Mitigating Data Loss During Partial Sync Failures

During a recent update, we encountered an issue where partial failures during data synchronization could lead to data loss. This post details the problem and the solution implemented to ensure data integrity.

The Problem

Our application relies on synchronizing data from an external source. The synchronization process involves fetching and updating various data points. Initially, we used a method that replaced the entire data structure with new information. However, if any sub-fetch within the synchronization process failed (e.g., due to a timeout or rate limit), previously fetched data would be silently deleted, leading to incomplete and inaccurate information.

To illustrate, consider a simplified example where we're updating user profile information. Initially, the code might have looked something like this (illustrative):

// WARNING: This code is vulnerable
$profile = Profile::updateOrCreate(
    ['user_id' => $userId],
    ['external_data' => $freshData]
);

In this simplified scenario, updateOrCreate would replace the entire external_data field. If any part of $freshData was missing due to a failed fetch, the corresponding information in the database would be lost.

The Solution

To address this, we modified the synchronization process to merge new data on top of existing data. This ensures that even if a sub-fetch fails, the previously fetched data remains intact. Here's a conceptual illustration of the improved approach:

// Safer approach: merge new data with existing data
$profile = Profile::firstOrNew(['user_id' => $userId]);
$existingData = $profile->external_data ?? [];
$mergedData = array_merge($existingData, $freshData);
$profile->external_data = $mergedData;
$profile->save();

In this revised approach, we first retrieve the existing data. Then, we use array_merge (or a similar merging function for more complex data structures) to combine the new data with the existing data. Finally, we save the merged data back to the database. This ensures that existing data is preserved even if parts of the new data are missing.

Additionally, we implemented proper exception handling to prevent the synchronization timestamp from being updated after failures. This allows the synchronization job to retry and attempt to fetch the missing data again.

Key Takeaways

When synchronizing data from external sources, it's crucial to handle partial failures gracefully. Avoid replacing entire data structures with new data, as this can lead to data loss. Instead, merge new data with existing data to ensure data integrity. Implement robust exception handling to allow for retries and prevent incomplete synchronizations from being considered successful.