Troubleshooting Silent Performance Alerts In Self-Hosted Sentry

Introduction

Hey guys, ever faced the head-scratching issue where your performance alerts in a self-hosted Sentry setup just won't trigger, even when the metrics are clearly there? It's like setting up a fire alarm, seeing the smoke, but the alarm stays silent. Super frustrating, right? In this article, we're diving deep into a specific scenario where performance alerts in Sentry aren't firing despite metrics being ingested successfully. We'll break down the problem, explore potential causes, and offer a step-by-step guide to troubleshoot this issue. We'll be focusing on a real-world case where a user experienced this exact problem after upgrading their Sentry instance. We'll explore the setup, the symptoms, and the troubleshooting steps taken to resolve the issue. So, whether you're a seasoned Sentry user or just starting out, this guide will equip you with the knowledge to tackle those silent alert mysteries. We’ll cover everything from verifying your setup to digging into the logs and configurations, ensuring you can get your alerts firing as expected. Let's get started and make sure those alerts wake up when they should!

The Bug: Performance Alerts Not Triggering

So, what’s the deal? Imagine this: You’ve set up a performance alert in your Sentry project, all ready to be notified when things go south. You're dealing with a system that's got traffic flowing, metrics being recorded, and everything seems to be in order. But here's the kicker – no alerts are triggered. Even when the metrics clearly exceed the thresholds you’ve set. It’s like shouting into the void. You check everything. Metrics are being ingested without a hitch. Your project has those crucial transaction.duration values sitting pretty in ClickHouse. The alert rule? Visible, active, and seemingly ready to pounce. Yet, silence. The logs are clean, the web UI is error-free, and still, nothing. This is the exact scenario we’re tackling today. It’s a tricky one because on the surface, everything looks fine. But the core functionality – alerting you to performance issues – is simply not working. We need to dig deeper to understand why these alerts are staying silent. We’re going to walk through the common culprits, the troubleshooting steps, and how to verify each component in your Sentry setup to pinpoint the root cause. Think of it as a detective story, where the missing alert is our case, and we're the detectives armed with knowledge and tools. Let’s get to solving this mystery!

Expected Behavior vs. Actual Behavior

Okay, let's get specific about what we expect versus what's actually happening in this scenario. Ideally, a performance alert should act like a vigilant watchdog, barking loudly when something exceeds its guard limits. In our case, the expected behavior is straightforward: When the avg(transaction.duration) goes over the defined threshold within the specified time window, an alert should fire. It's like setting a speed limit on the highway – when you exceed it, you expect a notification (in this case, an alert). But here’s the frustrating actual behavior: Despite the average transaction duration clearly exceeding the set threshold, no alert is triggered. It's as if the watchdog is asleep on the job. To make matters more perplexing, data exists in ClickHouse, both in metrics_raw_v2_local and transactions_local. This means Sentry is receiving and storing the performance data correctly. Manually querying the data confirms that the threshold is indeed being exceeded. So, the problem isn't a lack of data or a misconfigured threshold. Further investigation reveals that the AlertRule exists and is properly linked to a valid SnubaQuery. This eliminates the possibility of a broken rule or a query issue. Even the subscription entry exists, with a subscription_id that seems correct. The final head-scratcher? No visible errors in snuba-subscription-consumer-generic-metrics. This component is crucial for processing subscriptions and triggering alerts, but it’s showing no signs of trouble. This discrepancy between expected and actual behavior is the heart of our mystery. We need to figure out why Sentry isn’t reacting to the data it’s receiving. It's time to roll up our sleeves and dive into the troubleshooting process.

Initial Setup and Environment Details

To really crack this case, let's look closely at the initial setup and environment details. This is like examining the crime scene – every detail matters. Our user upgraded their Sentry instance from v24.6.0 to v25.7.0. It’s crucial to note that the issue existed before the upgrade, ruling out the upgrade itself as the sole culprit. The deployment is self-hosted, running on Docker Compose. This setup means we have full control over the environment but also the responsibility of managing all the components. Kafka and Redis, key players in Sentry’s architecture, are both running smoothly. Kafka handles the message queue, and Redis acts as a caching and data storage system. If either of these were down, we'd likely see broader issues. Relay, responsible for processing events, is in processing mode with both ingest: true and store: true. This confirms that Relay is actively receiving and storing events, which is essential for alerts to function. Now, let's talk specifics. The user created a performance alert with the following settings: Dataset: generic_metrics Aggregate: avg(transaction.duration) Query: (either empty or includes a filter like transaction:/create-payment/) Time Window: 60 seconds Threshold: 100ms Project: ID 32, slug app-pay They confirmed that real transactions exceeding 500ms were being sent to Sentry. Despite this, no alert triggered. These details paint a clear picture of the setup and the specific conditions under which the issue occurs. It's like having a suspect lineup – we know the environment, the alert configuration, and the expected behavior. Now, we need to start interrogating the components to find the one that's not playing its part. By understanding the environment, we can start to isolate potential problem areas. Is it a configuration issue? A data flow problem? Or something else entirely? Let’s keep digging.

Reproducing the Issue

To really understand a problem, you’ve gotta try and break it yourself, right? So, let’s talk about how to reproduce this performance alert issue. Imagine you're a detective reconstructing a crime scene – you need to walk in the suspect's shoes. Here’s how our user set things up, and how we can replicate their steps to see the silent alert in action. First, we need to create a performance alert. We're aiming for an alert that should fire, but doesn't. Set it up like this: Dataset: generic_metrics. This tells Sentry we're looking at generic metrics data. Aggregate: avg(transaction.duration). We want to monitor the average duration of transactions. Query: Leave this empty or include a filter like transaction:/create-payment/. This allows us to focus on specific transactions, or all of them. Time Window: 60 seconds. This is the period over which the average is calculated. Threshold: 100ms. The alert should trigger if the average duration exceeds this. Project: ID 32, slug app-pay. The project where we expect the alert to fire. Next, we confirm that real transactions with durations greater than 500ms are being sent to Sentry. This is crucial – we need to ensure there’s data that should trigger the alert. Now, the waiting game. We wait and observe... and see no alert. Despite the transactions clearly exceeding the 100ms threshold, silence. Reproducing the issue confirms it’s not a one-off fluke. It’s a consistent problem that needs a solution. By following these steps, we can reliably recreate the issue in our own environments, allowing us to test potential fixes and isolate the root cause. It's like having a controlled experiment – we know the inputs, we know the expected output, and we can see the failure firsthand. This is invaluable for troubleshooting.

Diving Deeper: Debugging Steps and Insights

Alright, let's put on our debugging hats and dive into the nitty-gritty. We've reproduced the issue; now it's time to start dissecting it. This is where we become Sentry surgeons, carefully examining each component to find the ailment. Our user provided some valuable insights from their initial investigation. They poked around in the Sentry Django shell – a powerful tool for interacting directly with Sentry's backend. Here's what they found: They retrieved the AlertRule object by its name (`