<page xmlns="http://projectmallard.org/1.0/" xmlns:its="http://www.w3.org/2005/11/its" type="topic" id="parallel-installability" xml:lang="de">

  <info>

    <link type="guide" xref="index#maintainer-guidelines"/>

    <credit type="author copyright">

      <name>Havoc Pennington</name>

      <email its:translate="no">hp@pobox.com</email>

      <years>2002</years>

           installability: http://ometer.com/parallel.html.

           Licence CC-BY-SA 3.0 confirmed by e-mail with him. -->

    </credit>

    <credit type="author copyright">

      <name>Philip Withnall</name>

      <email its:translate="no">philip.withnall@collabora.co.uk</email>

      <years>2015</years>

    </credit>

    <include xmlns="http://www.w3.org/2001/XInclude" href="cc-by-sa-3-0.xml"/>

    <desc>

      Writing libraries to be future proof through parallel installation

    </desc>

    <mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">

      <mal:name>Mario Blättermann</mal:name>

      <mal:email>mario.blaettermann@gmail.com</mal:email>

      <mal:years>2016</mal:years>

    </mal:credit>

    <mal:credit xmlns:mal="http://projectmallard.org/1.0/" type="translator copyright">

      <mal:name>Christian Kirbach</mal:name>

      <mal:email>christian.kirbach@gmail.com</mal:email>

      <mal:years>2016</mal:years>

    </mal:credit>

  </info>

  <title>Parallel Installability</title>

  <synopsis>

    <title>Zusammenfassung</title>

      If two packages can be parallel installed, then they have no filenames in

      common, and people developing against the package always compile against

      the version they expected. This applies to daemons, utility programs and

      configuration files as it does to header files and library binaries.

    <list>

      <item>

        Ensure all versions of a library are parallel installable.

        (<link xref="#justification"/>)

      
</item>

      <item>

        Version all files installed by a library.

        (<link xref="#solution"/>)

      
</item>

      <item>

        Keep package version numbers separate from soname or libtool version

        numbers. Be clear which part of the package version number changes with

        the API.

        (<link xref="#version-numbers"/>)

      
</item>

      <item>

        Install C header files to

        <file>$(includedir)/liblibrary-version/library/</file>.

        (<link xref="#header-files"/>)

      
</item>

      <item>

        Install library binaries to

        <file>$(libdir)/liblibrary-version.so.soname</file>.

        (<link xref="#libraries"/>)

      
</item>

      <item>

        Install pkg-config files to

        <file>$(libdir)/pkgconfig/library-version.pc</file>.

        (<link xref="#pkg-config"/>)

      
</item>

      <item>

        Make configuration files forwards and backwards compatible, or install

        them to

        <file>$(sysconfdir)/library-version/</file>.

        (<link xref="#configuration-files"/>)

      
</item>

      <item>

        Set GETTEXT_PACKAGE to

        library-version.

        (<link xref="#gettext"/>)

      
</item>

      <item>

        Include a version number in all D-Bus interface names, service names and

        object paths. For example:

        org.domain.LibraryVersion.Interface,

        org.domain.LibraryVersion and

        /org/domain/LibraryVersion/.

        (<link xref="#dbus"/>)

      
</item>

      <item>

        Install daemon binaries to

        <file>$(libexecdir)/library-daemon-version</file>.

        (<link xref="#programs"/>)

      
</item>

      <item>

        Install utility binaries to

        <file>$(bindir)/library-utility-version</file>

        and install symbolic links to <file>$(bindir)/library-utility</file>.

        (<link xref="#programs"/>)

      
</item>

    </list>

  </synopsis>

  <section id="justification">

    <title>Justification</title>

      All public libraries should be designed to be parallel installed to ease

      API breaks later in the life of the library. If a library is used by

      multiple projects, and wants to break API, either all of the projects must

      be ported to the new API in parallel, or some of them will no longer be

      installable at the same time as the others, due to depending on

      conflicting versions of this library.

      This is unmaintainable, and asking all the projects to port to a new API

      at the same time is hard to organize and demoralizing, as most API breaks

      do not bring large new features which would motivate porting.

      The solution is to ensure that all libraries are parallel installable,

      allowing the old and new versions of the API to be installed and compiled

      against at the same time, without conflicts. Building in support for this

      kind of parallel installation is much easier to do at the start of a

      project than it is to do retroactively.

      This eliminates the ‘chicken and egg’ problem of porting a collection of

      applications from one version of a library to the next, and makes breaking

      API a lot simpler for library maintainers, which can allow for more rapid

      iteration and development of new features if they desire.

      The alternative, and equally valid, solution is for the library to never

      break API — the approach taken by libc.

  </section>

  <section id="solution">

    <title>Läsung</title>

      The solution to the problem is essentially to rename the library, and in

      most cases the nicest way to do so is to include the version number in the

      path of every file it installs. This means multiple versions of the

      library can be installed at the same time.

      For example, say that library Foo traditionally installs

      these files:

    <list>

      <item>
<file>/usr/include/foo.h</file>
</item>

      <item>
<file>/usr/include/foo-utils.h</file>
</item>

      <item>
<file>/usr/lib/libfoo.so</file>
</item>

      <item>
<file>/usr/lib/pkgconfig/foo.pc</file>
</item>

      <item>
<file>/usr/share/doc/foo/foo-manual.txt</file>
</item>

      <item>
<file>/usr/bin/foo-utility</file>
</item>

    </list>

      You might modify Foo version 4 to install these files

      instead:

    <list>

      <item>
<file>/usr/include/foo-4/foo/foo.h</file>
</item>

      <item>
<file>/usr/include/foo-4/foo/utils.h</file>
</item>

      <item>
<file>/usr/lib/libfoo-4.so</file>
</item>

      <item>
<file>/usr/lib/pkgconfig/foo-4.pc</file>
</item>

      <item>
<file>/usr/share/doc/foo-4/foo-manual.txt</file>
</item>

      <item>
<file>/usr/bin/foo-utility-4</file>
</item>

    </list>

      It could then be parallel installed with version 5:

    <list>

      <item>
<file>/usr/include/foo-5/foo/foo.h</file>
</item>

      <item>
<file>/usr/include/foo-5/foo/utils.h</file>
</item>

      <item>
<file>/usr/lib/libfoo-5.so</file>
</item>

      <item>
<file>/usr/lib/pkgconfig/foo-5.pc</file>
</item>

      <item>
<file>/usr/share/doc/foo-5/foo-manual.txt</file>
</item>

      <item>
<file>/usr/bin/foo-utility-5</file>
</item>

    </list>

      This is easily supported using

      <link href="http://www.freedesktop.org/wiki/Software/pkg-config/">

      <cmd>pkg-config</cmd></link>: <file>foo-4.pc</file> would add

      <file>/usr/include/foo-4</file> to the include path and

      <file>libfoo-4.so</file> to the list of libraries to link;

      <file>foo-5.pc</file> would add <file>/usr/include/foo-5</file> and

      <file>libfoo-5.so</file>.

  </section>

  <section id="version-numbers">

    <title>Versionsnummern</title>

      The version number that goes in filenames is an ABI/API version.

      It should not be the full version number of your package — just the part

      which signifies an API break. If using the standard

      major.minor.micro scheme for

      project versioning, the API version is typically the major version number.

      Minor releases (typically where API is added but not changed or

      removed) and micro releases (typically bug fixes) do not affect

      <link xref="api-stability">API backwards compatibility</link> so do not

      require moving all the files.

      The examples in the following sections assume that the API version and

      soname are exported from <file>configure.ac</file> using the following

      code:

    <listing>

      <title>API-Versionierung in Autoconf</title>

      <desc>

        Code to export the API version and soname from <file>configure.ac</file>

      </desc>

      # Before making a release, the LIBRARY_LT_VERSION string should be modified.

# The string is of the form c:r:a. Follow these instructions sequentially:

#

#  1. If the library source code has changed at all since the last update,

#     then increment revision (‘c:r:a’ becomes ‘c:r+1:a’).

#  2. If any interfaces have been added, removed, or changed since the last update,

#     increment current, and set revision to 0.

#  3. If any interfaces have been added since the last public release,

#     then increment age.

#  4. If any interfaces have been removed or changed since the last public release,

#     then set age to 0.

AC_SUBST([LIBRARY_LT_VERSION],[1:0:0])

AC_SUBST([LIBRARY_API_VERSION],[4])

    </listing>

  </section>

  <section id="header-files">

    <title>C-Kopfdateien</title>

      Header files should always be installed in a versioned subdirectory that

      requires an <cmd>-I</cmd> flag to the C compiler. For example, if my

      header is <file>foo.h</file>, and applications do this:

    #include <foo/foo.h>

      then I should install these files:

    <list>

      <item>
<file>/usr/include/foo-4/foo/foo.h</file>
</item>

      <item>
<file>/usr/include/foo-5/foo/foo.h</file>
</item>

    </list>

      Applications should pass the flag <cmd>-I/usr/include/foo-4</cmd> or

      <cmd>-I/usr/include/foo-5</cmd> to the C compiler. Again, this is

      facilitated by using <cmd>pkg-config</cmd>.

      Note the extra <file>foo/</file> subdirectory. This namespaces the

      #include to avoid file naming collisions

      with other libraries. For example, if two different libraries install

      headers called <file>utils.h</file>, which one gets included when you

      use #include <utils.h>?

      There’s some temptation to keep one of the header files outside of any

      subdirectory:

    <list>

      <item>
<file>/usr/include/foo.h</file>
</item>

      <item>
<file>/usr/include/foo-5/foo.h</file>
</item>

    </list>

      The problem there is that users are always accidentally getting the wrong

      header, since <cmd>-I/usr/include</cmd> seems to find its way onto compile

      command lines with some regularity. If you must do this, at least add a

      check to the library that detects applications using the wrong header file

      when the library is initialized.

      Versioned header files can be installed from automake using the following

      code:

    <listing>

      <title>Header-Dateien in Automake</title>

      <desc>

        Code to install versioned header files from <file>Makefile.am</file>

      </desc>

      libraryincludedir = $(includedir)/liblibrary-@LIBRARY_API_VERSION@/library

library_headers = \

	liblibrary/example1.h \

	liblibrary/example2.h \

	$(NULL)

# The following headers are private, and shouldn't be installed:

private_headers = \

	liblibrary/example-private.h \

	$(NULL)

# The main header simply #includes all other public headers:

main_header = liblibrary/library.h

public_headers = \

	$(main_header) \

	$(library_headers) \

	$(NULL)

libraryinclude_HEADERS = $(public_headers)

    </listing>

      As well as correct versioning, all APIs in installed headers should be

      <link xref="namespacing">namespaced correctly</link>.

  </section>

  <section id="libraries">

    <title>Bibliotheken</title>

      Library object files should have a versioned name. For example:

    <list>

      <item>
<file>/usr/lib/libfoo-4.so</file>
</item>

      <item>
<file>/usr/lib/libfoo-5.so</file>
</item>

    </list>

      This allows applications to get exactly the one they want at compile time,

      and ensures that versions 4 and 5 have no files in common.

      Versioned libraries can be built and installed from automake using the

      following code:

    <listing>

      <title>Bibliotehken in Automake</title>

      <desc>

        Code to build and install versioned libraries from

        <file>Makefile.am</file>

      </desc>

      lib_LTLIBRARIES = liblibrary/liblibrary-@LIBRARY_API_VERSION@.la

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_SOURCES = \

	$(private_headers) \

	$(library_sources) \

	$(NULL)

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_CPPFLAGS = …

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_CFLAGS = …

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_LIBADD = …

liblibrary_liblibrary_@LIBRARY_API_VERSION@_la_LDFLAGS = \

	-version-info $(LIBRARY_LT_VERSION) \

	$(AM_LDFLAGS) \

	$(NULL)

    </listing>

    <section id="library-sonames">

      <title>Library sonames</title>

        Library sonames (also known as libtool version numbers) only address the

        problem of runtime linking previously-compiled applications. They don’t

        address the issue of compiling applications that require a previous

        version, and they don’t address anything other than libraries.

        For this reason, sonames should be used, but in addition to

        versioned names for libraries. The two solutions address different

        problems.

    </section>

  </section>

  <section id="pkg-config">

    <title>pkg-config-Dateien</title>

      pkg-config files should have a versioned name. For example:

    <list>

      <item>
<file>/usr/lib/pkgconfig/foo-4.pc</file>
</item>

      <item>
<file>/usr/lib/pkgconfig/foo-5.pc</file>
</item>

    </list>

      Since each pkg-config file contains versioned information about the

      library name and include paths, any project which depends on the library

      should be able to switch from one version to another simply by changing

      their pkg-config check from <file>foo-4</file> to <file>foo-5</file> (and

      doing any necessary API porting).

      Versioned pkg-config files can be installed from autoconf and automake

      using the following code:

    <listing>

      <title>pkg-config-Dateien in Autoconf und Automake</title>

      <desc>

        Code to install versioned pkg-config files from

        <file>configure.ac</file> and <file>Makefile.am</file>

      </desc>

      AC_CONFIG_FILES([

liblibrary/library-$LIBRARY_API_VERSION.pc:liblibrary/library.pc.in

],[],

[LIBRARY_API_VERSION='$LIBRARY_API_VERSION'])

      # Note that the template file is called library.pc.in, but generates a

# versioned .pc file using some magic in AC_CONFIG_FILES.

pkgconfigdir = $(libdir)/pkgconfig

pkgconfig_DATA = liblibrary/library-$(LIBRARY_API_VERSION).pc

DISTCLEANFILES += $(pkgconfig_DATA)

EXTRA_DIST += liblibrary/library.pc.in

    </listing>

  </section>

  <section id="configuration-files">

    <title>Konfigurationsdateien</title>

      From a user standpoint, the best approach to configuration files is to

      keep the format both <link xref="api-stability">forward and backward

      compatible</link> (both library

      versions understand exactly the same configuration file syntax and

      semantics). Then the same configuration file can be used for all versions

      of the library, and no versioning is needed on the configuration file

      itself.

      If you can’t do that, the configuration files should simply be renamed,

      and users will have to configure each version of the library separately.

  </section>

  <section id="gettext">

    <title>Gettext-Übersetzungen</title>

      If you use gettext for translations in combination with autoconf and

      automake, normally things are set up to install the translations to

      <file>/usr/share/locale/lang/LC_MESSAGES/package</file>.

      You’ll need to change package. The convention used in GNOME is

      to put this in <file>configure.ac</file>:

    GETTEXT_PACKAGE=foo-4

AC_SUBST([GETTEXT_PACKAGE])

AC_DEFINE_UNQUOTED([GETTEXT_PACKAGE],["$GETTEXT_PACKAGE"])

      Then use GETTEXT_PACKAGE as the package name to pass to

      bindtextdomain(),

      textdomain(), and

      dgettext().

  </section>

  <section id="dbus">

    <title>D-Bus-Schnittstellen</title>

      A D-Bus interface is another form of API, similar to a C API except that

      resolution of the version is done at runtime rather than compile time.

      Versioning D-Bus interfaces is otherwise no different to C APIs: version

      numbers must be included in interface names, service names and object

      paths.

      For example, for a service org.example.Foo exposing

      interfaces A and B on objects

      Controller and Client, versions 4 and 5 of the

      D-Bus API would look like this:

    <list>

      <title>Dienstnamen</title>

      <item>
org.example.Foo4
</item>

      <item>
org.example.Foo5
</item>

    </list>

    <list>

      <title>Interface Names</title>

      <item>
org.example.Foo4.InterfaceA
</item>

      <item>
org.example.Foo4.InterfaceB
</item>

      <item>
org.example.Foo5.InterfaceA
</item>

      <item>
org.example.Foo5.InterfaceB
</item>

    </list>

    <list>

      <title>Objektpfade</title>

      <item>
/org/example/Foo4/Controller
</item>

      <item>
/org/example/Foo4/Client
</item>

      <item>
/org/example/Foo5/Controller
</item>

      <item>
/org/example/Foo5/Client
</item>

    </list>

  </section>

  <section id="programs">

    <title>Programs, Daemons and Utilities</title>

      Desktop applications generally do not need to be versioned, as they are

      not depended on by any other modules. Daemons and utility programs,

      however, interact with other parts of the system and hence need

      versioning.

      Given a daemon and utility program:

    <list>

      <item>
<file>/usr/libexec/foo-daemon</file>
</item>

      <item>
<file>/usr/bin/foo-lookup-utility</file>
</item>

    </list>

      these should be versioned as:

    <list>

      <item>
<file>/usr/libexec/foo-daemon-4</file>
</item>

      <item>
<file>/usr/bin/foo-lookup-utility-4</file>
</item>

    </list>

      You may want to install a symbolic link from

      <file>/usr/bin/foo-lookup-utility</file> to the recommended versioned

      copy of the utility, to make it more convenient for users to use.

  </section>

</page>