Coming from a lot of frustrating times with WSDL/Soap, I very much like the REST paradigm, but am trying to solve two basic problems in our application, before moving over to REST. The first problem relates to the lack of an interface document. I think I finally see how to handle this situation: One can query his way down from a top-level "/resources" resource using various requests of GET, HEAD, and OPTIONS to find the one needed resource in the correct hypermedia format. Is this the idea? If so, the client need only be provided with a top-level resource URI: http://www.mywebservicesite.com/mywebservice/resources. He will then have to do some searching and possible keep track of what he is discovering, so that he can use the URIs again efficiently in future to do GETs, POSTs, PUTs, and DELETEs. Are there any thoughts on what should happen here?
The other problem is that we cannot use descriptive URLs like /resources/../customer/Madonna/phonenumber. We do have an implementation of opaque URLs we use in the context of a session, and I'm wondering how opaque URLs might be applied to REST. The general problem is how to keep domain-specific details out of URLs, and still benefit from what REST has to offer.
The other problem is that we cannot use descriptive URLs like /resources/../customer/Madonna/phonenumber.
I think you've misunderstood the point of opaque URIs. The notion of opaque URIs is with respect to clients: A client shall not decipher a URI to guess anything of semantic meaning from it. So a service may well have URIs like /resources/.../customer/Madonna/phonenumber, and that's quite a good idea. The URIs should be treated as opaque by clients: not infer from the URI that it represents Madonna's phone number, and that Madonna is a customer of some sort. That knowledge can only be obtained by looking inside the URI itself, or perhaps by remembering where the URI was discovered.
Edit:
A consequence of this is that navigation should happen by links, not by deconstructing the URI. So if you see /resouces/customer/Madonna/phonenumber (and it actually represents Customer Madonna's phone number) you should have links in that resource to point to the Madonna resource: e.g.
{
"phone_number" : "01-234-56",
"customer_URI": "/resources/customer/Madonna"
}
That's the only way to navigate from a phone number resource to a customer resource. An important aspect is that the server implementation might or might not have domain specific information in the URI, The Madonna record might just as well live somewhere else: /resources/customers/byid/81496237. This is why clients should treat URIs as opaque.
Edit 2:
Another question you have (in the comments) is then how a client, with the required no knowledge of the server's URIs is supposed to be able to find anything. Clients have the following possibilities to find resources:
Provide a search interface. This could be done by providing an OpenSearch description document, which tells clients how to search for items. An OpenSearch template can include several variables, and several endpoints, depending on what you're looking for. So if you have a "customer ID" that's unique, you could have the following template: /customers/byid/{proprietary:customerid}", the customerid element needs to be documented somewhere, inside the proprietary namespace. A client can then know how to use such a template.
Provide a custom form. This implies making a custom media type in which you explicitly define how (based on an instance of the document) a URI to a customer can be forged. <customers template="/customers/byid/{id}"/>. The documentation (for the media type) would have to state that the template attribute must be interpreted as a relative URI after the string substitution "{id}" to an actual customer ID.
Provide links to all resources. Some resources aren't innumerable, so you can simply make a link to each and every one of them, optionally including identifying information along with the links. This could also be done in a custom media type: <customer id="12345" href="/customer/byid/12345"/>.
It should be noted that #1 and #2 are two ways of saying the same thing: Clients are allowed to create URIs if they
haven't got the URI structure a priori
a media type exists for which the documentation states that URIs should be created
This is much the same way as a web browser has no idea of any URI structure on the web, except for the rules laid out in the definition of HTML forms, to add a ? and then all the query parameters separated by &.
In theory, if you have a customer with id 12345, then you could actually dispense with the href, since you could plug the customer id 12345 into #1 or #2. It's more common to actually provide real links between resources, rather than always relying on lookup or search techniques.
I haven't really used web RPC systems (WSDL/Soap), but i think the 'interface document' is there mostly to allow client libraries to create the service API, right? if so, REST shouldn't need it, because the verbs are already defined and don't really need to be documented again.
AFAIUI, the REST way is to document the structure of each resource (usually encoded in XML or JSON). In that document, you'll also have to document the relationship between those resources. In my case, a resource is often a container of other resources (sometimes more than one type), therefore the structure doc specifies what field holds a list of URLs pointing to the contained resources. Ideally, only one unique resource will need a single, fixed (documented) URL. everithing else follows from there.
The URL 'style' is meaningless to the client, since it shouldn't 'construct' an URL. Every URL it needs should be already constructed on a resource field. That let's you change the URL structure without changing the client (that has saved tons of time to me). Your URLs can be as opaque or as descriptive as you like. (personally, i don't like text keys or slugs; my keys are all BIGINTs or UUIDs)
I am currently building a REST "agent" that addresses the first part of your question. The agent offers a temporary bookmarking service. The client code that is interacting with the agent can request that an URL be bookmarked using some identifier. If the client code needs to retrieve that representation again, it simply asks the agent for the url that corresponds to the saved bookmark and then navigates to that bookmark. Currently those bookmarks are not persisted so they only last for the lifetime of the client application, but I have found it a useful mechanism for accessing commonly used resources. E.g. The root representation provides a login link. I bookmark that link and if the client ever receives a 401 then I can redirect to the "login" bookmark.
To address an issue you mentioned in a comment, the agent also has the ability to store retrieved representations in a dictionary. If it becomes necessary to aggregate and manipulate multiple representations at the same time then I can simply request that the agent store the current representation in a dictionary associated to a key and then continue navigating to the next resource. Once the client has accumulated all the necessary representation it can do what it needs to do.
Related
I'm trying to wrap my head around the difference between a "collection" and a "store" in REST. From what I've read so far,
a collection is:
"a server-managed directory of resources"
and a store is a:
"client-managed resource repository"
I found this post: How "store" REST archetype isn't creating a new resource and a new URI?
but it didn't really help me clarify the difference. I mean, I understand one is controlled by the server and the other by the client... but can someone give me a concrete example of what a store might be in a real world application?
I *think it's something like this:
GET http://myrestapplication.com/widgets/{widget_id} -- retrieves a widget from db
POST http://myrestapplication.com/widgets/{widget_id} -- creates a new widget from db
PUT http://myrestapplication.com/widgets/{widget_id},[list of updated parms & their vals] -- update widget
PUT http://myrestapplication.com/users/johndoe/mywishlist/{widget_id} -- updates john doe's profile to add a widget that already exists in the database... but links to it as his favorite one or one that he wants to buy
Is this correct?
if so, could the last PUT also be expressed as a POST somehow?
EDIT 1
I found an online link to the book i'm reading... where it makes the distinction between the two:
https://books.google.ca/books?id=4lZcsRwXo6MC&pg=PA16&lpg=PA16&dq=A+store+is+a+client-managed+resource+repository.+A+store+resource+lets+an+API+client:+put+resources+in,+get+them+back+out,+and+decide+when+to+delete+them&source=bl&ots=F4CkbFkweL&sig=H6eKZMPR_jQdeBZkBL1h6hVkK_E&hl=en&sa=X&ei=BB-vVJX6HYWvyQTByYHIAg&ved=0CB0Q6AEwAA#v=onepage&q=A%20store%20is%20a%20client-managed%20resource%20repository.%20A%20store%20resource%20lets%20an%20API%20client%3A%20put%20resources%20in%2C%20get%20them%20back%20out%2C%20and%20decide%20when%20to%20delete%20them&f=false
REST uses http verbs to manipulate resources. Full-Stop. That's it. To build some classes of browser-based application developers sometimes use local storage (a store), but that has absolutely nothing to do with REST (in fact, it's the opposite). Collections are a special consideration in REST-based API design because the REST principles place considerable constraints on how they are represented in the results of your queries -- special consideration also because there are no standards on how these things should be represented and access if you're using anything other than html as a resource type.
Edit:
REST suggests that when we ask for a resource we receive that resource and only that resource and that things referenced by that resource are returned as links, not as data. This mimics the http standard by which we return the requested page and links to other pages rather than embedding linked pages. So, our resources should return links to related resources, not the resources themselves.
So, what about collections?
Let's use as an example a college management system that has Course objects each of which contains a huge lists of Students.
When I GET the course I don't want to have the collection of students returned as an embedded list, because that could be huge and because my user might not be interested. Instead, I want to know that the course has a students collection and I want to be able to query that collection separately (when I need to) and I want to be able to page it on demand. For this to work, the course needs to link to the students collection URL (maybe with an appropriate type so that my code knows how to handle the link). Then, I want to use the given collection's url to request a paged list of resources. In this example, the collection's url could be something like: course/1/students, with the convention that I can add paging info to the search string to constrain the results with something like course/1/students?page=1&count=10. Embedding the students collection into the course resource would be a violation of REST. I would not be returning a course, I'd be returning course-and-students.
Pretty new to all this REST stuff.
I'm designing my API, and am not sure what I'm supposed to return from a search query. I was assuming I would just return all objects that match the query in their entirety, but after reading up a bit about HATEOAS I am thinking I should be returning a list of URI's instead?
I can see that this could help with caching of items, but I'm worried that there will be a lot of overhead generated by the subsequent multiple HTTP requests required to get the actual object info.
Am I misunderstanding? Is it acceptable to return object instances instead or URIs?
I would return a list of resources with links to more details on those resources.
From RESTFull Web Services Cookbook 2010 - Subbu Allamaraju
Design the response of a query as a representation of a collection
resource. Set the appropriate expiration caching headers. If the query
does not match any resources, return an empty collection.
IMHO it is important to always remember that "pure REST" and "real world REST" are two quite different beasts.
How are you returning the list of URIs from your query in the first place? If you return e.g. application/json, this certainly does not tell the client how it is supposed to interpret the content; therefore, the interaction is already being driven by out-of-band information (the client magically already knows where to look for the data it needs) in conflict with HATEOAS.
So, to answer your question: I find it quite acceptable to return object instances instead of URIs -- but be careful because in the general case this means you are generating all this data without knowing if the client is even going to use it. That's why you will see a hybrid approach quite often: the object instances are not full objects (i.e. a portion of the information the server has is not returned), but they do contain a unique identifier that allows the client to fetch the full representation of selected objects if it chooses to do so.
After reading an article on REST ("Restful Grails"), I have gotten the impression that it is not possible to truly conform to a REST style in a service that demands a lot of parameters. Is this so? All the examples I have seen so far seem to imply that true REST style services are "parameterless". Using parameters would be RPC-ish and not truly RESTful.
To be more specific, say we have a service that returns graph data for stock prices, and this service needs to know the start date, end date, the currency, stock name, and whatever else might be applicable. In any case, at least 4-5 parameters are needed to retrieve the information needed.
I would imagine the URL to be something like this : /stocks/YAHOO?startDate="2008-09-01"&endDate=...
("YAHOO" is here a made-up stock name).
Would this really be REST or is this more RPC-like, what the author of the aforementioned article calls "GETful" (i.e. just low ceremony rpc)?
You can see the querystring as a filter on the resource you are GETing. Here, your resource is the stock prices of yahoo. Doing a GET on that resource give you all the available data, or the most recents. The query string filter the prices you want. Content negociation allow you to change the representation, e.g. a png graph, a csv file, and so on. To add a price, simply POST a representation (e.g. CSV) to the same resource.
The "restfulness" is not realy in the URL itself, since URIs are obscures to client, but in the way you interact with resources themselves identified by their URI
Feel free to use as many parameters as you need to identify the resource you wish to access. REST doesn't care.
Why would you think it is not possible?
Google uses REST for their charts api, and they take alot of params:
http://chart.apis.google.com/chart?cht=bvg&chs=350x300&chd=t:20,35,10&chxr=1,0,40&chds=0,40&chco=FF0000|FFA000|00FF00&chbh=65,0,35&chxt=x,y,x&chxl=0:|High|Medium|Low|2:||Task+Priority||&chxs=2,000000,12&chtt=Tasks+on+my+To+Do+list&chts=000000,20&chg=0,25,5,5
This is one of those little detail (and possibly religious) questions. Let's assume we're constructing a REST architecture, and for definiteness lets assume the service needs three parameters, x, y, and z. Reading the various works about REST, it would seem that this should be expressed as a URI like
http://myservice.example.com/service/ x / y / z
Having written a lot of CGIs in the past, it seems about as natural to express this
http://myservice.example.com/service?x=val,y=val,z=val
Is there any particular reason to prefer the all-slashes form?
The reason is small but here it is.
Cool URI's Don't Change.
The http://myservice.example.com/resource/x/y/z/ form makes a claim in front of God and everybody that this is the path to a specific resource.
Note that I changed the name. There may be a service involved, but the REST principle is that you're describing a specific web resource, named /x/y/z/.
The http://myservice.example.com/service?x=val,y=val,z=val form doesn't make as strong a claim. It says there's a piece of code named service that will try to do some sort of query. No guarantees.
Query parameters are rarely "cool". Take a look at the Google Chart API. Should that use a /full/path/notation for all of the fields? Would each URL be cool if it did?
Query parameters are useful. Optional fields can be omitted. New keys can be added to support new functionality. Over time, old fields can be deprecated and removed. Doing this is clumsier with a /path/notation .
Quoting from http://www.xml.com/pub/a/2004/08/11/rest.html
URI Opacity [BP]
The creator of a URI decides the encoding
of the URI, and users should not derive
metadata from the URI itself. URI opacity
only applies to the path of a URI. The
query string and fragment have special
meaning that can be understood by users.
There must be a shared vocabulary between
a service and its consumers.
This sounds like query strings are what you want.
One downside to query strings is that the are unordered. The GET ending with "?x=1&y=2" is different than that ending with "?y=2&x=1". This means the browser and any other intermediate systems won't be able to cache it, because caching is done based on the full URL. If this is a concern, then generate the query string in a well-defined order.
While constructing URIs this is the priniciple I follow. I don't know whether it is perfectly acceptable in all cases
Say for instance, that I have to get the details of an employee, then the URI will be of the form:
GET /employees/1/ and not GET /employees?id=1 since I treat every employee as a resource and the whole URI "employees/{id}" is used in identification of the resource.
On the other hand, if I have algorithmic operations that do not identify a specific resource as such,but merely require inputs to the algorithm which in turn identify the resource, then I use query strings.
For instance GET /employees?empname='%Bob%'&maxResults=100 might give me all employees whose names have the word Bob in them, with the maximum results returned by the query limited to 100.
Hope this answers your question
URIs are strictly split into a hierarchical part (the path) and a non-hierarchical path (the query), and both serve to identify the resource
Tthe URI spec itself (RFC 3986) clearly sets the path and the query portion of a URI as equal.
Section 3.3:
The path component contains data [...] that along with [the] query component
serves to identify a resource.
Section 3.4:
The query component contains [...] data that, along with
[...] the path component serves to identify a resource
So your choice in using x/y/z versus x=val&y=val&z=val has mainly to do if x, y or z are hierarchical in nature or if they're non-hierarchical, and if you can perceive them as always being hierarchical or non-hierarchical for the foreseeable future, along with any technical limitations you might be having on selecting one over the other.
But to answer your question, as others have noted: Neither is more RESTful than the other, since they both end up identifying a resource.
If the resource is the service, independent of parameters, it should be
http://myservice.example.com/service?x=val&y=val&z=val
This is a GET query. One of the principles behind REST is that you GET to read (but not modify!) the resource; you can POST to modify a resource & get a response; you can PUT to write to a resource; and you can DELETE to remove a resource.
If the resource specific with those parameters is a persistent resource, it needs a name. You could (if you organized your webservice this way) POST to http://myservice.example.com/service?x=val&y=val&z=val to create a particular instance of the service and have it return an ID to name this instance, e.g.
http://myservice.example.com/service/12312549
then use GET/POST/PUT/DELETE to interact with that instance.
First of all, defining URIs as part of your API violates a constraint of the REST architecture. You cannot do that and call your API RESTful.
Secondly, the reason query parameters are bad for non-query resource access is that they are generally not cached. It is also a violation of HTTP standards.
A URL with slashes like /x/y/z/ would impose a hierarchy and is not suited for the exact case of just passing three parameters.
If, like you said, x y z are indeed just parameters and the order is not important, it would be more RESTful to use semicolons:
http://myservice.example.com/service/x;y;z/
If your "service" however is just an algorithm that works the same with different parameters, there would also be nothing unRESTful with using ?x=val format.
I have a resource at a URL that both humans and machines should be able to read:
http://example.com/foo-collection/foo001
What is the best way to distinguish between human browsers and machines, and return either HTML or a domain-specific XML response?
(1) The Accept type field in the request?
(2) An additional bit of URL? eg:
http://example.com/foo-collection/foo001 -> returns HTML
http://example.com/foo-collection/foo001?xml -> returns, er, XML
I do not wish to oblige machines reading the resource to parse HTML (or XHTML for that matter). Machines like the googlebot should receive the HTML response.
It is reasonable to assume I control the machine readers.
If this is under your control, rather than adding a query parameter why not add a file extension:
http://example.com/foo-collection/foo001.html - return HTML
http://example.com/foo-collection/foo001.xml - return XML
Apart from anything else, that means if someone fetches it with wget or saves it from their browser, it'll have an appropriate filename without any fuss.
My preference is to make it a first-class part of the URI. This is debatable, since there are -- in a sense -- multiple URI's for the same resource. And is "format" really part of the URI?
http://example.com/foo-collection/html/foo001
http://example.com/foo-collection/xml/foo001
These are very easy deal with in a web framework that has URI parsing to direct the request to the proper application.
If this is indeed the same resource with two different representations, the HTTP invites you to use the Accept-header as you suggest. This is probably a very reliable way to distinguish between the two different scenarios. You can be plenty sure that user agents (including search engine spiders) send the Accept-header properly.
About the machine agents you are going to give XML; are they under your control? In that case you can be doubly sure that Accept will work. If they do not set this header properly, you can give XML as default. User agents DO set the header properly.
I would try to use the Accept heder for this, because this is exactly what the Accept header is there for.
The problem with having two different URLs is that is is not automatically apparent that these two represent the same underlying resource. This can be bad if a user finds an URL in one program, which renders HTML, and pastes it in the other, which needs XML. At this point a smart user could probably change the URL appropriately, but this is just a source of error that you don't need.
I would say adding a Query String parameter is your best bet. The only way to automatically detect whether your client is a browser(human) or application would be to read the User-Agent string from the HTTP Request. But this is easily set by any application to mimic a browser, you're not guaranteed that this is going to work.