Loop inside a Step Function - amazon-web-services

Question

I am trying to call a couple of steps in my step function in a loop but I am unable to get my head around how I need to do this. Here's what I have till now: I need to add another lambda function(GetReviews) which will then call CreateReview, SendNotification in a loop. How would I go about doing this?
I am referring to the "Iterating a Loop Using Lambda" document, which shows it is possible.
Step function Defination:
{
"Comment": "Scheduling Engine",
"StartAt": "CreateReview",
"States": {
"CreateReview": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-west-2:529627678433:function:CreateReview",
"Next": "CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateReviewResult",
"OutputPath": "$"
},
"CreateNotification": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-west-2:529627678433:function:CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateNotificationResult",
"OutputPath": "$",
"End": true
}
}
}

Answer 1

I am contributing to this answer because I am using a little different approach to be able to loop inside of a step-function without having to rely on a lambda to increment. If someone in the future needs a generic solution, this can be a good reference.
Here is the example with code:
{
"Comment": "A description of my state machine",
"StartAt": "InitVariables",
"States": {
"InitVariables": {
"Type": "Pass",
"Parameters": {
"index": 0,
"incrementor": 1,
"ArrayLength.$": "States.ArrayLength($.inputArray)"
},
"ResultPath": "$.iterator",
"Next": "LoopChoice"
},
"LoopChoice": {
"Type": "Choice",
"Choices": [
{
"Variable": "$.iterator.ArrayLength",
"NumericGreaterThanPath": "$.iterator.index",
"Next": "IncrementVariable"
}
],
"Default": "End"
},
"IncrementVariable": {
"Type": "Pass",
"Parameters": {
"index.$": "States.MathAdd($.iterator.index, $.iterator.incrementor)",
"incrementor": 1,
"ArrayLength.$": "$.iterator.ArrayLength"
},
"ResultPath": "$.iterator",
"Next": "LoopChoice"
},
"End": {
"Type": "Pass",
"End": true
}
} }
This is the base for the loop, I use the States.MathAdd($.iterator.index, $.iterator.incrementor) intrinsic function to add two values, in this case, increment the index with a increment amount defined in the initVariables state. And also get the length of the array that I want to loop. You get the array length by also using a intrinsic function, States.ArrayLength("$.path.to.array"). The array is passed in the input.
To get the value of the array we can use the intrinsic function, States.ArrayGetItem($.inputArray, $.iterator.index).
All the custom logic should be put between the loopChoice state and the IncrementVariable State.
Hope this helps someone in the future.

Answer 2

Sorry for the late reply. You've probably solved it in between, but here you are
So, when looping in Step Functions, I quite simply add a Choice State (see Choice State Rules).
One of your State would need to output wether or not you have finished looping, or the number of items iterated on and the total number of items.
In the first case, it would be something like
{
"Comment": "Scheduling Engine",
"StartAt": "CreateReview",
"States": {
"GetReviews": {
whatever
"Next": "LoopChoiceState"
},
"LoopChoiceState": {
"Type" : "Choice",
"Choices": [
{
"Variable": "$.loopCompleted",
"BooleanEquals": false,
"Next": "GetReviews"
}
],
"Default": "YourEndState"
},
"CreateReview": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-westz2:529627678433:function:CreateReview",
"Next": "CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateReviewResult",
"OutputPath": "$"
},
"CreateNotification": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-west-2:529627678433:function:CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateNotificationResult",
"OutputPath": "$",
"End": true
}
}
}
Second case:
{
"Comment": "Scheduling Engine",
"StartAt": "CreateReview",
"States": {
"GetReviews": {
whatever
"Next": "LoopChoiceState"
},
"LoopChoiceState": {
"Type" : "Choice",
"Choices": [
{
"Variable": "$.iteratedItemsCount",
"NumericEquals": "$.totalItemsCount",
"Next": "CreateNotification"
}
],
"Default": "CreateReview"
},
"CreateReview": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-west-2:529627678433:function:CreateReview",
"Next": "CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateReviewResult",
"OutputPath": "$"
},
"CreateNotification": {
"Type": "Task",
"Resource": "arn:aws:lambda:us-west-2:529627678433:function:CreateNotification",
"InputPath": "$",
"ResultPath": "$.CreateNotificationResult",
"OutputPath": "$",
"End": true
}
}
}
You could also use indexes (current index and last index) instead of the the number of items iterated over; it would help you keep track of where you in create reviews.

Answer 3

Three options are presented below. Here is a visual summary:
#1 Map: Repeat a set of steps for each element of an array (without a loop, optionally concurrently)
Map State is an alternative to looping when you want to run a set of steps for each element of an input array. Each element runs in parallel by default. Set MaxConcurrency: 1 to mimic the serial execution of #NunoGamaFreire's loop-based solution.
{
"StartAt": "MockArray",
"States": {
"MockArray": {
"Type": "Pass",
"Result": [ { "name": "Zaphod" }, { "name": "Arthur" }, { "name": "Trillian" } ],
"ResultPath": "$.Items",
"Next": "MapState"
},
"MapState": {
"Type": "Map",
"ResultPath": "$.MapResult",
"End": true,
"Iterator": {
"StartAt": "MockWork",
"States": {
"MockWork": {
"Type": "Pass",
"Parameters": {
"output.$": "States.Format('Hello, {}!', $.name)"
},
"OutputPath": "$.output",
"End": true
}
}
},
"ItemsPath": "$.Items"
}
}
}
One output is produced for each element in the MockArray, processed concurrently:
"MapResult": [ "Hello, Zaphod!", "Hello, Arthur!", "Hello, Trillian!" ]
#2 Repeat a set of steps X times (loop without lambda, serially)
This option involves proper looping! Repeat a set of tasks serially X number of times until a Choice State determines that an incrementing counter variable has reached X. Use the new States.MathAdd intrinsic function to increment without a Lambda. This option is suited for custom retry logic or other cases when you may want to break the loop early.
{
"StartAt": "InitializeCounter",
"States": {
"InitializeCounter": {
"Type": "Pass",
"Comment": "Initialize the counter at 0. Move all inputs to Payload.Input",
"Parameters": {
"Counter": 0
},
"Next": "IncrementCounter"
},
"IncrementCounter": {
"Type": "Pass",
"Comment": "Increment the Counter by 1",
"Parameters": {
"Counter.$": "States.MathAdd($.Counter, 1)"
},
"Next": "MockWork"
},
"MockWork": {
"Type": "Pass",
"Comment": "Simulate some work. Optionally break early from the loop with ExitNow: true",
"Result": false,
"ResultPath": "$.ExitNow",
"Next": "Loop?"
},
"Loop?": {
"Type": "Choice",
"Choices": [{ "Or": [
{ "Variable": "$.Counter", "NumericGreaterThanEqualsPath": "$$.Execution.Input.workCount" },
{ "Variable": "$.ExitNow", "BooleanEquals": true } ],
"Next": "Success"
}
],
"Default": "IncrementCounter"
},
"Success": {
"Type": "Succeed"
}
},
"TimeoutSeconds": 3
}
Counter iterates by one for each loop. The loop breaks if a task returns ExitNow: true.
{ "Counter": 4, "ExitNow": false }
#3 Repeat a set of steps X times (without a loop, *concurrently*)
This option is a hybrid of the first two. Like #2, we start with a desired number of iterations from the $.workCount input . Like #1, we map over an array concurrently. This time, though, the state machine creates the array with another intrinsic function, States.ArrayRange(1, $.workCount, 1).
{
"StartAt": "Iterations",
"States": {
"Iterations": {
"Type": "Pass",
"Parameters": {
"Iterations.$": "States.ArrayRange(1, $.workCount, 1)"
},
"Next": "MapState"
},
"MapState": {
"Type": "Map",
"ResultPath": "$.MapResult",
"End": true,
"Iterator": {
"StartAt": "MockWork",
"States": {
"MockWork": {
"Type": "Pass",
"Parameters": {
"output.$": "States.Format('Hello from iteration #{}', States.JsonToString($))"
},
"OutputPath": "$.output",
"End": true
}
}
},
"ItemsPath": "$.Iterations"
}
}
}
Tasks run concurrently, once for each item in Iterations.
"Iterations": [ 1, 2, 3, 4 ],
"MapResult": [ "Hello from iteration #1", "Hello from iteration #2", "Hello from iteration #3", "Hello from iteration #4" ]