Cloud File API FAQ-2

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Cloud File API FAQ-2

alan henry 5

Subject: Issues Encountered During Development Based on the Windows Cloud Files API Framework

1. Copying large directories containing multi-gigabyte placeholders always fails. Based on our logs, each copy operation only triggers a single implicit hydration. Is this behavior expected? Does File Explorer have a default timeout mechanism for copying placeholder files? What is the recommended solution to resolve this failure?

2. Folder sync status UI remains "Sync Pending" after renaming a deep directory. In scenarios with deep directory hierarchies, after renaming the root directory and calling CfSetInSync (which returns success), The File Explorer UI still displays the "Sync Pending" icon. However, calling the CfGetPlaceholderInfo interface confirms that the file status is actually "In-Sync." How can we resolve this discrepancy? Is there a specific API available to proactively refresh the UI status?

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Jack Dang (WICLOUD CORPORATION) 18,310 Microsoft External Staff Moderator

Hi @alan henry ,

Thanks for reaching out.

1. Copying large directories with multi-GB placeholders

When File Explorer copies placeholder files, it may request hydration as part of normal file access. That said, the Cloud Files design generally expects the sync provider to own the hydration flow, rather than relying on File Explorer to manage large or long-running downloads. The guidance for building a cloud file sync engine highlights that providers should decide when and how content is brought local, particularly for larger files or more complex operations:

https://learn.microsoft.com/en-us/windows/win32/cfapi/build-a-cloud-file-sync-engine

Because of this, copy operations that rely only on implicit hydration can be less reliable when placeholders represent very large files.

2. Folder sync status UI remains "Sync Pending" after renaming a deep directory

Calling CfSetInSyncState does update the in-sync state of a placeholder from the platform’s point of view, which matches what you’re seeing when you query the state programmatically. The intended use of this API is described here:

https://learn.microsoft.com/en-us/windows/win32/api/cfapi/nf-cfapi-cfsetinsyncstate

However, setting the in-sync state doesn’t necessarily mean File Explorer will immediately refresh its visual indicators. Explorer updates its UI based on filesystem notifications and its own internal reconciliation, and there isn’t a documented API that explicitly forces a refresh of the sync icons. After larger structural changes - like renaming deep directory hierarchies - it’s possible for the UI to lag a bit and continue showing “Sync Pending,” even though the underlying placeholder state is already in sync.

In those cases, making sure all affected placeholders and their metadata are updated consistently, and giving Explorer time to process the changes, usually helps bring the UI back in line with the actual sync state.

Hope this helps! If my answer was helpful - kindly follow the instructions here so others with the same problem can benefit as well.

alan henry 5 Reputation points

2026-01-19T09:56:49.7566667+00:00
Thanks for the detailed reply! I still have some additional questions regarding this issue:__

Issue with Implicit Hydration not being triggered during large directory copies__ As a junior developer, I am looking for a feasible solution or recommendation regarding an issue where files fail to trigger "implicit hydration" when copying large directories. What is the best practice to ensure that placeholders are correctly hydrated during bulk copy operations?

2. Persistent inconsistency in file status icons/states Regarding the previously mentioned issue of inconsistent file status: even after waiting for a significant amount of time and attempting a system reboot, the file status still does not transition to the expected state.

Are there specific APIs available to proactively refresh or forcibly set/update the hydration or sync status?

Could this be caused by a functional defect in our implementation? What would be the best debugging/investigation strategy to identify the root cause?

3. Parent folder status inconsistency during batch uploads During testing of batch uploads for deep directory structures, we observed that while a file in a leaf directory is uploading (status: Sync Pending), its parent directories several levels up sometimes still display as In-Sync.

Is this behavior expected in the Windows Cloud Files (Placeholder) framework?

Does this stem from the same root cause as Question 2?

4. Missing file deletions during "Select All" (Ctrl+A) operations When selecting a large number of files using Ctrl+A and performing a bulk delete, we occasionally encounter cases where some files are "leaked" (not deleted). What could be causing these missing deletions in a cloud-backed file system environment?
Jack Dang (WICLOUD CORPORATION) 18,310 Reputation points Microsoft External Staff Moderator

2026-01-20T02:56:12.9133333+00:00

Hi @alan henry,

Thanks for the follow-up.

1. Issue with implicit hydration not being triggered during large directory copies

“What is the best practice to ensure that placeholders are correctly hydrated during bulk copy operations?”

During large directory copy operations, implicit hydration may occur when File Explorer accesses file data, but it isn’t guaranteed to trigger consistently for every placeholder. The Cloud Files design generally places responsibility for hydration decisions on the sync provider, especially for larger files or more complex workflows. This model is described in the guidance for building a cloud file sync engine:

https://learn.microsoft.com/en-us/windows/win32/cfapi/build-a-cloud-file-sync-engine

Because of this, relying entirely on copy-driven access to trigger hydration can be unreliable at scale. Many providers treat implicit hydration as opportunistic and ensure their implementation can explicitly make data available when required, rather than depending on the copy operation itself to drive downloads.

2. Persistent inconsistency in file status icons / states

“Are there specific APIs available to proactively refresh or forcibly set/update the hydration or sync status?”

Calling CfSetInSyncState updates the in-sync state of a placeholder from the platform’s perspective, which aligns with its documented purpose:

https://learn.microsoft.com/en-us/windows/win32/api/cfapi/nf-cfapi-cfsetinsyncstate

However, there is no documented Cloud Files API that explicitly forces File Explorer to refresh hydration or sync icons. The Explorer UI updates based on filesystem change notifications and its own internal reconciliation, so visual indicators may lag even when the underlying state is already correct.

“Could this be caused by a functional defect in our implementation? What would be the best debugging or investigation strategy to identify the root cause?”

If inconsistencies persist over time (including after a reboot), it’s reasonable to review provider behavior. Common investigation steps include validating placeholder state with CfGetPlaceholderInfo, logging state transitions, and checking the ordering of operations around renames, moves, or bulk updates. Issues in this area are often related to sequencing or incomplete updates rather than a single API call failing.

3. Parent folder status inconsistency during batch uploads

“Is this behavior expected in the Windows Cloud Files (Placeholder) framework?”

This behavior can occur. Folder sync status is not always a real-time aggregation of all descendant file states, particularly during active uploads in deep directory structures. Parent folders may continue to display their last-known state until the platform or provider updates them again.

“Does this stem from the same root cause as Question 2?”

Not necessarily. While both scenarios affect how sync status is surfaced in File Explorer, folder state updates and file-level icon refresh timing can be driven by different triggers and may not share the same underlying cause.

4. Missing file deletions during “Select All” (Ctrl+A) operations

“What could be causing these missing deletions in a cloud-backed file system environment?”

Bulk delete operations generate a high volume of filesystem events in a short time. In cloud-backed scenarios, missing deletions can occur if delete requests overlap with other state transitions, are processed concurrently, or encounter transient errors. Logging delete callbacks, validating error paths, and testing with smaller batches can help narrow down whether timing or concurrency plays a role.

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alan henry 5 Reputation points

2026-01-19T09:56:49.7566667+00:00

Thanks for the detailed reply! I still have some additional questions regarding this issue:__

Issue with Implicit Hydration not being triggered during large directory copies__ As a junior developer, I am looking for a feasible solution or recommendation regarding an issue where files fail to trigger "implicit hydration" when copying large directories. What is the best practice to ensure that placeholders are correctly hydrated during bulk copy operations?

2. Persistent inconsistency in file status icons/states Regarding the previously mentioned issue of inconsistent file status: even after waiting for a significant amount of time and attempting a system reboot, the file status still does not transition to the expected state.

Are there specific APIs available to proactively refresh or forcibly set/update the hydration or sync status?

Could this be caused by a functional defect in our implementation? What would be the best debugging/investigation strategy to identify the root cause?

3. Parent folder status inconsistency during batch uploads During testing of batch uploads for deep directory structures, we observed that while a file in a leaf directory is uploading (status: Sync Pending), its parent directories several levels up sometimes still display as In-Sync.

Is this behavior expected in the Windows Cloud Files (Placeholder) framework?

Does this stem from the same root cause as Question 2?

4. Missing file deletions during "Select All" (Ctrl+A) operations When selecting a large number of files using Ctrl+A and performing a bulk delete, we occasionally encounter cases where some files are "leaked" (not deleted). What could be causing these missing deletions in a cloud-backed file system environment?

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The behavior you are experiencing with copying large directories containing multi-gigabyte placeholders and the failure of the copy operation triggering only a single implicit hydration is indeed a known limitation. The Cloud File API does not inherently manage multiple hydration requests for large directories efficiently, which can lead to timeouts during the copy process. File Explorer does have a default timeout mechanism for operations involving placeholder files, but the specifics of this timeout are not detailed in the available documentation. To resolve this failure, consider implementing a more robust hydration strategy, such as breaking down the copy operation into smaller segments or ensuring that the hydration policy is set to a more aggressive level if possible.
The issue with the Folder sync status UI remaining "Sync Pending" after renaming a deep directory, despite the CfSetInSync call returning success, suggests a discrepancy between the internal state of the sync engine and the UI representation. While CfGetPlaceholderInfo confirms that the file status is "In-Sync," there may not be a direct API to refresh the UI status proactively. However, you can try invoking a refresh of the File Explorer view programmatically or ensuring that any relevant notifications are sent to the UI to update its state accordingly.

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Cloud File API FAQ-2

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