Document updated on Feb 7, 2023

Array manipulation in KrakenD API Gateway

The flatmap middleware allows you to manipulate collections (or arrays, or lists; you name it) or to flatten objects from the response.

While the basic manipulation operations allow you to work directly with objects, the collections require you to use this flatmap component. The flatmap also will enable you to extract or move nested objects to have a customized object structure.

When working with lists, KrakenD needs to flatten and expand array structures to objects to operate with them and vice versa. This process is automatically done by the flatmap component, letting you concentrate only on the type of operation you want to execute.

When to manipulate arrays

You can manipulate collections at two different stages:

• When the response of a backend is received (inside its backend section)
• After merging all the backend responses (inside the endpoint section)

You can do simultaneous combinations to output the desired result. For instance, declare an endpoint with three backends that apply transformations independently and a final change within the endpoint after merging the three.

When to manipulate objects with flatmap

The flatmap can be used on objects when the basic data manipulation options fall short. For instance, when you need to extract and rename nested objects to the root, append, and other more sophisticated operations.

Types of manipulations

There are different types of operations you can do:

• move: To move, rename, embed or extract items from one place to another (equivalent concepts to and allow)
• del: To delete specific items
• append: To append items from one list to the other

Flatmap configuration

Depending on the stage you want to do the manipulation, you will need an extra_config configuration inside your endpoint or backend section. For both cases, the namespace is proxy.

The component structure with three operations would be as follows:

{
    "extra_config": {
        "proxy": {
            "flatmap_filter": [
                {
                    "type": "move",
                    "args": ["target_in_collection", "destination_in_collection"]
                },
                {
                    "type": "del",
                    "args": ["target_in_collection"]
                },
                {
                    "type": "append",
                    "args": ["collection_to_append", "returned_collection"]
                }
            ]
        }
    }
}

Then the flatmap_filter is an array with the list of operations to execute sequentially (top-down). Each flatmap step takes the output of its previous execution, and every operation is defined with an object containing two properties:

Operations and arguments

The types of operations are defined as follows:

• Move: To move or rename a collection to another. It needs two arguments.
  • "type": "move"
  • "args": ["target_in_collection", "destination_in_collection"]
• Delete: To remove all matching patterns within a collection. It needs one or more arguments.
  • "type": "del"
  • "args": ["target_in_collection_to_delete", "another_collection_to_delete", "..."]
• Append: To append a collection after another and return only the latter. It needs two arguments.
  • "type": "append"
  • "args": ["collection_to_append", "returned_collection"]

The format of the arguments (args) to proceed with the operation is very simple. In short, object nesting is represented with dots, while the index of an array is represented with a number. Or all matching items with wildcards. So:

• The dot operator . indicates a new array nesting level
• The wildcard * matches any key (property name, collection key name, or index)
• A number identifies the Nth-1 member of a collection, being 0 its first item.

Operations always apply to ** the last item** in the arguments. So, for instance, the deletion of a.b.c deletes c but leaves a.b in the response.

Notation by example

We will use an elementary JSON structure as an example of data representation. See below:

{
    "a": [
        {
            "b1": [
                {
                    "c": 1,
                    "d": "foo"
                },
                {
                    "c": 2,
                    "d": "bar"
                }
            ],
            "b2": true
        },
        {
            "b1": [
                {
                    "c": 3,
                    "d": "vaz"
                }
            ]
        }
    ]
}

Observations

Notice from this example that…

• a and b1 contain arrays ([...]) with objects inside.
• b2, c and d are not arrays
• Since a is an array ("a": []), we need to use the flatmap component. If it were an object ("a": {}), we would use deny or allow

Representing some values

Now that we are familiar with the structure let’s represent some values:

Practical examples regarding operations

Some individual operations on the example structure above:

Configuration examples

The following examples demonstrate how to modify a collection or objects using flatmap.

Example: Extract objects to another level

We have a backend that provides the following response as Input for flatmap:

Input:

{
    "shipping_id": "f15f8c62-8c63-46de-a7f6-a08f131848c5",
    "zone": {
        "state": "NY",
        "zip": "10001"
    }
}

And we want to extract fields from zone, rename them, and place them in the root, like this:

Output:

{
    "shipping_id": "f15f8c62-8c63-46de-a7f6-a08f131848c5",
    "shipping_state": "NY",
    "shipping_zip": "10001"
}

Configuration:

{
  "backend": [{
      "url_pattern": "/shipping",
      "extra_config": {
          "proxy": {
              "flatmap_filter": [
                  { "type": "move", "args": ["zone.state","shipping_state"] },
                  { "type": "move", "args": ["zone.zip","shipping_zip"] },
                  { "type": "del","args": ["zone"] }
              ]
          }
      }
  }]
}

As you can see, we did three operations:

1. Move the state to the root with a new name
2. Move the zip to the root with a new name
3. Delete zone as it became a null object after emptying it.

Example: Moving around data in arrays

In this example we have a couple of arrays that we want to manipulate.

Input:

{
    "kindergarten": [
        { "name": "TEST Kinder" },
        { "name": "Lil' Elephants" },
        { "name": "Bright Rainbows" }
    ],
    "schools": [
        { "title": "Brookside Elementary" },
        { "title": "Oak Tree School" }
    ]
}

And we want an output where both arrays are merged, using consistent naming. And we also want to get rid of the first TEST element. As follows:

Output:

{
    "schools":[
        {"name":"Lil' Elephants"},
        {"name":"Bright Rainbows"},
        {"name":"Brookside Elementary"},
        {"name":"Oak Tree School"}
    ]
}

Then we need the following configuration:

{
    "endpoint": "/education-clean",
    "backend": [{
        "url_pattern": "/education",
        "extra_config": {
            "proxy": {
                "flatmap_filter": [
                    {
                        "type": "del",
                        "args": ["kindergarten.0"]
                    },
                    {
                        "type": "move",
                        "args": ["schools.*.title", "schools.*.name"]
                    },
                    {
                        "type": "append",
                        "args": ["kindergarten", "schools"]
                    }
                ]
            }
        }
    }]
}

What did we do here?

There is a sequence of three operations:

• Delete the first element (index 0) of kindergarten.
• Rename all title attributes of schools to name.
• Append all the kindergarten content to schools.

For more examples, see this test file.

Mixing flatmap with other manipulation operations

When the flatmap filter is enabled, the operations group and target keep their functionality, but allow, deny, and mapping are ignored.