## The Engine Container

### Table of Contents

- Introduction
- Attributes
  1. Common Attributes
  2. Standard Implementation
- Nested Components
- Special Features
  1. Logging
  2. Access Logs
  3. Lifecycle Listeners
  4. Request Filters

### Introduction

The Engine element represents the entire request processing machinery associated with a particular Catalina Service. It receives and processes all requests from one or more Connectors, and returns the completed response to the Connector for ultimate transmission back to the client.

Exactly one Engine element MUST be nested inside a Service element, following all of the corresponding Connector elements associated with this Service.

### Attributes

#### Common Attributes

All implementations of Engine support the following attributes:

Attribute | Description
--- | ---

`backgroundProcessorDelay`

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.

`className` | Java class name of the implementation to use. This class must implement the `org.apache.catalina.Engine` interface. If not specified, the standard value (defined below) will be used.
`defaultHost` | The default host name, which identifies the Host 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 `name` attributes of one of the Host elements nested immediately inside.

`jvmRoute`

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.

Note that the `jvmRoute` can also be set using the deprecated `jvmRoute` system property. The `jvmRoute` set in an `<Engine>` attribute will override any `jvmRoute` system property.

`name` | Logical name of this Engine, used in log and error messages. When using multiple Service elements in the same Server, each Engine MUST be assigned a unique name.

`startStopThreads`

The number of threads this Engine will use to start child Host elements in parallel. The special value of 0 will result in the value of `Runtime.getRuntime().availableProcessors()` being used. Negative values will result in `Runtime.getRuntime().availableProcessors() + value` 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 `ExecutorService`, the current thread will be used.

#### Standard Implementation

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

Attribute | Description
--- | ---

### Nested Components

You can nest one or more Host elements inside this Engine element, each representing a different virtual host associated with this server. At least one Host is required, and one of the nested Hosts MUST have a name that matches the name specified for the `defaultHost` attribute, listed above.

You can nest at most one instance of the following utility components by nesting a corresponding element inside your Engine element:

- Realm - Configure a realm that will allow its database of users, and their associated roles, to be shared across all Hosts and Contexts nested inside this Engine, unless overridden by a Realm configuration at a lower level.

### Special Features

#### Logging

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

#### Access Logs

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

You can ask Catalina to create an access log for all requests processed by an Engine, Host, or Context by nesting a Valve element like this:

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

See Access Logging Valves for more information on the configuration attributes that are supported.

#### Lifecycle Listeners

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

    <Engine name="Standalone" ...>
      ...
      <Listener className="com.mycompany.mypackage.MyListener" ... >
      ...
    </Engine>

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.

#### Request Filters

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

    <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>

See Remote Address Filter and Remote Host Filter for more information about the configuration options that are supported.