MODX Snippet Caching: Resource Update Invalidation Behavior

Question

MODX Revolution employs a snippet caching mechanism that stores execution results in the database to minimize CPU overhead. This system relies on a Time To Live (TTL) setting to determine when a snippet must re-run its internal logic.

A design uncertainty exists regarding the relationship between resource updates and the cache table. While the TTL governs automatic expiration, it is unclear how the system handles manual resource saves in relation to dependent cached snippets.

Constraints

Snippets may depend on data from multiple resources.
  The cache is stored as HTML fragments in the database.
  Placeholders allow for partial dynamic injection within cached blocks.

Does updating a MODX resource trigger an immediate purge of all cached snippets associated with that resource, or do those snippets continue to serve stale content until the TTL expires?

Tasadduq Burney · Answer

Updating a resource does not purge cached snippets
Updating a MODX resource does not trigger an immediate purge of cached snippets associated with that resource. Those snippets will continue to serve stale content until the defined Time To Live (TTL) expires or the cache is manually cleared.
Why the stale content remains
MODX's snippet caching mechanism (typically utilizing the modxcache table) is designed to reduce database overhead. When a snippet is called with a TTL, the system checks if a valid entry exists. The invalidation logic is primarily based on the timestamp of the cache entry rather than the state of the underlying resources.
Because the system does not maintain a granular dependency map between specific resource IDs and cached HTML fragments, it has no native way of knowing that a change in Resource A should invalidate a cached Snippet B.

Key Behaviors

TTL-Driven: The only automatic trigger is the expiration of the TTL period set in the snippet call.
  Placeholder Exception: If your cached block uses placeholders for dynamic injection, those specific values may update depending on how the placeholders are fetched, but the surrounding "cached" structural HTML will remain stale.
  Global Invalidation: Clearing the site cache via the System Manager is the only native way to force an immediate refresh, but this affects all cached elements globally.

How to handle manual invalidation
If your workflow requires immediate updates upon resource save, you must implement one of the following strategies:

Manual Clearing: Use the modCache::clear() method within an OnSave event hook to purge the cache when a specific resource is updated.
  Unique Cache Keys: Include a version number or a modified timestamp in the snippet call parameters (e.g., [[mySnippet?cache=60&v=[[[modified]]]]). When the timestamp changes, the system generates a new cache key, forcing a re-run.
  Shorten TTLs: Reduce the TTL to a duration that fits your tolerance for lag, though this increases database load.

MODX Snippet Caching: Resource Update Invalidation Behavior

Constraints

1 answer

Updating a resource does not purge cached snippets

Why the stale content remains

Key Behaviors

How to handle manual invalidation

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