Azure DevOps: Version Control Download Bug Returns Incorrect Files - Investigation

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This article addresses a specific issue encountered in Microsoft Team Foundation Server, now known as Azure DevOps Server, after restoring a Team Project Collection (TPC) from a backup. Specifically, it focuses on the problem where users might download incorrect file contents through version control operations following a TPC restoration. This situation can lead to significant confusion and potential data integrity issues within development teams relying on version control. Understanding the root cause and resolution is crucial for administrators managing Azure DevOps environments to ensure data consistency and prevent workflow disruptions.

Symptoms¶

The primary symptom of this issue manifests after a Team Project Collection (TPC) is restored from a backup. Users will observe that when they download files from Team Foundation Server (now Azure DevOps Server) using version control operations, the downloaded files may contain incorrect or outdated content. This discrepancy arises even though the version control system appears to be functioning correctly in terms of accessing and retrieving files. The files retrieved are not the latest versions as expected, but rather older or corrupted versions, leading to potential build failures, deployment errors, and general development inconsistencies. This issue can be intermittent and difficult to diagnose without understanding the underlying caching mechanism.

Cause¶

The root cause of this problem lies in the file caching mechanism employed by the Team Foundation Server application tier (AT) to optimize download performance. To enhance the speed of file retrieval in version control, the AT utilizes a cache to store files fetched from the data tier (DT). When files are initially checked into version control, they are assigned a unique FileID. This FileID is then used as an identifier when the file is cached on the AT. The problem emerges specifically in scenarios involving TPC restoration.

Consider this scenario: A backup of the TPC is taken at a certain point in time. After the backup, new files are added to version control. These newly added files are also cached on the AT and assigned FileIDs. If a TPC restore is performed using the older backup, the database reverts to the state at the time of the backup. However, if the version control cache on the AT is not purged before bringing the restored TPC back online, a critical conflict occurs. The AT cache might still contain entries for files added after the backup, using FileIDs assigned in the period between the backup and the restore. When users subsequently request files, the AT might incorrectly match FileIDs from the restored database with cached files from the pre-restore state. This results in “false positive cache-hits,” where the system mistakenly serves older, incorrect file contents from the cache instead of retrieving the correct versions from the restored database.

To illustrate this further, imagine a file “DocumentA.txt” is versioned and assigned FileID ‘123’ before the backup. After the backup, “DocumentA.txt” is modified and a new file “DocumentB.txt” is added, which gets FileID ‘123’ (because FileIDs can be reused or reassigned in certain scenarios especially after operations like restore). Then, the TPC is restored from the backup. The database now only knows about the older “DocumentA.txt” with FileID ‘123’. However, the AT cache might still hold the new “DocumentB.txt” associated with FileID ‘123’ (or even the newer version of “DocumentA.txt” if its FileID was somehow reassigned internally). When a user requests “DocumentA.txt” (FileID ‘123’), the AT cache might incorrectly serve “DocumentB.txt” or the newer version of “DocumentA.txt” from the cache, leading to the download of incorrect content.

This issue is primarily related to the timing and sequence of operations: backup, new file additions, and TPC restore, combined with the persistence of the AT version control cache across these operations. It highlights the importance of cache management in maintaining data consistency, especially after database restoration procedures.

Resolution¶

To effectively resolve this issue and prevent incorrect file downloads after a Team Foundation Server (Azure DevOps Server) database restoration (TPC restore or TPC Move), a crucial step is to purge the version control file cache. This purging action must be performed on all Team Foundation Server Application Tiers (ATs) and all Team Foundation Server proxy servers that are servicing the affected Team Foundation Server instance. This cache purging is a mandatory procedure that must be completed before bringing the restored TPC back online and allowing users to access it. Failing to purge the cache will very likely result in the symptoms described earlier, impacting users and potentially causing data integrity problems.

The cache purging process needs to be executed on each Application Tier server and each proxy server within your Azure DevOps Server environment. The steps to purge the cache are generally consistent across these server types.

Here’s a detailed procedure for purging the version control file cache on each Application Tier and proxy server:

1. Log on to the Server: Begin by logging onto the specific Application Tier machine or proxy server machine where you need to purge the cache. You will need administrative privileges on these servers to perform the necessary operations.

2. Navigate to the Cache Folder: Once logged in, you need to locate the version control cache folder. The default location for this cache folder is typically within the Team Foundation Server installation directory. The standard default path is:

   C:\Program Files\Microsoft Team Foundation Server \[Year]\Application Tier\Web Services\_tfs_data\%tfsInstanceGuid%\Proxy

   • [Year]: Replace [Year] with the specific year version of Team Foundation Server you are using (e.g., 2013, 2015, 2017, 2018, or Azure DevOps Server version).
   • %tfsInstanceGuid%: This represents the GUID (Globally Unique Identifier) of your Team Foundation Server instance. This GUID is unique to each TFS instance.

3. Determining the TFS Instance GUID (if unknown): If you are unsure of the GUID for your specific TPC, there is a simpler approach. Instead of trying to find the specific GUID, you can choose to purge the entire content within the _tfs_data folder. Navigating to and deleting everything under the _tfs_data folder will effectively clear the cache for all TFS instances managed by that Application Tier or proxy server. This is a safe and often quicker method if you are unsure of the exact GUID. The path to the _tfs_data folder is:

   C:\Program Files\Microsoft Team Foundation Server \[Year]\Application Tier\Web Services\_tfs_data\

   For proxy servers, the path might be similar but under the proxy installation directory.

4. Purge the Cache: Once you have located the correct cache folder (either the specific GUID folder under Proxy or the entire _tfs_data folder), you need to delete the contents of this folder. This effectively purges the cached files. Important: Ensure you are deleting the contents of the folder, not the folder itself, unless specifically instructed otherwise in more advanced scenarios. Deleting the contents is generally sufficient to clear the version control file cache.

5. Repeat for All ATs and Proxies: It is absolutely critical to repeat steps 1 through 4 for every Application Tier server and every proxy server in your Team Foundation Server deployment. If you have multiple ATs in a load-balanced setup or multiple proxy servers, you must perform the cache purge on each of them. Failure to purge the cache on even a single AT or proxy can lead to continued issues for users connecting through that specific server.

Important Considerations:

• Downtime: Purging the cache might temporarily impact performance immediately after the TPC is brought back online, as the cache will need to be rebuilt. However, this is a necessary step to ensure data consistency and prevent incorrect file downloads. Plan for a brief period of potentially reduced performance after the restore and cache purge.
• Permissions: Ensure the account you are using to perform these steps has sufficient permissions to delete files and folders within the Team Foundation Server installation directory. Typically, a local administrator account on the server will have the necessary permissions.
• Verification: After purging the cache and bringing the TPC back online, it is advisable to perform some basic verification. Have users attempt to download files from version control to confirm that they are now receiving the correct and latest versions.
• Automation: For environments with frequent TPC restores or moves, consider automating the cache purging process as part of your standard restoration procedures. Scripts can be developed to automatically locate and clear the cache folders on all relevant servers, reducing the risk of human error and ensuring consistency.

By diligently following these steps and purging the version control file cache on all Application Tiers and proxy servers after a TPC restore or move, you can effectively prevent the issue of users downloading incorrect file contents and maintain the integrity of your Azure DevOps Server version control system. This practice is a key component of proper Azure DevOps Server administration and disaster recovery planning.

We encourage you to share your experiences or questions in the comments below. Have you encountered this issue before? What strategies have you used to manage your Azure DevOps Server environment and prevent similar problems? Your insights can be valuable to the community!