As said, JSON is the format we will use as serialization format for answers given by the Open PHACTS LDA. The API actually supports XML, RDF, HTML, and TSV too, but I think JSON is a good balance between expressiveness and compactness. Moreover, and perhaps a much better argument, JSON works very well in a JavaScript environment: it is very easy to convert the serialization into a data model:

var jsonData = JSON.parse(jsonString);

Now, we previously covered maps. Maps have keys and values: the keys unlock a particular value. For example, take this JavaScript:

var map = { "key": "value", "key2": "value2" };

We define here a key-value object, and we can access the two values with the two keys:

map["key2"]; // == value2

These examples are JavaScript source code. Not a string. The content of the map variable is a data structure. But when we communicate with a web service, we need a (string) serialization of the data model, because we cannot send around memory pointers (which a variable is) because they are only valid on a single machine.

This is where the JSON format comes in. We can convert the content of the above map variable into a string representation with this code:

var mapStringified = JSON.stringify(map);

which gives us the following output:

{"key":"value","key2":"value2"}

This string looks an awful lot like the JavaScript code we wrote earlier.

And, likewise we can convert the JSON string back into a JavaScript data model again, with:

var mapAgain = JSON.parse(mapStringified);

Now, I did warn you earlier that values can be lists and maps itself again, so consider this JSON example from Wikipedia:

{
    "id": 1,
    "name": "Foo",
    "price": 123,
    "tags": [ "Bar", "Eek" ],
    "stock": {
        "warehouse": 300,
        "retail": 20
    }
}

Here we see that the value behind the stock key is another map, and the value behind the tags key is a list. This creates a quite flexible serialization format, which is happily used by Open PHACTS. (And for the semantic web readers, yes, we can make JSON more semantic. The Open PHACTS LDA supports a “rdfjson” format.)

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