Stale cache values after tag flush with file driver in Lumen

Answer

After Cache::tags('posts')->flush() the file driver may still return a value that was cached under that tag for a short period. On a typical local disk the window is on the order of a few milliseconds to about one second, depending on the operating‑system page‑cache eviction policy. On networked file systems (NFS, SMB, etc.) the delay can grow to several seconds or more.

The Redis driver deletes all keys belonging to the tag in a single transaction. Once the flush completes, any subsequent read will see the new value (or a miss) – the driver guarantees immediate consistency. A race can only occur if a write to the same key happens concurrently with the flush; otherwise Redis guarantees that the old value cannot be served.

Likely Explanation vs. Confirmed Facts

• Confirmed: The file cache driver removes the tag list file immediately, but the underlying data files may remain in the OS page cache until the filesystem clears them.
• Confirmed: Redis performs an atomic delete of all keys in the tag set, guaranteeing that a subsequent read will not hit a stale key unless a new write occurs after the flush.
• Likely: The stale‑read window for the file driver is typically <1 s on local disks; on networked filesystems it can be longer due to network latency and cache coherency delays.
• Likely: A concurrent write during the flush can create a brief race in both drivers, but the window is effectively the duration of the flush operation itself (a few milliseconds).

Verification Workflow (File Driver)

1. Cache a value with a tag:

   Cache::tags('posts')->put('posts.123', ['title' => 'Hello'], 3600);
   

2. Flush the tag:

   Cache::tags('posts')->flush();
   

3. Immediately read the key and note the result:

   $value = Cache::get('posts.123');
   var_dump($value);
   

4. Measure the time between the flush call and the read. If $value is not null, the stale window is at least that many milliseconds.

5. Repeat the test on a networked filesystem to compare delays.

Verification Workflow (Redis Driver)

1. Cache a value with a tag as above, using the Redis driver.

2. Flush the tag.

3. Immediately read the key. It should return null.

4. Insert a new value to the same key after the flush and read again – the new value should be returned, confirming that the flush erased the old entry.

Diagnostic Detail Needed

To refine the recommendation, please let us know whether your cache directory lives on a local disk or a networked filesystem (e.g., NFS, SMB). The type of storage can significantly affect the stale‑read window for the file driver.

Key Takeaways

• File driver: stale reads possible for < 1 s locally; longer on networked storage.
• Redis driver: immediate consistency after flush; race only if a write occurs during the flush.
• Measure the window in your environment to set appropriate cache TTLs or to decide on a more robust driver if consistency is critical.