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91 lines
3.9 KiB
Text
91 lines
3.9 KiB
Text
This tool is a component of a cross-platform auto-update system.
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It is responsible for performing the installation of an update after
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the necessary files have been downloaded to a temporary directory.
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This tool is responsible for:
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* Reading an XML file specifying the contents of an update.
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* Extracting and installing updated files from one or more compressed packages.
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* Removing any files which are no longer needed in the new version of the application.
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* Requesting elevated priviledges if required to install the update.
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* Displaying a simple updater UI and re-launching the main application
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once the update is installed.
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The tool consists of a single small binary which depends only on libraries
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that are part of the base system.
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The external dependencies of the updater binary are:
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* The C/C++ runtime libraries (Linux, Mac),
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* pthreads (Linux, Mac),
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* zlib (Linux, Mac)
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* native UI library (Win32 API on Windows, Cocoa on Mac, GTK on Linux if available)
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To perform an update, the application (or another separate tool) needs to download
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the updater binary, an XML file describing the update and one or more zip packages
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containing the files for the update to a temporary directory. It then needs
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to invoke the updater, specifying the installation directory, temporary package
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directory and path to the update script file. The updater then installs the
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update and restarts the application when done.
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Preparing an Update
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===================
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1. Create a directory containing your application's files,
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laid out and with the same permissions as they would be when installed.
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2. Create a config file specifying how the application's files should be
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partitioned into packages - see tools/config-template.json
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3. Use the tools/create-packages.rb script to create a file_list.xml file
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and a set of package files required for updates.
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4. Upload the file_list.xml file and packages to a server
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After step 4 is done, you need to notify existing installs that an update
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is available. The installed application then needs to download the
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relevant packages, file_list.xml file and updater binary to a temporary
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directory and invoke the updater.
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Delta Updates
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=============
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The simplest possible auto-update implementation is for existing installs
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to download a complete copy of the new version and install it. This is
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appropriate if a full download and install will not take a long time for most users
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(eg. if the application is small or they have a fast internet connection).
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To reduce the download size, delta updates can be created which only include
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the necessary files or components to update from the old to the new version.
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The file_list.xml file format can be used to represent either a complete
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install - in which every file that makes up the application is included,
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or a delta update - in which case only new or updated files and packages
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are included.
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There are several ways in which this can be done:
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Pre-computed Delta Updates
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- For each release, create a full update plus delta updates from the
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previous N releases. Users of recent releases will receive a small
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delta update. Users of older releases will receive the full update.
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Server-computed Delta Updates
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- The server receives a request for an update from client version X and in response,
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computes an update from version X to the current version Y, possibly
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caching that information for future use. The client then receives the
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delta file_list.xml file and downloads only the listed packages.
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Applications such as Chrome and Firefox use a mixture of the above methods.
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Client-computed Delta Updates
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- The client downloads the file_list.xml file for the latest version and
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computes a delta update file locally. It then downloads only the required
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packages and invokes the updater, which installs only the changed or updated
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files from those packages.
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This is similar to Linux package management systems.
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