Array manipulation - flatmap

Document updated on Jul 24, 2020

The flatmap middleware allows you to manipulate collections (or arrays, or lists, you name it). While the basic manipulation operations allow you to work directly with objects, the collections require a different approach: the flatmap component.

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 having merged all the backend responses (inside the endpoint section, starting at KrakenD 1.2)

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.

Types of manipulations

There are different types of operations you can do:

• Moving, embedding or extracting items from one place to another (equivalent concepts to mapping and allow)
• Deleting specific items (similar concept to deny)
• Appending 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 github.com/devopsfaith/krakend/proxy.

The component structure with three operations would be as follows:

    "extra_config": {
        "github.com/devopsfaith/krakend/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"]
                }
            ]
        }
    }

• flatmap_filter (list) The list of operations to execute sequentially (top down). Every operation is defined with an object containing two properties:
  • type (string) One of the recognized operation types
  • args (list) The arguments passed to the operation.

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. Needs one or more arguments.
  • "type": "del"
  • "args": ["target_in_collection_to_delete", "another_collection_to_delete", "..."]
• Append: To append a collection after another one, and return only the latter. Needs 2 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. For instance, a deletion of a.b.c deletes c but leaves a.b in the response.

Notation by example

We are going to 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 same values:

Practical examples regarding operations

Some individual operations on the example structure above:

Configuration example

The following example demonstrates how to modify a collection doing these operations:

    "extra_config": {
        "github.com/devopsfaith/krakend/proxy": {
            "flatmap_filter": [
                {
                    "type": "append",
                    "args": ["kindergarten", "schools"]
                },
                {
                    "type": "move",
                    "args": ["schools.42.students", "alumni"]
                },
                {
                    "type": "del",
                    "args": ["schools"]
                },
                {
                    "type": "del",
                    "args": ["alumni.*.password"]
                },
                {
                    "type": "move",
                    "args": ["alumni.*.PK_ID", "alumni.*.id"]
                }
            ]
        }
    }

What did we do here?

There is a sequence of 4 operations to:

• Extract all items inside kindergarten and append them to the students collection.
• Extract all students of the 43rd school (array starts at 0) and put them under a new property alumni
• Get rid of all the remaining schools
• Delete all items with a property password inside the array
• Rename all items with a property PK_ID to id

For more examples, see the 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.