Difference between revisions of "WebSockets"

From BaseX Documentation
Jump to navigation Jump to search
Line 30: Line 30:
 
=Annotations=
 
=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.  
+
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)==
+
==%ws:connect(path)==
  
Called directly after a successful WebSocket handshake. The <code>path</code> specifies the path which a client is connected to.
+
Called directly after a successful WebSocket handshake. The <code>path</code> specifies the path which a client is connected to.
You can specify here how to handle your users, f.e. save a name as a WebSocket attribute. Furthermore, you can check here some of the header-params for validity.  
 
  
==ws:message(path,message)==
+
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.
  
Called when a <code>message</code> arrives at the server. The <code>path</code> specifies the path which a client is connected to. The <code>message</code> is the <code>message</code> sent by the client. Could be a text-message or a binary-message. It should be ensured that the format of the message is correct.
+
==%ws:message(path, message)==
  
==ws:close(path)==
+
Called when a <code>message</code> arrives at the server. The <code>path</code> specifies the path which a client is connected to.
 +
 
 +
The <code>message</code> is the <code>message</code> sent by the client. It will be of type <code>xs:string</code> or <code>xs:base64Binary</code>. The client needs to take care of the message syntax, and it must not exceed the configured maximum size.
 +
 
 +
==%ws:close(path)==
  
 
Called when the WebSocket closes. The <code>path</code> specifies the path which a client is connected to.
 
Called when the WebSocket closes. The <code>path</code> 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.  
 
The WebSocket is already closed when this annotation is called so there can be no return.  
  
==ws:error(path, message)==
+
==%ws:error(path, message)==
 +
 
 +
Called when an error occurs. The <code>path</code> specifies the path which a client is connected to.
  
Called when an error has occurred. Usually, this happens because of bad/malformed incoming packets. The <code>path</code> specifies the path which a client is connected to. The <code>message</code> is the error message. The WebSocket connection gets closed after the error handling.
+
Usually, errors happen because of bad/malformed incoming packets. The <code>message</code> is the error message. The WebSocket connection gets closed after the error handling.
  
==ws:header-param(name,variable[,default])==
+
==%ws:header-param(name, variable[, default])==
  
For accessing specific parameters like the Http-Version or the Sec-WebSocket-Version.
+
For accessing connection-specific properties like the HTTP version. The value will be bound to the specified <code>variable</code>. If no value is available, an optional <code>default</code> value will be bound instead.
  
===Parameters===
+
The following parameters are available:
The following list shows the parameters of a WebSocket. You can access the parameters via the annotation <code>%ws:header-param(name,variable[,default])</code>
+
 
* Http-Version  -> f.e.: <code>%ws:param("Http-Version", "{$version}")</code>
+
{| class="wikitable"
* Origin
+
|- valign="top"
* Protocol-Version
+
! Name
* QueryString
+
! Description
* IsSecure
+
|- valign="top"
* RequestURI
+
| <code>http-version</code>
* Host
+
| The HTTP version used for the request.
* Sec-WebSocket-Version
+
|- valign="top"
* offset -> just for binary-Messages
+
| <code>origin</code>
* len -> just for binary-Messages
+
| The WebSocket origin.
 +
|- 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>is-secure</code>
 +
| Indicates if the connection is secure.
 +
|- valign="top"
 +
| <code>is-secure</code>
 +
| The Request URI to use for this request.
 +
|- valign="top"
 +
| <code>host</code>
 +
| The host of the request URI.
 +
|- valign="top"
 +
| <code>sec-websocket-version</code>
 +
| The websocket version.
 +
|}
  
 
=Tipps=
 
=Tipps=
  
* For interacting with other clients or manage specific clients you should check out the [[WebSocket Module]] as well.
+
* 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
+
* 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 possible to implement heart-beats  
+
* 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> for a secure WebSocket connection
+
* Use <code>wss</code> instead of <code>ws</code> for a secure WebSocket connection.
* If you use a proxy server, check its configuration if WebSocket support is enabled
+
* If you use a proxy server, check in the configuration if WebSockets are enabled.
  
 
=Example=
 
=Example=
The following chapter explains how to create a simple basic webapplication with websockets.
 
You can find another example in the BaseX source code.
 
First of all you have to ensure that the WsServlet is enabled in your <code>web.xml</code>. It will be enabled if you use the standard <code>web.xml</code>.
 
  
For establishing a connection to the WebSocket server it is necessary that the server provides at least one function annotated with a WebSocket annotation.  
+
The following chapter explains how to create a simple basic web application with WebSockets. You can find another example in the BaseX source code.
Lets start by using the annotation <code>%ws:connect('/')</code>.
+
 
In the connect function, specific WebSocket attributes like the id and name, emit a message to other connected users, write database entries, do nothing, ... can be set.
+
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 class="brush:xquery">
 
<pre class="brush:xquery">
Line 88: Line 114:
 
   %ws:connect('/')
 
   %ws:connect('/')
 
function example:connect() as empty-sequence() {
 
function example:connect() as empty-sequence() {
  ()
 
 
};
 
};
 
</pre>
 
</pre>
  
With the state until now you can create a connection between client and server.  
+
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>:
For doing sth. senseful with the WebSocket connection you should implement a function annotated with <code>%ws:message("/")</code>.
+
 
 
<pre class="brush:xquery">
 
<pre class="brush:xquery">
 
import module namespace ws = 'http://basex.org/modules/ws'
 
import module namespace ws = 'http://basex.org/modules/ws'
Line 99: Line 124:
 
   %ws:message('/', '{$message}')
 
   %ws:message('/', '{$message}')
 
function example:message(
 
function example:message(
   $message as xs:string
+
   $message as xs:string
 
) as empty-sequence() {
 
) as empty-sequence() {
 
   ws:emit($message)
 
   ws:emit($message)
 
};
 
};
 
</pre>
 
</pre>
In the function above the WebSocketModule function <code>emit</code> is used for forwarding the message to all connected clients. Notice that you have to import the [[WebSocket Module]] before using it.
 
  
With the code above it is possible to implement client-side functionality to connect to a WebSocket and send/receive messages via the WebSocket.
+
In the function above, the [[WebSocket Module]] is imported, and the function <code>ws:emit</code> is used for forwarding the message to all connected clients.
Following client-side code shows basic usage of the WebSocket connection:
+
 
 +
The following client-side code demonstrates a basic application of the WebSocket connection:
  
 
<pre>
 
<pre>
var ws = new WebSocket("wss://localhost:8984/ws");
+
var ws = new WebSocket("ws://localhost:8984/ws");
 +
 
 
ws.onmessage = function(event) {
 
ws.onmessage = function(event) {
 
   alert(event.data);
 
   alert(event.data);
Line 120: Line 146:
 
</pre>
 
</pre>
  
There are no heartbeats in this example. This means that the connection is terminated if nothing happens for 5 minutes (standard timeout).
+
The <code>send</code> function can be called to pass on a string (e. g. a chat message) to the server.
If you send a message which exceeds the textsize of 3kb defined in the <code>web.xml</code> the connection gets closed too.
+
 
 +
There are no heartbeats in this example. 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 size.

Revision as of 11:08, 22 October 2018

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.

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 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 web.xml 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 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 /, sending a message to the path /path, …), the module directory and its sub-directories will be traversed, and all XQuery files will be parsed for functions with WebSocket 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. For further information on cache handling, check out the RESTXQ introduction.

Configuration

  • The WebSocket servlet can be enabled and disabled in the web.xml configuration file. You can specify further configuration options, such as maxIdleTime, maxTextMessageSize, and maxBinaryMessageSize.
  • 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 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 ws:connect('/') will be executed if a client establishes a connection with the WebSocket root path (which is, by default, ws/). 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 path specifies the path which a client is connected to.

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 message arrives at the server. The path specifies the path which a client is connected to.

The message is the message sent by the client. It will be of type xs:string or xs:base64Binary. The client needs to take care of the message syntax, and it must not exceed the configured maximum size.

%ws:close(path)

Called when the WebSocket closes. The path 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.

%ws:error(path, message)

Called when an error occurs. The path specifies the path which a client is connected to.

Usually, errors happen because of bad/malformed incoming packets. The message is the error message. The WebSocket connection gets closed after the error handling.

%ws:header-param(name, variable[, default])

For accessing connection-specific properties like the HTTP version. The value will be bound to the specified variable. If no value is available, an optional default value will be bound instead.

The following parameters are available:

Name Description
http-version The HTTP version used for the request.
origin The WebSocket origin.
protocol-version The version of the used protocol (max. 1.3).
query-string The query string of the request URI.
is-secure Indicates if the connection is secure.
is-secure The Request URI to use for this request.
host The host of the request URI.
sec-websocket-version The websocket version.

Tipps

  • The WebSocket Module contains functions for interacting with other clients or manage specific clients.
  • The results of functions annotated with %ws:close or %ws:error 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 wss instead of ws for a secure WebSocket connection.
  • If you use a proxy server, check in the configuration if WebSockets are enabled.

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 WsServlet is enabled in your web.xml 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 %ws:connect('/'). 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.

declare
  %ws:connect('/')
function example:connect() as empty-sequence() {
};

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 %ws:message("/"):

import module namespace ws = 'http://basex.org/modules/ws'
declare
  %ws:message('/', '{$message}')
function example:message(
  $message  as xs:string
) as empty-sequence() {
  ws:emit($message)
};

In the function above, the WebSocket Module is imported, and the function ws:emit is used for forwarding the message to all connected clients.

The following client-side code demonstrates a basic application of the WebSocket connection:

var ws = new WebSocket("ws://localhost:8984/ws");

ws.onmessage = function(event) {
  alert(event.data);
};

function send(message) {
  ws.send(message);
};

The send function can be called to pass on a string (e. g. a chat message) to the server.

There are no heartbeats in this example. 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 size.