Windows® Internals, Sixth Edition, Part 1

Offline Files consists of the following components, as shown in Figure 7-23:

A user-mode agent (%SystemRoot%\System32\cscsvc.dll) running as a service in an SVCHOST process. This service is primarily concerned with maintaining synchronization between the cache and remote file systems. It also implements the COM interfaces used to interact with the Offline Files cache.

A remote file system driver (%SystemRoot%\System32\Drivers\csc.sys) that acts as both a MUP surrogate provider and a mini-redirector. This driver is responsible for controlling when I/O requests are sent to the cache or to the remote file system. The driver also implements the local cache, updating the cached data as appropriate based on the I/O requests seen.

An Explorer extension DLL (%SystemRoot%\System32\cscui.dll) for selecting which files, folders, or drives to pin in the Offline Files cache, and for displaying icon overlays to identify offline (cached) files. CSCUI links against %SystemRoot%\System32\cscobj.dll, which provides the interface to the Offline Files service.

A DLL (%SystemRoot%\System32\cscapi.dll) containing publicly available Win32 APIs for interacting with the Offline Files from applications.

An in-process COM object (%SystemRoot%\System32\cscobj.dll) used by application clients of Offline Files COM APIs.

Figure 7-23. Offline Files architecture

Caching Modes

Offline Files has five caching modes. The mode for an object is dependent upon the object’s connection status, which is determined by whether or not the local system has a network connection to the file server.

Online

This is the default mode for objects cached by Offline Files. In this mode, the server is available. The file system metadata operations and write operations flow to the server, and the cache state is updated as required. Read operations are serviced from the cache. When working online, Offline Files attempt to cache data only if the SMB client has been granted at least read-caching privileges from the file server.

Offline (Slow Connection)

To isolate the user from fluctuations in network performance, Offline Files transition into Offline (Slow Connection) mode when the network performance meets the configured slow-link latency or bandwidth thresholds. In Windows 7, a default slow-link latency threshold is configured at 80 milliseconds (ms). The latency and bandwidth thresholds can be controlled via the Group Policy editor (%SystemRoot%\gpedit.msc) via the Configure Slow-Link Mode policy.

When working in this mode, all file-system operations are serviced by the Offline Files cache. The data is synchronized back to the server every six hours by default, but this synchronization frequency can be controlled through Group Policy via the Configure Background Sync policy.

The Offline Files Service periodically checks the network performance of the shares in the Offline Files cache. If the network latency improves to be less than half the configured slow-link latency threshold, the user will transition back to working online.

The slow-link behavior can be controlled via the Group Policy editor (%SystemRoot%\gpedit.msc) as shown in Figure 7-24.

Figure 7-24. Offline Files Group Policy settings

Offline (Working Offline)

The user can force the client to work offline by clicking the Work Offline button in Explorer. When running in this mode, all file-system operations are satisfied from the cache. Periodic background synchronization of the data can be enabled in this mode through the Configure Background Sync policy, but by default they are not enabled. If the user wants to work online again, he must click the Work Online button in Explorer.

Offline (Not Connected)

A cached object is in Offline (Not Connected) mode when the server is not accessible. The transition to offline is transparently satisfied through the Offline Files cache, without the application knowing. When the network connection to the server is re-established, any changes written to the file are synchronized back to the server by the Offline Files agent. If a file is modified on both the client and the remote system while the file was offline, the conflict must be resolved by the user through Sync Center.

Offline (Need to Sync)

When a user transitions back online after making changes to the version of the file in the local cache, the status of this file will be Offline (Need to Sync) until the changes are synchronized back to the server. Offline Files keep the user working offline for the affected files until that synchronization is complete to ensure that the user sees a consistent view of the files, include the changes made while working offline.

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