The Unified Update Platform (UUP) was introduced to the Windows 10 Insider Preview with Build 14986 on December 11, 2016. It changes how Windows Update works in a profound and interesting way. This is explained in some detail in a Windows Insider blog post entitled "Introducing Unified Update Platform (UUP)" that posted on November 3, 2016, so I'll just hit some high points here. The original post is well worth a read, if you're into the nitty-gritty details.
What is UUP, and why is Microsoft using it for updates?
In the name for this new update, ‘Unified’ refers to the ability to service all potential Windows devices — PCs, tablets, smartphones, IoT devices and Hololens — from a single update platform. The impetus for this came from user feedback about how Windows Update would benefit from reducing consumption of local processing resources to improve battery life, and for download sizes to be smaller to complete more quickly and consume less bandwidth.
Thus, UUP is best understood as a technique designed to limit download size, since smaller downloads match those user desiderata quite nicely. The afore-linked blog post describes the files that get transferred via UUP as "a differential download package" instead of a "full build."
This means that Windows Update scans your PC's component store and packages up only those update files that need to be applied to your system before it does any downloads. Next, instead of downloading the same, single, large and monolithic download file to everybody, it downloads only those files for the target system for which updates are available.
Microsoft says its goal in using UUP is to reduce downloads by somewhere around 35 percent between a typical Windows 10 client and the corresponding electronic software download (ESD) or cabinet (CAB) file equivalents for the same update(s) downloaded the old-fashioned way. Microsoft is making progress but still has some ways to go to hit that target. Recently, I've compared Insider Preview major builds between the older ESD versions and the UUP versions and have seen differences of 11 to 12 percent (2.58 GB for UUP vs. 2.99 GB for ESD on 64-bit Build 15031 for example).
UUP has its ups and downs
UUP isn't scheduled to roll out to all Current Branch users until the Creators Update is released in April 2017, so recent exposure to and experience with UUP is limited to Insider Preview participants at present. Personally, I've been through four major update cycles with UUP for builds that required the Windows Installer to apply either a clean (new) install or upgrade install to a previous Insider Preview build.
I've had some problems with UUP, too. Of the eight installs (four versions on two PCs), I had to revert to ESD for two of them, to perform one clean install, and to perform one manual install of an ISO file instead of an automated install through Windows Update. The other four attempts worked as expected, without requiring extraordinary human intervention to complete successfully. You can read about a wide variety of issues associated with UUP in the hundreds of user responses to and reports about recent Insider Preview builds (15031, 15025 and 15002, for example) on TenForums.com. The issues range from the mildly irritating to the profoundly vexing to the outright bizarre, as is so often the case for software when in beta test.
These problems are not necessarily cause for concern, however. When a long-utilized system like Windows Update goes through major changes — and UUP is definitely a major reworking of the facility — it's inevitable that there will be problems along the way. As I've watched UUP roll out and be put through its paces over four major builds and numerous cumulative and other updates, I've observed that it's gotten faster, more robust and much more stable. By the time the Creators Update comes around, it should indeed be ready for prime time. (If not, Microsoft will probably revert to the older ESD and CAB file formats it's used for major and minor updates for years.)
Switching between UUP and ESD for major updates
Note: This section explains how current Windows 10 Insider Preview works, including how to switch from UUP to ESD for major updates, and back again, and how to capture UUP download files and transform them into an ISO file for standalone upgrade or clean install use. All of these technical details could, and very well may, change before or when the Creators Update goes live in April 2017.
At present, a minor registry hack may be used to turn UUP updates on and off, both within the WindowsUpdate key that resides in HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion. The EnableUUPScan key resides in the \Orchestrator subkey, while the SupportsUUP key resides in WindowsUpdate. Each value is a DWORD, where a setting of 1 means UUP is enabled, and 0 means UUP is disabled (the setting must be the same for both key values). If the values are set to 0, Windows Update will skip UUP and download an ESD file instead for major upgrades. For more details and instructions, see the TenForums post titled "Upgrading Windows 10 – ESD or UUP?" from ISO/ESD/UUP guru Kari.
If UUP is turned on and updates run really slowly (there have been reports of seven hours or longer to get through the download process), or if those downloads won't install, it's worth trying the aforementioned registry edits to see if they help. Hopefully, most users will neither face nor be forced to solve such problems.
A peek inside the UUP downloads
After downloading a UUP update, you can examine what gets downloaded if, instead of rebooting at the Windows Installer's prompt, you first do some poking around in the Windows file system. Any and all Windows Update downloads always go to the %Windir%\SoftwareDistribution\Downloads directory.
After Build 15031 appeared, I checked that directory as described and found that it contained 232 files, most of which were CAB files. They varied in size from a maximum of 71 MB to a minimum of just 7 KB (in files that didn't have an XML in their names before the CAB extension) or 5 KB (in files that did). There were also a couple of XML files in this mix, both of which were 3 KB in size. There was also a ‘Metadata’ directory that looked like it contained language-specific configuration files and other miscellaneous items, plus a couple of dynamic-link library (DLL) files. None of the 401 items in that directory was larger than 2 MB, and most ranged from 7 KB to 12 KB in size.
This boils down to a fair number (200 or so) CAB files of modest size, and about double that number of much smaller CAB and DLL files for metadata. Total size for the collection shows as 2.58 GB in File Explorer. By contrast, the ISO file I generated from that collection came out to 3.77 GB. The ESD equivalent is just under 3 GB in size, so the savings is 11.5 percent. But clearly, one trade-off between the switch from ESD to UUP is one that involves one great big honkin' file and a large number of smaller files, which are smaller in the aggregate than the ESD file they replace.
Those who are interested in working with the Insider Preview should also consult TenForums’ excellent tutorial (also by image/installer guru Kari) entitled "UUP to ISO — Create Bootable ISO from Windows 10 Build Upgrade Files." It takes you through the process of capturing and saving the UUP download files, then using them with a software tool called UUPtoISO.cmd to create a standard ISO-based Windows installer. Good stuff! It's what I used to install Build 15031 on the test machine when, for some odd reason, UUP would download the files correctly, but Windows Installer just couldn't get past the first restart during the Windows 10 installation process (which normally requires at least two restarts).