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Effectors
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Effectors perform an operation of some kind, carried out by a Brooklyn Entity. They can be manually invoked or triggered by a Policy.

Common uses of an effector include the following:

  • Perform a command on a remote machine.
  • Collect data and publish them to sensors.

Entities have default effectors, the lifecycle management effectors like start, stop, restart, and clearly more Effectors can be attached to them.

Off-the-Shelf Effectors

Effectors are highly reusable as their inputs, thresholds and targets are customizable.

SSHCommandEffector

An Effector to invoke a command on a node accessible via SSH.

It enables execution of a command in a specific execution director (executionDir) by using a custom shell environment (shellEnv). By default, the specified command will be executed on the entity where the effector is attached or on all children or all members (if it is a group) by configuring executionTarget.

There are a number of additional configuration keys available for the SSHCommandEffector:

Configuration Key Default Description
command command to be executed on the execution target
executionDir possible values: 'GET', 'HEAD', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS', 'TRACE'
shellEnv custom shell environment where the command is executed
executionTarget ENTITY possible values: 'MEMBERS', 'CHILDREN'

Here is a simple example of an SshCommandEffector definition:

{% highlight yaml %} brooklyn.initializers:

  • type: org.apache.brooklyn.core.effector.ssh.SshCommandEffector brooklyn.config: name: sayHiNetcat description: Echo a small hello string to the netcat entity command: | echo $message | nc $TARGET_HOSTNAME 4321 parameters: message: description: The string to pass to netcat defaultValue: hi netcat {% endhighlight %}

See here for more details.

HTTPCommandEffector

An Effector to invoke HTTP endpoints.

It allows the user to specify the URI, the HTTP verb, credentials for authentication and HTTP headers.

There are a number of additional configuration keys available for the HTTPCommandEffector:

Configuration Key Default Description
uri URI of the endpoint
httpVerb possible values: 'GET', 'HEAD', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS', 'TRACE'
httpUsername user name for the authentication
httpPassword password for the authentication
headers application/json It explicitly supports application/x-www-form-urlencoded
httpPayload The body of the HTTP request
jsonPath A jsonPath expression to extract values from a JSON object
jsonPathAndSensors A map where keys are jsonPath expressions and values the name of the sensor where to publish extracted values

When a the header HttpHeaders.CONTENT_TYPE is equals to application/x-www-form-urlencoded and the httpPayload is a map, the payload is transformed into a single string using URLEncoded.

{% highlight yaml %} brooklyn.initializers:

  • type: org.apache.brooklyn.core.effector.http.HttpCommandEffector brooklyn.config: name: request-access-token description: Request an access token for the Azure API uri: $brooklyn:formatString: - "https://login.windows.net/%s/oauth2/token" - $brooklyn:config("tenant.id") httpVerb: POST httpPayload: resource: https://management.core.windows.net/ client_id: $brooklyn:config("application.id") grant_type: client_credentials client_secret: $brooklyn:config("application.secret") jsonPathAndSensors: $.access_token: access.token headers: Content-Type: "application/x-www-form-urlencoded" {% endhighlight %}

See here for more details.

AddChildrenEffector

An Effector to add a child blueprint to an entity.

{% highlight yaml %} brooklyn.initializers:

  • type: org.apache.brooklyn.core.effector.AddChildrenEffector brooklyn.config: name: add_tomcat blueprint_yaml: | name: sample description: Tomcat sample JSP and servlet application. origin: http://www.oracle.com/nCAMP/Hand services: - type: io.camp.mock:AppServer name: Hello WAR wars: /: hello.war controller.spec: port: 80

      brooklyn.catalog:
  name: catalog-name
  type: io.camp.mock.MyApplication
  version: 0.9
  libraries:
  - name: org.apache.brooklyn.test.resources.osgi.brooklyn-test-osgi-entities
      version: 0.1.0
      url: classpath:/brooklyn/osgi/brooklyn-test-osgi-entities.jar

    auto_start: true {% endhighlight %}

One of the config keys BLUEPRINT_YAML (containing a YAML blueprint (map or string)) or BLUEPRINT_TYPE (containing a string referring to a catalog type) should be supplied, but not both.

See here for more details.

Writing an Effector

Your First Effector

Effectors generally perform actions on entities. Each effector instance is associated with an entity, and at runtime it will typically exectute an operation, collect the result and, potentially, publish it as sensor on that entity, performing some computation.

Writing an effector is straightforward. Simply extend AddEffector, providing an implementation for newEffectorBuilder and adding a constructor that consumes the builder or override an existing effector.

{% highlight java %}

public MyEffector(ConfigBag params) { super(newEffectorBuilder(params).build()); }

public static EffectorBuilder newEffectorBuilder(ConfigBag params) { EffectorBuilder eff = AddEffector.newEffectorBuilder(String.class, params); eff.impl(new Body(eff.buildAbstract(), params)); return eff; } {% endhighlight %}

and supply an EffectorBody similar to:

{% highlight java %}

protected static class Body extends EffectorBody { ...

@Override
public String call(final ConfigBag params) {
 ...
}

} {% endhighlight %}

Best Practice

The following recommendations should be considered when designing effectors:

Effectors should be small and composable

One effector which executes a command and emits a sensor, and a second effector which uses the previous sensor, if defined, to execute another operation.