Policies

Policies

Policies are an integral part when writing business logic in Strolch. In many cases it would suffice to write all such logic in Services and Commands, but as soon as behaviour can change, depending on the element being accessed, then this would quickly lead to many if/else blocks.

Since writing large if/else blocks is not maintainable in the long run, Strolch offers a different approach. All Strolch elements can store Policy definitions. This is a simple key/value store where the key defines the type of policy, and the value references the policy to use.

The PolicyHandler is Strolch’s mechanism for dependency injection and extensibility. It allows defining interchangeable logic that can be selected at runtime based on the data model.

Concepts

• Policy Definition (PolicyDef): A reference to a policy type and a key.
• Policy Implementation (StrolchPolicy): The actual Java class that implements the logic.
• Policy Configuration: Policies are configured in an XML file (e.g., StrolchPolicies.xml) or within the element’s XML definition.

Currently there are two ways to reference a policy in Strolch, either via a key which defines a further lookup in the PolicyHandler, or directly as the name of the class to instantiate.

Defining a Policy

Policies are registered on Resources, Orders, Activities and Actions. The following shows defining two policies on a Resource, a PlanningPolicy and an ExecutionPolicy in XML:

<Resource Id="myResource" Name="My Resource" Type="MyType">
    ...
    <Policies>
        <Policy Type="PlanningPolicy" Value="key:SimplePlanning"/>
        <Policy Type="ExecutionPolicy" Value="java:li.strolch.policytest.TestSimulatedExecutionPolicy"/>
    </Policies>
</Resource>

Implementing a Policy

To implement a policy, define an abstract class that extends StrolchPolicy and defines the API. A concrete class then extends this and implements the logic.

public class MyExecutionPolicy extends StrolchPolicy implements ExecutionPolicy {
	public MyExecutionPolicy(StrolchTransaction tx) {
		super(tx);
	}

	@Override
	public void execute(IActivityElement element) {
		// Implementation logic
	}
}

Configuration

To use the PolicyHandler, it must be configured in the StrolchConfiguration.xml:

<Component>
    <name>PolicyHandler</name>
    <api>li.strolch.policy.PolicyHandler</api>
    <impl>li.strolch.policy.DefaultPolicyHandler</impl>
    <Properties>
        <readPolicyFile>true</readPolicyFile>
        <policyConfigFile>StrolchPolicies.xml</policyConfigFile>
    </Properties>
</Component>

The StrolchPolicies.xml file defines the lookups:

<StrolchPolicies>
    <PolicyType Type="PlanningPolicy" Api="li.strolch.policytest.TestPlanningPolicy">
        <Policy Key="SimplePlanning" Class="li.strolch.policytest.TestSimplePlanningPolicy"/>
    </PolicyType>
    <PolicyType Type="ExecutionPolicy" Api="li.strolch.execution.policy.ExecutionPolicy">
        <Policy Key="SimulatedExecution" Class="li.strolch.execution.policy.RandomDurationExecution"/>
    </PolicyType>
</StrolchPolicies>

Using a Policy

Policies are retrieved within a transaction.

try (StrolchTransaction tx = openArgOrUserTx(arg)) {
    Resource res = tx.getResourceBy("MyType", "myTestResource");

    PlanningPolicy planningPolicy = tx.getPolicy(res, PlanningPolicy.class);
    planningPolicy.plan(...);

    ExecutionPolicy executionPolicy = tx.getPolicy(res, ExecutionPolicy.class);
    executionPolicy.toExecution(...);

    tx.commitOnClose();
}

Retrieving a Policy without an Element

Sometimes you need a policy that is not directly attached to a model element. In this case, you can create a PolicyDef manually.

PolicyDef policyDef = PolicyDef.getKeyPolicy(ExecutionPolicy.class, "DefaultExecution");
ExecutionPolicy policy = tx.getPolicy(ExecutionPolicy.class, policyDef);

Alternatively, you can use PolicyDef.valueOf():

PolicyDef policyDef = PolicyDef.valueOf(ExecutionPolicy.class, "key:DefaultExecution");

Or reference a Java class directly:

PolicyDef policyDef = PolicyDef.getJavaPolicy(ExecutionPolicy.class, MyExecutionPolicy.class);