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Apache Celix Introduction

What is Apache Celix

Apache Celix is an implementation of a dynamic service framework inspired by the OSGi specification and adapted to C and C++ (C++17). It is a framework to develop dynamic modular software applications using component and in-process service-oriented programming.

Apache Celix core is written in C and has a C++17 header-only API on top of the C API.

Building applications with Apache Celix can be done by creating bundles which provide and use services; These services can be used directly or in a declarative way using components. To run a selection of bundles, an Apache Celix container executable can be created. An Apache Celix containter will
start an Apache Celix framework and install and start the provided bundles.

Bundles

An Apache Celix Bundle is a zip file which contains a collection of shared libraries, configuration files and optional an activation entry. Bundles can be dynamically installed and started in an Apache Celix framework.

Services

An Apache Celix Service is a pointer registered to the Apache Celix framework under a set of properties (metadata). Services can be dynamically registered into and looked up from the Apache Celix framework.

By convention a C service in Apache Celix is a pointer to struct of function pointers and a C++ service is a pointer (which can be provided as a std::shared_ptr) to an object implementing a (pure) abstract class.

Components

Apache Celix also offers a way to create components which interact with dynamic services in declarative way.

This removes some complexity of dynamic services by declaring service dependency and configuring the dependency as required or optional.

Apache Celix components can be created using the built-in C and C++ dependency manager.

Note that the dependency manager is not part of the OSGi standard, and it is inspired by the Apache Felix Dependency Manager.

Containers

Apache Celix Containers are executables which starts an Apache Celix framework, with a set of preconfigured properties and a set of preconfigured bundles. Although it is also possible to create and start a Celix framework in code, the benefit of a Celix container is that this can be done with a single add_celix_container Apache Celix CMake command.

C++ Support

One of the reasons why C was chosen as implementation language is that C can act as a common denominator for (service oriented) interoperability between a range of languages.

C++ (C++17) support is build on top of the C API and is realized using a header only implementation. This means that all the binary artifact for the Apache Celix framework and util library are pure C and do not depend on libstdc++.

Apache Celix also offers some C++ only libraries and bundles. The C++ libraries are also header only, but the C++ bundles contains binaries depending on the stdlibc++ library.

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