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<!DOCTYPE document [
  <!ENTITY project SYSTEM "project.xml">
]>
<document url="engine.html">

  &project;

  <properties>
    <author email="craigmcc@apache.org">Craig R. McClanahan</author>
    <title>The Engine Container</title>
  </properties>

<body>

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

<section name="Introduction">

  <p>The <strong>Engine</strong> element represents the entire request
  processing machinery associated with a particular Catalina
  <a href="service.html">Service</a>.  It receives and processes
  <em>all</em> requests from one or more <strong>Connectors</strong>,
  and returns the completed response to the Connector for ultimate
  transmission back to the client.</p>

  <p>Exactly one <strong>Engine</strong> element MUST be nested inside
  a <a href="service.html">Service</a> element, following all of the
  corresponding Connector elements associated with this Service.</p>

</section>


<section name="Attributes">

  <subsection name="Common Attributes">

    <p>All implementations of <strong>Engine</strong>
    support the following attributes:</p>

    <attributes>

      <attribute name="backgroundProcessorDelay" required="false">
        <p>This value represents the delay in seconds between the
        invocation of the backgroundProcess method on this engine and
        its child containers, including all hosts and contexts.
        Child containers will not be invoked if their delay value is not
        negative (which would mean they are using their own processing
        thread). Setting this to a positive value will cause
        a thread to be spawn. After waiting the specified amount of time,
        the thread will invoke the backgroundProcess method on this engine
        and all its child containers. If not specified, the default value for
        this attribute is 10, which represent a 10 seconds delay.</p>
      </attribute>

      <attribute name="className" required="false">
        <p>Java class name of the implementation to use.  This class must
        implement the <code>org.apache.catalina.Engine</code> interface.
        If not specified, the standard value (defined below) will be used.</p>
      </attribute>

      <attribute name="defaultHost" required="true">
        <p>The default host name, which identifies the
        <a href="host.html">Host</a> that will process requests directed
        to host names on this server, but which are not configured in
        this configuration file.  This name MUST match the <code>name</code>
        attributes of one of the <a href="host.html">Host</a> elements
        nested immediately inside.</p>
      </attribute>

      <attribute name="jvmRoute" required="false">
        <p>Identifier which must be used in load balancing scenarios to enable
        session affinity. The identifier, which must be unique across all
        Tomcat servers which participate in the cluster, will be appended to
        the generated session identifier, therefore allowing the front end
        proxy to always forward a particular session to the same Tomcat
        instance.</p>
        <p>
            Note that the <code>jvmRoute</code> can also be set using the
            <code>jvmRoute</code> system property. The <code>jvmRoute</code>
            set in an <code>&lt;Engine&gt;</code> attribute will override
            any <code>jvmRoute</code> system property.
        </p>
      </attribute>

      <attribute name="name" required="true">
        <p>Logical name of this Engine, used in log and error messages. <em>When
        using multiple <a href="service.html">Service</a> elements in the same
        <a href="server.html">Server</a>, each Engine MUST be assigned a unique
        name.</em></p>
      </attribute>

      <attribute name="startStopThreads" required="false">
        <p>The number of threads this <strong>Engine</strong> will use to start
        child <a href="host.html">Host</a> elements in parallel. The special
        value of 0 will result in the value of
        <code>Runtime.getRuntime().availableProcessors()</code> being used.
        Negative values will result in
        <code>Runtime.getRuntime().availableProcessors() + value</code> being
        used unless this is less than 1 in which case 1 thread will be used. If
        not specified, the default value of 1 will be used. If 1 thread is
        used then rather than using an <code>ExecutorService</code> the current
        thread will be used.</p>
      </attribute>

    </attributes>

  </subsection>


  <subsection name="Standard Implementation">

    <p>The standard implementation of <strong>Engine</strong> is
    <strong>org.apache.catalina.core.StandardEngine</strong>.
    It supports the following additional attributes (in addition to the
    common attributes listed above):</p>

    <attributes>

    </attributes>

  </subsection>


</section>


<section name="Nested Components">

  <p>You can nest one or more <a href="host.html">Host</a> elements inside
  this <strong>Engine</strong> element, each representing a different virtual
  host associated with this server.  At least one <a href="host.html">Host</a>
  is required, and one of the nested <a href="host.html">Hosts</a> MUST
  have a name that matches the name specified for the
  <code>defaultHost</code> attribute, listed above.</p>

  <p>You can nest at most one instance of the following utility components
  by nesting a corresponding element inside your <strong>Engine</strong>
  element:</p>
  <ul>
  <li><a href="realm.html"><strong>Realm</strong></a> -
      Configure a realm that will allow its
      database of users, and their associated roles, to be shared across all
      <a href="host.html">Hosts</a> and <a href="context.html">Contexts</a>
      nested inside this Engine, unless overridden by a
      <a href="realm.html">Realm</a> configuration at a lower level.</li>
  </ul>

</section>


<section name="Special Features">


  <subsection name="Logging">

    <p>An engine is associated with the
       <code>org.apache.catalina.core.ContainerBase.[enginename]</code>
       log category.  Note that the brackets are actually part of the name,
       don't omit them.</p>

  </subsection>


  <subsection name="Access Logs">

    <p>When you run a web server, one of the output files normally generated
    is an <em>access log</em>, which generates one line of information for
    each request processed by the server, in a standard format.  Catalina
    includes an optional <a href="valve.html">Valve</a> implementation that
    can create access logs in the same standard format created by web servers,
    or in any number of custom formats.</p>

    <p>You can ask Catalina to create an access log for all requests
    processed by an <a href="engine.html">Engine</a>,
    <a href="host.html">Host</a>, or <a href="context.html">Context</a>
    by nesting a <a href="valve.html">Valve</a> element like this:</p>

<source><![CDATA[<Engine name="Standalone" ...>
  ...
  <Valve className="org.apache.catalina.valves.AccessLogValve"
         prefix="catalina_access_log" suffix=".txt"
         pattern="common"/>
  ...
</Engine>]]></source>

    <p>See <a href="valve.html#Access_Logging">Access Logging Valves</a>
    for more information on the configuration attributes that are
    supported.</p>

  </subsection>


  <subsection name="Lifecycle Listeners">

    <p>If you have implemented a Java object that needs to know when this
    <strong>Engine</strong> is started or stopped, you can declare it by
    nesting a <strong>Listener</strong> element inside this element.  The
    class name you specify must implement the
    <code>org.apache.catalina.LifecycleListener</code> interface, and
    it will be notified about the occurrence of the corresponding
    lifecycle events.  Configuration of such a listener looks like this:</p>

<source><![CDATA[<Engine name="Standalone" ...>
  ...
  <Listener className="com.mycompany.mypackage.MyListener" ... >
  ...
</Engine>]]></source>

    <p>Note that a Listener can have any number of additional properties
    that may be configured from this element.  Attribute names are matched
    to corresponding JavaBean property names using the standard property
    method naming patterns.</p>

  </subsection>


  <subsection name="Request Filters">

    <p>You can ask Catalina to check the IP address, or host name, on every
    incoming request directed to the surrounding
    <a href="engine.html">Engine</a>, <a href="host.html">Host</a>, or
    <a href="context.html">Context</a> element.  The remote address or name
    will be checked against configured "accept" and/or "deny"
    filters, which are defined using <code>java.util.regex</code> Regular
    Expression syntax.  Requests that come from locations that are
    not accepted will be rejected with an HTTP "Forbidden" error.
    Example filter declarations:</p>

<source><![CDATA[<Engine name="Standalone" ...>
  ...
  <Valve className="org.apache.catalina.valves.RemoteHostValve"
         allow=".*\.mycompany\.com|www\.yourcompany\.com"/>
  <Valve className="org.apache.catalina.valves.RemoteAddrValve"
         deny="192\.168\.1\.\d+"/>
  ...
</Engine>]]></source>

  <p>See <a href="valve.html#Remote_Address_Filter">Remote Address Filter</a>
  and <a href="valve.html#Remote_Host_Filter">Remote Host Filter</a> for
  more information about the configuration options that are supported.</p>

  </subsection>


</section>


</body>


</document>
