AWS step function: how to pass InputPath to OutputPath unchanged in Fargate task - json

Question

I have an AWS steps function defined using this Serverless plugin with 3 steps (FirstStep -> Worker -> EndStep -> Done):
stepFunctions:
stateMachines:
MyStateMachine:
name: "MyStateMachine"
definition:
StartAt: FirstStep
States:
FirstStep:
Type: Task
Resource:
Fn::GetAtt: [ FirstStep, Arn ]
InputPath: $
OutputPath: $
Next: Worker
Worker:
Type: Task
Resource: arn:aws:states:::ecs:runTask.sync
InputPath: $
OutputPath: $
Parameters:
Cluster: "#{EcsCluster}"
TaskDefinition: "#{EcsTaskDefinition}"
LaunchType: FARGATE
Overrides:
ContainerOverrides:
- Name: container-worker
Environment:
- Name: ENV_VAR_1
'Value.$': $.ENV_VAR_1
- Name: ENV_VAR_2
'Value.$': $.ENV_VAR_2
Next: EndStep
EndStep:
Type: Task
Resource:
Fn::GetAtt: [ EndStep, Arn ]
InputPath: $
OutputPath: $
Next: Done
Done:
Type: Succeed
I would like to propagate the InputPath unchanged from Worker step (Fargate) to EndStep, but when I inspect step input of EndStep from AWS management console I see that data associated with Fargate task is passed instead:
{
"Attachments": [...],
"Attributes": [],
"AvailabilityZone": "...",
"ClusterArn": "...",
"Connectivity": "CONNECTED",
"ConnectivityAt": 1619602512349,
"Containers": [...],
"Cpu": "1024",
"CreatedAt": 1619602508374,
"DesiredStatus": "STOPPED",
"ExecutionStoppedAt": 1619602543623,
"Group": "...",
"InferenceAccelerators": [],
"LastStatus": "STOPPED",
"LaunchType": "FARGATE",
"Memory": "3072",
"Overrides": {
"ContainerOverrides": [
{
"Command": [],
"Environment": [
{
"Name": "ENV_VAR_1",
"Value": "..."
},
{
"Name": "ENV_VAR_2",
"Value": "..."
}
],
"EnvironmentFiles": [],
"Name": "container-worker",
"ResourceRequirements": []
}
],
"InferenceAcceleratorOverrides": []
},
"PlatformVersion": "1.4.0",
"PullStartedAt": 1619602522806,
"PullStoppedAt": 1619602527294,
"StartedAt": 1619602527802,
"StartedBy": "AWS Step Functions",
"StopCode": "EssentialContainerExited",
"StoppedAt": 1619602567040,
"StoppedReason": "Essential container in task exited",
"StoppingAt": 1619602553655,
"Tags": [],
"TaskArn": "...",
"TaskDefinitionArn": "...",
"Version": 5
}
Basically, if the initial input is
{
"ENV_VAR_1": "env1",
"ENV_VAR_2": "env2",
"otherStuff": {
"k1": "v1",
"k2": "v2"
}
}
I want it to be passed as is to FirstStep, Worker and EndStep inputs without changes.
Is this possible?

Answer 1

Given that you invoke the step function with an object (let's call that A), then a task's...
...InputPath specifies what part of A is handed to your task
...ResultPath specifies where in A to put the result of the task
...OutputPath specifies what part of A to hand over to the next state
Source: https://docs.aws.amazon.com/step-functions/latest/dg/input-output-example.html
So you are currently overwriting all content in A with the result of your Worker state (implicitly). If you want to discard the result of your Worker state, you have to specify:
ResultPath: null
Source: https://docs.aws.amazon.com/step-functions/latest/dg/input-output-resultpath.html#input-output-resultpath-null