PSR-14 EventDispatcher vs Custom CompilerPass for Listener Ordering

The Resolution

To restore deterministic ordering across independent extensions in TYPO3 v12 without introducing tight coupling, the PSR-14 priority attribute remains the primary architectural tool, but it must be used with a strict, documented priority scale. If two extensions must maintain a specific sequence and cannot agree on a priority value, a Custom CompilerPass is the technically correct way to enforce order at the container level without hard-coding dependencies in your PHP logic.

Likely Explanation of the Conflict

In TYPO3 v12, the EventDispatcher sorts listeners first by their assigned priority (descending) and then by their service ID. Because the service ID sequence is influenced by the order in which extensions are loaded into the container, listeners with identical priorities exhibit non-deterministic behavior. This creates a race condition where Extension A might run before Extension B in one environment, but after it in another, despite both having the same priority.

Implementation Paths

1. The Priority Scale (Recommended)

Avoid using generic priorities like 0 or 100. Instead, establish a wide-gap priority scale (e.g., increments of 100 or 1000). This allows third-party extensions to "slot in" between your required sequence without requiring a container rebuild or a code change.

2. Custom CompilerPass (For Strict Enforcement)

If you must ensure Extension A precedes Extension B regardless of priority or load order, use a CompilerPass to manipulate the tagged services before the container is frozen. This avoids the "Orchestrator Pattern" because the services remain independent; only the container's internal map is modified.

1. Create a class implementing Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface.
2. Locate the services tagged with the event listener tag.
3. Reorder the collection of service definitions based on your specific business logic.
4. Register the pass in your Services.yaml or via the kernel.

Verification Steps

To verify that the ordering is truly deterministic and not dependent on the load order, perform the following checks:

• Container Inspection: Dump the compiled container to verify the sequence of service IDs assigned to the event.
• Execution Logging: Implement a temporary logger in each listener to capture the exact timestamp and sequence of execution across multiple system restarts.
• Load Order Shuffle: Change the extension load order in the TYPO3 Install Tool and verify the execution sequence remains unchanged.

Diagnostic Detail Required: Are the listeners in question modifying the same object property? If so, the recommendation shifts toward a Chain of Responsibility pattern rather than a simple event dispatcher.