Correct way to pass multiple values for same parameter name in GET request

Indeed, there is no defined standard. To support that information, have a look at wikipedia, in the Query String chapter. There is the following comment:

While there is no definitive standard, most web frameworks allow multiple values to be associated with a single field.[3][4]

Furthermore, when you take a look at the RFC 3986, in section 3.4 Query, there is no definition for parameters with multiple values.

Most applications use the first option you have shown: http://server/action?id=a&id=b. To support that information, take a look at this Stackoverflow link, and this MSDN link regarding ASP.NET applications, which use the same standard for parameters with multiple values.

However, since you are developing the APIs, I suggest you to do what is the easiest for you, since the caller of the API will not have much trouble creating the query string.

I am describing a simple method which worked very smoothly in Python (Django Framework).

1. While sending the request, send the request like this

http://server/action?id=a,b

2. Now in my backend, I split the value received with a split function which always creates a list.

id_filter = id.split(',')

Example: So if I send two values in the request,

http://server/action?id=a,b

then the filter on the data is

id_filter = ['a', 'b']

If I send only one value in the request,

http://server/action?id=a

then the filter outcome is

id_filter = ['a']

3. To actually filter the data, I simply use the 'in' function

queryset = queryset.filter(model_id__in=id_filter)

which roughly speaking performs the SQL equivalent of

WHERE model_id IN ('a', 'b')

with the first request and,

WHERE model_id IN ('a')

with the second request.

This would work with more than 2 parameter values in the request as well !

I would suggest looking at how browsers handle forms by default. For example take a look at the form element <select multiple> and how it handles multiple values from this example at w3schools.

<form action="/action_page.php">
<select name="cars" multiple>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
  <option value="audi">Audi</option>
</select>
<input type="submit">
</form>

For PHP use:

<select name="cars[]" multiple>

Live example from above at w3schools.com

From above if you click "saab, opel" and click submit, it will generate a result of cars=saab&cars=opel. Then depending on the back-end server, the parameter cars should come across as an array that you can further process.

Hope this helps anyone looking for a more 'standard' way of handling this issue.