"The only reason you'd have only a SOAP API is because you hate 80% of your addressable market." - @sramji
There's usually little argument that a REST API is easier to use than a SOAP API.
But how important is it to be 'truly' or 'strictly' RESTful? That is, adhering to standard HTTP operations or 'verbs' - GET, PUT, DELETE, POST - on well defined resources, as opposed to the common practice of embedding 'verbs' or operations as methods in a GET URL.
However, a truly RESTful API may help you boost developer adoption. For example, imagine a 'shopping cart' API:
|Insert new item into the cart||POST||http://api.shopping.com/InsertNewItem|
|Delete item from the cart||POST||http://api.shopping.com/DeleteItem|
|List everything in the cart||GET||http://api.shopping.com/ListCart?cartId=X|
|Get an item in the cart||GET||http://api.shopping.com/ShowItem?cartId=X&itemId=Y|
|Delete the whole cart||POST||http://api.shopping.com/DeleteCart|
While the above API isn't 'truly RESTful', it's not that hard to use. But you do have to learn the individual operations and this can get cumbersome if there are a lot of them or as the API evolves.
Instead, this 'true REST API' may be easier to learn and predict as you use more features.
|Insert new item into the cart||POST *||http://api.shopping.com/carts/X.xml|
|Delete item from the cart||DELETE||http://api.shopping.com/carts/X/item/Y.xml|
|List everything in the cart||GET||http://api.shopping.com/carts/X.xml|
|Get an item in the cart||GET||http://api.shopping.com/carts/X/item/Y.xml|
|Delete the whole cart||DELETE||http://api.shopping.com/carts/|
What if we want to list all the shopping carts in the system at any one time? We would add that via an HTTP GET to:
Query parameters can still serve a purpose - making it possible to specify additional options. For instance, imagine a very large shopping cart, and you want to "paginate" the results. To look at items 20-29, you might use a URL like:
Bonus: True REST makes analytics easier
It's a lot easier to build reports that segregate and analyze traffic by URL than to build logic that tries to do this by methods or combination of methods. Good API analytics helps you optimize features, debug problems, and weed out traffic that can slow down your service.
What if your 'non-strict' REST API is already out there?
It might not be that big a deal if your API is very simple or 'read-only' API with information that isn't too sensitive (such as a free search API). Or you can map a non-RESTful API into a 'truly RESTful" API with custom code or API management tools thatperform API transformations.
* A note on POST VS. PUT
* One way to insert an item in the shopping cart is to use POST to update the shopping cart by sending it a new item. In this case, we are using POST to send the server an instruction that essentially tells it to insert some new content to the existing resource. This is why we use POST -- it is like an "update" in a database.
Alternately, we could add the item by using PUT to a new URL, such as:
But if we do this, then we need to somehow give our item some sort of unique URL by picking a value for "Y". This is kind of a strange thing to do, so it may be more natural to use POST and have the response include the URL for the new item, so that we may retrieve it later using GET or delete it using DELETE. Still, sometimes using PUT like this makes sense.
This all comes down to the difference between PUT and POST in the HTTP spec. POST modifies something that already exists, and how that thing is modified is up to the server. PUT replaces the entire contents of the URL with new data. Plus, like GET, HEAD, and DELETE, PUT is idempotent, which means that if you call it more than once, it has the same result every time, whereas POST may keep doing what you ask it to do over and over again