Python Test Micrometer Testing Metrics

Micrometer Testing Metrics

Table of Contents

Quick Start → What Is This | When to Use | Simple Example

How to Implement → Step-by-Step | Test Registry Setup | Examples

Help → Anti-Patterns | Requirements

Reference → Related Skills

Purpose

Testing metrics requires explicit registry setup since Micrometer doesn't auto-wire in unit tests. This skill shows how to verify that custom metrics are correctly recorded in unit tests (SimpleMeterRegistry), integration tests (@AutoConfigureMetrics), and percentile/histogram calculations.

When to Use

Use this skill when:

• Writing unit tests for services that record metrics
• Validating metric values after business operations
• Testing percentiles and histogram configurations
• Asserting metric behavior without full Spring context
• Verifying counter increments, timer durations, or gauge values
• Testing integration with Spring Boot Actuator
• Debugging why metrics aren't being recorded in tests

Quick Start

For unit tests, use SimpleMeterRegistry:

class ChargeServiceTest {

    private SimpleMeterRegistry registry;
    private ChargeService chargeService;

    @BeforeEach
    void setUp() {
        registry = new SimpleMeterRegistry();
        chargeService = new ChargeService(registry);
    }

    @Test
    void testChargeProcessingRecordsMetrics() {
        Charge charge = new Charge("SUP123", BigDecimal.valueOf(150.00));

        chargeService.processCharge(charge);

        Counter counter = registry.get("charge.processed").counter();
        assertThat(counter.count()).isEqualTo(1);
    }
}

Instructions

Step 1: Choose the Right Registry for Your Test

SimpleMeterRegistry (unit tests, no Spring):

• Use for testing service logic in isolation
• Fast, no Spring context startup
• Meters accessible immediately after recording
• No percentile calculations (use assertions instead)

MeterRegistry with @AutoConfigureMetrics (integration tests):

• Use when testing with Spring context
• Spring auto-configures actual metrics
• Slower but tests real behavior
• Supports percentiles if configured

CompositeMeterRegistry (multi-backend testing):

• Use when testing with multiple backends
• Rarely needed in unit tests

// ❌ Wrong: no registry
class ServiceTest {
    private Service service = new Service(); // Needs registry!
}

// ✅ Unit test: SimpleMeterRegistry
class ServiceTest {
    private SimpleMeterRegistry registry = new SimpleMeterRegistry();
    private Service service = new Service(registry);
}

// ✅ Integration test: Spring auto-configures
@SpringBootTest
@AutoConfigureMetrics
class ServiceIntegrationTest {
    @Autowired private MeterRegistry registry;
    @Autowired private Service service;
}

Step 2: Set Up Test Fixtures

Create test fixtures for common patterns:

// Base test class for metrics testing
class MetricsTestBase {

    protected SimpleMeterRegistry registry;

    @BeforeEach
    void setUp() {
        registry = new SimpleMeterRegistry();
    }

    /**
     * Assert that a counter with given name and tags exists and has expected count.
     */
    protected void assertCounterValue(String name, long expectedValue, Tag... tags) {
        Counter counter = registry.get(name)
            .tags(tags)
            .counter();

        assertThat(counter.count())
            .as("Counter %s with tags %s", name, Arrays.asList(tags))
            .isEqualTo(expectedValue);
    }

    /**
     * Assert that a timer with given name has recorded expected count.
     */
    protected void assertTimerCount(String name, long expectedCount, Tag... tags) {
        Timer timer = registry.get(name)
            .tags(tags)
            .timer();

        assertThat(timer.count())
            .as("Timer %s count with tags %s", name, Arrays.asList(tags))
            .isEqualTo(expectedCount);
    }

    /**
     * Assert that a distribution summary has expected count and total.
     */
    protected void assertDistributionSummary(
            String name,
            long expectedCount,
            double expectedTotal,
            Tag... tags) {

        DistributionSummary summary = registry.get(name)
            .tags(tags)
            .summary();

        assertThat(summary.count())
            .as("DistributionSummary %s count", name)
            .isEqualTo(expectedCount);

        assertThat(summary.totalAmount())
            .as("DistributionSummary %s total", name)
            .isCloseTo(expectedTotal, within(0.01));
    }

    /**
     * List all metrics for debugging.
     */
    protected void printMetrics() {
        registry.getMeters().forEach(meter -> {
            System.out.println(meter.getId().getName() +
                             " [" + meter.getId().getTags() + "]: " +
                             meter.measure());
        });
    }
}

Step 3: Test Counter Metrics

Test that counters increment correctly:

class ChargeServiceTest extends MetricsTestBase {

    private ChargeService chargeService;

    @BeforeEach
    void setUp() {
        super.setUp();
        chargeService = new ChargeService(registry);
    }

    @Test
    void testChargeProcessingIncrementsCounter() {
        Charge charge = new Charge("SUP123", BigDecimal.valueOf(150.00));

        chargeService.processCharge(charge);

        assertCounterValue("charge.processed", 1);
    }

    @Test
    void testMultipleChargesIncrementCounter() {
        chargeService.processCharge(new Charge("SUP1", BigDecimal.valueOf(100)));
        chargeService.processCharge(new Charge("SUP2", BigDecimal.valueOf(200)));
        chargeService.processCharge(new Charge("SUP3", BigDecimal.valueOf(300)));

        assertCounterValue("charge.processed", 3);
    }

    @Test
    void testChargeRejectionRecordsWithReason() {
        chargeService.rejectCharge("validation");
        chargeService.rejectCharge("duplicate");
        chargeService.rejectCharge("validation");

        assertCounterValue("charge.rejected", 2, Tag.of("reason", "validation"));
        assertCounterValue("charge.rejected", 1, Tag.of("reason", "duplicate"));
    }
}

Step 4: Test Timer Metrics

Test that timers record duration:

class ChargeProcessingTest extends MetricsTestBase {

    private ChargeService service;

    @BeforeEach
    void setUp() {
        super.setUp();
        service = new ChargeService(registry);
    }

    @Test
    void testChargeApprovalRecordsDuration() {
        Charge charge = new Charge("SUP123", BigDecimal.valueOf(150.00));

        service.processCharge(charge);

        Timer timer = registry.get("charge.approval.duration").timer();

        assertThat(timer.count()).isEqualTo(1);
        assertThat(timer.totalTime(TimeUnit.MILLISECONDS)).isGreaterThan(0);
    }

    @Test
    void testTimerRecordsAllOperations() {
        for (int i = 0; i < 5; i++) {
            service.processCharge(new Charge("SUP" + i, BigDecimal.ONE));
        }

        Timer timer = registry.get("charge.approval.duration").timer();

        assertThat(timer.count()).isEqualTo(5);
        assertThat(timer.totalTime(TimeUnit.MILLISECONDS))
            .isGreaterThan(timer.count() * 1); // At least 1ms per operation
    }

    @Test
    void testTimerTracksMaxDuration() {
        service.recordApprovalTime(Duration.ofMillis(10));
        service.recordApprovalTime(Duration.ofMillis(100));
        service.recordApprovalTime(Duration.ofMillis(50));

        Timer timer = registry.get("charge.approval.duration").timer();

        assertThat(timer.max(TimeUnit.MILLISECONDS)).isEqualTo(100);
    }
}

Step 5: Test Distribution Summary Metrics

Test value distributions:

class ChargeValueMetricsTest extends MetricsTestBase {

    private ChargeService service;

    @BeforeEach
    void setUp() {
        super.setUp();
        service = new ChargeService(registry);
    }

    @Test
    void testChargeValueDistribution() {
        service.recordCharge(new Charge("SUP1", BigDecimal.valueOf(100)));
        service.recordCharge(new Charge("SUP2", BigDecimal.valueOf(200)));
        service.recordCharge(new Charge("SUP3", BigDecimal.valueOf(300)));

        DistributionSummary summary = registry.get("charge.value").summary();

        assertThat(summary.count()).isEqualTo(3);
        assertThat(summary.totalAmount()).isEqualTo(600.0);
        assertThat(summary.mean()).isCloseTo(200.0, within(0.1));
    }

    @Test
    void testChargeValueMax() {
        service.recordCharge(new Charge("SUP1", BigDecimal.valueOf(500)));
        service.recordCharge(new Charge("SUP2", BigDecimal.valueOf(1000)));

        DistributionSummary summary = registry.get("charge.value").summary();

        assertThat(summary.max()).isEqualTo(1000.0);
    }
}

Step 6: Test Gauge Metrics

Test gauges that fetch values on demand:

class SupplierMetricsTest extends MetricsTestBase {

    private SupplierService service;
    private SupplierRepository repository;

    @BeforeEach
    void setUp() {
        super.setUp();
        repository = mock(SupplierRepository.class);
        service = new SupplierService(registry, repository);
    }

    @Test
    void testActiveSupplierCountGauge() {
        // Mock repository to return supplier count
        when(repository.countActive()).thenReturn(5);

        // Gauge is lazy-evaluated on access
        Gauge gauge = registry.get("supplier.active.count").gauge();

        assertThat(gauge.value()).isEqualTo(5.0);
    }

    @Test
    void testGaugeReflectsChanges() {
        when(repository.countActive())
            .thenReturn(5)      // First call
            .thenReturn(8);     // Second call

        Gauge gauge = registry.get("supplier.active.count").gauge();

        assertThat(gauge.value()).isEqualTo(5.0);

        // Gauge re-evaluates on each access
        assertThat(gauge.value()).isEqualTo(8.0);
    }
}

Step 7: Test with Spring Integration Tests

Use @AutoConfigureMetrics for testing with Spring context:

@SpringBootTest
@AutoConfigureMetrics
@AutoConfigureMockMvc
class ChargeControllerIntegrationTest {

    @Autowired
    private MockMvc mockMvc;

    @Autowired
    private MeterRegistry registry;

    @BeforeEach
    void setUp() {
        registry.clear();
    }

    @Test
    void testChargeCreationRecordsMetrics() throws Exception {
        mockMvc.perform(post("/charges")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""
                    {
                        "supplierId": "SUP123",
                        "amount": 150.00
                    }
                    """))
            .andExpect(status().isCreated());

        // Verify HTTP metrics
        Timer httpTimer = registry.get("http.server.requests")
            .tag("uri", "/charges")
            .tag("status", "201")
            .timer();

        assertThat(httpTimer.count()).isEqualTo(1);

        // Verify business metrics
        Counter chargeCounter = registry.get("charge.processed")
            .counter();

        assertThat(chargeCounter.count()).isEqualTo(1);
    }

    @Test
    void testHttpMetricsIncludeException() throws Exception {
        mockMvc.perform(post("/charges")
                .contentType(MediaType.APPLICATION_JSON)
                .content("""
                    {
                        "supplierId": null,
                        "amount": 0
                    }
                    """))
            .andExpect(status().isBadRequest());

        Timer httpTimer = registry.get("http.server.requests")
            .tag("uri", "/charges")
            .tag("status", "400")
            .timer();

        assertThat(httpTimer.count()).isEqualTo(1);
    }
}

Step 8: Test Percentiles

Test that percentiles are calculated correctly:

class PercentileTest {

    private SimpleMeterRegistry registry;

    @BeforeEach
    void setUp() {
        registry = new SimpleMeterRegistry();
    }

    @Test
    void testTimerPercentiles() {
        Timer timer = Timer.builder("test.timer")
            .publishPercentiles(0.5, 0.95, 0.99)
            .register(registry);

        // Record ordered samples
        long[] samples = {10, 50, 100, 200, 500, 1000};
        for (long sample : samples) {
            timer.record(sample, TimeUnit.MILLISECONDS);
        }

        // Get snapshot with percentiles
        Snapshot snapshot = timer.takeSnapshot();

        // Verify percentiles (note: exact values depend on implementation)
        assertThat(snapshot.percentileValues())
            .extracting(ValueAtPercentile::percentile)
            .contains(0.5, 0.95, 0.99);

        assertThat(snapshot.percentileValues()[0].value(TimeUnit.MILLISECONDS))
            .isBetween(50.0, 150.0); // P50 should be around median
    }

    @Test
    void testHistogramBuckets() {
        Timer timer = Timer.builder("test.timer")
            .serviceLevelObjectives(
                Duration.ofMillis(10).toNanos(),
                Duration.ofMillis(100).toNanos(),
                Duration.ofMillis(500).toNanos()
            )
            .register(registry);

        timer.record(5, TimeUnit.MILLISECONDS);
        timer.record(50, TimeUnit.MILLISECONDS);
        timer.record(200, TimeUnit.MILLISECONDS);
        timer.record(1000, TimeUnit.MILLISECONDS);

        List<Measurement> measurements = timer.measure();

        // Verify count
        long count = measurements.stream()
            .filter(m -> m.getStatistic() == Statistic.COUNT)
            .mapToLong(m -> (long) m.getValue())
            .sum();
        assertThat(count).isEqualTo(4);
    }
}

Examples

Example 1: Complete Service Test

class InvoiceServiceTest extends MetricsTestBase {

    private InvoiceService service;
    private InvoiceRepository repository;

    @BeforeEach
    void setUp() {
        super.setUp();
        repository = mock(InvoiceRepository.class);
        service = new InvoiceService(registry, repository);
    }

    @Test
    void testInvoiceGenerationRecordsMetrics() {
        List<Charge> charges = List.of(
            new Charge("SUP1", BigDecimal.valueOf(100)),
            new Charge("SUP2", BigDecimal.valueOf(200))
        );

        Invoice invoice = service.generateInvoice(charges);

        // Verify counter
        assertCounterValue("invoice.generated", 1);

        // Verify amount distribution
        assertDistributionSummary(
            "invoice.amount",
            1,
            300.0
        );

        // Verify generation time
        Timer generationTimer = registry.get("invoice.generation.duration")
            .timer();
        assertThat(generationTimer.count()).isEqualTo(1);
    }

    @Test
    void testInvoiceSentRecordsMetric() {
        Invoice invoice = service.generateInvoice(List.of());

        service.sendInvoice(invoice);

        assertCounterValue("invoice.sent", 1, Tag.of("channel", "email"));
    }
}

Example 2: Test Metric Cardinality

Ensure tags stay bounded:

class CardinalityTest extends MetricsTestBase {

    private ChargeService service;

    @BeforeEach
    void setUp() {
        super.setUp();
        service = new ChargeService(registry);
    }

    @Test
    void testChargeRejectionReasonsCategorized() {
        // Record many different rejection reasons
        service.rejectCharge("validation_error_supplier_id_null");
        service.rejectCharge("validation_error_amount_zero");
        service.rejectCharge("duplicate_within_24h");
        service.rejectCharge("supplier_inactive");

        // Should map to bounded categories
        Set<Tag> uniqueTags = registry.get("charge.rejected")
            .getMeterRegistry()
            .getMeters()
            .stream()
            .filter(m -> m.getId().getName().equals("charge.rejected"))
            .flatMap(m -> m.getId().getTags().stream())
            .collect(Collectors.toSet());

        // Only a few categories, not 4+ unique values
        assertThat(uniqueTags)
            .extracting(Tag::getValue)
            .containsOnly("validation", "duplicate", "supplier_issue");
    }

    @Test
    void testNoHighCardinalityTags() {
        // Try to create metrics with unbounded tags
        // Should be rejected or normalized by MeterFilter

        int initialMeterCount = registry.getMeters().size();

        // This would normally create unbounded metrics
        // But service should normalize...
        // (actual implementation depends on service)

        int finalMeterCount = registry.getMeters().size();

        assertThat(finalMeterCount)
            .isLessThan(initialMeterCount + 100); // Reasonable growth
    }
}

Requirements

• Spring Boot 2.1+ with spring-boot-starter-actuator
• Micrometer core library (included with Spring Boot)
• JUnit 5 with AssertJ for assertions
• Mockito for mocking repositories/services
• Java 11+

Anti-Patterns to Avoid

// ❌ Wrong: not clearing registry between tests
@Test
void test1() {
    registry.get("counter").counter().increment();
}

@Test
void test2() {
    // counter from test1 still exists!
    registry.get("counter").counter(); // Wrong count
}

// ✅ Right: clear registry in @BeforeEach
@BeforeEach
void setUp() {
    registry = new SimpleMeterRegistry(); // Fresh registry
}

// ❌ Wrong: ignoring registry failures
Counter counter = registry.get("metric").counter(); // May not exist!

// ✅ Right: assert meter exists with proper tags
Counter counter = registry.get("metric")
    .tag("expected", "value")
    .counter();

// ❌ Wrong: testing without proper registry injection
class ServiceTest {
    private Service service = new Service(); // Registry is null!
}

// ✅ Right: inject registry in constructor
class ServiceTest {
    private SimpleMeterRegistry registry;
    private Service service;

    @BeforeEach
    void setUp() {
        registry = new SimpleMeterRegistry();
        service = new Service(registry);
    }
}

See Also

• python-micrometer-business-metrics - Creating business metrics
• python-micrometer-cardinality-control - Managing metric cardinality
• python-micrometer-metrics-setup - Initial configuration