What is REST, in brief (incomplete)

What exactly IS a REST API. What makes it RESTful? What is an average use case for this in a program? My knowledge of APIs is… Well, not that good, but a lot of stuff I’ve seen talks about REST and I don’t really understand it.

Not really relevant to python, but here is the thing. The idea behind REST is that everything is a resource or can be represented as a resource, and must be presented to the outside world as such. Each resource is represented by one URL (actually, a URI which also happens to be a URL), which contains a unique ID (either a numeric value, or a uuid, are generally used). You can create, read, update, or delete these resources using the standard http verbs. You create with POST, you read with GET, you update with PUT (or POST) and you delete with DELETE. When you create the object with POST, you also deliver a payload, in whatever format you want, that describes the content. The most trivial example would be a collection of users, you create a new user like this, supposing the payload is in xml. You send this to the webserver

POST /users/
<user><name>John Doe</name></user>

The response generally contains the new id, suppose it’s 2453. now you have a resource at http://example.com/users/2453/ and if you GET it, you obtain back your xml, or another representation, if you want

GET /users/2453/
<user><name>John Doe</name</user>

The idea is that you always create resources, and all operations are changes in the state of these resources. You never perform actions on these resources on the webserver, meaning that some patterns that rely on “remote procedure call” like behavior, such as when you call stuff like http://example.com/?action=changeName&id=2453&newName=John%20Deer is a no-no in REST. Instead, you would do

PUT /users/2453/
<user><name>John Deer</name</user>

And you would delete with

DELETE /users/2453/

Most of the difficulty of using REST is to convert your problem as being representable as a resource. For example, one problem I had was to create a queuing system. In that case, you have a queue, and create a resource for a new submission. This submission payload contains the relevant information to start the job. When you query the resource, it contains the info plus a flag that says if it’s submitted, errored, in the queue, or running, meaning that the webserver does not necessarily act just as a repository of info you stuff in. It can also add new info. If you want to understand more, the best resource is definitely Roy Fielding’s thesis (not all of it, just the relevant parts), and the o’Reilly book RESTful web services. It’s rather short, but clean and concise.

Edit: A rather challenging part of REST is to perform transactions. Suppose you have to perform multiple operations to keep the resource set consistent. How do you do it, considering that you are operating on one resource at a time? In that case, you first create a transaction resource, e.g. in /transactions/. Then, you perform the operations on the individual resources, and inside the payload you specify the transaction id you got. GET operations on the resources while you are performing the transaction return the old payload. When you are done, you PUT the transaction with a payload flag “state: committed” and the resources are synchronized. Alternatively, you can create the transaction with the payload describing the operations to perform and on which resources, and then the webserver applies atomically all the changes. It’s mostly a matter of taste, but the point is that, as I said, everything is seen as a resource, even transactions.