Document updated on Feb 7, 2023
Array manipulation
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 andallow
)del
: To delete specific itemsappend
: To append items from one list to the other
mapping
, target
, deny
or allow
over flatmap whenever you work with objects as their computational cost is lower. Reserve the flatmap component for collections or operations that basic manipulation can’t do (such as flattening objects)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
.
endpoint
level unless there is more than one backend configured, as its purpose is to manipulate responses after the merge operation. Therefore, use it in the backend
if you only have one.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:
Fields of Flatmap 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. 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, being0
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
andb1
contain arrays ([...]
) with objects inside.b2
,c
andd
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:
Notation | Value |
---|---|
a | The content of a: [{"b1": [{"c": 1,"d": "foo"},{"c": 2,"d": "bar"}],"b2": true}, {"b1": [{"c": 3,"d": "vaz"}]}] |
a.1 | Second object of a key: {"b1": [ { "c": 3, "d": "vaz" } ]} (first objects starts at 0) |
a.0.b1.0.d | foo |
a.1.b1.0.d | vaz |
a.*.b1.*.d | 3 matches of d in this path: foo , bar , vaz |
a.*.*.*.d | 3 matches of d in this path: foo , bar , vaz |
Practical examples regarding operations
Some individual operations on the example structure above:
Target | Destination | Correct? | Comments |
---|---|---|---|
"a.*.b1.*.c" | "a.*.b1.*.d" | Yes | Rename c to d |
"a.*.b1.*.c" | "a.*.c" | No | Missing level |
"a.b1.c" | "c" | No | Missing array after a |
"a.0.b1.0.c" | "c" | Yes | Extract only c from the first and first items |
"a.*.b1.c" | "c" | No | Incorrect target, b1 has an array surrounding c |
"a.*.b1.c" | "a.*.b1.*.d.*.e" | No | Incorrect target, b1 has an array surrounding c |
"a.*.b1.*.c" | "a.*.b1.*.c.d.e.f.g" | Yes | Add additional levels |
"a.*.b1.*.c" | "a.*.x.*.c" | No | Incorrect, renaming to an element x that is not in the last position |
"a.*.b1.*.c" | "a.*.x.*.c.d.e.f.g" | No | Incorrect, renaming to an element x that is not in the last position |
"a.*.b1.*.c" | "a.*.b1.*.d.*.e" | No | Incorrect, destination path has more wildcards than source path |
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:
- Move the state to the root with a new name
- Move the zip to the root with a new name
- 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
) ofkindergarten
. - Rename all
title
attributes ofschools
toname
. - Append all the
kindergarten
content toschools
.
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.
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