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Tue, 14 April 2020 13:49

Eclipse User

Hello,

I'd like to receive JSON-LD encoded messages as I work with linked data (RDF) at the moment.

Unfortunately, I couldn't find any RDF-related encodings (e.g. turtle or JSON-LD).

So my first simple try was to use the JSON encoding built into Titan, but of course, due to the invalid token "@" that is used in JSON-LD for certain keywords, the decoding failed.

Is there any straightforward way to make Titan handle Turtle or JSON-LD encodings?

Thank you in advance.

Best regards
Sascha

Wed, 15 April 2020 06:16

Eclipse User

The JSON encoder of the TITAN can deal with the special "@..." names of the JSON-LD

Either you can use the generic JSON structure defined in:
https://github.com/eclipse/titan.ProtocolModules.JSON_v07_2006

Or you can specify the exact type definitions and use the "name as " attribute the define the attribute names with @

Example:

{
"@id" : "http://manu.sporny.org/about#manu"
}

type set r1 {
charstring id
} with {
variant (id) "name as '@id'"
}

Result:
{
id := "http://manu.sporny.org/about#manu"
}

Wed, 15 April 2020 07:31

Eclipse User

Thanks for your reply, Gábor.

Actually, that's what I tried. For that, I reused the example from here: https://www.eclipse.org/forums/index.php/t/1068704/

Here the excerpt of my custom type:

type set ConnectorSelfDescription {
  charstring connector_type optional,
  ... some more fields ...
 } with {
  variant (connector_type) "name as '@type'"
}

The result is the following:

index.php/fa/37834/0/

As you can see, the

"@type" : "ids:BaseConnector"

is the first field.

But the result is an error with the following error message:
index.php/fa/37835/0/

Is there anything else I need to consider?

Attachment: json-ld_result.PNG
(Size: 21.50KB, Downloaded 312 times)
Attachment: json-ld_result_error.PNG
(Size: 9.86KB, Downloaded 302 times)

[Updated on: Wed, 15 April 2020 07:33] by Moderator

Wed, 15 April 2020 08:29

Eclipse User

Hi Sascha,

indeed, the @ character is no accepted in JSON encoding variants; we will look into the possibility to relax this restriction.

As a quick and dirty hack I can propose you the following:

-when encoding, use a token different from @, but easy to detect
- post encoding , replace the token with @

-when receiving the JSON instance, replace @ with token and then decode

(see attached code)

The downside of this-besides being a hack- is that the token will become a restricted expression, i.e. cannot be used in other parts of the JSON; this is of little practical impact, if the token is carefully chosen ("atsupercalifragistic" in my example, to be replaced with "@" back and forth).
For replacement of substrings, functions from TCCConversion_Functions library can be used.

The other option mentioned by Gabor is also valid, I have attached an example as well. JSON will be decoded into a generic TTCN-3 structure that
unfortunately is quite ugly; nonetheless this requires the least initial investment as there is no need of any further TTCN-3 types.

As an example , this JSON-LD

{
      "@context": {
        "name": "http://xmlns.com/foaf/0.1/name",
        "knows": "http://xmlns.com/foaf/0.1/knows"
      },
      "@id": "http://me.markus-lanthaler.com/",
      "name": "Markus Lanthaler",
      "knows": [
        {
          "name": "Dave Longley"
        }
      ]
    }

is decoded as below:

 JSON_object := {
        {
            key := "@context",
            val := {
                JSON_object := {
                    {
                        key := "name",
                        val := {
                            JSON_string := "http://xmlns.com/foaf/0.1/name"
                        }
                    },
                    {
                        key := "knows",
                        val := {
                            JSON_string := "http://xmlns.com/foaf/0.1/knows"
                        }
                    }
                }
            }
        },
        {
            key := "@id",
            val := {
                JSON_string := "http://me.markus-lanthaler.com/"
            }
        },
        {
            key := "name",
            val := {
                JSON_string := "Markus Lanthaler"
            }
        },
        {
            key := "knows",
            val := {
                JSON_array := {
                    {
                        JSON_object := {
                            {
                                key := "name",
                                val := {
                                    JSON_string := "Dave Longley"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

Best regards
Elemer

Attachment: JSON_test-200415-1406.tgz
(Size: 11.48KB, Downloaded 105 times)

Wed, 15 April 2020 08:36

Eclipse User

Maybe something is wrong with the type definition. The error message indicates that JSON_PDU doesn't have any field named as "@type"

Is it possible to share the whole type definition and the value to be decoded?

Try to decode the ConnectorSelfDescription directly.

Wed, 15 April 2020 08:53

Eclipse User

Hi Sascha, Gabor ,

please disregard my previous observation about the @ being restricted; it's not , and it should work as Gabor described, see new attachment. Hence no need to rename it.

I agree with Gabor, likely the structure does not tally.

BR

Elemer

Attachment: JSON_test-200415-1448.tgz
(Size: 11.02KB, Downloaded 103 times)

Thu, 16 April 2020 10:47

Eclipse User

Hi Elemer, hi Gábor,

thank you for your help. I managed to decode the JSON-LD.

In fact, the issue was a connection between using the variant extension wrongly and a false defined JSON structrure. But thanks to you I knew it should work with JSON-LD messages.

Best regards,
Sascha

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