I'm struggling with something that seems like it should be simple, but the 'obvious' approach isn't working :-|. I have an AWS Step Function Lambda step. The input to that step (and function) looks like so:
{ "foo":"aaa" , "bar":"bbb" }
The Lambda's output looks like so:
{
// a bunch of Lambda execution details here
"Payload" : { "baz":"ccc" , "qux":"ddd" }
}
I'd like for the step's output to be:
{ "foo":"aaa" , "bar":"bbb" , "fred":"ccc" }
... i.e. I want to overlay the step's input with a new value at key "fred" referencing the Lambda output's value at "baz".
My first attempt (which still seems the most logical) is to 'extract' only "ccc" from the Lambda output and inject it into the step's output, like so:
"ResultSelector": "$.Payload.baz",
"ResultPath": "$.fred"
This puts the literal string "$.Payload.baz" into the output, not the dereferenced value.
It seems ResultSelector wants to build an object, so then I tried:
"ResultSelector": { "fred.$":"$.Payload.baz" },
"ResultPath": "$"
This replaces the entire step's input object, yielding just:
{ "fred":"ccc" }
And finally, the hybrid of those two:
"ResultSelector": { "fred.$":"$.Payload.baz" },
"ResultPath": "$.fred"
... yields:
{ "foo":"aaa" , "bar":"bbb", "fred":{"fred":"ccc"} }
Close (but no cigar)!
I'm a bit stumped for how to craft ResultSelector and ResultPath to achieve this simple overlay, and I feel like there's just some simple syntax trick that somehow I haven't found yet in the docs ... any ideas?
Note: As @Maurice points out, this solution works in cases where the task input is equal to (or is a subset of) the execution input. The execution input is exposed as $$.Execution.Input, which we can reference in ResultSelector to get the desired output.
To output the union of the task (execution) input and the task output, use JsonMerge in ResultSelector. That intrinsic function shallowly merges two objects. Reference the execution input from the context object and the output from the task payload. ResultSelector always expects an object, so we need a temporary key. I used merged, which I get rid of with OutputPath.
"ResultSelector": {
"merged.$": "States.JsonMerge($$.Execution.Input, $.Payload, false)",
},
"OutputPath": "$.merged",
🤔 Output:
{
"bar": "bbb",
"baz": "ccc",
"qux": "ddd",
"foo": "aaa"
}
That's close, but not exactly what you wanted. You want a *partial* union of the task input and output, dropping qux and renaming baz. To do that, we do the JsonMerge thing, but with a subset of the task output constructed with the Format and StringToJson intrinsic functions:
"ResultSelector": {
"merged.$": "States.JsonMerge($$.Execution.Input, States.StringToJson(States.Format('\\{\"fred\":\"{}\"\\}', $.Payload.baz)), false)"
},
"OutputPath": "$.merged",
✅ Output:
{
"bar": "bbb",
"foo": "aaa",
"fred": "ccc",
}