Passive Component Registration

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UNO components in the various language bindings (dynamic libraries for binary/C++ UNO, jars for Java UNO, etc.) have mechanisms to actively report information about the services and singletons they offer (exported symbol component_writeInfo for dynamic libraries, public static __writeRegistryServiceInfo method for jars, etc.). This active component registration has various drawbacks:

  • Calls to actively register the components (regcomp) need to be made in carefully crafted environments, ensuring that the relevant UNO components can actually be loaded and their code executed. Especially for Java and Python components, this is nontrivial.
    • At OOo build time, when a services.rdb is assembled in instsetoo_native.
    • At OOo installation time, when bundled extensions that contain UNO components are registered.
    • At OOo run time, when extensions that contain UNO components are registered or revoked.
  • Loading the UNO components to actively execute their code can be time consuming (especially if the components' functionality is not otherwise needed, like when assembling a services.rdb at build time).

Therefore, it is sought to replace active component registration with a passive approach, where a UNO component is accompanied by a representation of the relevant information that can be obtained without running the component's code.

An XML Format

The canonic choice for representing such information these days is XML. The necessary information is encoded as follows:

An XML document encoding information about multiple UNO components (i.e., corresponding to a services.rdb) has a root element with local name components and namespace URI

The components root element contains zero or more elements with local name component and namespace URI, each with an attribute with unprefixed name uri, the value of which is a URI (in OOo-internal form) denoting the relevant UNO component (and which may be a URL), and an attribute with unprefixed name loader, the value of which is the name of the relevant UNO service with which to load the component (e.g., or Each component element in turn contains zero or more service elements, followed by zero or more singleton elements.

Each element with local name service or singleton and namespace URI has an attribute with unprefixed name name, the value of which is the name of the respective service or singleton.

(The information provided by active component registration contains redundancies that are dropped from this XML format. For one, UNO components are listed in an IMPLEMENTATIONS section and each service and singleton is again listed in a SERVICES resp. SINGLETONS section. For another, each singleton specifies the service implementing it.)

There was already a somewhat similar textual format for regcomplazy in use, which however was based on ini-file syntax (but with irregularities, containing lines that do not match the key=value format) instead of XML. This has been superseded by the approach described here, and regcomplazy has been removed.

As we will see later, within the OOo build system there are also XML files describing single UNO components, and which have a component element instead of a components element as their root element, but otherwise use the same structure as the above XML format.

XML at Run Time

There was a choice whether to either take the XML files and inject them into registry-format services.rdb files (i.e., to still call regcomp resp. unopkg at build resp. installation time), or to use the XML files directly at run time. I decided for the latter.

The configmgr re-write showed that it is acceptable performance-wise to read a handful of XML files during OOo start up, using an XML reader specifically written for that task. So it should be acceptable to replace the existing registry-format services.rdb files (for a start, the URE layer services.rdb and the basis layer services.rdb and legacy_binfilters.rdb) with XML files.

The advantages are easier and potentially faster creation (no need for regcomp like code any more), and easier manipulation (due to the textual format). Also, the registry format has a known design defect, in that it cannot handle arbitrary combinations of key names.

Work in Progress

Work is done on CWS sb129  , currently as a stack of MQ patches:

simpleregistry-cleanup cleans up the stoc/source/simpleregistry code so that subsequent patches can better modify it.

xmlreader extracts the XmlReader code from configmgr to a new xmlreader URE module, so that it can be reused from stoc. An open problem is how to version its C++ ABI (currently done via symbol visibility, which does not allow for versioning).

textualservices extends the implementation of in stoc/source/simpleregistry, so that it can internally also read XML files in addition to registry files, and present them to its clients as if they had the structure of registry-format services.rdb files. This appeared to be the simplest approach for now, given how deep the knowledge about registry-format files is rooted in the code base (see, for example, the functionality in cppuhelper/bootstrap.hxx that bootstraps a UNO environment based on rdb files); it is still subject to change, however.

passive replaces the active registration of all of OOo's UNO components with passive registration. The resulting XML files are still called services.rdb etc. (instead of, say, services.xml), mainly because [ure/source/README] declares the URE services.rdb as part of the published URE interface (acknowledging only its existence, not its internal format, and especially warning against registering anything into it).

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