Context Object - AWS Step Functions

Context Object

The context object is an internal JSON structure that is available during an execution, and contains information about your state machine and execution. This allows your workflows access to information about their specific execution. You can access the context object from the following fields:

  • InputPath

  • OutputPath

  • ItemsPath (in Map states)

  • Variable (in Choice states)

  • ResultSelector

  • Variable to variable comparison operators

Context Object Format

The context object includes information about the state machine, state, execution, and task. This JSON object includes nodes for each type of data, and is in the following format.

{ "Execution": { "Id": "String", "Input": {}, "StartTime": "Format: ISO 8601" }, "State": { "EnteredTime": "Format: ISO 8601", "Name": "String", "RetryCount": Number }, "StateMachine": { "Id": "String" }, "Task": { "Token": "String" } }

During an execution, the context object is populated with relevant data for the Parameters field from where it is accessed. The value for a Task field is null if the Parameters field is outside of a task state.

Content from a running execution includes specifics in the following format.

{ "Execution": { "Id": "arn:aws:states:us-east-1:123456789012:execution:stateMachineName:executionName", "Input": { "key": "value" }, "Name": "executionName", "RoleArn": "arn:aws:iam::123456789012:role...", "StartTime": "2019-03-26T20:14:13.192Z" }, "State": { "EnteredTime": "2019-03-26T20:14:13.192Z", "Name": "Test", "RetryCount": 3 }, "StateMachine": { "Id": "arn:aws:states:us-east-1:123456789012:stateMachine:stateMachineName", "Name": "name" }, "Task": { "Token": "h7XRiCdLtd/83p1E0dMccoxlzFhglsdkzpK9mBVKZsp7d9yrT1W" } }
Note

For context object data related to Map states, see Context Object Data for Map States.

Accessing the Context Object

To access the context object, first specify the parameter name by appending .$ to the end, as you do when selecting state input with a path. Then, to access context object data instead of the input, prepend the path with $$.. This tells AWS Step Functions to use the path to select a node in the context object.

This example task state uses a path to retrieve and pass the execution Amazon Resource Name (ARN) to an Amazon Simple Queue Service (Amazon SQS) message.

{ "Order Flight Ticket Queue": { "Type": "Task", "Resource": "arn:aws:states:::sqs:sendMessage", "Parameters": { "QueueUrl": "https://sqs.us-east-1.amazonaws.com/123456789012/flight-purchase", "MessageBody": { "From": "YVR", "To": "SEA", "Execution.$": "$$.Execution.Id" } }, "Next": "NEXT_STATE" } }
Note

For more information about using the task token when calling an integrated service, see Wait for a Callback with the Task Token.

Context Object Data for Map States

There are two additional items available in the context object when processing a Map state: Index and Value. The Index contains the index number for the array item that is being processed in the current iteration. Within a Map state, the context object includes the following.

"Map": { "Item": { "Index": "Number", "Value": "String" } }

These are available only in a Map state, and can be specified in the Parameters field, before the Iterator section.

Note

You must define parameters from the context object in the Parameters block of the main Map state, not within the states included in the Iterator section.

Given a state machine with a simple Map state, we can inject information from the context object as follows.

{ "StartAt": "ExampleMapState", "States": { "ExampleMapState": { "Type": "Map", "Parameters": { "ContextIndex.$": "$$.Map.Item.Index", "ContextValue.$": "$$.Map.Item.Value" }, "Iterator": { "StartAt": "TestPass", "States": { "TestPass": { "Type": "Pass", "End": true } } }, "End": true } } }

If you execute the previous state machine with the following input, Index and Value are inserted in the output.

[ { "who": "bob" }, { "who": "meg" }, { "who": "joe" } ]

The output for the execution is the following.

[ { "ContextValue": { "who": "bob" }, "ContextIndex": 0 }, { "ContextValue": { "who": "meg" }, "ContextIndex": 1 }, { "ContextValue": { "who": "joe" }, "ContextIndex": 2 } ]