Rest API design with multiple unique ids

Currently, we are developing an API for our system and there are some resources that may have different kinds of identifiers. For example, there is a resource called orders, which may have an unique order number and also have an unique id. At the moment, we only have URLs for the id, which are these URLs:

GET /api/orders/{id}
PUT /api/orders/{id}
DELETE /api/orders/{id}

But now we need also the possibility to use order numbers, which normally would result into:

GET /api/orders/{orderNumber}
PUT /api/orders/{orderNumber}
DELETE /api/orders/{orderNumber}

Obviously that won't work, since id and orderNumber are both numbers.

I know that there are some similar questions, but they don't help me out, because the answers don't really fit or their approaches are not really restful or comprehensible (for us and for possible developers using the API). Additionally, the questions and answers are partially older than 7 years.

To name a few:

1. Using a query param

One suggests to use a query param, e.g.

GET /api/orders/?orderNumber={orderNumber}

I think, there are a lot of problems. First, this is a filter on the orders collections, so that the result should be a list as well. However, there is only one order for the unique order number which is a little bit confusing. Secondly, we use such a filter to search/filter for a subset of orders. Additionally, a query params is some kind of a second-class parameter, but should be first-class in this case. This is even a problem, if I the object does not exist. Normally a get would return a 404 (not found), but a GET /api/orders/?orderNumber=1234 would be an empty array, if the order 1234 does not exist.

2. Using a prefix

Some public APIs use some kind of a discriminator to distinguish between different types, e.g. like:

GET /api/orders/id_1234
GET /api/orders/ordernumber_367652

This works for their approach, because id_1234 and ordernumber_367652 are their real unique identifiers that are also returned by other resources. However, that would result in a response object like this:

{
  "id": "id_1234",
  "ordernumber": "ordernumber_367652"
  //...
}

This is not very clean, because the type (id or order number) is modelled twice. And apart from the problem of changing all identifiers and response objects, this would be confusing, if you e.g. want to search for all order numbers greater than 67363 (thus, there is also a string/number clash). If the response does not add the type as a prefix, a user have to add this for some request, which would also be very confusing (sometime you have to add this and sometimes not...)

3. Using a verb

This is what e.g. Twitter does: their URL ends with show.json, so you can use it like:

GET /api/orders/show.json?id=1234 
GET /api/orders/show.json?number=367652

I think, this is the most awful solution, since it is not restful. Furthermore, it has some of the problems that I mentioned in the query param approach.

4. Using a subresource

Some people suggest to model this like a subresource, e.g.:

GET /api/orders/1234 
GET /api/orders/id/1234   //optional
GET /api/orders/ordernumber/367652

I like the readability of this approach, but I think the meaning of /api/orders/ordernumber/367652 would be "get (just) the order number 367652" and not the order. Finally, this breaks some best practices like using plurals and only real resources.

So finally, my questions are: Did we missed something? And are there are other approaches, because I think that this is not an unusual problem?

to me, the most RESTful way of solving your problem is using the approach number 2 with a slight modification.

From a theoretical point of view, you just have valid identification code to identify your order. At this point of the design process, it isn't important whether your identification code is an id or an order number. It's something that uniquely identify your order and that's enough.

The fact that you have an ambiguity between ids and numbers format is an issue belonging to the implementation phase, not the design phase.

So for now, what we have is:

GET /api/orders/{some_identification_code}

and this is very RESTful.

Of course you still have the problem of solving your ambiguity, so we can proceed with the implementation phase. Unfortunately your order identification_code set is made of two distinct entities that share the format. It's trivial it can't work. But now the problem is in the definition of these entity formats.

My suggestion is very simple: ids will be integers, while numbers will be codes such as N1234567. This approach will make your resource representation acceptable:

{
  "id": "1234",
  "ordernumber": "N367652"
  //...
}

Additionally, it is common in many scenarios such as courier shipments.

Here is an alternate option that I came up with that I found slightly more palatable.

GET /api/orders/1234
GET /api/orders/1234?idType=id //optional
GET /api/orders/367652?idType=ordernumber

The reason being it keeps the pathing consistent with REST standards, and then in the service if they did pass idType=orderNumber (idType of id is the default) you can pick up on that.

Rest API design with multiple unique ids

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