first of all I am new to this...REST, RDF, LDP etc.
I could able to get an understanding about REST and RDF in a vague manner:
REST is a framework where everything is a resource and complex client side requests are converted to URI based structural requests and
using HTTP methods, we will get the results in RDF resource format:
XML or json format.
RDF is a framework to explain the relational structure or in other words, conceptual model of a web resource.
LDP seems to be same as REST, uses HTTP protocols to interact with RDF resources. What I understand is HTTP protocols are used to communicate with web services and get the result in HTML, jpeg, png or any other format, even XML too. Then what is LDP? - Does it somehow updates the XML using the HTTP methods.
Can't that be done in normal architecture. other than LDP?
LDP, Linked Data Platform, is a W3C specification defining a standard way for servers and clients to interact with resources (primarily RDF resources) over HTTP. In particular, it introduces the notion of Containers, RDFSources, and Non-RDFSources (or binaries).
It may help to think of an RDFSource as a document, kind of like an HTML web page. Only, the content is not HTML, it's a graph (a set of RDF triples) sharing the same subject URI. Together, the triples in this document would typically describe or make up a given entity or object. So, those could be thought of as properties of the object. The document could be expressed in RDF XML, in Turtle, JSON-LD, or possibly other formats. These properties may be literal values or they may be links to other resources.
LDP implements the RESTful architecture, so how you view this RDFSource depends on how you ask for it in your request to the server. If you ask for the resource to be expressed in JSON-LD, you should get back a JSON-LD representation of the resource. If you ask for it as Turtle, you should get back a Turtle representation. This is done by passing certain HTTP headers in the request. Additionally, the RESTful nature of an LDP allows you to use HTTP methods (GET,POST,PUT,DELETE) to interact with the resources in various ways.
A Container is also an RDFSource, but it allows you to manage membership. Containers can have member resources. You could think of a Container kind of like a folder. Only, it doesn't physically contain RDFSources or documents. Instead, it has a set of triples (membership triples) that define or link to its members. You can view a Container as a container or as an RDFSource depending on the preferred interaction model that you specify in a request header.
So, basically, you can think of an LDP as a way of interacting with RDF resources in a way that is similar to a web site with folders and documents. Only everything is RDF, not XHTML. On the back-end, the server may actually manage each resource as a real document (file). Or, as is the case with Carbon LDP, for example, it may put everything in a triplestore (an RDF store / database). Then it just gives you back a set of triples that look like a "document" because they share the same subject URI, which is what you used when making the RESTful request. Since Carbon LDP manages all these "documents" in a triplestore, it can also provide SPARQL query support across everything (though SPARQL support is not part of the LDP spec).
So, in essence, an LDP enables a very "webby" (or RESTful) way of working with RDF data. You make HTTP requests to URI's and they get resolved to resources (Containers or RDFSources), which you can then consume to get at all the triples. And of course you can create resources, update them, list members of a container, etc. In this way, you can build web applications, that use RESTful requests (perhaps async JavaScript or AJAX requests).
One advantage you win is that even though the data you're working with may be very specific to any given application your building on LDP, the REST API you use to work with that data is standard and consistent.
Another advantage is that you're working with RDF, so the properties of your objects, the predicates, can link data across your enterprise or the World Wide Web. This can help you incorporate data and discover things that your app may not have been specifically designed to support. And also, because you're working with the RDF data model, you can use pre-existing vocabularies for your triples, but you don't have near as much hassle with schemas.
In RDF, you can add new triples (new properties or links) without having to update some database schema and the associated code required to interpret it. LDP deals with RDF resources in a very generic way - it doesn't care what the triples that define or make up the resources actually are. When you build an LDP app, you can extend that sort of generic quality into the app in such a way that your data can keep changing and evolving without imposing as heavy costs on the maintenance and evolution of the app itself.
This kind of technology helps you bridge the gap between the current web of hyperlinked documents to a web of linked data, which is easier for computers to understand and interoperate with. For a little more info about RDF and the big difference between a hyperlink and a linked data link, see The Awesome Power of the Link in Linked Data.
You can also find a somewhat technical introduction to LDP in Introduction to: Linked Data Platform, an article I wrote a while back for Dataversity.
Related
I created a few Rest apis right now and I always preferred a solution, where I created an endpoint for each resource.
For example:
GET .../employees/{id}/account
GET .../supervisors/{id}/account
and the same with the other http methods like put, post and delete. This blows up my api pretty much. My rest apis in general preferred redundancy to reduce complexity but in this cases it always feels a bit cumbersome. So I create another approach where I work with inheritance to keep the "dry" principle.
In this case there is a base class User and via inheritance my employee and supervisor model extends from it. Now I only need one endpoint like
GET .../accounts/{id}
and the server decides which object is returned. Also while this thins out my api, it increases complexity and in my api documentation ( where I use spring rest docs ) I have to document two different Objects for the same endpoint.
Now I am not sure about what is the right way to do it ( or at least the better way ). When I think about Rest, I think in resources. So my employees are a seperate resource as well as my supervisors.
Because I always followed this approach, I tink I might be mentally run in it and maybe lost the objectivity.
It would be great if you can give my any objective advice on how this should be handled.
I built an online service that deals with this too. It's called Wirespec:
https://wirespec.dev
The backend automatically creates the url for users and their endpoints dynamically with very little code. The code for handling the frontend is written in Kotlin while the backend for generating APIs for users is written in Node.js. In both cases, the amount of code is very negligible and self-maintaining, meaning that if the user changes the name of their API, the endpoint automatically updates with the name. Here are some examples:
API: https://wirespec.dev/Wirespec/projects/apis/Stackoverflow/apis/getUserDetails
Endpoint: https://api.wirespec.dev/wirespec/stackoverflow/getuserdetails?id=100
So to answer your question, it really doesn't matter where you place the username in the url.
Try signing in to Wirespec with your Github account and you'll see where your Github username appears in the url.
There is, unfortunately, no wright or wrong answer to this one and it soley depends on how you want to design things.
With that being said, you need to distinguish between client and server. A client shouldn't know the nifty details of your API. It is just an arbitrary consumer of your API that is fed all the information it needs in order to make informed choices. I.e. if you want the client to send some data to the server that follows a certain structure, the best advice is to use from-like representations, such as HAL forms, Ion or even HTML. Forms not only teach a client about the respective properties a resource supports but also about the HTTP operation to use, the target URI to send the request to as well as the representation format to send the data in, which in case of HTML is application/x-www-form-urlencoded most of the time.
In regards to receiving data from the server, a client shouldn't attempt to extract knowledge from URIs directly, as they may change over time and thus break clients that rely on such a methodology, but rely on link relation names. Per URI there might be multiple link relation names attached to that URI. A client not knowing the meaning of one should simply ignore it. Here, either one of the standardized link relation names should be used or an extension mechanism as defined by Web linking. While an arbitrary client might not make sense from this "arbitrary string" out of the box, the link relation name may be considered the predicate in a tripple often used in ontologies where the link relation name "connects" the current resource with the one the link relation was annotated for. For a set of URIs and link relation names you might therefore "learn" a semantic graph over all the resources and how they are connected to each other. I.e. you might annotate an URI pointing to a form resource with prefetch to hint a client that it may load the content of the referenced URI if it is IDLE as the likelihood is high that the client will be interested to load that resource next anyway. The same URI might also be annotated with edit-form to hint a client that the resource will provide an edit form to send some data to the server. It might also contain a Web linking extension such as https://acme.org/ref/orderForm that allows clients, that support such a custom extension, to react to such a resource accordingly.
In your accounts example, it is totally fine to return different data for different resources of the same URI-path. I.e. resource A pointing to an employee account might only contain properties name, age, position, salery while resource B pointing to a supervisor could also contain a list of subordinates or the like. To a generic HTTP client these are two totally different resources even though they used a URI structure like /accounts/{id}. Resources in a REST architecture are untyped, meaning they don't have a type ouf of the box per se. Think of some arbitrary Web page you access through your browser. Your browser is not aware of whether the Web page it renders contains details about a specific car or about the most recent local news. HTML is designed to express a multitude of different data in the same way. Different media types though may provide more concrete hints about the data exchanged. I.e. text/vcard, applciation/vcard+xml or application/vcard+json all may respresent data describing an entity (i.e. human person, jusistic entity, animal, ...) while application/mathml+xml might be used to express certain mathematical formulas and so on. The more general a media type is, the more wiedspread usage it may find. With more narrow media types however you can provide more specific support. With content type negotiation you also have a tool at your hand where a client can express its capabilities to servers and if the server/API is smart enough it can respond with a representation the client is able to handle.
This in essence is all what REST is and if followed correctly allow the decoupling of clients from specific servers. While this might sound confusing and burdensome to implement at first, these techniques are intended if you strive for a long-lasting environment that still is able to operate in decateds to come. Evolution is inherently integrated into this phiolosophy and supported by the decoupled design. If you don't need all of that, REST might not be the thing you want to do actually. Buf if you still want something like REST, you for sure should design the interactions between client and server as if you'd intereact with a typical Web server. After all, REST is just a generalization of the concepts used on the Web quite successfully for the past two decades.
We are building set of new REST APIs.
Let's say we have a resource /users with the following fields:
{
id: 1
email: "test#user.com"
}
Clients implement this API and can then update this resource by sending a new resource representation to PUT /users/1.
Now let's say we add a new property name to the model like so:
{
id: 1
email: "test#user.com"
name: "test user"
}
If the models the existing clients are using are to call our API not updated, then calls to PUT /users/1 will remove the new name property since PUT is supposed to replace the resource. I know that the clients could work straight with the raw json to ensure they always receive any new properties that are added in the API, but that is a lot of extra work, and under normal circumstances clients are going to create their own model representations of the API resources on their side. This means that any time any new property is added, all clients need to update the code/models on their side to make sure they aren't accidentally removing properties. This creates unneeded coupling between systems.
As a way to solve this problem, we are considering not implementing PUT operations at all and switching updates to PATCH where properties that aren't passed in are simply not changed. That seems technically correct, but might not be in the spirit of REST. I am also slightly concerned about client support for the PATCH verb.
How are others solving this problem? Was is the best practice here?
You are in a situation where you need some form of API versioning. The most appropriate way is probably using a new media-type every time you make a change.
This way you can support older versions and a PUT would be perfectly legal.
If you don't want this and just stick to PATCH, PATCH is supported everywhere except if you use ancient browsers. Not something to worry about.
Switching from PUT to PATCH will not fix your problem, IMO. The root cause, IMO, is that clients already consider the data being returned for a representation to follow a certain type. According to Fielding
A REST API should never have “typed” resources that are significant to the client.
(Source)
Instead of using typed resources clients should use content-type negotiation to exchange data. Here, media-type formats that are generic enough to gain widespread adoption are for sure beneficial, certain domains may however require a more specific representation format.
Think of a car-vendor Web page where you can retrieve the data from your preferred car. You, as a human, can easily identify that the data depicts a typical car. However, the media-type you most likely received the data in (HTML) does not state by its syntax or the semantics of its elements that the data describes a car, unless some semantic annotation attributes or elements are present, though you might be able to update the data or use the data elsewhere.
This is possible as HTML ships with a rich specification of its elements and attributes, such as Web forms that not only describe the supported or expected input parameters but also the URI where to send the data to, the representation format to use upon sending (implicitly given by application/x-www-form-urlencoded; may be overwritten by the enctype attribute though) or the HTTP method to use, which is fixed to either GET or POST in HTML. Through this, a server is able to teach a client on how a request needs to be built. As a consequence the client does not need to know anything else besides having to understand the HTTP, URI and HTML specifications.
As Web pages are usually filled with all kinds of unrelated stuff, such as adds, styling information or scripts, and the XML(-like) syntax, which is not every ones favourite, as it may increase the size of the actual payload slightly, most so-called "REST" APIs do want to exchange JSON-based documents. While plain JSON is not an ideal representation format, as it does not ship with link-support at all, it is though very popular. Certain additions such as JSON Hyper-Schema (application/schema+json hyper-schema) or JSON Hypertext Application-Language (HAL) (application/hal+json) add support for links and link-relation. These can be used to render data received from the server as-is. However, if you want a response to automatically drive your application state (i.e. to dynamically draw the GUI with the processed data) a more specific representation format is needed, that can be parsed by your client and act accordingly as it understands what the server wants it to do with it (= affordance). If you like to instruct a client on how to build a request support for other media-types such as hal-forms or ion need to be supported. Certain media-types furthermore allow you to use a concept called profiles, that allow you to annotate a resource with a semantic type. HAL JSON i.e. does support something like that where the Content-Type header may now contain a value such as application/hal+json;profile=http://schema.org/Car that hints the media-type processor that the payload follows the definition of the given profile and may thus apply further validity checks.
As the representation format should be generic enough to gain widespread usage, and URIs itself shouldn't hint a client as well what kind of data to expect, an other mechanism needs to be used. Link relation names are basically an annotation for URIs that tell a client about the purpose of a certain link. A pageable collection might return links annotated with first, prev, next and last which are pretty obvious what they do. Other links might be hinted with prefetch, that hint a client that a resource can be loaded right after loading the current resource finished as it is very likely that the client will retrieve this resource next. Such media-types, however, should be either standardized (defined in a proposal or RFC and registerd with IANA) or follow the schema proposed by Web linking, (i.e. as used by Dublin Core). A client that just uses the URI for an invoked link-relation name will still work in case the server changes its URI scheme instead of attempting to parse some parameters from the URI itself.
In regards to de/coupling in a distributed system a certain amount of coupling has to exist otherwise parties wont be able to communicate at all. Though the point here is, the coupling should be based on well-defined and standardized formats that plenty of clients may support instead of exchanging specific representation formats only a very limited number of clients support (in worst case only the own client). Instead of directly coupling to the API and using an undefined JSON-based syntax (maybe with external documentation of the semantics of the respective fields) the coupling should now occur on the media-types parties can use to exchange the format. Here, not the question of which media-type to support should be asked but how many you want to support. The more media-types your client or server supports, the more likely it is to interact with other peers in the distributed system. On the grand-scheme of things, you want a server to be able to server a plethora of clients while a single client should be able to interact with (in best case) every server without the need for constant adoptions.
So, if you really want to decouple clients from servers, you should take a closer look at how the Web actually works and try to mimic its interaction model onto your application layer. As "Uncle Bob" Robert C. Martin mentioned
An architecture is about intent! (Source)
and the intention behind the REST architecture is the decoupling of clients from servers/services. As such, supporting multiple media-types (or defining your own-one that is generic enough to reach widespread adoption), looking up URIs just via their accompanying link-relation names and relying on content-type negotiation as well as relying only on the provided data may help you to achieve the degree of decoupling you are looking for.
All nice and well in theory, but so far every rest api I encountered in my career had predefined contracts that changed over time.
The problem here is, that almost all of those so called "REST APIs" are RPC services at its heart which should not be termed "REST" to start with - this is though a community issue. Usually such APIs ship with external documentation (i.e. Swagger) that just re-introduce the same problems classical RPC solutions, such as CORBA, RMI or SOAP, suffer from. The documentation may be seen as IDL in that process without the strict need for skeleton classes, though most "frameworks" use some kind of typed data classes that will either ignore the recently introduced field (in best case) or totally blow up on invocation.
One of the problems REST suffers from is, that most people haven't read Fieldings thesis and therefore don't see the big picture REST tries to establish but claim to know what REST is and therefore mix up things and call their services RESTful which lead to a situation where REST != REST. The ones pointing out what a REST architecture is and how one might achieve it are called out as dreamers and unworldly when the ones proclaiming the wrong term (RPC over HTTP = REST) continue to do so adding to the confusion of especially the ones just learning the whole matter.
I admit that developing a true REST architecture is really, really hard as it is just too easy to introduce some form of coupling. Hence, a very careful design needs to be done that needs time and also costs money. Money plenty of companies can't or don't want to spend, especially in a domain where new technologies evolve on a regular basis and the ones responsible for developing such solutions often leave the company before the whole process had finished.
Just saying it shouldn’t be ‘typed’ is not really a viable solution
Well, how often did you need to change your browser as it couldn't interact with a Web page? I don't talk about CSS-stuff or browser-specific CSS or JS stuff. How often needed the Web to change in the last 2-3 decades? Similar to the Web, the REST architecture is intended for long-lasting applications for years to come, that supports natural evolution by design. For simple frontend-2-backend systems it is for sure overkill. It starts to shine especially in cases where there are multiple peers not under your control you can interact with.
My understanding of SOAP vs REST:
REST = JSON, simple consistent interface, gives you CRUD access to 'entities' (Abstractions of things which are not necessarily single DB rows), simpler protocol, no formally enforced 'contract' (e.g. the values an endpoint returns could change, though it shouldn't)
SOAP = XML, more complex interface, gives you access to 'services' (specific operations you can apply to entities, rather than allowing you to CRUD entities directly), formally enforced, pre-stated 'contract' (like a WSDL, where e.g. the return types are predefined and formalized)
Is that a broadly correct assessment?
What about a mixture?
If so, what do I call an API that is a mixture?
For example, If we have what at surface level looks like a REST API (returns JSON, no WSDL or formalized contract defined - but instead of giving you access to the 'entities' that the system manages (User, product, comment, etc) it instead gives you specific access to services and complex operations (/sendUserAnUpdate/1111, /makeCommentTextPurple/3333, /getAllCommentsByUserThisYear/2222) without having full coverage?
The 'services' already exist internally, and the team simply publishes access to them on a request by request basis, through what would otherwise look like a REST API.
Question:
What is the 'mixture' typically referred to as (besides, maybe, a bad API). Is there a word for it? or a concept I can refer to that'll make most developers understand what I'm referring to, without having to say the entire paragraph I did above?
Is it just "JSON SOAP API?", "A Service-based REST API?" - what would you call it?
Thanks!
Thanks!
If you take a look at all those so-called REST-APIs your observation might seem true, though REST actually is something completely different. It describes an architecture or a philosophy whose intent it is to decouple clients from servers, allowing the latter one to evolve in future without breaking clients. It is quite similar to the typical Web page interaction in that a server will teach a client on what it needs and only reacts on client-triggered requests. One has to be pretty careful and pendant when designing REST services as it is too easy to include a coupling that may affect clients when a change is introduced, especially with all the pragmatism around in (commercial) software engineering. Stefan Tilkov gave a great talk on REST back in 2014 that, alongside with Jim Webber or Asbjørn Ulsberg, can be used as introduction lectures to what REST is at its core.
The general premise in REST should always be that a server teaches clients what they need and what a server expects and offers choices to the client via links. If the server expects to receive data from the client it will send a form-esque representation to inform the client about the respective fields it supports and based on the affordance of the respective elements contained in the form a client knows whether to select one or multiple options, enter some free text or enter a date value and such. Unfortunately, most of the media-type formats that attempt to mimic HTML's forms are still in draft versions.
If you take a look at HTML forms in particular you might sense what I'm refering to. Each of the elements that may occur inside a form are well defined to avoid abmiguity and improve interoperability. This is defacto the ultimate goal in REST, having one client that is able to interact with a sheer amount of other services without having to be adapted to each single API explicitely.
The beauty of REST is, it isn't limited to a single representation form, i.e. JSON, in fact there is almost an infinite number of possible representation formats that could be exchanged in a REST environment. Plain application/json is a terrible media-type for REST applications IMO as it doesn't include any defintions in regards to links and forms and doesn't describe the semantics of certain fields that may be shipped in requests and responses. The lack of semantical description usually leads to typed resources where a recipient expects that receiving data from i.e. /api/users returns some specific user data, that may differ from host to host. If you skim through IANA's media type registry you will find a couple of media-type formats you could have used to transfer user-related data and any client supporting these representation formats whold be able to interact with this enpoint without any issues. Fielding himself claimed that
A REST API should spend almost all of its descriptive effort in defining the media type(s) used for representing resources and driving application state, or in defining extended relation names and/or hypertext-enabled mark-up for existing standard media types. Any effort spent describing what methods to use on what URIs of interest should be entirely defined within the scope of the processing rules for a media type (and, in most cases, already defined by existing media types). (Source)
Through content-type negotiation client and server will negotiate about a representation format both support and understand. The question therefore shouldn't be which one to support but how many you want to support. The more media-type your API or client is able to exchange payloads for, the more likely it will be to interact with other participants.
Most of those so-called REST APIs are in reality just RPC services exposed via HTTP that may or may not respect and support certain HTTP operations. HTTP thereby is just a transport layer whose domain is the transfer of files or data over the Web. Plenty of people still believe that you shouldn't put verbs in URIs when in reality a script or process usually doesn't (and shouldn't) care whether a URI contains a verb or not. The URI itself is just a pointer a client will follow and invoke when it is interested in receiving the payload. We humans are also not that much interested in the URI itself in regards to the content it may return after invoking that URI. The same holds true for arbitrary clients. It is more important what you ship along with that URI. On the Web a link can be annotated with certain text and/or link relation names that set the links content in relation to the current page. It may hint a client that certain content may be invoked before the whole response was parsed as it is quite likely that the client will also want to know about that. preload i.e. is such a link-relation name that hints the client about that. If certain domain-specific terms exist one might use an extension scheme as defined by Web linking or reuse common knowlege or special microformats.
The whole interaction in a REST environment is similar to playing a text-based computer game or following a certain process flow (i.e. ordering and paying produts) defined by an application domain protocol, that can be designed as a state machine. The client is therefore guided through the whole process. It basically just follows the orders the server gave it, with some choices to break out of the process (i.e. cancel the order before paying).
SOAP on the otherhand is, as you've stated, an XML-based RPC protocol reusing a subset of HTTP to exchange requests and responses. The likelihood that when you change something within your WSDL plenty of clients have to be adapted and recompiled are quite high. SOAP even defines its own security mechanism instead of reusing TLS, which requires explicit support by the clients therefore. As you have a one-to-one communication model due to the state that may be kept in process, scaling SOAP services isn't that easy. In a REST environment this is just a matter of adding a load-balancer before the server and then mirroring the server n-times. The load-balancer can send the request to any of the servers due to the stateless constraint
What is the 'mixture' typically referred to as (besides, maybe, a bad API). Is there a word for it? or a concept I can refer to that'll make most developers understand what I'm referring to, without having to say the entire paragraph I did above?
Is it just "JSON SOAP API?", "A Service-based REST API?" - what would you call it?
The general term for an API that communicates on top of HTTP would be Web API or HTTP API IMO. This article also uses this term. It also lists XML-RPC and JSON-RPC besides SOAP. I do agree with Voice though that you'll receive 5 answers on asking 4 people about the right term to use. While it would be convenient to have a respective term available everyone would agree upon, the reality shows that people are not that interested in a clear separation. Just look here at SO on the questions taged with rest. There is nothing wrong with not being "RESTful", though one should avoid the term REST for truly RPC services. Though I think we are already in a situation where the term REST can't be rescued from misusage and marketing purposes.
For something that requires external documentation to use and that ships with its own custom, non-standardized representation format or that just exposes CRUD for domain objects I'd add -RPC to it, as this is more or less what it is at its heart. So if the API sends JSON and the representation to expect is documented via Swagger or some other external documentationJSON-RPC would probably the most fitting name IMO.
To sum up this post, I hope I could shed some light on what REST truly is and how your observation is flawed by all those pragmatic attempts that unfortunately are RPC through and through. If you change something within their implementation, how many clients will break? In addition to that you can't reuse the client that you've implemented for API A to interact with API B (of a different company or vendor) out of the box and therefore have to either adapt your client or create a new one solely for that API. This is true RPC and therfore should be reflected in the name somehow to hint developers about future expectations. Unfortunately, the process of naming things propperly, especially in regards to REST, seems already lost. There is a fine but tiny group who attempt to spread the true meaning, like Voice, Cassio and some others, though it is like fighting windmills. The best advice here would be to first discuss the naming conventions and what each participant understand on which term and then agree on a naming scheme everyone agrees on to avoid future confusion.
My understanding of SOAP vs REST
...
Is that a broadly correct assessment?
No.
REST is an "architectural style", which is to say a coordinated collection of architectural constraints. The World Wide Web is an example of an application built using the REST architectural style.
SOAP is a transport agnostic message protocol specification, based on XML Information Set
If so, what do I call an API that is a mixture?
I don't think you are going to find an authoritative terminology here. Colloquially, you are likely to hear the broad umbrella term "web api" to describe an HTTP API that isn't "RESTful".
The whole space is rather polluted by semantic diffusion.
I read that HATEOAS links are the one that separates a REST API from a normal http API. In that case, does REST need a separate name? I wonder what all this hype about REST API is about. It seems to be just a http method with one extra rule in the response.
Q) What other differences exist?
I read that HATEOAS links are the one that separates a REST API from a normal http API.
That's probably a little bit of an understatement. When Leonard Richardson (2008) described the "technology stack" of the web, he listed:
URI
HTTP
HTML
A way of exploring the latter is to consider how HTML, as a media type, differs from a text document with URI in it. To my mind, the key element is links and forms -- standardized ways of encoding into the representation the semantics of a URI (this is a link to another page, this is an embedded image, this is an embedded script, this is a form...).
Mike Admundsen, 2010:
Hypermedia Types are MIME media types that contain native hyper-linking semantics that induce application flow. For example, HTML is a hypermedia type; XML is not.
Atom Syndication/Atom Publishing is a good demonstration for defining a REST API.
Can you throw some light on what REST actually means and how it differs from normal http?
Have you noticed that websites don't normally use plain text for the representations of the information that they share? It's something of a dead end -- raw text doesn't have any hypermedia semantics built into it, so a generic client can't do anything more interesting than search for sequences that might be URI.
On the other hand, with HTML we have link semantics: we can include references to images, to style sheets, to scripts, as well as linking to other documents. We can describe forms, that allow the creation of parameterized HTTP requests.
Additionally, that means that if some relation shouldn't be used by the client, the server can easily change the representation to remove the link.
Furthermore, the use of the hypermedia representation allows the server to use a richer description of which request message should be sent by the client.
Consider, for example, Google. They can use the form to control whether search requests use GET or POST. They can remove the "I Feel Lucky" option, or arrange that it redirects to the main experience. They can embed additional information in to the fields of the form, to track what is going on. They can choose which URI targets are used in the search results, directing the client to send to Google another request which gets redirected to the actual target, with additional meta data embedded in the query parameters, all without requiring any special coordination with the client used.
For further discussion, see Leonard Richardson's slide deck from QCon 2008, or Phil Sturgeon's REST and Hypermedia in 2019.
Does n't think the client need to read the documentation if the HATEOAS link is a POST API? HATEOAS links will only guide you to an API but will not throw any light on how its request body needs to be filled....GET won't have request body. So, not much or a problem. but POST API?
Sort of - here's Fielding writing in 2008:
REST doesn’t eliminate the need for a clue. What REST does is concentrate that need for prior knowledge into readily standardizable forms.
On the web, the common use case is agents assisting human beings; the humans can resolve certain ambiguities on their own. The result is a separation of responsibilities; the humans decode the domain specific semantics of the messages, the clients determine the right way to describe an interaction as an HTTP request.
If we want to easily replace the human with a machine, then we'll need to invest extra design capital in a message schema that expresses the domain specific semantics as clearly as we express the plumbing.
To me, REST is an ideology you want to aim for if you have a system that should last for years to come which has the freedom to evolve freely without breaking stuff on parts you can't control. This is very similar to the Web where a server can't control browsers directly though browsers are able to cooperate with any changes done to Web site representations returned by the server.
I read that HATEOAS links are the one that separates a REST API from a normal http API. In that case, does REST need a separate name?
REST does basically what its name implies, it transfers the state of a resource representation. If so, we should come up with a new name for such "REST" APIs that are truly RPC in the back, to avoid confusion.
If you read through the Richardson Maturity Model (RMM) you might fall under the impression that links or hypermedia controls as Fowler named it, which are mandatory at Level 3, are the feature that separates REST from normal HTTP interaction. However, Level 3 is just not enough to reach the ultimate goal of decoupling.
Most so called "REST APIs" do put a lot of design effort into pretty URIs in a way to express meaning of the target resource to client developer. They come up with fancy documentation generated by their tooling support, such as Swagger or similar stuff, which the client developer has to follow stringent or they wont be able to interact with their API. Such APIs are RPC though. You won't be able to point the same client that interacts with API A to point to API B now and still work out of the box as they might use completely different endpoints and return different types of data for almost the same named resource endpoint. A client that is attempting to use a bit more of dynamic behavior might learn the type from parsing the endpoint and expect a URI such as .../api/users to return users, when all of a sudden now the API changed its URI structure to something like .../api/entities. What would happen now? Most of these clients would break, a clear hint that the whole interaction model doesn't follow the one outline by a REST architecture.
REST puts emphasis on link relation names that should give clients a stable way of learning the URIs intent by allowing a URI to actually change over time. A URI basically is attached to a link relation name and basically represents an affordance, something that is clear what it does. I.e. the affordance of a button could be that you can press it and something would happen as a result. Or the affordance of a light switch would be that a light goes on or off depending on the toggled state of the light switch.
Link relation names now express such an affordance and are a text-based way to represent something like a trash bin or pencil symbol next to table entry on a Web page were you might figure out that on clicking one will delete an entry from the table while the other symbol allows to edit that entry. Such link relation names should be either standardized, use widely accepted ontologies or use custom link-relation extensions as outlined by RFC 8288 (Web Linking)
It is important to note however, that a URI is just a URI which should not convey a semantic meaning to a client. This does not mean that a URI can't have a semantic meaning to the server or API, but a client should not attempt to deduce one from the URI itself. This is what the link-relation name is for, which provides the infrequently changing part of that relation. An endpoint might be referenced by multiple, different URIs, some of which might use different query parameters used for filtering. According to Fielding each of these URIs represent different resources:
The definition of resource in REST is based on a simple premise: identifiers should change as infrequently as possible. Because the Web uses embedded identifiers rather than link servers, authors need an identifier that closely matches the semantics they intend by a hypermedia reference, allowing the reference to remain static even though the result of accessing that reference may change over time. REST accomplishes this by defining a resource to be the semantics of what the author intends to identify, rather than the value corresponding to those semantics at the time the reference is created. It is then left to the author to ensure that the identifier chosen for a reference does indeed identify the intended semantics. (Source 6.2.1)
As URIs are used for caching results, they basically represent the keys used for caching the response payload. As such, it gets obvious that on adding additional query parameters to URIs used in GET requests, you end up bypassing caches as the key is not stored in the cache yet and therefore get the result of a different resource, even though it might be identical (also in response representation) as the URI without that additional parameter.
I wonder what all this hype about REST API is about. It seems to be just a http method with one extra rule in the response.
In short, this is what those self- or marketing-termed pseudo "REST APIs" do convey and many people seem to understand.
The hype for "REST" arose from the inconveniences put onto developers on interacting with other interop-solutions such as Corba, RMI or SOAP where often partly-commercial third-party libraries and frameworks had to be used in order to interact with such systems. Most languages supported HTTP both as client and server out of the box removing the requirement for external libraries or frameworks per se. In addition to that, RPC based solution usually require certain stub- or skeleton-classes to be generated first, which was usually done by the build pipeline automatically. Upon updates of the IDL, such as WSDL linking or including XSD schemata, the whole stub-generation needed to be redone and the whole code needed to looked through in order to spot whether a breaking change was added or not. Usually no obvious changelog was available which made changing or updating such stuff a pain in the ...
In those pseudo "REST" APIs plain JSON is now pretty much the de facto standard, avoiding the step of generating stub classes and the hazzle of analyzing the own code to see whether some of the forced changes had a negative impact on the system. Most of those APIs use some sort of URI based versioning allowing a developer to see based on the URI whether something breaking was introduced or not, mimicking some kind of semantic versioning.
The problem with those solution though is, that not the response representation format itself is versioned but the whole API itself leading to common issues when only a change on a part of the API should be introduced as now the whole API's version needs to be bumped. In addition to that, to URIs such as .../api/v1/users/1234 and .../api/v2/users/1234 may represent the same user and thus the same resource though are in fact different by nature as the URI is different.
Q) What other differences exist?
While REST is just an architecture model that can't force you to implement it stringent, you simply will not benefit from its properties if you ignore some of its constraints. As mentioned above, HATEOAS support is therefore not yet enough to really decouple all clients from an API and thus allow to benefit from the REST architecture.
RMM unfortunately does not talk about media types at all. A media type basically specifies how a received payload should be processed and defines the semantics and constraints of each of the elements used within that payload. I.e. if you look at text/html registered in IANA's media type registry, you can see that it points to the published specification, which always references the most recent version of HTML. HTML is designed in a way to stay backwards compatible so no special versioning stuff is required.
HTML provides, IMO, two important things:
semi-structured content
form support
The former one allows to structure data, giving certain segments or elements the possibility to express different semantics defined in the media type. I.e. a browser will handle an image differently than a div element or an article element. A crawler might favor links and content contained in an article element and ignore script and image elements completely. Based on the existence or absence of certain elements even certain processing differences may occur.
Including support for forms is a very important thing in REST actually as this is the feature which allows a server to teach a client on what a server needs as input. Most so called "REST APIs" just force a developer to go through their documentation, which might be outdated, incorrect or incomplete, and send data to a predefined endpoint according to the documentation. In case of outdated or incomplete documentation, how should a client ever be able to send data to the server? Moreover, a server might never be able to change as basically the documentation is now the truth and the API has to align with the documentation.
Unfortunately, form-support is still a bit in its infancy. Besides HTML, which provides <form>...</form>, you have a couple of JSON based form attempts such as hal-forms, halo-json (halform), Ion or hydra. None of these have yet wide library or framework support yet as some of these form representations still have not finalized their specification on how to support forms more effectively.
Other media-types, unfortunately, might not use semi-structured content or provide support for forms that teach a client on the needs of a server, though they are still valuable to REST in general. First, through Web linking link support can be added to media types that do not naturally support those. Second, the data itself does not really need to be text-based at all in order for an application to use it further. I.e. pictures an videos usually are encoded and byte based anyways still a client can present them to users.
The main point about media-types though is, as Fielding already pointed out in one of his cited blog posts, is, that representations shouldn't be confused with types. Fielding stated that:
A REST API should never have “typed” resources that are significant to the client. Specification authors may use resource types for describing server implementation behind the interface, but those types must be irrelevant and invisible to the client. The only types that are significant to a client are the current representation’s media type and standardized relation names.
Jørn Wildt explained in an excellent blog post what a "typed" resource is and why a REST architecture shouldn't use such types. Basically, to sum the blog post up, a client expecting a ../api/users endpoint to return a pre-assumed data payload might break if the server adds additional, unexpected fields, renames existing fields or leave out expected fields. This coupling can be avoided by using simple content-type negotiation where a client informs a server on which capabilities the client supports and the client will chose the representation that best fits the target resource. If the server can't support the client with a representation the client supports the server should respond with a failure (or a default representation) the client might log to inform the user.
This in essence is exactly what the name REST stands for, the transfer of a resource's state representation where the representation may differ depending on the representation format defined by the selected media type. While HATEOAS may be one of the most obvious changes between REST and a non-REST based HTTP solution, this for sure is not the only factor that makes up a payload in REST. I hope I could shed some light on the decoupling intention and that a server should teach clients what the server expects through forms and that the affordance of URIs is captured by link-relation names. All these tiny aspects in sum make up REST, and you will only benefit from REST, unfortunately, if you respect all of its constraints and not only those that are either easy to obtain or what you have the mood for implementing.
a basic REST question.. I design a REST API and would like to be able to get a list of book recommendations based on a book id (i.e. client sends book id=w to server and server replies with a list of recommended books, id=x,y,z).
I see two ways to do this:
/recommendation?bookId=thetitle
/recommendation/thetitle
Option 2 seems a bit cleaner to me but I'm not sure if it would be considered good REST design? Because /recommendation/thetitle looks like an element URI, not a collection URI (although in this case it would return a collection). Also, the first part of the resource (/recommendation) would not make any sense by itself.
Thankful for any advice.
URL patterns of this kind have nothing to do with REST. None of the defining properties of REST requires readable URLs.
At the same time, one of the core principles (HATEOAS), if followed properly, allows API clients (applications, not people!) to browse the API and obtain every link required to perform a desired transition of application state or resource state based on a well known message format.
If you feel your API must have readable URLs, it's a good sign that its design probably isn't RESTful at all. This implies the need for a developer to understand the URL structure and hardcode it somewhere in a client application. Something that REST is supposed to avoid by principle.
To quote Roy Fielding's blog post on the subject:
A REST API must not define fixed resource names or hierarchies (an obvious coupling of client and server). Servers must have the freedom to control their own namespace. Instead, allow servers to instruct clients on how to construct appropriate URIs, such as is done in HTML forms and URI templates, by defining those instructions within media types and link relations. [Failure here implies that clients are assuming a resource structure due to out-of band information, such as a domain-specific standard, which is the data-oriented equivalent to RPC’s functional coupling].
Obviously, nothing stops you from actually making URLs meaningful regardless of how RESTful your API actually is. Even if it's for a purpose not dictated by REST itself (viewing the logs left by a client of a properly RESTful API could be easier for a human if they're readable, off the top of my head).
Finally, if you're fine with developing a Web API that's not completely RESTful and you expect developers of clients to read this kind of docs and care about path building, you might actually benefit from comprehensible URLs. This can be very useful in APIs of the so-called levels 0-3, according to Richardson's maturity model.
What's important in terms of REST is how you're leveraging the underlying protocol (HTTP in this case) and what it allows you to do. If we consider your examples from this perspective, /recommendation/thetitle seems preferable. This is because the use of query parameters may prevent responses from being cached by browsers (important if you're writing a JS client) or proxies, making it harder to reuse existing tools and infrastructure.