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Called when a <codepre lang='xquery'>declare %ws:message<('/code> arrives at the server. The <code>path', '{$info}') function local:message($info) { };</codepre> specifies the path which a client is connected to.
The WebSocket is already closed when this annotation is called so there can be no return. <pre lang='xquery'>declare %ws:error('/', '{$error}') function local:error($error) { };</pre>
==%ws:Usually, errors happen because of bad/malformed incoming packets. The WebSocket connection gets closed after the error(path, message)==handling.
Called when an error occurs. The <code>==ws:close(path</code> specifies the path which a client is connected to.)==
Usually, errors happen because of bad/malformed incoming packetsCalled when the WebSocket closes. The <code>messagepath</code> specifies the path which a client is the error message. The WebSocket connection gets closed after the error handling.connected to:
* The [[WebSocket Module]] contains functions for interacting with other clients or manage specific clients.* The results of functions annotated with <code>%ws:close</code> or <code>%ws:error</code> will not be transmitted to the client.* For keeping the connection alive it is recommendable to use heart-beats, and send regular pings to the server.* Use <code>wss</code> instead of <code>ws</code> There is no ideal timespan for a secure WebSocket connection.* If sending pings: It should not be sent too often, but you use a proxy server, check in the configuration if WebSockets are enabledshould also consider possible network latencies.
There are no heartbeats in this exampleWebSockets werre introduced with Version 9. This means that the connection is terminated if nothing happens for 5 minutes (standard timeout). It will also be closed if you send a message that exceeds the standard text size1.
Text replacement - "syntaxhighlight" to "pre"
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)polling]. 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.
There are a bunch of annotations depending to WebSockets for annotating XQuery functions. When a WebSocket message 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 subdirectories will be traversed, and all [[XQuery Extensions#Suffixes|XQuery files]] will be parsed for functions with WebSocket annotations. Sub-directories Subdirectories 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]].
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.: <pre lang='xquery'>declare %ws:connect('/') function local:connect() { };</pre>
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:
The <code>message</code> is the <code>message</code> sent by the client. It value will be of type <code>xs:string</code> or <code>xs:base64Binary</code>. The As there is no fixed message protocol, the client needs to take care of the message syntax, and it must not exceed the configured maximum size.
==%ws:closeerror(path, message)==
Called when the WebSocket closesan 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:
<pre lang=='xquery'>declare %ws:header-paramclose('/') function local:connect(name, variable[, default])=={ };</pre>
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 is available, an optional <code>default</code> value will be bound assigned instead.: <pre lang='xquery'>declare %ws:close('host', '{$host}') %ws:header-param('host', '{$host}')function local:close($host) { admin:write-log('Connection was closed: ' || $host)};</pre>
The following parameters are available:
! 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>
|- valign="top"
| <code>protocol-version</code>
| The version of the used protocol (max. 1.3).
|- valign="top"
| <code>query-string</code>
| The query string of the request URI.
|- valign="top"
| <code>isrequest-secure</code>| Indicates if the connection is secure.|- valign="top"| <code>is-secureuri</code>
| The Request URI to use for this request.
|- valign="top"
| <code>hostsub-protocols</code>| The host List of the request URI.|- valign="top"| <code>sec-websocketconfigured sub-version</code>| The websocket versionprotocols.
|}
General information on the request can be retrieved via the [[Request Module]]. =TippsWriting 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.
If your HTTP connection is secure, you should use the <code>wss</code> instead of the <code>ws</code> scheme. If you get the <code>[basex:ws] WebSocket connection required</code> error, you may be attempting to call WebSocket functions from a non-WebSocket context. If you use a proxy server, check in the configuration if WebSockets are enabled. =Examples= ==Basic Example==
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 <code>WsServlet</code> is enabled in your <code>web.xml</code> file. It will be enabled if you use the standard 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:connect('/')</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 message can be emitted to other connected users, and so on.
<pre classlang="brush:'xquery"'>
declare
%ws:connect('/')
The connect function is sufficient for creating the persistent client/server connection. In order to something sensible with the connection, you should implement a function annotated with <code>%ws:message("/")</code>:
<pre classlang="brush:'xquery"'>import module namespace ws = 'http://basex.org/modules/ws';
declare
%ws:message('/', '{$message}')
The following client-side code demonstrates a basic application of the WebSocket connection:
<pre classlang="brush:javscriptjavascript">var ws = new WebSocket("ws://localhost:89848080/ws");
ws.onmessage = function(event) {
</pre>
The <code>send</code> function can be called to pass on a string to the server. There are no heart-beats in this example. This means that the connection is terminated if nothing happens for 5 minutes (estandard timeout). gIt will also be closed if you send a message that exceeds the standard text size. ==Chat Application== In the full distributions of BaseX, you will find a little self-contained chat message) to the serverapplication that demonstrates how WebSockets can be used in practice. =Changelog=
