The Resource System

The C++ Micro Services library provides a generic resource system that allows you to:

  • Embed resources in a bundle.
  • Access resources at runtime.

The features and limitations of the resource system are described in more detail in the following sections.

Embedding Resources in a Bundle

Resources are embedded into a bundle’s shared or static library (or into an executable) by using the usResourceCompiler3 executable. It will create a ZIP archive of all input files and can append it to the bundle file with a configurable compression level. See usResourceCompiler3 for the command line reference.

Accessing Resources at Runtime

Each bundle provides individual resource lookup and access to its embedded resources via the Bundle class which provides methods returning BundleResource objects. The BundleResource class provides a high-level API for accessing resource information and traversing the resource tree.

The BundleResourceStream class provides a std::istream compatible object for the seamless usage of embedded resource data in third-party libraries.

Resources are managed in a tree hierarchy, modeling the original parent-child relationship on the file-system.

The following example shows how to retrieve a resource from each currently installed bundle whose path is specified by a bundle property:

  // Check if a bundle defines a "service-component" property
  // and use its value to retrieve an embedded resource containing
  // a component description.
  for (auto const & bundle : bundleCtx.GetBundles()) {
    if (bundle.GetState() == Bundle::STATE_UNINSTALLED)
      continue;
    auto headers = bundle.GetHeaders();
    auto iter = headers.find("service-component");
    std::string componentPath =
      (iter == headers.end()) ? std::string() : iter->second.ToString();
    if (!componentPath.empty()) {
      BundleResource componentResource = bundle.GetResource(componentPath);
      if (!componentResource.IsValid() || componentResource.IsDir())
        continue;

      // Create a std::istream compatible object and parse the
      // component description.
      BundleResourceStream resStream(componentResource);
      parseComponentDefinition(resStream);
    }
  }

This example could be enhanced to dynamically react to bundles being started and stopped, making use of the popular extender pattern from OSGi.

Runtime Overhead

The resource system has the following runtime characteristics:

  • During bundle install, the bundle’s ZIP archive header data (if available) is parsed and stored in memory.
  • Querying Bundle or BundleResource objects for resource information will not extract the embedded resource data and hence only has minimal runtime and memory overhead.
  • Creating a BundleResourceStream object will allocate memory for the uncompressed resource data and inflate it. The memory will be free’ed after the BundleResourceStream object is destroyed.

Conventions and Limitations

  • Resources have a size limitation of 2GB due to the use of the ZIP format.
  • Resource entries are stored with case-insensitive names. On case-sensitive file systems, adding resources with the same name but different capitalization will lead to an error.
  • Looking up resources by name at runtime is case sensitive.
  • The CppMicroServices library will search for a valid zip file inside a shared library, starting from the end of the file. If other zip files are embedded in the bundle as well (e.g. as an additional resource embedded via the Windows RC compiler or using other techniques), it will stop at the first valid zip file and use it as the resource container.