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Overview

The metrics middleware collects custom application metrics including request counts, latencies, and error rates. Use it when you need:
  • Application monitoring
  • Custom metric collection
  • Performance tracking

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Installation

import "github.com/go-mizu/mizu/middlewares/metrics"

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Quick Start

app := mizu.New()

m := metrics.New()
app.Use(m.Middleware())

// Expose metrics endpoint
app.Get("/metrics", m.Handler())

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Configuration

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Options

OptionTypeDefaultDescription
Namespacestring"http"Metric namespace
Buckets[]float64StandardLatency buckets

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Examples

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Basic Metrics

m := metrics.New()
app.Use(m.Middleware())
app.Get("/metrics", m.Handler())

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Custom Namespace

m := metrics.New(metrics.Options{
    Namespace: "myapp",
})

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Custom Buckets

m := metrics.New(metrics.Options{
    Buckets: []float64{0.01, 0.05, 0.1, 0.5, 1, 5},
})

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Add Custom Metrics

m := metrics.New()

// Counter
m.Counter("jobs_processed", "Total jobs processed")
m.Inc("jobs_processed")

// Gauge
m.Gauge("active_connections", "Active connections")
m.Set("active_connections", 42)

// Histogram
m.Histogram("job_duration", "Job duration", buckets)
m.Observe("job_duration", 1.5)

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Default Metrics

MetricTypeDescription
http_requests_totalCounterTotal requests
http_request_duration_secondsHistogramRequest latency
http_requests_in_flightGaugeActive requests

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API Reference

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Functions

// New creates metrics collector
func New(opts ...Options) *Metrics

// Middleware returns metrics middleware
func (m *Metrics) Middleware() mizu.Middleware

// Handler returns metrics HTTP handler
func (m *Metrics) Handler() mizu.Handler

// Custom metrics
func (m *Metrics) Counter(name, help string)
func (m *Metrics) Gauge(name, help string)
func (m *Metrics) Histogram(name, help string, buckets []float64)
func (m *Metrics) Inc(name string)
func (m *Metrics) Set(name string, value float64)
func (m *Metrics) Observe(name string, value float64)

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Technical Details

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Core Components

The metrics middleware consists of the following key components:

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Metrics Structure

  • RequestCount: Total number of HTTP requests processed (atomic int64)
  • ErrorCount: Total number of errors (4xx and 5xx status codes, atomic int64)
  • TotalDuration: Cumulative duration of all requests in nanoseconds (atomic int64)
  • ActiveRequests: Number of currently in-flight requests (atomic int64)
  • statusCodes: Map tracking counts for each HTTP status code
  • pathCounts: Map tracking request counts per URL path

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Thread Safety

All counters use atomic operations (atomic.AddInt64, atomic.LoadInt64, atomic.StoreInt64) to ensure thread-safe concurrent access without explicit locking for the main counters. Maps (statusCodes and pathCounts) are protected by a sync.RWMutex to safely handle concurrent reads and writes.

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Status Capture Mechanism

The middleware uses a custom statusCapture wrapper that implements http.ResponseWriter to intercept the status code before writing the response. This allows tracking of status codes even when they’re set by downstream handlers.

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Metrics Calculation

  • Average Duration: Calculated as TotalDuration / RequestCount and converted to milliseconds
  • Error Detection: Requests are counted as errors when err != nil or statusCode >= 400

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Output Formats

The middleware supports two output formats:
  1. JSON Format (Handler()): Returns statistics as JSON with fields:
    • request_count, error_count, active_requests
    • average_duration_ms, status_codes, path_counts
  2. Prometheus Format (Prometheus()): Exports metrics in Prometheus text format:
    • http_requests_total (counter)
    • http_errors_total (counter)
    • http_active_requests (gauge)

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Implementation Notes

  • Uses custom itoa() function for integer to string conversion to avoid allocations
  • Metrics can be reset using Reset() method which clears all counters and maps
  • The middleware does not affect the response or request flow, only observes them

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Best Practices

  • Use meaningful metric names
  • Add labels for dimensions
  • Set appropriate histogram buckets
  • Monitor metric cardinality

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Testing

The metrics middleware includes comprehensive test coverage for all functionality:
Test CaseDescriptionExpected Behavior
TestNewBasic initialization and request countingCreates middleware, counts single request correctly
TestMetrics_StatusCodesHTTP status code trackingTracks 200 and 500 status codes separately, counts 500 as error
TestMetrics_PathCountsURL path request countingCounts requests per path (3 to /api/users, 1 to /api/posts)
TestMetrics_HandlerJSON metrics endpointReturns metrics as JSON with correct Content-Type header
TestMetrics_PrometheusPrometheus format exportReturns Prometheus-formatted metrics with http_requests_total
TestMetrics_ResetMetrics reset functionalityResets all counters to zero after processing requests
TestMetrics_ConcurrentThread-safe concurrent accessCorrectly counts 100 concurrent requests without race conditions
TestMetrics_AverageDurationDuration measurementCalculates positive average duration for requests
TestMetrics_JSONJSON serializationMarshals stats to JSON with request_count field