Representational State Transfer (REST) is an architectural style that is a coordinated set of constraints which also apply to the web. It aims at simplifying component implementations, reducing the complexity of distributed software elements, improving the performance, and increasing the scalability. In relation to the definition of a RESTful application programming interface (API) the uniform interface constraint is of high importance. It simplifies and decouples the architecture and makes REST components independent. The constraints for a uniform interface can be reduced to: the identification of resources, the self-descriptive representation of resources, and the self-descriptive manipulation of resources.
REST systems typically communicate over the Hypertext Transfer Protocol (HTTP) and are gaining large acceptance due to its growing support and its simplicity for implementation. RESTful web services are in this context a simpler alternative to SOAP  and WSDL-based Web services which are specified for the use with XMPP in SOAP over XMPP (XEP-0072)  and also a more powerful alternative to XML-RPC  which are specified as XMPP extension in Jabber-RPC (XEP-0009) .
The HTTP Over XMPP Transport (XEP-0332)  allows for designing REST services in the context of XMPP, but requires an implementation of both protocols: XMPP and HTTP. Furthermore, HTTP was selected in the past because of its degree of popularity, but has some drawbacks like the lack of discoverability of services. The REST with XMPP extension is a powerful protocol for cloud services that has several advantages in contrast to the traditional HTTP-based REST approach:
The REST with XMPP protocol makes use of the <iq/> stanza in order to enable access, to create, to delete, or to modify resources within a XMPP network overlay. This specification defines two XML Schema files: one for exploring the capabilities of a resource and one for performing actions on a resource.
The author has designed the REST with XMPP protocol with the following requirements in mind:
REST with XMPP should be easy to implement such as it is with REST over HTTP.
This specification should apply the REST architectural style to XMPP and should eliminate limitations of HTTP.
Resources should be linkable in terms of static connections as well as link targets used for resource access and modifications.
The number of request and response parameters, representations and links should be unbounded.
The number of operations should be unlimited as in contrast to HTTP's GET, POST, PUT, DELETE methods.
In order to explore the capabilities of a resource, the <iq/> stanza type "get" have to be used. The returned <iq/> stanza is either of type "error" or "result". If it is of type "result", the returned content has to be complied with the xwadl schema of this specification. The xwadl schema has been designed for providing a machine process-able description of a resource. It was inspired by the Web Application Description Language (WADL) standard delivered by the World Wide Web Consortium (W3C).
An <iq/> stanza of type "get" is returning an <iq/> stanza of type "result" that describes all actions that the requesting party can perform. The following example shows an exploration of a cloud provider's REST based interface for handling compute services.
In order to explore a resource, only the path to a resource is required. The counter party has to answer of such a request with a response that exposes all possible actions which can be performed on the resource located at the specified path. The following example illustrates a response that exposes all actions for this resource.
This response exposes two methods that can be performed on the resource located at "/compute". The first method "create" can be used to create one or more virtual machines (VM). This method has three input parameters in its request and one output parameter in its response. If a client would like to perform this method, at least only a link to the location of an image is required. The other two parameters are optional. The server will respond to this method with a list of links to the instantiated VMs. A detailed example of how to access this method is illustrated in the Resource Access section.
The following subsections describe each component of a xwadl document in detail.
The "resource_type" element forms the root of a xwadl document and MAY comprises the following sub-elements: "doc", "grammars", and "method"
Each xwadl-defined element down to the "param" can have one or more child "doc" elements that can be used to document that element. The doc element has "title" attributes which is a short plain text description of the element being documented. The "doc" element can have mixed content and may contain text and zero or more child elements.
The "grammars" element acts as a container for definitions of the format of data exchanged during execution of the protocol described by the xwadl document and SHOULD be according to the XML Schema definition.
A method element describes the specific actions that can be performed on a resource targeted by the "path" attribute of the "resource_type" element. A method element is a child of a "resource_type" element and has a "name" attribute that identifies this method.
Additionally, each "method" MUST have one "request" and one "response" element which can be empty or be used to expose the optional parameters that this method can execute.
The "request" and the "response" element are according to the xwadl schema of type "call". They are identical by definition and describe the input and output data for accessing a resource.
Both elements describe the information to be included when applying a method to a resource. Both elements have no attributes and may contain one or more "param" elements as child elements.
A "param" element describes a parameterized component of its parent element (either a request or a response). It can be identified by its "name" attribute and MUST have a minimum of one "option" element that defines one of a set of possible values for the parameter. In order to parameterize a component, the "param" element SHOULD specify a combination of the following attributes:
Some combinations of attributes do not make sense, e.g. the specification of "default=?" and "repeating=true", and SHOULD be considered application specific.
An "option" element defines one of a set of possible types, representations, link type, or also values for the parameter. An "option" element MUST have one of the following attributes:
The "type" attribute indicates one possible type of the parameter as an XML qualified name, defaults to xs:string. It SHOULD specify the type of a single optional value. Multiple options with the same type but different values SHOULD specify a set of possible values which are acceptable as input for the parent parameter.
The "mediaType" attribute indicates that the parent parameter acts as a media type selector for requests or responses. The value of the attribute is the media type that is expected. If a representation of an OPTIONAL media type is exposed, this representation can act as a template for manipulating a resource.
The "link" attribute is used to identify links to resources. It can have the value local, remote, or list. A local link links to another resource located at the same server entity. A remote link links to a resource located at another server entity anywhere in the XMPP network overlay. A link with a list value indicates a list of remote links that can be used for discovery or linking to a set of resources.
In order to access a resource, the <iq/> stanza type "set" has to be used. The returned <iq/> stanza is either of type "error" or "result". If it is of type "result", the returned content has to be complied with the xml-rest schema of this specification. The xml-rest schema has been designed for providing a xml-rest encoded payload for accessing a resource. An <iq/> stanza MUST NOT contain more than one method element with one request and one response. The following example illustrates how the create method of the previous example is requested. Here, the client requests three VMs which are based on an image that is available as resource on the client's side.
In order to make sure that both parties have a common understanding, the requester specifies also the expected responds type which has been exposed during the exploration step. The counter party has to answer such a request with the same request element and an extended complitation of the response element as illustrated in the example below.
The following subsections describe each component of a xml-rest document in detail.
The "resource" element forms the root of an xml-rest document and MUST comprise only a single "method" sub-element. In contrast to the xwadl description, no further documentation or grammers are possible in order to keep the number of bytes as low as possible. In order to specify the resource to access, the "path" attribute is required.
The "method" element MUST have one request element and one response element. Additionally, the "name" attribute is required in order to identify the action that has to be performed on the resource.
The "request" and the "response" element are according to the xml-rest schema of type "call". They are identical by definition and including the information for applying the method of a resource. Both elements have no attributes and may contain one or more "param" elements as child elements.
Each "param" element has a "name" attribute to identify the parameter and a single sub-element that includes the data. Possible sub-elements are value, representation, link, or resourceList.
The "value" element has a "type" attribute that specifies the type of the value.
The "representation" element has a "mediaType" attribute that specifies the media type of the representation.
A "link" element or a "resourceList" element have no attributes. While a link targets to a single local or remote location, a resource list is a set of links which are targeting to any resource in an XMPP overlay network. A link can be expressed in two forms: as XMPP resource link with a "to" (e.g. <to>firstname.lastname@example.org/rest-client</to>) and a "path" (e.g. <path>/images/myLinuxImage</path>) element or as URI (<uri>xmpp://email@example.com/rest-client?/images/myLinuxImage</uri>).
The REST with XMPP protocol enables a multi-dimensional resource placement. The following examples show how different resources can be placed within a single server entity:
All examples above are accessing different resources. A variation of the path attribute of the resource element would also combine the presented JIDs and referring to different resources. Further examples are possible to show how different resources can be located at a single server entity.
If an entity supports the REST with XMPP protocol, it SHOULD advertise that fact in response to Service Discovery (XEP-0030)  information ("diso#info") requests by returning an identity of "automation/rest" and a feature of "jabber:iq:rest":
Determining when and how a resource can be accessed or modified based on permissions or rights are considered outside the scope of this document. Although such mechanisms SHOULD be considered specifically to the application and/or implementation of this document, future specifications may address these concerns.
The XMPP Registrar includes a Service Discovery type of "rest" within the "automation" category in its registry of service discovery identities.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <email@example.com>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
7. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/