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Apache Karaf Documentation

repository·main·Indexed 20 days ago

https://github.com/apache/karaf

A modular runtime environment for hosting and managing enterprise applications using frameworks such as Spring Boot, CXF, and Camel. It features hot deployment, dynamic configuration, a centralized logging system via Log4J, and an extensible shell console. The runtime supports Java SE 11 or higher and provides specialized distributions including a minimal version and an integration platform pre-configured with Apache Camel and Apache ActiveMQ. Documentation covers installation, Docker image builds, and examples for Blueprint and bundle deployment.

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What's inside Apache Karaf

  1. Overview of Apache Karaf features

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    Apache Karaf is a modular runtime that supports various frameworks and programming models (REST/API, web, Spring Boot, etc.). It can be deployed as a mutable or immutable application.

    Key capabilities include:

    • Hot deployment: Deploy applications by placing them in the deploy directory.
    • Dynamic configuration: Centralized configuration is managed in the etc directory using various formats (properties, JSON, etc.).
    • Logging System: Centralized logging via Log4J, supporting multiple APIs (JDK 1.4, JCL, SLF4J, Avalon, Tomcat, etc.).
    • Provisioning: Libraries and applications can be downloaded, installed, and started automatically via a resolver that handles required components.
    • Extensible Shell console: A text-based console for managing services, installing applications, and managing state. It can be extended by deploying new commands dynamically.
    • Remote access: Connect to the kernel using any SSH client to issue console commands.
    • Security & ACL: Role-Based Access Control (RBAC) provided via a JAAS-based framework.
    • Instance Management: Commands are available within the console to create, delete, start, and stop Karaf instances.
    • Enterprise features: Built-in support for JDBC, JPA, JTA, JMS, etc.
    • HTTP Service: A full-featured web container for deploying web applications.
    • REST & Services: Support for service frameworks like Apache CXF.
    • Extensibility: The runtime can be extended via Karaf subprojects (Decanter, Cellar, Cave, etc.) or third-party projects like Apache Camel.

  2. Overview of Karaf Redis Example artifacts

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    The Redis example is composed of the following Maven artifacts:

    • karaf-example-redis-api: Contains the BookingService interface and the Booking POJO.
    • karaf-example-redis-service: The implementation of BookingService that uses Redis as the underlying data storage.
    • karaf-example-redis-command: Provides the Karaf shell commands to interact with the BookingService.
    • karaf-example-redis-features: Provides the Karaf feature definitions required for deployment.

  3. Overview of Apache Karaf Integration Distribution

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    Apache Karaf Integration is a specialized Karaf distribution designed to function as an integration platform. It is similar to Apache ServiceMix, providing a complete Karaf environment pre-configured with essential integration features such as Apache Camel and Apache ActiveMQ "out of the box". This distribution is intended to be customized to serve as a foundation for specific integration use cases.

  4. What is Apache Karaf?

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    Apache Karaf is a modular runtime that supports multiple frameworks and programming models, including REST/API, web, and Spring Boot. It is designed to host applications as either mutable or immutable units and provides turnkey enterprise features.

    Key capabilities include:

    • Hot deployment: Deploy applications by placing them in the deploy directory.
    • Dynamic configuration: Centralized configuration managed in the etc directory (supporting properties and JSON formats).
    • Provisioning: Automatic downloading, installation, and starting of libraries/applications via a resolver that handles dependencies.
    • Extensible Shell: A text-based console for managing services, applications, and state, accessible remotely via any SSH client.
    • Enterprise Services: Built-in support for JDBC, JPA, JTA, JMS, and an HTTP service for web applications.
    • Service Frameworks: Support for frameworks like Apache CXF to implement REST and other services.

  5. Explore Apache Karaf usage examples

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    The Apache Karaf Examples repository contains various starting points and implementations of concrete use cases. These examples demonstrate how to leverage specific Apache Karaf capabilities.

    To understand the goal of a specific example and how to execute it, you must refer to the README.md file located within each individual example directory.

  6. Understand the Apache Karaf directory structure

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    A Karaf installation is organized into several key directories that separate configuration, binaries, deployment, and runtime data. Understanding this layout is essential for managing configurations, deploying bundles, and troubleshooting runtime issues.

    Key directories include:

    • /bin: Control scripts for lifecycle management (e.g., start, stop, login).
    • /etc: Central location for all configuration files.
    • /deploy: The hot deployment directory where you can drop bundles to be automatically installed.
    • /lib: Contains libraries, including /lib/boot (bootstrap libraries), /lib/endorsed, and /lib/ext (JRE extensions).
    • /system: The OSGi bundles repository, structured as a Maven 2 repository.
    • /instances: Contains Karaf instances.
    • /examples: Sample configurations and use cases.
    • /data: The working directory for the runtime (see 'Reset Karaf to a clean state' for details).

  7. What is a Karaf instance and how does it work?

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    An instance is a complete, isolated Apache Karaf runtime that runs in its own JVM. Instead of a full distribution, an instance contains only the necessary configuration files and a data folder (logs, temporary files, etc.). This allows you to manage multiple independent Karaf runtimes from a single 'root' instance without the overhead of multiple full installations.

  8. What is a KAR (KAraf aRchive)?

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    A KAR is a special type of artifact used in Apache Karaf to package a set of feature descriptors (XML or JSON) and all the resources (bundles, configuration files, etc.) described in those features.

    Conceptually, a KAR is a zip file containing:

    • A repository folder: contains feature repository files and artifacts (JAR files) following the Maven directory structure (groupId/artifactId/version/artifactId-version.type).
    • An optional resources folder: contains files like scripts or configuration files that should be included in the Karaf distribution.

    Because it is a zip file, it does not strictly require a META-INF/MANIFEST.MF, though the karaf-maven-plugin includes one by default.

  9. What is Apache Karaf Blueprint?

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    Blueprint is a dependency injection and service orchestration container for OSGi. It allows you to describe beans and services using an XML descriptor.

    Key characteristics:

    • XML Descriptors: You can define service wiring and bean configurations in XML files.
    • Automation: You can write XML manually or use annotations combined with the blueprint-maven-plugin to generate the XML descriptors automatically.

  10. Configure JMX Role-Based Access Control (RBAC)

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    Karaf uses Role-Based Access Control to restrict JMX operations. Access lists are defined in files within the etc folder, prefixed with jmx.acl.

    Configuration Hierarchy (from most specific to most general):

    1. etc/jmx.acl.[ObjectName].cfg (e.g., etc/jmx.acl.foo.bar.Test.cfg)
    2. etc/jmx.acl.[domain].cfg (e.g., etc/jmx.acl.foo.bar.cfg)
    3. etc/jmx.acl.cfg (Global configuration)

    Matching Logic for Invocations: When an operation is called, Karaf searches for matches in this order:

    1. Specific match: test(int)["17"] = role1
    2. Regex match: test(int)[/[0-9]/] = role2
    3. Signature match: test(int) = role3
    4. Method name match: test = role4
    5. Wildcard match: te* = role5 (The longest wildcard match wins).

    Example: Restricting Bundle Stop operations To allow only admin to stop bundles with IDs 0-49, and manager to stop all other bundles:

    # etc/jmx.acl.org.apache.karaf.bundles.cfg
stop(java.lang.String)[/([1-4])?([0-9]/] = admin
stop = manager
    # etc/jmx.acl.org.apache.karaf.bundles.cfg
stop(java.lang.String)[/([1-4])?([0-9]/] = admin
stop = manager

  11. Understand the FeatureMBean data model

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    The FeatureMBean provides a programmatic way to inspect the state of features within Karaf. Each feature object contains the following attributes:

    • Name: The name of the feature.
    • Version: The version of the feature.
    • Installed: A boolean indicating if the feature is currently installed.
    • Bundles: A tabular data set of all bundles (including bundle URLs) described in the feature.
    • Configurations: A tabular data set of all configurations described in the feature.
    • Configuration Files: A tabular data set of all configuration files described in the feature.
    • Dependencies: A tabular data set of all dependent features described in the feature.