How does Windows Update work?

The Windows Update workflow has four core areas of functionality:

Download

Install

Commit

How updating works

During the updating process, the Windows Update Orchestrator operates in the background to scan, download, and install updates. It does these actions automatically, according to your settings, and silently so that doesn’t disrupt your computer usage.

Scanning updates

The Windows Update Orchestrator on your PC checks the Microsoft Update server or your WSUS endpoint for new updates at random intervals. The randomization ensures that the Windows Update server isn’t overloaded with requests all at the same time. The Update Orchestrator searches only for updates that have been added since the last time updates were searched, allowing it to find updates quickly and efficiently.

When checking for updates, the Windows Update Orchestrator evaluates whether the update is appropriate for your device. It uses guidelines defined by the publisher of the update, for example, Microsoft Office including enterprise group policies.

Make sure you’re familiar with the following terminology related to Windows Update scan:

Term	Definition
Update	We use this term to mean several different things, but in this context it’s the actual updated code or change.
Bundle update	An update that contains 1-N child updates; doesn’t contain payload itself.
Child update	Leaf update that’s bundled by another update; contains payload.
Detector update	A special «update» that contains «IsInstalled» applicability rule only and no payload. Used for prereq evaluation.
Category update	A special «detectoid» that has an IsInstalled rule that is always true. Used for grouping updates and to allow the device to filter updates.
Full scan	Scan with empty datastore.
Delta scan	Scan with updates from previous scan already cached in datastore.
Online scan	Scan that uses the network and to check an update server.
Offline scan	Scan that doesn’t use the network and instead checks the local datastore. Only useful if online scan has been performed before.
CatScan	Category scan where caller can specify a categoryId to get updates published under that categoryId.
AppCatScan	Category scan where caller can specify an AppCategoryId to get apps published under that appCategoryId.
Software sync	Part of the scan that only checks for software updates (both the apps and the operating system).
Driver sync	Part of the scan that checks driver updates only. This sync is optional and runs after the software sync.
ProductSync	A sync based on attributes, in which the client provides a list of device, product, and caller attributes ahead of time to allow service to check applicability in the cloud.

How Windows Update scanning works

Windows Update does the following actions when it runs a scan.

Starts the scan for updates

When users start scanning in Windows Update through the Settings panel, the following occurs:

• The scan first generates a “ComApi” message. The caller (Microsoft Defender Antivirus) tells the Windows Update engine to scan for updates.
• «Agent» messages: queueing the scan, then actually starting the work:

  Updates are identified by the different IDs («ID = 10», «ID = 11») and from the different thread ID numbers.

  Windows Update uses the thread ID filtering to concentrate on one particular task.

  

  Identifies service IDs

  Service IDs indicate which update source is being scanned.

  The Windows Update engine treats every service as a separate entity, even though multiple services may contain the same updates.

  Common service IDs

  ServiceId here identifies a client abstraction, not any specific service in the cloud. No assumption should be made of which server a serviceId is pointing to. It’s totally controlled by responses from the Service Locator Service.

Service	ServiceId
Unspecified / Default	WU, MU, or WSUS 00000000-0000-0000-0000-000000000000
Windows Update	9482F4B4-E343-43B6-B170-9A65BC822C77
Microsoft Update	7971f918-a847-4430-9279-4a52d1efe18d
Store	855E8A7C-ECB4-4CA3-B045-1DFA50104289
OS Flighting	8B24B027-1DEE-BABB-9A95-3517DFB9C552
WSUS or Configuration Manager	Via ServerSelection::ssManagedServer 3DA21691-E39D-4da6-8A4B-B43877BCB1B7
Offline scan service	Via IUpdateServiceManager::AddScanPackageService

Finds network faults

Common update failure is caused due to network issues. To find the root of the issue:

Look for «ProtocolTalker» messages to see client-server sync network traffic.

«SOAP faults» can be either client- or server-side issues; read the message.

The Windows Update client uses the Service Locator Service to discover the configurations and endpoints of Microsoft network update sources: Windows update, Microsoft Update, or Flighting.

If the search is against WSUS or Configuration Manager, you can ignore warning messages for the Service Locator Service.

On sites that only use WSUS or Configuration Manager, the Service Locator Service might be blocked at the firewall. In this case the request will fail, and though the service can’t scan against Windows Update or Microsoft Update, it can still scan against WSUS or Configuration Manager, since it’s locally configured.

Downloading updates

Once the Windows Update Orchestrator determines which updates apply to your computer, it will begin downloading the updates, if you have selected the option to automatically download updates. It does operation in the background without interrupting your normal use of the device.

To ensure that your other downloads aren’t affected or slowed down because updates are downloading, Windows Update uses Delivery Optimization, which downloads updates and reduces bandwidth consumption.

Installing updates

When an update is applicable, the «Arbiter» and metadata are downloaded. Depending on your Windows Update settings, when downloading is complete, the Arbiter will gather details from the device, and compare that with the downloaded metadata to create an «action list».

The action list describes all the files needed from Windows Update, and what the installation agent (such as CBS or Setup) should do with them. The action list is provided to the installation agent along with the payload to begin the installation.

Committing Updates

When the option to automatically install updates is configured, the Windows Update Orchestrator, in most cases, automatically restarts the device for you after installing the updates. It has to restart the device because it might be insecure, or not fully updated, until it restarts. You can use Group Policy settings, mobile device management (MDM), or the registry (not recommended) to configure when devices will restart after a Windows 10 update is installed.

Troubleshoot problems updating Windows 10

In Windows 10, you decide when and how to get the latest updates to keep your device running smoothly and securely. This article will help you answer questions and troubleshoot common problems with your Windows 10 update.

If you’re looking for info on how to activate Windows 10 or to verify that your installation of Windows 10 is activated, see Activate Windows 10.

Have issues installing Windows updates? Start here.

Your device may not be able to update to the latest version of Windows 10 for a few reasons. The following tips can help you pinpoint the issue affecting your device.

Before you begin, make sure your device is plugged into a power source and connected to the internet. Then, try the steps listed below to get your PC to update.

Some updates require administrator access. If your account doesn’t have administrator access, see Create a local user or administrator account in Windows 10. Or if someone else at home or in your office has an administrator account on your device, try asking them to install the updates.

Before trying any of the solutions below, make sure you back up your personal files. You can use File History to back up your files to another drive, or insert a USB drive and use File Explorer to drag and copy important files to the USB drive. If you’re signing into Windows with a Microsoft account, your system settings will be automatically restored after updating, once you’re connected to the internet.

You can also back up your files with OneDrive. For more info, go to Back up your Documents, Pictures, and Desktop folders with OneDrive.

Make sure that your device has enough space. Your device requires at least 16 GB of free space to upgrade a 32-bit OS, or 20 GB for a 64-bit OS. If your device has a small hard drive, you may need to insert a USB drive to update it.

If your PC is running low on storage, try the techniques at Free up drive space in Windows 10.

Many updates require you to restart your device. Save your work and close all open applications. Then, select Start > Power , and select either Update and restart or Update and shut down.

Even if you have downloaded some updates, there may be more available. After trying the preceding steps, run Windows Update again by selecting Start > Settings > Update & Security > Windows Update > Check for updates. Download and install any new updates.

Remove any external storage devices and drives, docks, and other hardware plugged into your device that aren’t needed for basic functionality. Then try to run updates again and see if this resolved your issue. If it didn’t, continue to the next tip.

If you’ve added hardware to your device, check for third-party driver updates and installation instructions on the hardware manufacturer’s website. Update any drivers, then try to run updates again and see if this resolved your issue. If it didn’t, continue to the next tip.

In the search box on the taskbar, type device manager, then select Device Manager from the list of results.

In the window that appears, look for any device with a yellow exclamation mark next to it. (You may have to select each category to view the list of devices.) Select the device name.

Select Action, and then choose either Update driver or Uninstall to correct the errors.

Common troubleshooting questions

If you are receiving an error message with a specific error code, try running the Windows Update Troubleshooter. If that doesn’t resolve the issue, see Fix Windows Update errors and follow the instructions to help resolve common update issues.

The time required to download and install updates depends on connection speed, network settings, and the size of the update. If the installation remains stuck at the same percentage, try checking for updates again or running the Windows Update Troubleshooter.

To check for updates, select Start > Settings > Update & Security > Windows Update > Check for updates.

To get help, in the search box on the task bar, type get help, then select Get Help from the list of results. To leave feedback if you run into further problems, in the search box on the taskbar, type feedback hub, then select Feedback Hub from the list of results.

Windows 10 is the most secure Windows ever, and we recommend you stay updated to protect your PC from security risks and to keep it running smoothly. However, you can temporarily pause updates as well as schedule a restart to install updates at a time that’s convenient for you.
Learn how to temporarily pause updates
Find out how to schedule a restart to install updates at a convenient time

You will only receive a notification when it’s time to restart your computer to apply the installed updates.

If you need to know where you left off in your work after an update, use Timeline to jump back in.

To get started with Timeline, select Task View on the taskbar, or press the Windows logo key +Tab and select the activity you want to resume.

Monthly quality updates on Windows 10 are quite small and have a minimal impact on data usage. Feature updates typically happen twice per year, and you can control when to install them. They require more data but are less frequent.

If you’re on a metered connection, some updates for Windows won’t be installed automatically. Metered connections in Windows 10

Your computer might be slow for a number of reasons, but updating to the latest version of Windows 10 can improve its performance. For help with a slow computer, see Tips to improve PC performance in Windows 10.

If you’re having trouble finding your files after an upgrade, see Find lost files after the upgrade to Windows 10 for other things to try.

Warning: Microsoft only recommends trying the procedures in this section if you’re comfortable working in the command line. These procedures require administrator permissions on your device.

In the search box on the taskbar, type command prompt, then select Run as administrator from the list of options. Select Yes. Next, in the window that appears, type the following commands one at a time, including the spaces as shown. After you type each command, press Enter. Then let each command complete before you type the next command.

net stop wuauserv

ren %systemroot%\softwaredistribution softwaredistribution.bak

ren %systemroot%\system32\catroot2 catroot2.bak

net start wuauserv

After all these commands are completed, close the Command Prompt window and restart your computer.

In some instances, third-party antivirus or security software can cause errors when you try to update to the latest version of Windows 10. You can temporarily uninstall this software, update your PC, and then reinstall the software after your device is up to date. We recommend using third-party antivirus or security software that’s compatible with the latest version of Windows 10. You can check compatibility by visiting the software manufacturer’s website.

Note: Before uninstalling software, make sure you know how to reinstall your programs and that you have any necessary product keys.

In the search box on the taskbar, type command prompt, and then select Run as administrator from the list of options. Select Yes. In the window that appears, type the following command, including the space as shown:
chkdsk/f C:
and then press Enter. Repairs will automatically start on your hard drive, and you’ll be asked to restart your device.

A clean restart starts Windows with a minimal set of drivers and startup programs. This helps eliminate software conflicts that occur when you install a program or an update that may cause issues updating your PC.
Learn how to do a clean restart

In the search box on the taskbar, type command prompt, then select Run as administrator from the list of options.

In the window that appears, type this command including spaces as shown:
DISM.exe /Online /Cleanup-image /Restorehealth

Press Enter. When the command operation has run successfully, you’ll see a confirmation message from Command Prompt that says, “The restore operation completed successfully” or “The operation completed successfully”.
Note: If you don’t see a confirmation message, retype the command and try again.

Next, type this command, including the space as shown:
sfc /scannow

Press Enter. Wait until the sfc scan verification reaches 100% completion, and then close Command Prompt.

Try running the Windows Update again.

Go to the software download page and select Download tool now to download the clean installation tool. Make sure to carefully read the notes on the software download page before using the tool.

Still stuck?

Contact Microsoft support if you continue to have problems with your Windows updates.