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<!DOCTYPE document [
  <!ENTITY project SYSTEM "project.xml">
]>
<document url="web-socket-howto.html">

    &project;

    <properties>
      <title>WebSocket How-To</title>
    </properties>

<body>

<section name="Table of Contents">
<toc/>
</section>

<section name="Overview">
<p>Tomcat provides support for WebSocket as defined by
   <a href="https://tools.ietf.org/html/rfc6455">RFC 6455</a>.</p>
</section>

<section name="Application development">
<p>Tomcat implements the Java WebSocket 1.1 API defined by <a
   href="https://www.jcp.org/en/jsr/detail?id=356">JSR-356</a>.</p>

<p>There are several example applications that demonstrate how the WebSocket API
   can be used. You will need to look at both the client side <a
   href="https://svn.apache.org/viewvc/tomcat/trunk/webapps/examples/websocket/">
   HTML</a> and the server side <a
   href="https://svn.apache.org/viewvc/tomcat/trunk/webapps/examples/WEB-INF/classes/websocket/">
   code</a>.</p>
</section>

<section name="Production usage">
<p>Although the WebSocket implementation does work with any of the HTTP
connectors, it is not recommended to the WebSocket with the BIO HTTP connector
as the typical uses of WebSocket (large numbers of mostly idle connections) is
not a good fit for the HTTP BIO connector which requires that one thread is
allocated per connection regardless of whether or not the connection is idle.
</p>

<p>It has been reported (<bug>56304</bug>) that Linux can take large numbers of
minutes to report dropped connections. When using WebSocket with the BIO HTTP
connector this can result in threads blocking on writes for this period. This is
likely to be undesirable. The time taken for the connection to be reported as
dropped can be reduced by using the kernel network parameter
<code>/proc/sys/net/ipv4/tcp_retries2</code>. Alternatively, one of the other
HTTP connectors may be used as they utilise non-blocking IO enabling Tomcat to
implement its own timeout mechanism to handle these cases.</p>
</section>

<section name="Tomcat WebSocket specific configuration">

<p>Tomcat provides a number of Tomcat specific configuration options for
   WebSocket. It is anticipated that these will be absorbed into the WebSocket
   specification over time.</p>

<p>The write timeout used when sending WebSocket messages in blocking mode
   defaults to 20000 milliseconds (20 seconds). This may be changed by setting
   the property <code>org.apache.tomcat.websocket.BLOCKING_SEND_TIMEOUT</code>
   in the user properties collection attached to the WebSocket session. The
   value assigned to this property should be a <code>Long</code> and represents
   the timeout to use in milliseconds. For an infinite timeout, use
   <code>-1</code>.</p>

<p>If the application does not define a <code>MessageHandler.Partial</code> for
   incoming binary messages, any incoming binary messages must be buffered so
   the entire message can be delivered in a single call to the registered
   <code>MessageHandler.Whole</code> for binary messages. The default buffer
   size for binary messages is 8192 bytes. This may be changed for a web
   application by setting the servlet context initialization parameter
   <code>org.apache.tomcat.websocket.binaryBufferSize</code> to the desired
   value in bytes.</p>

<p>If the application does not define a <code>MessageHandler.Partial</code> for
   incoming text messages, any incoming text messages must be buffered so the
   entire message can be delivered in a single call to the registered
   <code>MessageHandler.Whole</code> for text messages. The default buffer size
   for text messages is 8192 bytes. This may be changed for a web application by
   setting the servlet context initialization parameter
   <code>org.apache.tomcat.websocket.textBufferSize</code> to the desired value
   in bytes.</p>

<p>The Java WebSocket specification 1.0 does not permit programmatic deployment
   after the first endpoint has started a WebSocket handshake. By default,
   Tomcat continues to permit additional programmatic deployment. This
   behavior is controlled by the
   <code>org.apache.tomcat.websocket.noAddAfterHandshake</code> servlet context
   initialization parameter. The default may be changed by setting the
   <code>org.apache.tomcat.websocket.STRICT_SPEC_COMPLIANCE</code> system
   property to <code>true</code> but any explicit setting on the servlet context
   will always take priority.</p>

<p>The Java WebSocket 1.0 specification requires that callbacks for
   asynchronous writes are performed on a different thread to the thread that
   initiated the write. Since the container thread pool is not exposed via the
   Servlet API, the WebSocket implementation has to provide its own thread pool.
   This thread pool is controlled by the following servlet context
   initialization parameters:</p>
   <ul>
     <li><code>org.apache.tomcat.websocket.executorCoreSize</code>: The core
         size of the executor thread pool. If not set, the default of 0 (zero)
         is used. Note that the maximum permitted size of the executor thread
         pool is hard coded to <code>Integer.MAX_VALUE</code> which effectively
         means it is unlimited.</li>
     <li><code>org.apache.tomcat.websocket.executorKeepAliveTimeSeconds</code>:
         The maximum time an idle thread will remain in the executor thread pool
         until it is terminated. If not specified, the default of 60 seconds is
         used.</li>
   </ul>

<p>When using the WebSocket client to connect to server endpoints, the timeout
   for IO operations while establishing the connection is controlled by the
   <code>userProperties</code> of the provided
   <code>javax.websocket.ClientEndpointConfig</code>. The property is
   <code>org.apache.tomcat.websocket.IO_TIMEOUT_MS</code> and is the
   timeout as a <code>String</code> in milliseconds. The default is 5000 (5
   seconds).</p>

<p>When using the WebSocket client to connect to secure server endpoints, the
   client SSL configuration is controlled by the <code>userProperties</code>
   of the provided <code>javax.websocket.ClientEndpointConfig</code>. The
   following user properties are supported:</p>
   <ul>
     <li><code>org.apache.tomcat.websocket.SSL_CONTEXT</code></li>
     <li><code>org.apache.tomcat.websocket.SSL_PROTOCOLS</code></li>
     <li><code>org.apache.tomcat.websocket.SSL_TRUSTSTORE</code></li>
     <li><code>org.apache.tomcat.websocket.SSL_TRUSTSTORE_PWD</code></li>
   </ul>
   <p>The default truststore password is <code>changeit</code>.</p>

<p>If the <code>org.apache.tomcat.websocket.SSL_CONTEXT</code> property is
   set then the <code>org.apache.tomcat.websocket.SSL_TRUSTSTORE</code> and
   <code>org.apache.tomcat.websocket.SSL_TRUSTSTORE_PWD</code> properties
   will be ignored.</p>

<p>For secure server end points, host name verification is enabled by default.
   To bypass this verification (not recommended), it is necessary to provide a
   custom <code>SSLContext</code> via the
   <code>org.apache.tomcat.websocket.SSL_CONTEXT</code> user property. The
   custom <code>SSLContext</code> must be configured with a custom
   <code>TrustManager</code> that extends
   <code>javax.net.ssl.X509ExtendedTrustManager</code>. The desired verification
   (or lack of verification) can then be controlled by appropriate
   implementations of the individual abstract methods.</p>

<p>When using the WebSocket client to connect to server endpoints, the number of
   HTTP redirects that the client will follow is controlled by the
   <code>userProperties</code> of the provided
   <code>javax.websocket.ClientEndpointConfig</code>. The property is
   <ocde>org.apache.tomcat.websocket.MAX_REDIRECTIONS</ocde>. The default value
   is 20. Redirection support can be disabled by configuring a value of zero.</p>

</section>

</body>
</document>