RESTXQ

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This page presents one of the Web Application services. It describes how to use the RESTXQ API of BaseX.

RESTXQ, introduced by Adam Retter, is a new API that facilitates the use of XQuery as a Server Side processing language for the Web. RESTXQ has been inspired by Java’s JAX-RS API: it defines a pre-defined set of XQuery 3.0 annotations for mapping HTTP requests to XQuery functions, which in turn generate and return HTTP responses.

Note that various details of the specification may be subject to change due to the early state of the API. Next, with Version 7.7, the RESTXQ prefix has been changed from restxq to rest.

Usage

Since Version 7.7, the RESTXQ service is by default available at http://localhost:8984/.

All RESTXQ annotations are assigned to the http://exquery.org/ns/restxq namespace, which is statically bound to the restxq prefix (and since Version 7.7: rest). A Resource Function is an XQuery function that has been marked up with RESTXQ annotations. When an HTTP request comes in, a resource function will be invoked that matches the constraints indicated by its annotations.

Whenever a RESTXQ URL is requested, the RESTXQPATH module directory and its sub-directories will be parsed for library modules (detected by the extension .xqm) and functions with RESTXQ annotations. Since Version 7.7, also main modules (detected by .xq) will be parsed. All modules will be cached and parsed again when their timestamp changes.

A simple RESTXQ module is shown below. It is part of a clean installation and available at http://localhost:8984/ .

(: simplified version of the function found in webapp/restxq.xqm :)
module namespace page = 'http://basex.org/examples/web-page';

declare %rest:path("hello/{$world}")
        %rest:GET
        %rest:header-param("User-Agent", "{$agent}")
        function page:hello($world as xs:string, $agent as xs:string*) {
  <response>
    <title>Hello { $world }!</title>
    <info>You requested this page with { $agent }.</info>
  </response>
};

If the URI http://localhost:8984/hello/world is accessed, the result will be similar to:

<response>
  <title>Hello World!</title>
  <time>The current time is: 18:42:02.306+02:00</time>
</response>

The RESTXQ module contains yet another function:

declare
  %rest:path("/form")
  %rest:POST
  %rest:form-param("message","{$message}", "(no message)")
  %rest:header-param("User-Agent", "{$agent}")
  function page:hello-postman(
    $message as xs:string,
    $agent   as xs:string*)
    as element(response)
{
  <response type='form'>
    <message>{ $message }</message>
    <user-agent>{ $agent }</user-agent>
  </response>
};

If you post something (e.g. using curl or the embedded form at http://localhost:8984/)...

curl -i -X POST --data "content='CONTENT'" http://localhost:8984/form

...you will receive something similar to the following result:

<response type="form">
  <message>CONTENT</message>
  <user-agent>Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML
    like Gecko) Chrome/28.0.1500.72 Safari/537.36</user-agent>
</response>

Annotations

This section lists all annotations provided by RESTXQ.

Constraints

Constraints restrict the HTTP requests that a resource function may process.

Paths

A resource function must have a single Path Annotation with a single string as argument. The function will be called if a URL matches the path segments and templates of the argument. Path templates contain variables in curly brackets, and map the corresponding segments of the request path to the arguments of the resource function.

The following example contains a path annotation with three segments and two templates. One of the function arguments is further specified with a data type, which means that the value for $variable will be cast to an xs:integer before being bound:

declare %rest:path("/a/path/{$with}/some/{$variable}")
  function page:test($with, $variable as xs:integer) { ... };

HTTP Methods

The HTTP method annotations are equivalent to all HTTP request methods except for TRACE and CONNECT. Zero or more methods may be used on a function; if none is specified, the function will be invoked for each method.

The following function will be called if GET or POST is used as request method:

declare %rest:GET %rest:POST %rest:path("")
  function page:root() { <html/> };

The POST and PUT annotations may optionally take a string literal, which will be mapped to a named function parameter. Once again, the target variable must be embraced by curly brackets:

declare %rest:PUT("{$data}") %rest:path("")
  function page:put($data) { "Data: " || $data };

Content Negotiation

Two following annotations can be used to restrict functions to specific content types:

  • HTTP Content Types: a function will only be invoked if the HTTP Content-Type header of the request matches one of the given mime types. Example:
%rest:consumes("application/xml", "text/xml")
  • HTTP Accept: a function will only be invoked if the HTTP Accept header of the request matches one of the defined mime types. Example:
%rest:produces("application/atom+xml")

By default, both mime types are */*. Note that this annotation will not affect the content-type of the HTTP response. Instead, you will need to add a %output:media-type annotation.

Parameters

Parameters are optional annotations that can be used to bind additional values to function arguments:

Query Parameters

The value of the first parameter, if found in the query string, will be assigned to the variable specified as second parameter. If no value is specified in the HTTP request, all additional parameters, if available, will be bound to the variable:

%rest:query-param("parameter", "{$value}", "default")
%rest:query-param("answer", "{$answer}", 42, 43, 44)
%rest:query-param("search", "{$search-param}")

HTML Form Fields

Form parameters are specified the same way as query parameters. Their values are extracted from GET or POST requests.

%rest:form-param("parameter", "{$value}", "default")

HTTP Headers

Header parameters are specified the same way as query parameters:

%rest:header-param("User-Agent", "{$user-agent}")
%rest:header-param("Referer", "{$referer}", "none")

Cookies

Cookie parameters are specified the same way as query parameters:

%rest:cookie-param("username", "{$user}")
%rest:cookie-param("authentication", "{$auth}", "no_auth")

Response

By default, a successful request is answered with the HTTP status code 200 (OK), and is followed by the given content; an erroneous request leads to an error code and an optional error message (e.g. 404 for “resource not found”).

Custom Responses

Custom responses can be built from within XQuery by returning an rest:response element, an http:response child node that matches the syntax of the EXPath HTTP Client Module specification, and more optional child nodes that will be serialized as usual. A function that reacts on an unknown resource may look as follows:

declare %rest:path("") function page:404() {
  <rest:response>
    <http:response status="404" message="I was not found.">
      <http:header name="Content-Language" value="en"/>
      <http:header name="Content-Type" value="text/html; charset=utf-8"/>
    </http:response>
  </rest:response>
};

Forwards and Redirects

The two XML elements rest:forward and rest:redirect can be used in the context of Web Applications, precisely in the context of RESTXQ. These nodes allow e.g. multiple XQuery Updates in a row by redirecting to the RESTXQ path of updating functions. Both wrap a URL to a RESTXQ path. The wrapped URL should be properly encoded via fn:encode-for-uri().

Note that, currently, these elements are not part of RESTXQ specification.

rest:forward

Usage: wrap the location as follows

<rest:forward>{ $location }</rest:forward>

This results in a server-side forwarding, which as well reduces traffic among client and server. A forwarding of this kind will not change the URL seen from the client's perspective.

As an example, returning

<rest:forward>/hello/universe</rest:forward>

would internally forward to http://localhost:8984/hello/universe

rest:redirect

<rest:redirect>{ $location }</rest:redirect>

…is basically an abbreviation for…

<rest:response>
  <http:response status="302" message="Temporary Redirect">
    <http:header name="location" value="{ $location }"/>
  </http:response>
</rest:response>

The client decides whether to follow this redirection. Browsers usually will, tools like curl won’t unless -L is specified.

Output

Similar to the REST interface, result serialization can be modified via XQuery 3.0 serialization parameters; in RESTXQ, serialization parameters may be specified in the query prolog, via annotations, or within REST response element. Global parameters are overwritten by more local parameters.

Query Prolog

In main modules, serialization parameters may be specified in the query prolog. These parameters will then apply to all functions in a module. In the following example, the content type of the response is overwritten with the media-type parameter:

declare option output:media-type 'text/plain';

declare %rest:path("version1") function page:version1() {
  'Keep it simple, stupid'
};

Annotations

The serialization can also be parameterized via annotations:

declare %output:media-type("text/plain") %rest:path("version2") function page:version2() {
  'Still somewhat simple.'
};

Response Element

The following example demonstrates how serialization parameters can be dynamically set within a query:

declare %rest:path("version3") function page:version3() {
  <rest:response>
    <output:serialization-parameters>
      <output:media-type value='text/plain'/>
    </output:serialization-parameters>
  </rest:response>,
  'Well, not that simple anymore'
};

The content type can also be overwritten by specifying an output method. The following method mappings are available:

  • xmlapplication/xml
  • xhtmltext/html
  • htmltext/html
  • texttext/plain
  • rawapplication/octet-stream
  • json or jsonmlapplication/json

By default, application/xml is returned as content type. In the following example, XHTML headers will be generated, and text/html will be set as content type:

declare
  %rest:path("")
  %output:method("xhtml")
  %output:omit-xml-declaration("no")
  %output:doctype-public("-//W3C//DTD XHTML 1.0 Transitional//EN")  
  %output:doctype-system("http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd")
  function page:start()
{
  <html xmlns="http://www.w3.org/1999/xhtml">
    <body>done</body>
  </html>
};

Error Handling

Introduced with Version 7.7:

XQuery runtime errors can be processed via error annotations. A single argument must be supplied, which represents the QName of the error to be caught. A wildcard * may be specified to catch all possible errors. A function can only have a single error annotation:

declare %rest:error("*") function page:error() {
  "An error occurred while processing your RESTXQ code!"
};

The XQuery try/catch construct assigns error information to a number of pre-defined variables (code, description, value, module, line-number, column-number, additional). These variables can be bound to variables via error parameter annotations, which are specified the same way as query parameters.

Errors may occur unexpectedly. However, they can also be triggered by a query, as the following example shows:

declare %rest:path("/check/{$user}") function page:check($user) {
  if($user = ('jack', 'lisa'))
  then 'User exists'
  else fn:error(xs:QName('err:user'), $user)
};

declare %rest:error("err:user") %rest:error-param("description", "{$user}")
  function page:user-error($user) {
  'User "' || $user || '" is unknown'
};

Errors that occur outside RESTXQ can be caught by adding error-page elements with an error code and a target location to the web.xml configuration file (find more details in the Jetty Documentation):

<error-page>
  <error-code>404</error-code>
  <location>/error404</location>
</error-page>

The target location may be another RESTXQ function. The request:attribute function can be used to request details on the caught error:

declare %rest:path("/error404") function page:error404() {
  "URL: " || request:attribute("javax.servlet.error.request_uri") || ", " || 
  "Error message: " || request:attribute("javax.servlet.error.message")
};

Functions

The Request Module contains functions for accessing data related to the current HTTP request. Two additional modules exist for setting and retrieving server-side session data of the current user (Session Module) and all users known to the HTTP server (Sessions Module). With Version 7.7, a RESTXQ Module was added that provides functions for requesting RESTXQ base URIs and generating a WADL description of all services. Please note that the namespaces of all of these modules must be explicitly specified via module imports in the query prolog.

The following example returns the current host name:

import module namespace request = "http://exquery.org/ns/request";

declare %rest:path("/host-name") function page:host() {
  'Remote host name: ' || request:remote-hostname()
};

File Upload

Introduced with Version 7.7:

By using the content type multipart/form-data uploaded files can be handled at server-side using RestXQ. A user is therefore able to upload files using a standard file upload input element. Suppose you have a simple form:

<form name="myForm" action="/upload" method="POST" enctype="multipart/form-data">
  <input type="hidden" name="example1" value="test" />
  <input type="file" name="files[]" multiple="multiple" />
</form>

All input elements will be put into a map by using the element name as key. File elements will be treated differently as multiple files can be uploaded at once using the multiple attribute. All files will be put into a map using the file name as key and this map is accessible using files as key of the original map. See the following example to store all uploaded files into a specific directory:

declare
  %restxq:POST("{$data}")
  %restxq:path("/upload")
  function page:upload($data)
{
  let $example1 := $data('example1')
  let $files := $data('files')
  for $name in map:keys($files)
  let $content := $files($name)
  let $size := count(convert:binary-to-bytes($content))
  return file:write-binary("files/", $content)
};

The value of the hidden form element example1 is stored into $example1. Additionally, you can calculate the size of the uploaded file using count(convert:binary-to-bytes($content)).

References

RESTXQ has been proposed by Adam Retter. More information on all specifics can be found in the following two documents:

Changelog

Version 7.7
  • Updated: RESTXQ function may now also be specified in main modules (suffix: *.xq).
  • Updated: the RESTXQ prefix has been changed from restxq to rest.
  • Added: Error Handling
Version 7.5
  • Added: new XML elements <rest:redirect/> and <rest:forward/>