When I set “output_encoding”: “json-collection”, the returned data is an empty array []

[Xuqy3]

Environment info:

• KrakenD version: devopsfaith/krakend:2.7.2
• System info: docker
• Backend technology: Java

Describe what are you trying to do:
When my backend api returns a data structure of

{
	"ret": 1,
	"data": [{
		"id": "f4376e0ab6454cbc8c865205bca7eff2"
	}]
}

I would like to return only the data for data to the user, i.e.

[{
	"id": "f4376e0ab6454cbc8c865205bca7eff2"
}]

When I set “target”: “data” and “output_encoding”: “json-collection”, krakend always returns an empty result with only “[]”, when I remove “target”: “data” it will When I remove the “target”: “data” and set "output_encoding": "json", it returns the full data, but I only want the “data” field.

I've compared to other api's and when the backend returns a data structure as

{
	"ret": 1,
	"data": {
		"total": 1,
		"id": "f87ce065898c40bb91b113bcb8748fd8"
	}
}

In this case “target”: “data” will extract the “data” correctly and return it to the user. The difference between them is that one of the structs of “data” is “{}”, while the other is a collection “[]”.

Your configuration file:

{
    "$schema": "https://www.krakend.io/schema/v3.json",
    "version": 3,
    "name": "KrakenD Gateway",
    "port": 8081,
    "timeout": "3000ms",
    "cache_ttl": "300s",
    "endpoints": [
        {
            "endpoint": "/tree",
            "method": "GET",
            "backend": [
                {
                    "url_pattern": "/tree",
                    "extra_config": {},
                    "encoding": "json",
                    "method": "GET",
                    "target": "data",
                    "host": [
                        "http://127.0.0.1:9998"
                    ]
                }
            ],
            "input_query_strings": [
                "*"
            ],
            "input_headers": [
                "*"
            ],
            "output_encoding": "json-collection"
        },
        {
            "endpoint": "/meetings",
            "output_encoding": "json",
            "method": "GET",
            "backend": [
                {
                    "url_pattern": "/meetings",
                    "extra_config": {},
                    "encoding": "json",
                    "sd": "static",
                    "method": "GET",
                    "target": "data",
                    "host": [
                        "http://127.0.0.1:9999"
                    ]
                }
            ],
            "input_query_strings": [
                "from",
                "to"
            ],
            "input_headers": [
                "*"
            ]
        }
    ]
}

Answers would be greatly appreciated.

[alombarte]

Hello @Xuqy3 ,

The json-collection encoding expects that the response from the backend is an object named collection that contains inside the array you want to return. Instead of doing a target, what you need is to rename the data property to collection and remove the rest of the noise. This is a working configuration you can try locally that does what you need:

{
    "version": 3,
    "$schema": "https://www.krakend.io/schema/krakend.json",
    "endpoints": [
        {
            "endpoint": "/collection",
            "output_encoding": "json-collection",
            "backend": [
                {
                    "url_pattern": "/data",
                    "host": [
                        "http://localhost:8080"
                    ],
                    "allow": [
                        "data"
                    ],
                    "mapping": {
                        "data": "collection"
                    }
                }
            ]
        },
        {
            "endpoint": "/data",
            "extra_config": {
                "proxy": {
                    "static": {
                        "strategy": "always",
                        "data": {
                            "ret": 1,
                            "data": [
                                {
                                    "id": "f4376e0ab6454cbc8c865205bca7eff2"
                                }
                            ]
                        }
                    }
                }
            },
            "backend": [
                {
                    "url_pattern": "/aaa"
                }
            ]
        }
    ]
}

[Xuqy3]

Thank you very much, I think my problem is solved!