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Small cross-platform software update installer

README.md

This tool is a component of a cross-platform auto-update system. It is responsible for performing the installation of an update after the necessary files have been downloaded to a temporary directory.

It was originally written for use with Mendeley Desktop (see www.mendeley.com)

The tool consists of a single small binary which performs update installation, an XML file format describing the contents of an update (an 'update script') and a tool to create update scripts from a directory containing an installed application.

To perform an update, the application (or another separate tool) needs to download the updater binary, an update script and one or more compressed packages containing the files for the update to a temporary directory. It then needs to invoke the updater, specifying the location where the application is installed, the location of the compressed packages and the path to the update script.

Once the updater has been started, it:

  1. Waits for the application to exit
  2. Acquires the necessary priviledges to install the updates, prompting the user if necessary.
  3. Installs the updates, displaying progress to the user in a small dialog
  4. Performs cleanup and any additional actions required as part of the update
  5. Starts the new version of the main application.

    In the event of a failure during the update, the installation is rolled back to its previous state and a message is presented to the user.

Building the Updater

Create a new directory for the build and from that directory run:

cmake <path to source directory>
make

The updater binary will be built in the src/ directory.

You should also run the tests in src/tests to verify that the updater is functioning correctly.

Preparing an Update

  1. Create a directory containing your application's files, laid out in the same way and with the same permissions as they would be when installed.
  2. Create a config file specifying how the application's files should be partitioned into packages - see tools/config-template.json
  3. Use the tools/create-packages.rb script to create a file_list.xml file and a set of package files required for updates.
  4. Upload the file_list.xml file and packages to a server

    After step 4 is done, you need to notify existing installs that an update is available. The installed application then needs to download the relevant packages, file_list.xml file and updater binary to a temporary directory and invoke the updater.

    See doc/update-hosting for more details on hosting and delivering the updates.

Invoking the Updater

Once the application has downloaded an update, it needs to invoke it. The syntax is:

updater --install-dir <install-dir> --package-dir <package-dir> --script <script file>

Where <install-dir> is the directory which the application is installed into, <package-dir> is the directory containing the packages required for the update and <script> is the file_list.xml file describing the update.

Once the updater has run, it will launch the file specified in the file_list.xml file as being the main application binary.

See the updater test in src/tests/test-update.rb for an example of how to invoke the updater.

You should design the process used to download and launch the updater so that new versions of the updater itself can be delivered as part of the update if necessary.

Customizing the Updater

To customize the application name, organization and messages displayed by the updater:

  1. Edit the AppInfo class (in AppInfo.h, AppInfo.cpp) to set the name of the application and associated organization.
  2. Replace the icons in src/resources
  3. Change the product name and organization in src/resources/updater.rc
  4. If you are building the updater on Windows and have a suitable Authenticode certificate, use it to sign the Windows binary. This will make the application show a less scary UAC prompt if administrator permissions are required to complete the installation.

Updater Dependencies

The external dependencies of the updater binary are:

  • The C/C++ runtime libraries (Linux, Mac),
  • pthreads (Linux, Mac),
  • zlib (Linux, Mac)
  • native UI library (Win32 API on Windows, Cocoa on Mac, GTK on Linux if available)

Full and Delta Updates

The simplest auto-update implementation is for existing installs to download a complete copy of the new version and install it. This is appropriate if a full download and install will not take a long time for most users (eg. if the application is small or they have a fast internet connection).

With this tool, a full-update involves putting all files in a build of the application into a single package.

To reduce the download size, delta updates can be created which only include the necessary files or components to update from the old to the new version.

The file_list.xml file format can be used to represent either a complete install - in which every file that makes up the application is included, or a delta update - in which case only new or updated files and packages are included.

There are several ways in which this can be done:

  • Pre-computed Delta Updates For each release, create a full update plus delta updates from the previous N releases. Users of recent releases will receive a small delta update. Users of older releases will receive the full update.

  • Server-computed Delta Updates The server receives a request for an update from client version X and in response, computes an update from version X to the current version Y, possibly caching that information for future use. The client then receives the delta file_list.xml file and downloads only the listed packages.

    Applications such as Chrome and Firefox use a mixture of the above methods.

  • Client-computed Delta Updates The client downloads the file_list.xml file for the latest version and computes a delta update file locally. It then downloads only the required packages and invokes the updater, which installs only the changed or updated files from those packages.

    This is similar to Linux package management systems.

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