Bug: Metrics Not Flushed On Kernel.terminate Event

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Introduction

In the context of OpenTelemetry and Symfony, it has been observed that metrics are not flushed when the kernel.terminate event is triggered. This issue arises when using metrics within a request lifecycle, resulting in the loss of recorded data. In this article, we will delve into the description of the problem, steps to reproduce, expected and actual behavior, and a possible fix.

Description

The OpenTelemetry bundle, designed to provide a seamless integration of OpenTelemetry with Symfony, appears to have a critical flaw. When the kernel.terminate event is triggered, the bundle fails to flush metrics, leading to their loss. This issue is particularly concerning when using metrics, such as counters and histograms, within a request lifecycle.

Steps to Reproduce

To reproduce this issue, follow these steps:

  1. Install and Configure the OpenTelemetry Bundle: Begin by installing the OpenTelemetry bundle in your Symfony project. Ensure that you have properly configured the bundle to work in conjunction with your application.
  2. Register and Use a Metric: Within your request lifecycle, register and use a metric (e.g., counter, histogram). This will help you understand the issue at hand.
  3. Ensure Request Completion and kernel.terminate Triggering: Complete the request and ensure that the kernel.terminate event is triggered. This event is responsible for flushing metrics, but in this case, it fails to do so.
  4. Check if Metric is Flushed and Exported: Verify that the metric is not flushed and exported. This will confirm that the issue is indeed related to the kernel.terminate event.

Expected Behavior

The expected behavior is that metrics should be flushed and exported when the kernel.terminate event is triggered. This ensures that all recorded data is properly sent before the request cycle ends. In a standard Symfony request flow, metrics should be exported, and this behavior is expected.

Actual Behavior

Unfortunately, the actual behavior is that metrics are not flushed, meaning they are effectively lost when using the bundle in a standard Symfony request flow. This issue is critical, as it results in the loss of valuable data.

Possible Fix

A potential fix for this issue would be to register an EventListener on the kernel.terminate event that explicitly flushes all MeterProvider instances available in the container. This would ensure that metrics are properly exported before the kernel lifecycle ends.

Implementation

To implement this fix, follow these steps:

  1. Create an Event Listener: Create a new class that implements the EventListener interface. This class will be responsible for flushing metrics when the kernel.terminate event is triggered.
  2. Flush MeterProvider Instances: Within the event listener, fetch all MeterProvider instances available in the container. Then, explicitly flush each instance to ensure that metrics are exported.
  3. Register the Event Listener: Register the event listener with the kernel.terminate event. This will ensure that the event listener is triggered when the kernel.terminate event is fired.

Example Code

Here is an example implementation of the event listener:

use OpenTelemetry\SDK\Meter\MeterProvider;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;

class MetricFlushListener implements EventSubscriberInterface
{
    public function onKernelTerminate(KernelEvents::TERMINATE, GetResponseForControllerResultEvent $event)
    {
        $container = $event->getContainer();
        $meterProviders = $container->get('meter_provider');

        foreach ($meterProviders as $meterProvider) {
            $meterProvider->flush();
        }
    }

    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::TERMINATE => ['onKernelTerminate', 0],
        ];
    }
}

Conclusion

Introduction

In our previous article, we discussed the issue of metrics not being flushed on the kernel.terminate event in the OpenTelemetry bundle for Symfony. We also provided a possible fix by registering an EventListener on the kernel.terminate event that explicitly flushes all MeterProvider instances available in the container. In this article, we will address some frequently asked questions (FAQs) related to this issue.

Q: What is the kernel.terminate event?

A: The kernel.terminate event is a Symfony event that is triggered after the request has completed and the kernel has finished processing. This event is responsible for flushing metrics, but in the case of the OpenTelemetry bundle, it fails to do so.

Q: Why are metrics not being flushed on the kernel.terminate event?

A: The OpenTelemetry bundle does not explicitly flush metrics on the kernel.terminate event. This results in metrics being lost when using the bundle in a standard Symfony request flow.

Q: How can I reproduce this issue?

A: To reproduce this issue, follow these steps:

  1. Install and configure the OpenTelemetry bundle.
  2. Register and use a metric (e.g., counter, histogram) within a request lifecycle.
  3. Ensure that the request completes and the kernel.terminate event is triggered.
  4. Check if the metric is flushed and exported.

Q: What is the expected behavior?

A: The expected behavior is that metrics should be flushed and exported when the kernel.terminate event is triggered. This ensures that all recorded data is properly sent before the request cycle ends.

Q: What is the actual behavior?

A: The actual behavior is that metrics are not flushed, meaning they are effectively lost when using the bundle in a standard Symfony request flow.

Q: How can I fix this issue?

A: A possible fix for this issue is to register an EventListener on the kernel.terminate event that explicitly flushes all MeterProvider instances available in the container.

Q: What is the implementation of the event listener?

A: The implementation of the event listener involves creating a new class that implements the EventListener interface. Within the event listener, fetch all MeterProvider instances available in the container and explicitly flush each instance to ensure that metrics are exported.

Q: What is the example code for the event listener?

A: Here is an example implementation of the event listener:

use OpenTelemetry\SDK\Meter\MeterProvider;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;

class MetricFlushListener implements EventSubscriberInterface
{
    public function onKernelTerminate(KernelEvents::TERMINATE, GetResponseForControllerResultEvent $event)
    {
        $container = $event->getContainer();
        $meterProviders = $container->get('meter_provider');

        foreach ($meterProviders as $meterProvider) {
            $meterProvider->flush();
        }
    }

    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::TERMINATE => ['onKernelTerminate', 0],
        ];
    }
}

Q: How can I register the event listener?

A: To register the event listener, add the following code to your services.yml file:

services:
    app.metric_flush_listener:
        class: App\MetricFlushListener
        tags:
            - { name: kernel.event_listener, event: kernel.terminate, method: onKernelTerminate }

Conclusion

In conclusion, the issue of metrics not being flushed on the kernel.terminate event is a critical problem that affects the OpenTelemetry bundle in Symfony. By registering an EventListener on the kernel.terminate event that explicitly flushes all MeterProvider instances available in the container, we can ensure that metrics are properly exported before the kernel lifecycle ends. This fix provides a solution to the issue at hand and ensures that valuable data is not lost.