Using JSLTTransformJSON (an alternative to JoltTransformJSON)

As of Apache NiFi 1.19.0, there is a processor called JSLTTransformJSON. This is similar in function to the JoltTransformJSON processor, but it allows you to use JSLT as the Domain-Specific Language (DSL) to transform one JSON to another. This processor was added because some users found JOLT too complex to learn, as it uses a "tree-walking" pattern to change the JSON structure versus a more programmatic DSL (From the docs, JSLT is based on jq, XPath, and XQuery). For an example of using JSLT, check out the demo playground which has a ready-made example.

For this post, I'll show a couple of examples including this following one that shows the Inception example from the JOLT playground, just as a comparison between the JOLT and JSLT expressions:

Input:

{
  "rating": {
    "primary": {
      "value": 3
    },
    "quality": {
      "value": 3
    }
  }
}

JOLT Transform:

[
  {
    "operation": "shift",
    "spec": {
      "rating": {
        "primary": {
          "value": "Rating"
        },
        "*": {
          "value": "SecondaryRatings.&1.Value",
          "$": "SecondaryRatings.&1.Id"
        }
      }
    }
  },
  {
    "operation": "default",
    "spec": {
      "Range": 5,
      "SecondaryRatings": {
        "*": {
          "Range": 5
        }
      }
    }
  }
]

Equivalent JSLT Transform:

{
 "Rating" : .rating.primary.value,
 "SecondaryRatings" : 
  [for (.rating) {
    .key : { 
        "Id": .key,
        "Value" : .value.value,
        "Range": 5
     }
   }
   if (.key != "primary")
  ],
  "Range": 5
}

These both kind of "walk the structure", but in my opinion the JSLT is more succinct and more readable for at least these reasons:

• There is no need for a "default" spec like JOLT
• There are fewer levels of JSON objects in the specification
• You don't have to walk the structure one level at a time because you can refer to lower levels using the dot-notation  
• You don't have to "count the levels" to use the & notation to walk back up the tree

Learning JSLT may not be easier than JOLT but you may find it easier if you are familiar with the tools JSLT is based on. At the end of the day they are both designed to modify the structure of input JSON, and just approach the DSL in different ways. And NiFi offers the choice between them :)

As always I welcome all comments, suggestions, and questions. Cheers!