Difference between revisions of "RESTXQ"
Line 376: | Line 376: | ||
==Query Execution== | ==Query Execution== | ||
− | In many | + | In many web search scenarios, user input from browser forms is processed and search results are returned. Such operations can be made more interactive by sending a new search request to the server with each key click. However, this may lead to many parallel server-side requests, from which only the result of the last request will be relevant for the client. |
− | With the <code>%rest:single</code> annotation, it can be enforced that only one instance of a function will | + | With the <code>%rest:single</code> annotation, it can be enforced that only one instance of a function will run at the same time and for the same client. If the same function will be called for the second time, a currently executed query will be stopped, and the HTTP error code {{Code|460}} will be returned instead: |
<syntaxhighlight lang="xquery"> | <syntaxhighlight lang="xquery"> | ||
Line 394: | Line 394: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
− | By | + | By adding a string value to with the annotation, functions can be bundled together, and a running query can be canceled by calling another one that has the same annotation value. This is shown by another example, in which the first function can be interrupted by the second one. If you call both functions in separate browser tabs, you will note that the first tab will return <code>460</code>, and the second one will return <xml>stopped</xml>. |
− | |||
− | This is shown by another example, in which the first function can be interrupted by the second one. If you call both functions in separate browser tabs, you will note that the first tab will return <code>460</code>, and the second one will return <xml>stopped</xml>. | ||
<syntaxhighlight lang="xquery"> | <syntaxhighlight lang="xquery"> |
Revision as of 15:42, 10 May 2021
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 an API that facilitates the use of XQuery as a server-side processing language for the Web. It has been inspired by the Java JAX-RS API: It provides a pre-defined set of XQuery 3.0 annotations for mapping HTTP requests to XQuery functions, which in turn generate and return HTTP responses.
Please note that BaseX provides various extensions to the original draft of the specification:
- Multipart types are supported, including
multipart/form-data
- A
%rest:error
annotation can be used to catch XQuery errors - Servlet errors can be redirected to other RESTXQ pages
- A RESTXQ Module provides some helper functions
- Parameters are implicitly cast to the type of the function argument
- The Path Annotation can contain regular expressions
%input
annotations, support for input-specific content-type parameters%rest:single
annotation to cancel running RESTXQ functions- Quality factors in the Accept header will be evaluated
- Support for server-side quality factors in the
%rest:produces
annotation - Better support for the OPTIONS and HEAD methods
Contents
Introduction
Preliminaries
The RESTXQ service is accessible via http://localhost:8984/
.
All RESTXQ annotations are assigned to the http://exquery.org/ns/restxq
namespace, which is statically bound to the rest
prefix. 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.
If a RESTXQ URL is requested, the RESTXQPATH
module directory and its sub-directories will be traversed, and all XQuery files will be parsed for functions with RESTXQ annotations. Sub-directories that include an .ignore
file will be skipped.
To speed up processing, the functions of the existing XQuery modules are automatically cached in main memory:
- Functions will be invalidated and parsed again if the timestamp of their module changes.
- File monitoring can be adjusted via the
PARSERESTXQ
option. In productive environments with a high load, it may be recommendable to change the timeout, or completely disable monitoring. - If files are replaced while the web server is running, the RESTXQ module cache should be explicitly invalidated by calling the static root path
/.init
or by calling the rest:init function.
Examples
A first RESTXQ function is shown below:
<syntaxhighlight lang="xquery"> module namespace page = 'http://basex.org/examples/web-page';
declare %rest:path("hello/{$who}") %rest:GET function page:hello($who) {
<response> <title>Hello { $who }!</title> </response>
}; </syntaxhighlight>
If the URI http://localhost:8984/hello/World is accessed, the result will be:
<syntaxhighlight lang="xml"> <response>
<title>Hello World!</title>
</response> </syntaxhighlight>
The next function demonstrates a POST request:
<syntaxhighlight lang="xquery"> declare
%rest:path("/form") %rest:POST %rest:form-param("message","{$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>
}; </syntaxhighlight>
If you post something (e.g. using curl or the embedded form at http://localhost:8984/)...
<syntaxhighlight lang="shell"> curl -i -X POST --data "message='CONTENT'" http://localhost:8984/form </syntaxhighlight>
...you will receive something similar to the following result:
<syntaxhighlight lang="shell"> HTTP/1.1 200 OK Content-Type: application/xml; charset=UTF-8 Content-Length: 107 Server: Jetty(8.1.11.v20130520) </syntaxhighlight>
<syntaxhighlight lang="xml"> <response type="form">
<message>'CONTENT'</message> <user-agent>curl/7.31.0</user-agent>
</response> </syntaxhighlight>
Request
This section shows how annotations are used to handle and process HTTP requests.
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 first slash in the path is optional.
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 $variable
will be cast to an xs:integer
before being bound:
<syntaxhighlight lang="xquery"> declare %rest:path("/a/path/{$with}/some/{$variable}")
function page:test($with, $variable as xs:integer) { ... };
</syntaxhighlight>
Variables can be enhanced by regular expressions:
<syntaxhighlight lang="xquery"> (: Matches all paths with "app" as first, a number as second, and "order" as third segment :) declare %rest:path("app/{$code=[0-9]+}/order")
function page:order($code) { ... };
(: Matches all other all paths starting with "app/" :) declare %rest:path("app/{$path=.+}")
function page:others($path) { ... };
</syntaxhighlight>
If multiple path candidates are found for the request, the one with more segments will be preferred.
Content Negotiation
Functions can be restricted to specific Media Types. The default type is */*
. Multiple types can either be specified by a single or by multiple annotations.
Consuming Data
A function will only be taken into consideration if the HTTP Content-Type
header of the request matches one of the given types:
<syntaxhighlight lang="xquery"> declare
%rest:POST("{$body}") %rest:path("/xml") %rest:consumes("application/xml") %rest:consumes("text/xml")
function page:xml($body) { $body }; </syntaxhighlight>
Producing Data
A function will only be chosen if the HTTP Accept
header of the request matches one of the given types:
<syntaxhighlight lang="xquery"> declare
%rest:path("/xml") %rest:produces("application/xml", "text/xml")
function page:xml() { <xml/> }; </syntaxhighlight>
Note that the annotations will not affect the type of the actual response: You will need to supply an additional %output:media-type
annotation or (if a single function may produce results of different types) generate an apt Custom Response.
Quality Factors
A client can supply quality factors to influence the server-side function selection process. If a client sends the following HTTP header with quality factors…
Accept: */*;q=0.5,text/html;q=1.0
…and if two RESTXQ functions exist for the addressed path with two different annotations for producing data…
<syntaxhighlight lang="xquery"> declare function %rest:produces("text/html") ... ... declare function %rest:produces("*/*") ... </syntaxhighlight>
…the first of these function will be chosen, as the quality factor for text/html
documents is highest.
As we cannot ensure that the client may supply quality factors, the selection process can also be controlled server-side. The qs
parameter can be attached server-side to the Media Type. If multiple functions are left in the selection process, the one with the highest quality factor will be favored:
<syntaxhighlight lang="xquery"> declare function %rest:produces("application/json;qs=1") ... ... declare function %rest:produces("*/*;qs=0.5") ... </syntaxhighlight>
HTTP Methods
Default Methods
The HTTP method annotations are equivalent to all HTTP request methods except 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:
<syntaxhighlight lang="xquery"> declare %rest:GET %rest:POST %rest:path("/post")
function page:post() { "This was a GET or POST request" };
</syntaxhighlight>
The POST and PUT annotations may optionally take a string literal in order to map the HTTP request body to a function argument. Once again, the target variable must be embraced by curly brackets:
<syntaxhighlight lang="xquery"> declare %rest:PUT("{$body}") %rest:path("/put")
function page:put($body) { "Request body: " || $body };
</syntaxhighlight>
Custom Methods
Custom HTTP methods can be specified with the %rest:method
annotation. An optional body variable can be supplied as second argument:
<syntaxhighlight lang="xquery"> declare
%rest:path("binary-size") %rest:method("SIZE", "{$body}")
function page:patch(
$body as xs:base64Binary
) {
"Request method: " || request:method(), "Size of body: " || bin:length($body)
}; </syntaxhighlight>
If an OPTIONS request is received, and if no function is defined, an automatic response will be generated, which includes an Allow
header with all supported methods.
If a HEAD request is received, and if no function is defined, the corresponding GET function will be processed, but the response body will be discarded.
Content Types
The body of a POST or PUT request will be converted to an XQuery item. Conversion can be controlled by specifying a content type. It can be further influenced by specifying additional content-type parameters:
Content-Type | Parameters (;name=value )
|
Type of resulting XQuery item |
---|---|---|
text/xml , application/xml
|
document-node()
| |
text/*
|
xs:string
| |
application/json
|
JSON Options | document-node() or map(*)
|
text/html
|
HTML Options | document-node()
|
text/comma-separated-values
|
CSV Options | document-node() or map(*)
|
others | xs:base64Binary
| |
multipart/*
|
sequence (see next paragraph) |
For example, if application/json;lax=yes
is specified as content type, the input will be transformed to JSON, and the lax QName conversion rules will be applied, as described in the JSON Module.
Input options
Conversion options for JSON
, CSV
and HTML
can also be specified via annotations with the input
prefix. The following function interprets the input as text with the CP1252 encoding and treats the first line as header:
<syntaxhighlight lang="xquery"> declare
%rest:path("/store.csv") %rest:POST("{$csv}") %input:csv("header=true,encoding=CP1252")
function page:store-csv($csv as document-node()) {
"Number of rows: " || count($csv/csv/record)
}; </syntaxhighlight>
Multipart Types
The single parts of a multipart message are represented as a sequence, and each part is converted to an XQuery item as described in the last paragraph.
A function that is capable of handling multipart types is identical to other RESTXQ functions:
<syntaxhighlight lang="xquery"> declare
%rest:path("/multipart") %rest:POST("{$data}") %rest:consumes("multipart/mixed") (: optional :)
function page:multipart($data as item()*) {
"Number of items: " || count($data)
}; </syntaxhighlight>
Parameters
The following annotations can be used to bind request values to function arguments. Values will implicitly be cast to the type of the argument.
Query Parameters
The value of the first parameter, if found in the query component, will be assigned to the variable specified as second parameter. If no value is specified in the HTTP request, all additional parameters will be bound to the variable (if no additional parameter is given, an empty sequence will be bound):
<syntaxhighlight lang="xquery"> declare
%rest:path("/params") %rest:query-param("id", "{$id}") %rest:query-param("add", "{$add}", 42, 43, 44)
function page:params($id as xs:string?, $add as xs:integer+) {
<result id="{ $id }" sum="{ sum($add) }"/>
}; </syntaxhighlight>
HTML Form Fields
Form parameters are specified the same way as query parameters. Their values are the result of HTML forms submitted with the content type application/x-www-form-urlencoded
.
<syntaxhighlight lang="xquery"> %rest:form-param("parameter", "{$value}", "default") </syntaxhighlight>
File Uploads
Files can be uploaded to the server by using the content type multipart/form-data
(the HTML5 multiple
attribute enables the upload of multiple files):
<syntaxhighlight lang="xml"> <form action="/upload" method="POST" enctype="multipart/form-data">
<input type="file" name="files" multiple="multiple"/> <input type="submit"/>
</form> </syntaxhighlight>
The file contents are placed in a map, with the filename serving as key. The following example shows how uploaded files can be stored in a temporary directory:
<syntaxhighlight lang="xquery"> declare
%rest:POST %rest:path("/upload") %rest:form-param("files", "{$files}")
function page:upload($files) {
for $name in map:keys($files) let $content := $files($name) let $path := file:temp-dir() || $name return ( file:write-binary($path, $content), <file name="{ $name }" size="{ file:size($path) }"/> )
}; </syntaxhighlight>
HTTP Headers
Header parameters are specified the same way as query parameters:
<syntaxhighlight lang="xquery"> %rest:header-param("User-Agent", "{$user-agent}") %rest:header-param("Referer", "{$referer}", "none") </syntaxhighlight>
Cookies
Cookie parameters are specified the same way as query parameters:
<syntaxhighlight lang="xquery"> %rest:cookie-param("username", "{$user}") %rest:cookie-param("authentication", "{$auth}", "no_auth") </syntaxhighlight>
Query Execution
In many web search scenarios, user input from browser forms is processed and search results are returned. Such operations can be made more interactive by sending a new search request to the server with each key click. However, this may lead to many parallel server-side requests, from which only the result of the last request will be relevant for the client.
With the %rest:single
annotation, it can be enforced that only one instance of a function will run at the same time and for the same client. If the same function will be called for the second time, a currently executed query will be stopped, and the HTTP error code 460
will be returned instead:
<syntaxhighlight lang="xquery"> (: If fast enough, returns the result. Otherwise, if called again, raises 460 :) declare
%rest:path("/search") %rest:query-param("term", "{$term}") %rest:single
function page:search($term as xs:string) {
- {
for $result in db:open('large-db')//*[text() = $term]
return
- { $result } }
}; </syntaxhighlight>
By adding a string value to with the annotation, functions can be bundled together, and a running query can be canceled by calling another one that has the same annotation value. This is shown by another example, in which the first function can be interrupted by the second one. If you call both functions in separate browser tabs, you will note that the first tab will return 460
, and the second one will return <xml>stopped</xml>.
<syntaxhighlight lang="xquery"> declare
%rest:path("/compute") %rest:single("EXPENSIVE")
function local:compute() {
(1 to 100000000000000)[. = 0]
};
declare
%rest:path("/stop") %rest:single("EXPENSIVE")
function local:stop() {
<xml>stopped</xml>
}; </syntaxhighlight>
The following things should be noted:
- If a query will be canceled, there will be no undesirable side effects. For example, it won’t be possible to abort a query if it is currently updating the database or performing any other I/O operations. As a result, the termination of a running query can take some more time as expected.
- The currently executed function is bound to the current session. This way, a client will not be able to cancel requests from other clients. As a result, functions can only be stopped if there was at least one previous successful response, in which initial session data was returned to the client.
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 Response
Custom responses can be generated in XQuery by returning an rest:response
element, an http:response
child node that matches the syntax of the EXPath HTTP Client Module specification, and optional child nodes that will be serialized as usual. A function that yields a response on an unknown resource may look as follows:
<syntaxhighlight lang="xquery"> declare %output:method("text") %rest:path("") function page:error404() {
<rest:response> <http:response status="404"> <http:header name="Content-Language" value="en"/> <http:header name="Content-Type" value="text/plain; charset=utf-8"/> </http:response> </rest:response>, "The requested resource is not available."
}; </syntaxhighlight>
Forwards and Redirects
Redirects
The server can invite the client (e.g., the web browser) to make a second request to another URL by sending a 302 response:
<syntaxhighlight lang="xml"> <rest:response>
<http:response status="302"> <http:header name="Location" value="new-location"/> </http:response>
</rest:response> </syntaxhighlight>
The convenience function web:redirect
can be called to create such a response.
In the XQuery context, redirects are particularly helpful if Updates are performed. An updating request may send a redirect to a second function that generates a success message, or evaluates an updated database:
<syntaxhighlight lang="xquery"> declare %updating %rest:path('/app/init') function local:create() {
db:create('app', <root/>, 'root.xml'), db:output(web:redirect('/app/ok'))
};
declare %rest:path('/app/ok') function local:ok() {
'Stored documents: ' || count(db:open('app'))
}; </syntaxhighlight>
Forwards
A server-side redirect is called forwarding. It reduces traffic among client and server, and the forwarding will not change the URL seen from the client’s perspective:
<syntaxhighlight lang="xml"> <rest:forward>new-location</rest:forward> </syntaxhighlight>
The fragment can also be created with the convenience function web:forward
.
Output
The content-type of a response can be influenced by the user via Serialization Parameters. The steps are described in the REST chapter. In RESTXQ, serialization parameters can be specified in the query prolog, via annotations, or within the REST response element:
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:
<syntaxhighlight lang="xquery"> declare option output:media-type 'text/plain';
declare %rest:path("version1") function page:version1() {
'Keep it simple, stupid'
}; </syntaxhighlight>
Annotations
Global serialization parameters can be overwritten via %output
annotations. The following example serializes XML nodes as JSON, using the JsonML format:
<syntaxhighlight lang="xquery"> declare
%rest:path("cities") %output:method("json") %output:json("format=jsonml")
function page:cities() {
element cities { db:open('factbook')//city/name }
}; </syntaxhighlight>
The next function, when called, generates XHTML headers, and text/html
will be set as content type:
<syntaxhighlight lang="xquery"> declare
%rest:path("done") %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:html() {
<html xmlns="http://www.w3.org/1999/xhtml"> <body>done</body> </html>
}; </syntaxhighlight>
Response Element
Serialization parameters can also be specified in a REST reponse element in a query. Serialization parameters will be overwritten:
<syntaxhighlight lang="xquery"> declare %rest:path("version3") function page:version3() {
<rest:response> <output:serialization-parameters> <output:media-type value='text/plain'/> </output:serialization-parameters> </rest:response>, 'Not that simple anymore'
}; </syntaxhighlight>
Error Handling
Template:Mark Status code 400
changed to 500
; omit stack trace.
If an error is raised when RESTXQ code is parsed, compiled or evaluated, an HTTP response with the status code 500 is generated.
By default, all server-side errors will be passed on to the client. This is particularly helpful during the development process. In a productive environment, however, it is advisable not to expose errors to the client. This can be realized via the RESTXQERRORS
option. If disabled,
- XQuery modules that cannot be parsed will be ignored and
- full error messages and stack traces will be suppressed and not included in the HTTP response.
The full error information can still be looked up in the database logs.
Raise Errors
With web:error
, you can abort query evaluation, enforce a premature HTTP response and report errors back to the client:
<syntaxhighlight lang="xquery"> declare
%rest:path("/teapot")
function page:teapot() {
web:error(418, "I'm a pretty teapot")
}; </syntaxhighlight>
In contrast to the standard fn:error
function, a status code can be supplied, and the response body will only contain the specified error message and no stack trace.
Catch XQuery Errors
XQuery runtime errors can be processed via error annotations. Error annotations have one or more arguments, which represent the error codes to be caught. The codes equal the names of the try/catch construct:
Precedence | Syntax | Example |
---|---|---|
1 | prefix:name Q{uri}name
|
err:FORG0001 Q{http://www.w3.org/2005/xqt-errors}FORG0001
|
2 | prefix:* Q{uri}*
|
err:* Q{http://www.w3.org/2005/xqt-errors}*
|
3 | *:name
|
*:FORG0001
|
4 | *
|
*
|
All error codes that are specified for a function must have the same precedence. The following rules apply when catching errors:
- Codes with a higher precedence (smaller number) will be given preference.
- A global RESTXQ error will be raised if two functions with conflicting codes are found.
Similar to try/catch, the pre-defined variables (code
, description
, value
, module
, line-number
, column-number
, additional
) 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 demonstrated by the following example:
<syntaxhighlight lang="xquery"> 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'
}; </syntaxhighlight>
Catch HTTP Errors
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):
<syntaxhighlight lang="xml"> <error-page>
<error-code>404</error-code> <location>/error404</location>
</error-page> </syntaxhighlight>
The target location may be another RESTXQ function. The request:attribute function can be used to request details on the caught error:
<syntaxhighlight lang="xquery"> declare %rest:path("/error404") function page:error404() {
"URL: " || request:attribute("javax.servlet.error.request_uri") || ", " || "Error message: " || request:attribute("javax.servlet.error.message")
}; </syntaxhighlight>
User Authentication
If you want to provide restricted access to parts of a web applications, you will need to check permissions before returning a response to the client. The Permissions layer is a nice abstraction for defining permission checks.
Functions
The Request Module contains functions for accessing data related to the current HTTP request. Two 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). The RESTXQ Module 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:
<syntaxhighlight lang="xquery"> import module namespace request = "http://exquery.org/ns/request";
declare %rest:path("/host-name") function page:host() {
'Remote host name: ' || request:remote-hostname()
}; </syntaxhighlight>
References
Documentation:
- RESTXQ Specification, First Draft
- RESTful XQuery, Standardised XQuery 3.0 Annotations for REST. Paper, XMLPrague, 2012
- RESTXQ. Slides, MarkLogic User Group London, 2012
- Web Application Development. Slides from XMLPrague 2013
Examples:
- Sample code combining XQuery and JavaScript: Materials and paper from Amanda Galtman, Balisage 2016.
- DBA: The Database Administration interface, bundled with the full distributions of BaseX.
Changelog
- Version 9.5
- Updated: Raise Errors: Status code
400
changed to500
, omit stack trace.
- Version 9.3
- Updated: Custom Methods: Better support for the OPTIONS and HEAD methods.
- Updated: XQuery Errors: Suppress stack trace and error code in the HTTP response.
- Removed:
rest:redirect
element (web:redirect
can be used instead)
- Version 9.2
- Updated: Ignore XQuery modules that cannot be parsed
- Version 9.0
- Added: Support for server-side quality factors in the
%rest:produces
annotation - Updated: Status code
410
was replaced with460
- Removed:
restxq
prefix
- Version 8.4
- Added:
%rest:single
annotation
- Version 8.1
- Added: support for input-specific content-type parameters
- Added:
%input
annotations
- Version 8.0
- Added: Support for regular expresssions in the Path Annotation
- Added: Evaluation of quality factors that are supplied in the Accept header
- Version 7.9
- Updated: XQuery Errors, extended error annotations
- Added:
%rest:method
- Version 7.7
- Added: Error Handling, File Uploads, Multipart Types
- Updated: RESTXQ function may now also be specified in main modules (suffix:
*.xq
). - Updated: the RESTXQ prefix has been changed from
restxq
torest
. - Updated: parameters are implicitly cast to the type of the function argument
- Updated: the RESTXQ root url has been changed to
http://localhost:8984/
- Version 7.5
- Added: new XML elements
<rest:redirect/>
and<rest:forward/>