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This page presents one of the [[Web Application]] services. It describes how to use the WebSockets API of BaseX. WebSocket is a communication protocol for providing '''full-duplex ''' communication: Data can be sent in both directions and simultaneously.

After an initial HTTP-request it communicates over a single TCP connectionPlease note that the current WebSocket implementation relies on Jetty’s WebSocket servlet API. The WebSocket protocol was standardized Other web servers may be supported in RFC 6455 by the IETF. It creates the possibility to transponse real-time data from the server to the client and the client to the serverfuture versions.

In contrast =Introduction= ==Protocol== Use WebSockets if you have to exchange data with a high frequency or if you have to the HTTP protocol it enables send messages from the server to send unsolicited data to the clientwithout techniques like [polling https://en. For establishing a WebSocket connection there is wikipedia.org/wiki/Polling_(computer_science)]. In contrast to REST, WebSockets use a WebSocket handshake request, send by single URL for the clientwhole communication.  The server returns a WebSocket handshake responseprotocol was standardized in [https://tools. This will be handled ietf.org/html/rfc6455 RFC 6455] by jettythe IETF. After an initial HTTP request, all communication takes place over a successful handshakesingle TCP connection. Unlike the HTTP protocol, the annotation <code>ws:connect</code> a connection will be called. The persistent connection is now open until an the client or the server closes itkept alive, an error occures or and a timeout happens. How long this timeout goes server can be set in send unsolicited data to the web.xml. It is possible to transmit all kind of data, binary or textclient.

For further information about establishing a WebSocket connection, a handshake request is sent by the client. The web server returns a handshake response. If the handshake is successful, the persistent connection will be open until the client or the server closes it, an error occurs or a timeout happens. It is possible to transmit all kind of data, binary or text. '''The BaseX WebServer handles the handshake completely.''' You just have to define some limits of the connection in the <code>web.xml</code> and specify functions for WebSocket protocol check out the RFC6455 specevents like ''onConnect'' and ''onMessage''.

=Introduction=Notice that there is no specification of a message protocol. The WebSocket protocol just specifies the message architecture but not how the payload of the messages is formatted. To agree on a format between the server and the client one can use sub-protocols. Some older browsers don’t support the WebSocket protocol. Therefore you can use fallback options like Ajax. JavaScript client libraries like SockJS can be used for building client applications. The library takes care of how to establish the real-time connection. If the WebSocket protocol isn’t supported, it uses polling. You have to provide server functions for the fallback solutions if you have to support fallbacks.

If a WebSocket function is requested (like connecting to the path '<code>/'</code>, sending a message to the path '<code>/path'</code>, ...…), the module directory and its sub-directories will be traversed, and all [[XQuery Extensions#Suffixes|XQuery files]] will be parsed for functions with WebSocket annotations. Sub-directories that include an {{Code|.ignore}} file will be skipped. To speed up processing, the functions of the existing XQuery modules are automatically cached in main memory. For further information on cache handling, check out the [[RESTXQ#Introduction|RESTXQ introduction]]. ==Configuration== * The WebSocket servlet can be enabled and disabled in the <code>web.xml</code> configuration file . You can specify further configuration options, such as <code>maxIdleTime</code>, <code>maxTextMessageSize</code>, and <code>maxBinaryMessageSize</code>.* The default limit for messges is 64 KB. If you a message exceeds the default or the specified limit, an error will be raised and the connection will be skippedclosed. =Annotations=

To speed up processing, tag functions as WebSocket functions you have to use [[XQuery 3.0#Annotations|annotations]]. The annotation is written after the keyword ''declare'' and before the keyword ''function''. For the functions context of WebSockets there are some annotations listed below. Functions which are annotated with a WebSocket annotation will be called if the appropriate event occurs. For example, the existing XQuery modules are automatically cached in main memoryfunction annotated with <code>ws:* Functions connect('/')</code> will be invalidated executed if a client establishes a connection with the WebSocket root path (which is, by default, <code>ws/</code>). By using annotations, it’s easy to provide an API for your WebSocket connection. You just have to specify what to do when a WebSocket Event occurs, annotate it with the corresponding annotation and parsed again if the timestamp of their module changesServlet will do the rest for you.

==Examples%ws:connect(path)==

Called directly after a successful WebSocket handshake. The <code>path</code> specifies the path which a client is connected to: <pre classsyntaxhighlight lang="brushxquery">declare %ws:connect('/') function local:connect() { };</syntaxhighlight> You can specify here how to handle your users, e. g. save a name as a WebSocket attribute. Furthermore, you can check header parameters for validity.  ==%ws:message(path, message)== Called when a client message arrives at the server. The <code>path</code> specifies the path which a client is connected to. The <code>message</code> string contains the name of the variable to which the message will be bound: <syntaxhighlight lang="xquery">module namespace page declare %ws:message('/', '{$info}') function local:message($info) { };</syntaxhighlight> The value will be of type <code>xs:string</code> or <code>xs:base64Binary</code>. As there is no fixed message protocol, the client needs to take care of the message syntax. ==%ws:error(path, message)== Called when an error occurs. The <code>path</code> specifies the path which a client is connected to. The <code>message</code> string contains the name of the variable to which the message will be bound: <syntaxhighlight lang= "xquery">declare %ws:error('/', '{$error}'http) function local:error($error) { };</syntaxhighlight> Usually, errors happen because of bad/basexmalformed incoming packets. The WebSocket connection gets closed after the error handling. ==%ws:close(path)== Called when the WebSocket closes.orgThe <code>path</modulescode> specifies the path which a client is connected to: <syntaxhighlight lang="xquery">declare %ws:close('/web-page') function local:connect() { };</syntaxhighlight> The WebSocket is already closed when this annotation is called so there can be no return.  ==%ws:header-param(name, variable[, default])== For accessing connection-specific properties like the HTTP version. The value will be bound to the specified <code>variable</code>. If the property has no value, an optional <code>default</code> value will be assigned instead: <syntaxhighlight lang="xquery">

%ws:connectclose("/"'host', '{$host}') %ws:header-param('host', '{$host}')function wsdbalocal:connectclose($host) { admin:write-log('Connection was closed: Do something after a client connects to the path "/" :' || $host)

</presyntaxhighlight> The following parameters are available: {| class="wikitable" |- valign="top"! Name! Description|- valign="top"| <code>host</code>| The host of the request URI.|- valign="top"| <code>http-version</code>| The HTTP version used for the request.|- valign="top"| <code>is-secure</code>| Indicates if the connection is secure.|- valign="top"| <code>origin</code>| The WebSocket origin.|- valign="top"| <code>protocol-version</code>| The version of the used protocol.|- valign="top"| <code>query-string</code>| The query string of the request URI.|- valign="top"| <code>request-uri</code>| The Request URI to use for this request.|- valign="top"| <code>sub-protocols</code>| List of configured sub-protocols.|} General information on the request can be retrieved via the [[Request Module]]. =Writing Applications= The [[WebSocket Module]] contains functions for interacting with other clients or manage specific clients. For example, you can store and access client-specific properties for a WebSocket connection or close the connection of clients. Note that one WebSocket connection can be opened per browser tab. In contrast, only one HTTP session exists for multiple tabs in in a browser. If you want to keep client-specific data on the web server, you can either store them in HTTP sessions or in the WebSocket connection.

Note further that the results of functions annotated with <pre class="brush:xquery"code>module namespace page = 'http%ws:close<//basex.org/modules/web-page';declare code> or <code>%ws:message("/","{$message}")function chat:message( $message as xs:string) { (: Do something if a message arrives at the server :)};error</precode>will not be transmitted to the client. Both annotations have rather been designed to gracefully close connections, write log data, remove clients from session data, etc.

<pre class="brush:xquery">module namespace page = 'http://basexFor keeping the connection alive it is recommendable to use heart-beats, and send regular pings to the server.org/modules/web-page';import module namespace ws = "httpThere is no ideal timespan for sending pings://basexIt should not be sent too often, but you should also consider possible network latencies.org/modules/Websocket";

declare %ws:close("/")function chat:close() { (: Build a close message with the id of If your HTTP connection is secure, you should use the client who closes the connection :) let $client-id := ws:id() let $msg := json:serialize( <json type="object"code> <type>WebsocketClosedwss</typecode> <idThatClosed>{$client-id}instead of the </idThatClosedcode> </json> ) (: Broadcast the message to all connected users except the client who closes the connection :) return ws:broadcast($msg) };</precode>scheme.

* Enable the WebSocket servlet in the web.xml. You can set here the maxIdleTime, maxTextMessageSize and maxBinaryMessageSize too.* If you get a message that exceeds the maxTextMessageSize/maxBinaryMessageSize or, if not set, the default messageSize of Jetty of 65 536 bytes (64 kB) then the connection will close. In this case, the ws:error annotation will be called. <pre class="brush:xml"><servlet> <servlet-name>wsservlet</servlet-name> <servlet-class>org.basex.http.ws.WsServlet</servlet-class> <init-param> <param-name>maxIdleTime</param-name> <param-value>100000</param-value> </init-param> <init-param> <param-name>maxTextMessageSize</param-name> <param-value>3000</param-value> </init-param></servlet><servlet-mapping> <servlet-name>wsservlet</servlet-name> <url-pattern>/ws/*</url-pattern></servlet-mapping></pre>* Annotate your specific XQuery-Functions with WebSocketAnnotations.Examples=

=Annotations=Basic Example==

Called when a client WebSocket connection successfully connected First of all, you have to ensure that the server. The <code>pathWsServlet</code> is enabled in your <code>web.xml</code> specifies file. It will be enabled if you use the path the client connected tostandard configuration of BaseX.

==For establishing a connection to the WebSocket server, it is necessary that the server provides at least one function annotated with a WebSocket annotation. Let’s start by using the annotation <code>%ws:messageconnect(path'/')</code>. In the connect function, a bidirectional communication with the client can be initialized: attributes such as the id and name of a client can be set,or a welcome messagecan be emitted to other connected users, and so on. <syntaxhighlight lang="xquery">declare %ws:connect('/')==function example:connect() as empty-sequence() {};</syntaxhighlight>

Called when a <code>message<The connect function is sufficient for creating the persistent client/code> arrives at the serverconnection. The <code>path</code> specifies In order to something sensible with the connection, you should implement a function annotated with <code>path</code> the client is connected to. The <code>%ws:message<("/code> is the <code>message")</code> sent by the client. Could be a text-message or a binary-message.:

<syntaxhighlight lang="xquery">import module namespace ws ='http://basex.org/modules/ws:close(path)=='

Called when the WebSocket closes. The <code>path<declare %ws:message('/code> specifies the <code>path', '{$message}')function example:message( $message as xs:string) as empty-sequence() { ws:emit($message)};</codesyntaxhighlight> the client is connected to.The WebSocket is already closed when this annotation is called so there can be no return.

==ws:error(pathIn the function above,message)==Called when an error occurred. Usuallythe [[WebSocket Module]] is imported, this happens because of bad/malformed incoming packets. The <code>path</code> specifies and the function <code>path</code> the client is connected to. The <code>messagews:emit</code> is used for forwarding the error-message.The WebSocket gets closed after an error occurredto all connected clients.

==wsThe following client-side code demonstrates a basic application of the WebSocket connection:path(name,variable[,default]==

The value of the first parameter will be assigned to the variable specified as the second parameter<syntaxhighlight lang="javascript">var ws = new WebSocket("ws://localhost:8984/ws"); ws. The third parameter can be a default valueonmessage = function(event) { alert(event. data);}; function send(message) { ws.send(message);};</syntaxhighlight>

For accessing specific parameters like There are no heart-beats in this example. This means that the Http-Version or connection is terminated if nothing happens for 5 minutes (standard timeout). It will also be closed if you send a message that exceeds the Sec-WebSocket-Versionstandard text size.

=Parameters=Chat Application==

* HttpIn the full distributions of BaseX, you will find a little self-Version -> f.econtained chat application that demonstrates how WebSockets can be used in practice.: ```%ws:param("Http-Version", "{$version}")```* Origin* Protocol-Version* QueryString* IsSecure* RequestURI* Host* Sec-WebSocket-Version* offset -> just for binary-Messages* len -> just for binary-Messages

=TippsChangelog=

* For interacting WebSockets werre introduced with other clients or manage specific clients you should check out the [[WebSocket Module]] as wellVersion 9.1.