Designing a Domain‐Specific Language for Process Control - wwestlake/Labyrinth GitHub Wiki

Designing a Domain-Specific Language (DSL) for Process Control

Introduction

A Domain-Specific Language (DSL) is a specialized language tailored to a specific domain, providing abstractions and constructs that make expressing domain concepts easier and more concise. For the ProcessEngine, we aim to create a DSL that facilitates defining and controlling processes, particularly for criteria and actions within process steps. This DSL will be parsed using FParsec in F#, producing an Abstract Syntax Tree (AST) that the ProcessEngine can execute.

The purpose of this paper is to explore the design considerations and propose a structure for a DSL suited to process control, including the syntax, semantics, and potential execution model.

Objectives of the DSL

The primary objectives of the DSL for the ProcessEngine are:

  1. Expressiveness: Enable clear and concise definitions of process criteria and actions, making it easier for non-technical stakeholders to understand and modify process rules.
  2. Parameterization: Allow criteria and actions to accept parameters, enabling dynamic and context-sensitive behavior.
  3. Integration with Server Actions: Facilitate interactions with server-side operations, such as database queries, external API calls, or conditional logic execution.
  4. Extensibility: Support the addition of new keywords, operations, and integrations as process requirements evolve.
  5. Safety and Security: Ensure that the language prevents unsafe operations, enforces security policies, and is resilient to injection attacks or misuse.

DSL Design Considerations

1. Syntax and Semantics

The DSL should be human-readable and align closely with the domain of process management. The syntax must support defining criteria (conditions that need to be met) and actions (tasks to be performed).

Example Syntax:

  • Criteria: Defines conditions under which actions should be executed.
  • Actions: Defines tasks to be performed when criteria are met.

Example Criteria:

IF User.Age >= 18 AND NOT User.EmailExists(User.Email) THEN APPROVE ELSE REJECT "User must be 18 or older and have a unique email."

Example Actions:

SEND_EMAIL TO User.Email WITH Template.Welcome LOG "User registration approved for {User.Id}"

2. Core Language Constructs

To implement a DSL for process control, we need to define several core constructs:

  • Variables: Representation of entities like User, Admin, and System with properties and methods (e.g., User.Age, User.EmailExists()).
  • Operators: Logical (AND, OR, NOT), comparison (=, !=, >, <), and arithmetic operators to evaluate expressions.
  • Control Structures: IF, ELSE, and THEN for conditional logic.
  • Functions: Predefined functions like SEND_EMAIL, LOG, APPROVE, and REJECT that perform specific actions.

3. Parameters and Parameterization

Parameters allow criteria and actions to be dynamic and context-sensitive:

  • Positional Parameters: Ordered parameters that are passed to functions (e.g., User.EmailExists(User.Email)).
  • Named Parameters: Key-value pairs that provide more flexibility (e.g., SEND_EMAIL TO User.Email WITH Template.Welcome).

Parameter Examples:

Criteria with parameters:

IF CheckUserAge(minAge=18) AND CheckUniqueEmail(email=User.Email) THEN APPROVE

Actions with parameters:

REJECT WITH MESSAGE "User does not meet the requirements."

4. Interaction with Server-Side Operations

The DSL must support executing actions that interact with server-side operations:

  • Database Queries: Fetching or updating records (e.g., checking if a user email exists).
  • API Calls: Interacting with external services (e.g., sending emails or notifications).
  • Conditional Logic: Implementing complex business logic based on user data or system state.

The DSL should provide built-in functions or commands to interact with these operations securely and efficiently.

5. Extensibility

The DSL should be designed to support future extensions, including:

  • New Keywords and Functions: Adding new actions or criteria as business needs change.
  • Custom Logic: Allowing developers to define custom logic blocks that can be reused across processes.
  • Plugin Architecture: Supporting plugins or modules that extend the DSL’s capabilities.

6. Safety and Security

To ensure the DSL is safe and secure:

  • Input Validation: Strict input validation to prevent SQL injection, cross-site scripting (XSS), or other vulnerabilities.
  • Execution Sandbox: Running the DSL in a controlled environment (sandbox) to prevent unauthorized access to sensitive data or system resources.
  • Error Handling: Comprehensive error handling to manage runtime exceptions and provide meaningful feedback.

Proposed DSL Structure

1. Grammar and Parsing with FParsec

FParsec is an F# library for building parsers. We can use FParsec to define the grammar for the DSL and generate an AST. The grammar will be defined to parse:

  • Expressions: Mathematical, logical, and comparison operations.
  • Commands: Instructions like SEND_EMAIL, APPROVE, REJECT.
  • Statements: Combinations of expressions and commands to form meaningful business logic.

2. Abstract Syntax Tree (AST) Design

The AST represents the parsed structure of the DSL code and includes:

  • Nodes for Expressions: Representing operations and comparisons.
  • Nodes for Commands: Representing actions like sending emails or logging.
  • Control Flow Nodes: Representing conditional structures (IF, THEN, ELSE).

3. Execution Model

The execution model defines how the AST is processed:

  1. Interpretation: The ProcessEngine interprets the AST nodes, evaluating expressions and executing commands.
  2. Context Management: Maintaining context (state) across steps, such as variables, user inputs, and process state.
  3. Action Execution: Performing actions like sending emails or updating the database.

4. Example DSL Definition

Example DSL definition for a user registration process:

IF User.Age >= 18 AND NOT User.EmailExists(User.Email) THEN APPROVE SEND_EMAIL TO User.Email WITH Template.Welcome LOG "User registration approved for {User.Id}" ELSE REJECT "User must be 18 or older and have a unique email." LOG "User registration rejected for {User.Id} due to unmet criteria."

This definition checks if the user meets the age requirement and has a unique email before approving registration and sending a welcome email.

Conclusion

Designing a DSL for process control requires careful consideration of syntax, semantics, extensibility, and security. The proposed DSL aims to provide a clear and concise way to define process criteria and actions, with the flexibility to handle dynamic conditions and integrate with server-side operations. Using FParsec in F# allows for robust parsing and AST generation, enabling the ProcessEngine to execute complex workflows efficiently and securely. By following these guidelines, we can create a powerful tool for managing automated processes in a variety of business contexts.