A REST primer – FHIR style – part 1.

In this post I’m going to talk a little about REST – REpresentational State Transfer – in the context of FHIR. The FHIR specification goes into some detail about how REST and FHIR work together, so go there for more details.

As mentioned in a previous post, FHIR works across many different interoperability paradigms, but REST is the best established at this point in time – and by far the easiest to use (as you will see). It’s the one used by companies such as google, amazon and twitter to provide access to their services (as well as 37 signals – which was the original inspiration for FHIR).

Despite the complicated sounding name, it’s actually quite simple – in fact, it’s the way the web works. The term REST was first used by a chap called Roy Feilding – one of those uber-smart guys that defined many of the basic web protocols that we take for granted today. In a PhD dissertation, he took some of the basic web concepts, and described an ‘architectural style’ that took advantage of those concepts in an easy to use way. (sounds like the goals of FHIR – right)?

The basics are really quite simple.

  • REST is built on top of the HTTP protocol – the same protocol you use when requesting a web page in your browser. HTTP (short for HyperText Transfer Protocol) is simply a set of rules that both server and client (in this case a browser) use to exchange data over the web. HTTP is a ‘request/response’ protocol – you make a request (which can contain data), the server responds and that’s it.
  • REST thinks in terms of resources, each of which has an identity by which it can be manipulated. This is the URI (Uniform Resource Identifier), and uniquely specifies the location of a resource. It is specific to a particular server.
  • When manipulating resources, there are a fixed number of methods that you can use. The ones that are of particular interest to FHIR include:
    • GET – retrieve a particular resource based on its URI (server location + id). FHIR uses the GET method both to retrieve a single resource, and to perform a search which could potentially return multiple resources.
    • POST – create a new resource at a specified endpoint. In REST the server will assign an id, and the combination of server location and id becomes the URI for that resource.
    • PUT – update an existing resource. The client that is executing the PUT needs to know the URI of the resource (which, in FHIR, means that they know the id of the resource as well as the server where it is stored). In FHIR, this will create a new version of the resource.
    • DELETE – remove a resource. Again the URI must be known. It is recommended (though not mandated) in FHIR that a copy of the resource is maintained, and potentially reachable using a version aware read, and the history by a history request, but a plain GET should not return the resource any more. (It will, however, return a status code telling the user that a resource was deleted – read on!)
    • OPTIONS – this is a ‘special’ method in FHIR, that is directed to the root of the server, and returns a conformance resource that informs the client what the capabilities of the server are. I’ll write about this some other time – it’s actually quite useful, though not necessary to know for normal operation.
  • When a server responds to an HTTP REST request, it processes the request according to the method and its internal business logic, and then returns a response to the client. The response has a number of parts to it:
    • The body of the response is what the client is requesting from the server – generally in response to a GET request. Most of the other methods do not return a response. (Note: in the current version of the spec they actually do – but that will be removed soon).
    • The server always returns a status code – which informs the client what the outcome of the request was – did it succeed, or did it fail – with some further details about the nature of the outcome. The status code is a number, which has a well defined meaning within the HTTP protocol. FHIR defines an operationOutcome resource, which the server can use to give the client detailed information about any failure.
    • Both the request and the response can contain headers. These provide extra information about the request or response to assist the server or client to properly process the transaction. Examples include:
      • Both request and response use the Content-type header to tell the other party whether a resource is in xml or json format.
      • A client will use the Accept header to tell the server what format (xml/json) it would prefer to get back when making a GET (though the server is not obligated to honor this)
      • The server will use the Location header to tell the client the URI of a newly created resource
      • And so on.

    Well, that’s enough background – lets actually do something in FHIR. Let’s get a patient resource from one of the on line FHIR test servers.

    Paste the following command (which is actually a URI) into your browser:

    http://fhir.healthintersections.com.au/open/Patient/100

    What this does is to instruct your browser to make a GET request to the specified URI, and display the results in your browser. You should see an XML version of the Patient resource that has the id of 100. Simple – eh?

    Dissecting the instruction a bit:

    • HTTP:// – specified the HTTP protocol
    • hl7connect.healthintersections.com.au/svc/fhir/ – the endpoint on Grahames server that responds to FHIR requests
    • Patient/ – specified that the operation was to be on the Patient resource
    • 100 – specified the patient with the id of 100.

    A couple of notes:

    • I needed to know what the Id was. It was fairly safe in this case because Grahames is a test server with some fixed resources, and unless someone else deletes that resource it should be available (try another number if that happens). You could use this technique to retrieve any type of resource from any FHIR server – provided that you knew the Id – and passed any security mechanisms.
    • The only thing you can do through the browser (at least this way) is to issue GET requests.
    • Grahames server ‘wraps’ the actual resource in a User Interface – you won’t see this UI when making requests directly from a client (Like POSTMan) – only the xml in the middle.
    • I haven’t talked at all about security.

    In the next post I’m going to delve into REST a bit further – how to search for resources if you don’t know the id, how to change existing ones or create new one. And, if there is time, how to do queries across resources – e.g. all the conditions associated with a specific patient.

About David Hay
I'm a Product Strategist at Orion Health, Chair emeritus of HL7 New Zealand and co-Chair of the FHIR Management Group. I have a keen interest in health IT, especially health interoperability with HL7 and the new FHIR standard.

8 Responses to A REST primer – FHIR style – part 1.

  1. Firebug says:

    Do you have PUT and POST around the wrong way? PUT to create and then POST to update?

  2. Firebug says:

    Then is it not possible to create a resource with an id chosen by the client (because the server always assigns the id)?

  3. David Hay says:

    It is certainly possible for the client to assign the ID – in FHIR, if a client PUT’s a resource to a URI (i.e. includes the ID) and there is no resource already there, then the server will create the resource using that ID (it returns a 201 status to indicate that this has happened). The issue with this of course is in potential ID collisions – what prevents 2 clients using the same ID for different resources?

    However, FHIR takes the view that this is a deployment decision, and up to the implementers and trading partners using the server. They could insist on GUID’s as the IDs for example. I’m just finishing a post that goes into resource manipulation a bit more deeply – I’ll make sure to cover this off in a little more detail. Thanks for the question!

  4. Pingback: Manipulating a single FHIR Patient Resource | Hay on FHIR

  5. Pingback: FHIR: A question of identity | Hay on FHIR

  6. Dr. Mark G. Janczewski says:

    David:

    This is a nice intro in FHIR and REST! However, I tried connecting to the link you gave at http://hl7connect.healthintersections.com.au/svc/fhir/Patient/@100 to get a patient resource, but nothing came back. Is the link defunct?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: