WebSockets

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 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  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  , …), 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  configuration file. You can specify further configuration options, such as ,  , and.
 * 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  will be executed if a client establishes a connection with the WebSocket root path (which is, by default,  ). 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  specifies the path which a client is connected to:

 declare %ws:connect('/') function local:connect { };

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  specifies the path which a client is connected to. The  string contains the name of the variable to which the message will be bound:

 declare %ws:message('/', '{$info}') function local:message($info) { };

The value will be of type  or. 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  specifies the path which a client is connected to. The  string contains the name of the variable to which the message will be bound:

 declare %ws:error('/', '{$error}') function local:error($error) { };

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  specifies the path which a client is connected to:

 declare %ws:close('/') function local:connect { };

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. If the property has no value, an optional  value will be assigned instead:

 declare %ws:close('host', '{$host}') %ws:header-param('host', '{$host}') function local:close($host) { admin:write-log('Connection was closed: ' || $host) };

The following parameters are available:

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  or   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  instead of the   scheme.

If you get the  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  is enabled in your   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. 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 :

 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  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  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 (standard timeout). It 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 application that demonstrates how WebSockets can be used in practice.

=Changelog=

WebSockets werre introduced with Version 9.1.