "key_value": "<QUERY_KEY>",
        "attribute_value": "<ATTRIBUTE_VALUE>"
      }
    }
```
The next step is the actual query to DynamoDB. You can also use GetItem instead of Query if you have the record keys.
```
"Type": "Task",
      "Parameters": {
        "TableName": "$.tableName",
        "IndexName": "<INDEX_NAME_IF_NEEDED>",
        "KeyConditionExpression": "#n1 = :v1",
        "FilterExpression": "#n2.#n3 = :v2",
        "ExpressionAttributeNames": {
          "#n1": "<KEY_NAME>",
          "#n2": "<ATTRIBUTE_NAME>",
          "#n3": "<NESTED_ATTRIBUTE_NAME>"
        },
        "ExpressionAttributeValues": {
          ":v1": {
            "S.$": "$.key_value"
          },
          ":v2": {
            "S.$": "$.attribute_value"
          }
        },
        "ScanIndexForward": false
      },
      "Resource": "arn:aws:states:::aws-sdk:dynamodb:query",
      "ResultPath": "$.ddb_record",
      "ResultSelector": {
        "result.$": "$.Items[0]"
      },
      "Next": "Check for DDB Object"
    }
```
The above example seems a bit complicated, using both ExpressionAttributeNames and ExpressionAttributeValues. However, it makes it possible to query on nested attributes such as item.id.

In this example, we only take the first item response with $.Items[0]. However, you can take all the results if you need more than one.

The next step is to check if the query returned a record or not.
```
"Check for DDB Object": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.ddb_record.result",
          "IsNull": false,
          "Comment": "Found Context Object",
          "Next": "Parse DDB Object"
        }
      ],
      "Default": "Do Nothing"
    }
```
And lastly, to answer your original question, we can parse the query result, in case that we have one:
```
"Parse DDB Object": {
      "Type": "Pass",
      "Parameters": {
        "string_object.$": "$.ddb_record.result.string_object.S",
        "bool_object.$": "$.ddb_record.result.bool_object.Bool",
        "dict_object": {
          "nested_dict_object.$": "$.ddb_record.result.item.M.name.S",
        },
        "dict_object_full.$":   "States.StringToJson($.ddb_record.result.JSON_object.S)"
      },
      "ResultPath": "$.parsed_ddb_record",
      "End": true
    }
```
Please note that:
- Simple strings are easily converted by "string_object.$": "$.ddb_record.result.string_object.S"
- The same for numbers or booleans by "bool_object.$": "$.ddb_record.result.bool_object.Bool")
- Nested objects are parsing the map object ("item.name.$": "$.ddb_record.result.item.M.name.S", for example)
- Creation of a JSON object can be achieved by using States.StringToJson
- The parsed object is added as a new entry on the flow using "ResultPath": "$.parsed_ddb_record"
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