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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.
Please note that the current WebSocket implementation relies on Jetty’s WebSocket servlet API. Other web servers may be supported in future versions.
=Introduction=
 
==Protocol==
 
Use WebSockets if you have to exchange data with a high frequency or if you have to send messages from the server to the client without techniques like [polling https://en.wikipedia.org/wiki/Polling_(computer_science)]. In contrast to REST, WebSockets use a single URL for the whole communication.
 
The WebSocket protocol was standardized in [https://tools.ietf.org/html/rfc6455 RFC 6455] by the IETF. After an initial HTTP request, all communication takes place over a single TCP connection. Unlike the HTTP protocol, a connection will be kept alive, and a server can send unsolicited data to the client.
 
For 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 events like ''onConnect'' and ''onMessage''.
 
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.
==Preliminaries==
There are a bunch of Annotations annotations depending to WebSockets for annotating XQuery functions. When a WebSocket-Message comes inmessage arrives at the server, an XQuery function will be invoked that matches the constraints indicated by its annotations. 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 closed. =Annotations= To 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 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 function annotated with <code>ws:connect('/')</code> will be 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 the Servlet will do the rest for you.  ==%ws:connect(path)== Called directly after a successful WebSocket handshake. The <code>path</code> specifies the path which a client is connected to: <syntaxhighlight lang="xquery">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">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('/', '{$error}') function local:error($error) { };</syntaxhighlight> Usually, errors happen because of bad/malformed incoming packets. The WebSocket connection gets closed after the error handling. ==%ws:close(path)== Called when the WebSocket closes. The <code>path</code> specifies the path which a client is connected to: <syntaxhighlight lang="xquery">declare %ws:close('/') 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">declare %ws:close('host', '{$host}') %ws:header-param('host', '{$host}')function local:close($host) { admin:write-log('Connection was closed: ' || $host)};</syntaxhighlight> 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 <code>%ws:close</code> or <code>%ws:error</code> 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. For keeping the connection alive it is recommendable to use heart-beats, and send regular pings to the server. There is no ideal timespan for sending pings: It should not be sent too often, but you should also consider possible network latencies. If your HTTP connection is secure, you should use the <code>wss</code> instead of the <code>ws</code> scheme.
If a you get the <code>[basex:ws] WebSocket Function is requested (like connecting to the Path 'connection required</'code> error, message you may be attempting to the Path '/path', ...), the module directory and its subcall WebSocket functions from a non-directories will be traversed, and all [[XQuery Extensions#Suffixes|XQuery files]] will be parsed for functions with WebSocket annotationscontext. Sub-directories that include an {{Code|.ignore}} If you use a proxy server, check in the file will be skippedconfiguration if WebSockets are enabled.
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.=Examples=
==ExamplesBasic Example==
A first WebSocket function is shown below:The following chapter explains how to create a simple basic web application with WebSockets. You can find another example in the BaseX source code.
First of all, you have to ensure that the <pre class="brush:xquery"code>WsServlet</code> is enabled in your <code>web.xml</code>file. It will be enabled if you use the standard configuration of BaseX.module namespace page = 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:connect('http:/')</basexcode>.org/modules/web-page';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 message can be emitted to other connected users, and so on. import module namespace websocket <syntaxhighlight lang= "http://basex.org/modules/Websocketxquery";>
declare
%ws:connect("'/"') function pageexample:connect( ) { websocket:broadcast( json:serializeas empty-sequence( <json type="object"> <type>Connect</type> </json> ) ){ };</presyntaxhighlight>
=Usage=* Enable The connect function is sufficient for creating the WebSocket Servlet in the webpersistent client/server connection.xml. You can set here In order to something sensible with the maxIdleTimeconnection, maxTextMessageSize and maxBinaryMessageSize too.* If you get should implement a function annotated with <code>%ws: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 websocket:error annotation will be called. <pre class="brush:xml/"><servlet> <servlet-name>wsservlet)</servlet-namecode> <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.:
<syntaxhighlight lang=Annotations"xquery">import module namespace ws ='http://basex.org/modules/ws' ==websocketdeclare %ws:connectmessage(path'/', '{$message}')==Called when a Clientfunction example:message( $message as xs:string) as empty-sequence() { ws:emit($message)};</syntaxhighlight> In the function above, the [[WebSocketModule]] is imported, and the function <code>ws:emit</code> is used for forwarding the message to all connected clients. The following client-Connection successfully connected to side code demonstrates a basic application of the serverWebSocket connection: <syntaxhighlight lang="javascript">var ws = new WebSocket("ws://localhost:8984/ws"); ws.onmessage = function(event) { alert(event.data);}; function send(message) { ws. send(message);};</syntaxhighlight> The <code>pathsend</code> specifies function can be called to pass on a string to the path the Client connected toserver.
==websocket:message(path,message)==Called when a <code>message</code> arrives at the ServerThere are no heart-beats in this example. The <code>path</code> specifies This means that the <code>path</code> the Client connection is connected toterminated if nothing happens for 5 minutes (standard timeout). The <code>message</code> is the <code>It will also be closed if you send a message</code> sent by that exceeds the Client. Could be a Text-Message or a Binary-Messagestandard text size.
==websocket:close(path)Chat Application==Called when the WebSocket closes. The <code>path</code> specifies the <code>path</code> the Client is connected to.The WebSocket is already closed when this Annotation is called so there can be no return.
==websocket:error(path,message)==Called when an Error occurred. UsuallyIn the full distributions of BaseX, this happens because of bad/malformed incoming packets. The <code>path</code> specifies the <code>path</code> the Client is connected to. The <code>message</code> is the Erroryou will find a little self-Message.The WebSocket gets closed after an error occurredcontained chat application that demonstrates how WebSockets can be used in practice.
=Changelog=websocket:path(name,variable[,default]==The value of the first parameter will be assigned to the variable specified as the second parameter. The third parameter can be a default value.
=Parameters=* Http-WebSockets werre introduced with Version -> f9.e1.: ```%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
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