Validation Plugins in Fabric

Motivation and Design

Validation plugins in Fabric provide a flexible and extensible mechanism for checking the compatibility and correctness of Fabric plans before execution. By decoupling validation logic from the core, Fabric can support multiple validation strategies and policies without requiring changes to the main system.

• Early error detection: Catch configuration and compatibility issues before plan execution.
• Extensibility: New validation plugins can be added to enforce custom rules or policies.
• Runtime selection: A validation plugin is selected at runtime through the compatibility_validation_plugin parameter.

ValidationBase Interface

All validation plugins inherit from the ValidationBase abstract class, which defines the required interface:

• initialize(const rclcpp::Node::SharedPtr&): Prepares the plugin with the ROS2 node context.
• validate(fabric::Plan&, std::vector<CapabilityInfo>&): Validates the plan against available capabilities.

This ensures that all plugins are interchangeable and can be managed uniformly by the Fabric core.

CompatibilityValidation: Reference Implementation

The CompatibilityValidation plugin is the default validator for plan compatibility. Its key features include:

• Interface/provider matching: Ensures every connection source in the parsed plan references a valid capability interface and provider.
• Provider flexibility: Supports both default and alternative providers for redundancy.
• Error reporting: Throws on the first unmatched interface/provider combination, preventing invalid plans from executing.

How CompatibilityValidation Works

1. Initialization: The plugin is initialized with the ROS2 node context.
2. Validation: For each connection in the plan:
   • Checks whether the source node's interface and provider match any entry in the provided capabilities list.
   • If not matched, checks the interface's alt_providers list.
   • If still unmatched, throws an exception to halt processing.
3. Result: Only plans with valid capability assignments are allowed to proceed to execution.

Main Method

Method	Purpose
validate(plan, capabilities)	Validates each connection's interface/provider against available capabilities.

Example Usage

• The Fabric server loads the validation plugin at startup.
• After parsing a plan, the server calls validate(plan, capabilities) before proceeding to capability allocation and execution.
• If validation fails, the server marks the plan as VALIDATION_FAILED and logs the exception.

Extending Validation

To support new validation policies:

1. Implement a new validation plugin inheriting from ValidationBase.
2. Register the plugin so it can be discovered and loaded by Fabric.
3. Configure the compatibility_validation_plugin parameter to load the new plugin.

The current server loads a single validation plugin instance.

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Summary:

• The validation plugin system allows Fabric to swap validation strategies without changing the server.
• The CompatibilityValidation plugin demonstrates how to enforce capability compatibility and prevent invalid plans from executing.
• New validation policies can be supported by adding new plugins, keeping the core stable and extensible.