Fixing Prometheus Metrics Cardinality Error In Grove Path

by Omar Yusuf 58 views

Hey guys! Today, we're diving deep into a critical issue we encountered during load testing on pocketd: a pesky Prometheus metrics cardinality error in the Shannon protocol metrics. This error was causing our Gateway pods to crash, which is definitely not what we want, especially when things get busy.

Understanding the Problem

During a local load test, we stumbled upon a panic in github.com/buildwithgrove/path/metrics/protocol/shannon. The shannon_relay_total metric was expecting four labels but was only receiving three. This mismatch led to an inconsistent label cardinality error, resulting in those dreaded pod restarts. Let's break down the error message to understand it better:

panic: inconsistent label cardinality: expected 4 label values but got 3 in prometheus.Labels{"error_type":"SHANNON_REQUEST_ERROR_INTERNAL_DELEGATED_FETCH_APP", "service_id":"anvil", "success":"false"}

goroutine 10030 [running]:
github.com/prometheus/client_golang/prometheus.(*CounterVec).With(0x371e4c0?, 0xc002efe960?)
    /home/runner/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/counter.go:296 +0x51
github.com/buildwithgrove/path/metrics/protocol/shannon.recordRelayTotal({0x44b48a0, 0xc0015e0380}, 0xc003f44540)
    /home/runner/work/path/path/metrics/protocol/shannon/metrics.go:247 +0x2eb
github.com/buildwithgrove/path/metrics/protocol/shannon.PublishMetrics({0x44b48a0, 0xc0015e0380}, 0x2?)
    /home/runner/work/path/path/metrics/protocol/shannon/metrics.go:219 +0xdd
github.com/buildwithgrove/path/metrics/protocol.PublishMetrics({0x44b48a0, 0xc0015e0380}, 0xc00297cbc0)
    /home/runner/work/path/path/metrics/protocol/protocol.go:24 +0x113
github.com/buildwithgrove/path/metrics.(*PrometheusMetricsReporter).Publish(0xc0014f58a0, 0xc005332900)
    /home/runner/work/path/path/metrics/prometheus_reporter.go:42 +0x8b
github.com/buildwithgrove/path/gateway.(*requestContext).BroadcastAllObservations.func1()
    /home/runner/work/path/path/gateway/request_context.go:359 +0x25a
created by github.com/buildwithgrove/path/gateway.(*requestContext).BroadcastAllObservations in goroutine 6496
    /home/runner/work/path/path/gateway/request_context.go:327 +0x4f
[event: pod path1-649ffb7c65-hvn4k] Back-off restarting failed container path1 in pod path1-649ffb7c65-hvn4k_default(acc07aed-9b8c-4393-8aa3-ac99f2743607)

The error clearly states that the CounterVec expected four label values but only received three. This discrepancy originated in the metrics/protocol/shannon/metrics.go file, specifically around line 247. The pod restart events further confirm the severity of the issue, indicating a critical need for a fix. Ensuring consistent Prometheus metrics labeling is crucial for maintaining the stability and reliability of our system.

Why is Label Cardinality Important?

Before we dive into the solution, let's quickly touch on why label cardinality matters in Prometheus metrics. Cardinality refers to the uniqueness of label combinations. If a metric has too many unique label combinations, it can lead to performance issues in Prometheus, such as high memory usage and slow query times. In our case, the inconsistent cardinality was causing panics and pod crashes, highlighting the importance of managing it effectively. Maintaining observability without breaking metric collection is a key goal for us, ensuring we can monitor system performance without causing instability.

Goals and Deliverables

Our primary goal here is to ensure consistent Prometheus metrics labeling across all Shannon protocol metrics. This consistency is crucial to prevent Gateway pod crashes, especially during high-load scenarios. We also need to maintain observability without disrupting our metric collection processes. To achieve this, we've outlined a few key deliverables:

  • Identify the missing label: Pinpoint the exact label that's causing the cardinality mismatch in the shannon_relay_total metric.
  • Fix the cardinality mismatch: Implement the necessary changes in metrics/protocol/shannon/metrics.go:247 to correct the label count.

What We're Not Focusing On

To keep our efforts targeted and efficient, we've also defined some non-goals:

  • We won't be redesigning the entire metrics collection system. Our focus is solely on fixing the immediate cardinality issue.
  • We're not planning to change existing metric names or introduce breaking changes. Backwards compatibility is important to us.
  • We won't be adding any new metrics beyond what's necessary to resolve the cardinality problem.

Diving into the Solution

So, how do we tackle this? The first step is to identify the missing label. We need to carefully examine the shannon_relay_total metric definition and the context in which it's being used. By comparing the expected labels with the actual labels being passed, we can pinpoint the discrepancy.

Once we've identified the missing label, we'll need to modify the code to include it. This might involve adding a new label value in the recordRelayTotal function or adjusting how the labels are constructed. The key is to ensure that the shannon_relay_total metric always receives the expected four labels. The fix will be implemented in metrics/protocol/shannon/metrics.go:247.

General Deliverables: Ensuring Quality and Maintainability

Beyond the immediate fix, we're committed to ensuring the long-term quality and maintainability of our codebase. This means we'll be focusing on the following general deliverables:

  • Comments: We'll add or update TODOs and comments in the source code to make it easier for others (and our future selves) to understand the changes and the reasoning behind them. Clear comments are essential for maintaining a healthy codebase.
  • Testing: We'll add new tests, both unit and end-to-end (E2E), to our test suite. These tests will help us verify that the fix is working correctly and prevent regressions in the future. Comprehensive testing is crucial for building confidence in our code.
  • Documentation: We'll update any relevant architectural or development READMEs to reflect the changes. Where appropriate, we'll use mermaid diagrams to illustrate complex concepts or relationships. Good documentation is vital for onboarding new team members and maintaining a shared understanding of the system.

Steps to Fix the Cardinality Mismatch

  1. Analyze the Code: Start by examining the metrics/protocol/shannon/metrics.go file, specifically the recordRelayTotal function and the definition of the shannon_relay_total metric. Identify the expected labels and the labels that are actually being passed.
  2. Identify the Missing Label: Determine which label is missing from the set of labels being passed to the metric.
  3. Implement the Fix: Modify the code to include the missing label. This might involve adding a new label value or adjusting the label construction logic.
  4. Add Comments and TODOs: Add comments to explain the changes and any relevant context. Use TODOs to highlight any areas that might need further attention or future work.
  5. Write Tests: Create unit tests and/or E2E tests to verify that the fix is working correctly and prevent regressions.
  6. Update Documentation: Update any relevant documentation, such as READMEs or architectural diagrams, to reflect the changes.
  7. Test Thoroughly: Run all tests to ensure that the fix has not introduced any new issues.

Example of Code Modification

Let's say, for instance, that the missing label is delegated_service. You might need to modify the recordRelayTotal function to include this label:

func (m *Metrics) recordRelayTotal(serviceID string, errorType shannon.ErrorType, success bool, delegatedService string) {
    m.relayTotal.With(prometheus.Labels{
        "service_id":      serviceID,
        "error_type":      string(errorType),
        "success":         strconv.FormatBool(success),
        "delegated_service": delegatedService, // Added missing label
    }).Inc()
}

This is just an illustrative example, and the actual code modification might vary depending on the specific context.

Conclusion

Fixing the Prometheus metrics cardinality error in the Shannon protocol metrics is crucial for maintaining the stability and reliability of our Gateway pods. By systematically identifying the missing label, implementing the necessary code changes, and ensuring thorough testing and documentation, we can resolve this issue and prevent future occurrences. This effort not only addresses the immediate problem but also contributes to the overall health and maintainability of our codebase. So, let's get to it and make our system more robust!

By addressing this issue, we ensure that our monitoring and alerting systems remain effective, allowing us to respond quickly to any performance degradations or errors. The stability of our Gateway pods is paramount for smooth operation, and correcting this metric cardinality error is a significant step in that direction. The focus on testing and documentation further ensures that the fix is robust and maintainable in the long run. Let's keep building a reliable and observable system!