An EPL module file is a plain text file in which EPL statements appear separated by the semicolon (;) character. It bundles EPL statements with optional deployment instructions. A service provider instance keeps track of the known and/or deployed EPL modules and makes it easy to add, remove, deploy and undeploy EPL modules.

The synopsis of an EPL module file is:

[module module_name;]
[uses module_name; | import import_name;] [uses module_name; | import import_name;] [...]
[epl_statement;] [epl_statement;] [...]

Use the module keyword followed a module_name identifier or a package (identifiers separated by dots) to declare the name of the module. The module name declaration must be at the beginning of the file, comments and whitespace excluded. The module name serves to check uses-dependences of other modules.

If a module file requires certain constructs that may be shared by other module files, such as named windows, variables, event types, variant streams or inserted-into streams required by statements, a module file may specify zero to many dependent modules with the uses keyword. At deployment time the engine checks the uses-dependencies and ensures that a module of that name is already deployed or will be deployed as part of the deployments. The deployment API supports ordering modules according to their uses-relationship.

If the EPL statements in the module require Java classes such as for underlying events or user-defined functions, use the import keyword followed by the fully-qualified class name or package name in the format package.*. The uses and import keywords are optional and must occur after the module declaration.

Following the optional deployment instructions are any number of epl_statement EPL statements that are separated by semicolon (;).

The following is a sample EPL module file explained in detail thereafter:

// Declare the name for the module
module org.myorganization.switchmonitor;

// Declare other module(s) that this module depends on
uses org.myorganization.common;

// Import any Java/.NET classes in an application package 
import org.myorganization.events.*;

// Declare an event type based on a Java class in the package that was imported as above
create schema MySwitchEvent as MySwitchEventPOJO;

// Sample statement
@Name('Off-On-Detector')
insert into MyOffOnStream
select * from pattern[every-distinct(id) a=MySwitchEvent(status='off') 
  -> b=MySwitchEvent(id=a.id, status='on')];

// Sample statement
@Name('Count-Switched-On')
@Description('Count per switch id of the number of Off-to-On switches in the last 1 hour')
select id, count(*) from MyOffOnStream.win:time(1 hour) group by id;

The example above declares a module name of org.myorganization.switchmonitor. As defined by the uses keyword, it ensures that the org.myorganization.common module is already deployed. The example demonstrates the import keyword to make a package name known to the engine for resolving POJO class names, as the example assumes that MySwitchEventPOJO is a POJO event class. In addition the example module contains two statements separated by semicolon characters.

Your application code may, after deployment, look up a statement and attach listeners as shown here:

epService.getEPAdministrator().getStatement("Count-Switched-On").addListener(...);

The com.espertech.esper.client.deploy.EPDeploymentAdmin service available from the EPAdministrator interface by method getDeploymentAdmin provides the functionality available to manage packaging and deployment. Please consult the JavaDoc documentation for more information.

The deployment API allows to read resources and parse text strings to obtain an object representation of the EPL module, the Module. A Module object can also be simply constructed.

After your application obtains a Module instance it may either use deploy to deploy the module directly, starting all statements of the module. Alternatively your application may add a module, making it known without starting statements for later deployment. In each case the module is assigned a deployment id, which acts as a unique primary key for all known modules. Your application may assign its own deployment id or may have the engine generate a deployment id (two footprints for add and deploy methods).

A module may be in two states: undeployed or deployed. When calling add to add a module, it starts life in the undeployed state. When calling deploy to deploy a module, it starts life in the deployed state. A module may be transitioned by providing the deployment id and by calling the deploy or undeploy methods.

Your code can remove a module in undeployed state using the remove method or the undeployRemove method. If the module is in deployed state, use undeployRemove to undeploy and remove the module.

The DeploymentOptions instance that can be passed to the deploy method when validating or deploying modules controls validation, fail-fast, rollback and the isolated service provider, if any, for the deployment. Also use DeploymentOptions to set a user object per statement or to set a statement name per statement.

We also provide additional sample code to read and deploy modules as part of the J2EE considerations below.

Esper can well be deployed as part of a J2EE web or enterprise application archive to a web application server. When designing for deployment into a J2EE web application server, please consider the items discussed here.

We provide a sample servlet context listener in this section that uses the deployment API to deploy and undeploy modules as part of the servlet lifecycle.

The distribution provides a message-driven bean (MDB) example that you may find useful.

Esper does not have a dependency on any J2EE or Servlet APIs to allow the engine to run in any environment or container.