Helidon Documentation

Helidon is a set of Java libraries designed for writing microservices.

Key features of Helidon 4 include:

• Níma WebServer: A high-performance web server built from the ground up for Java 21 Virtual Threads.
• Thread-per-request style: Provides high throughput (similar to reactive servers) while maintaining the simplicity of blocking, thread-per-request programming.
• Simplified SE API: In Helidon 4, the Helidon SE API has transitioned from asynchronous to blocking to leverage virtual threads, making code easier to write, maintain, and debug.

The Helidon DB Client provides a unified API for data access and querying. It acts as a layer above JDBC or the MongoDB Java Driver, allowing support for any relational database with a JDBC driver or MongoDB.

Key features include:

• Unified API: Consistent interface for different database types.
• Observability: Built-in support for health checks, metrics, and tracing.
• Portability: By defining native database statements in configuration files rather than hardcoding them, you can switch between different relational database drivers by only changing the configuration.

Helidon Declarative is a declarative programming model for Helidon SE. It allows developers to declare intention by annotating elements, reducing the amount of manual programming required for common microservice tasks.

Key principles:

• Uses a "nested" annotation approach (e.g., @Http.Path).
• Relies on code generation instead of reflection to minimize runtime overhead and lookups.
• All features are configured through the service registry.
• Features are organized into namespace classes (e.g., Http, Ft for Fault Tolerance, Secured for Security).

Helidon provides two specialized generators via the OpenAPI Generator tool to create code that integrates smoothly with Helidon applications:

• Helidon Client Generator (java-helidon-client): Creates a Helidon SE client based on Helidon WebClients. It provides an abstraction similar to Remote Procedure Calls (RPC), allowing you to call remote service endpoints by invoking local methods with POJO business objects or Java types.
• Helidon Server Generator (java-helidon-server): Creates server endpoint stubs. You implement the business logic by extending a generated class or implementing a generated interface, reducing the need for resource boilerplate.

For Helidon SE, use the se library with each generator.

Helidon Security provides the following core capabilities:

• Authentication: Authenticates incoming requests and creates a security Subject containing a Principal (representing the user or service) and Grants (representing roles or scopes). Supported default principals include UserPrincipal and ServicePrincipal.
• Authorization: Supports Attribute-Based Access Control (ABAC) and Role-Based Access Control (RBAC). RBAC can be implemented using the @RolesAllowed annotation in supported integrations.
• Outbound Security: Propagates identity or secures outbound requests via the OutboundSecurityProvider SPI.
• Audit: Audits security events (authentication, authorization, outbound) through the AuditProvider SPI. By default, it logs to a Java Util Logging (JUL) logger named AUDIT.

The concurrency-limits module allows you to restrict the number of concurrent, in-progress operations (e.g., within a WebServer). Helidon provides three primary implementation types that can be selected via configuration:

• fixed: A semaphore-based limit that supports queuing.
• aimd: An Additive-Increase/Multiplicative-Decrease limit based on TCP congestion control algorithms, providing variable concurrency limits between a fixed minimum and maximum.
• throughput: A rate-based limit that throttles requests to a specific amount over a defined duration, also supporting queuing.

Helidon Data Codegen provides a framework for generating repository and persistence-related code. It is divided into two main functional areas:

1. Common Codegen: Contains the foundational logic for code generation, including base generators for persistence, repositories, and repository interfaces, as well as blueprints for method parameters and repository information.
2. Helidon Data Codegen: Provides specialized generators for Helidon Data, including CRUD repository interfaces, JPQL query methods, and name-based query methods.

Helidon uses three primary environments for code processing and generation:

1. Annotation processors: Preferred method for source code generation and processing.
2. Classpath scanning: Used for analyzing external libraries or arbitrary code (e.g., via Maven plugins or CLI tools).
3. Reflection: Limited to specific modules intentionally designed for reflection, such as the Config object mapping module.

Helidon prioritizes annotation processing and source code generation whenever possible.