How can I specify an optional value in a tasks result selector?

Question

I would like to create a dynamodb step using the resource arn:aws:states:::dynamodb:getItem where some attributes may not be available.

The relevant portion of my task is as follows:

"Get Config": {
  "Type": "Task",
  "Resource": "arn:aws:states:::dynamodb:getItem",
  "Next": "DummyPass",
  "Parameters": {
    "Key": {
      "pk": { "S.$": "$.someKey)" },
      "sk": { "S": "A" }
    },
    "TableName": "test_table"
  },
  "ResultPath": "$.config",
  "ResultSelector": {
    "foo.$": "$.Item.foo.S",
    "bar.$": "$.Item.bar.S"
  },
}

Within this task I am attempting to select the DynamoDB typed annotated items and flatten into a JSON structure of just the keys and values.

There appears to not be any intrinsic functions available that could achieve this.

The workaround is to create a lambda, though it defeats the purpose of services now being supported by step functions.

Answer 1

I've resolved this by creating a lambda that exposes jmespath that takes a Data and Expression attribute:

const jmespath = require("jmespath");

// convenience that ensures correct sync vs async lambda definition.
const { wrappedApply } = require("./lambda-tools.js");

function jmespathHandler (options) {
  return jmespath.search(options.Data, options.Expression);
}

module.exports = {
  jmespathHandler: wrappedApply(jmespathHandler)
};

Given an input of the following:

{
  "x": {
    "foo": "FOO",
    "bar": "BAR",
  },
  "y": {
    "bat": "BAT"
  }
}

I can now call this function with a definition like:

{
  "Type": "Task"
  "Resource": "arn:aws:states:::lambda:invoke",
  "End": true,
  "ResultSelector": {
    "FormatOutput.$": "$.Payload"
  },
  "OutputPath": "$.FormatOutput",
  "Parameters": {
    "FunctionName": "arn:aws:lambda:REGION:ACCOUNT_ID:function:FUNCTION_NAME",
    "Payload": {
      "Data.$": "States.JsonMerge($.x, $.y, false)",
      "Expression": "{Foo: foo, Bar: bar, Bat: bat, Baz: baz}"
    }
  },
}

Which will now output the following json.

{
  "Foo": "FOO",
  "Bar": "BAR",
  "Bat": "BAT",
  "Baz": null
}

You can see here that I've renamed the keys but also included a missing key Baz that because selected will become null.

Note that I've demonstrated using States.JsonMerge which could have been achieved within jmespath directly, however I just used this to demonstrate another tool you may find useful.

For completeness I'm including wrappedApply here:

function wrappedApply (action) {
  if (action.constructor.name === "AsyncFunction") {
    return action;
  }
  /* eslint consistent-return: "off" */
  return function (data, ctx, callback) {
    if (typeof callback !== "function") {
      return action(data, ctx, callback);
    }
    try {
      const res = action(data, ctx, callback);
      callback(null, res);
    } catch (e) {
      callback(e);
    }
  };
}

This can then be called to