Section 1: Update Architecture Overview

The Big Picture: A Delivery Service for Your Computer

Think of a magazine subscription. A publisher prints new issues, a delivery service brings them to your door, and you (or a helper) put them on the shelf. Windows Update works the same way. Microsoft publishes fixes, a client on your PC checks for new "issues", download helpers carry them in, and an installer puts them in the right place.

Why do updates exist at all? Windows has tens of millions of lines of code. Nobody writes that much code without mistakes. Updates fix them over time.

1.1 Windows Update Service Role

What it is: The Windows Update client is the part of Windows that asks Microsoft what is new, then fetches and installs it. It is not one program. It is a team of services (Chapter 12) that work together.

Exact service names and which one does what can differ between Windows versions. Treat the table as a map, not a law. You can see the real ones on your PC with Get-Service wuauserv,UsoSvc,BITS,TrustedInstaller,DoSvc.

Analogy: UsoSvc is the manager with the calendar. wuauserv is the clerk who checks the catalog. BITS and DoSvc are the delivery vans. TrustedInstaller is the builder who puts things on the shelves.

Most of these are set to Manual or Automatic (Trigger Start). They wake up when needed and go back to sleep. That is why you often see them Stopped when you look. That is normal (Chapter 12).

 YOUR PC                                  MICROSOFT
+------------------------------+        +-------------------------------+
| UsoSvc: "time to check"      |        | Windows Update / Microsoft    |
|   |                          |        | Update servers                |
|   v                          |  ask   | (update catalog + metadata)   |
| wuauserv: "what is new" -----+------->|                               |
|   ^                          |  list  |                               |
|   +--------------------------+<-------+                               |
| BITS / DoSvc: download <-----+--------+ content servers (the files)   |
|   |                          |        |                               |
|   v                          |        +-------------------------------+
| TrustedInstaller: install    |
+------------------------------+

Figure 35.1 — The PC asks Microsoft which updates apply, receives a list, downloads the files with BITS or Delivery Optimization, and hands them to the installer.

Two names, one idea. You may see Windows Update and Microsoft Update. Windows Update covers Windows itself. Microsoft Update also covers other Microsoft products such as Office. In modern Settings you can switch on "Receive updates for other Microsoft products".

1.2 Update Sources (Microsoft servers, WSUS, Windows Update for Business)

What it is: The place your PC gets its updates from. At home it is almost always Microsoft directly. Companies often put a middle layer in between so they can test and control updates.

Why do companies bother? One bad update on 5,000 PCs is a disaster. IT teams test on a few PCs first, then roll out in waves. A company PC may therefore show "Some settings are managed by your organization" in Windows Update. That message means a policy is in charge (policies are stored in the registry, see Chapter 17 — Windows Registry).

How does Microsoft decide what your PC needs? Each update is published with metadata: a small description that says which Windows versions it applies to, what it replaces, and which files it changes. During a scan, your PC sends facts about itself (Windows version and build, installed updates, hardware and driver IDs) and the service answers: "These apply to you." You will see this step in Section 3.

1.3 Downloading: BITS and Delivery Optimization

What BITS does. BITS is a polite downloader. It uses spare network capacity, slows down when you stream a video, and resumes after a disconnect or restart. That is why a big update can pause and continue without starting over.

What Delivery Optimization does. Big updates are large (hundreds of megabytes or more). Instead of every PC pulling every byte from Microsoft, Delivery Optimization (DO) lets PCs share pieces of an already-downloaded update with each other, like neighbors who split a bulk order.

You find it in Settings → Windows Update → Advanced options → Delivery Optimization. The switch "Allow downloads from other PCs" can be turned off if you do not want sharing, for example on a slow upload line. The pieces are checked (Section 3.2), so another PC cannot hand you a tampered update.

Metered connections. If you mark a network as metered (Settings → Network & internet → Wi-Fi → your network), Windows downloads fewer updates automatically. This protects a small mobile-data plan. Networking basics are in Chapter 28 — Windows Networking Internals.

1.4 The Whole Picture in One Table

Services were covered in Chapter 12 — Windows Services. Section 2 shows what an update actually looks like as a package.

Summary