java 50 lines · 7 steps

A custom replica health check in Spring

A Spring Actuator HealthIndicator that probes a read replica and reports UP, DEGRADED, or DOWN based on latency.

Explained by highlit

Spring Intermediate health-check actuator monitoring datasource latency

1package com.example.monitoring;
2 
3import org.springframework.boot.actuate.health.Health;
4import org.springframework.boot.actuate.health.HealthIndicator;
5import org.springframework.stereotype.Component;
6 
7import javax.sql.DataSource;
8import java.sql.Connection;
9import java.time.Duration;
10import java.time.Instant;
11 
12@Component("replicaDatabase")
13public class ReplicaDatabaseHealthIndicator implements HealthIndicator {
14 
15    private static final Duration SLOW_THRESHOLD = Duration.ofMillis(250);
16 
17    private final DataSource replicaDataSource;
18 
19    public ReplicaDatabaseHealthIndicator(DataSource replicaDataSource) {
20        this.replicaDataSource = replicaDataSource;
21    }
22 
23    @Override
24    public Health health() {
25        Instant start = Instant.now();
26        try (Connection connection = replicaDataSource.getConnection()) {
27            boolean valid = connection.isValid((int) SLOW_THRESHOLD.toSeconds() + 1);
28            Duration elapsed = Duration.between(start, Instant.now());
29 
30            if (!valid) {
31                return Health.down()
32                        .withDetail("reason", "replica connection failed validation")
33                        .build();
34            }
35 
36            Health.Builder builder = elapsed.compareTo(SLOW_THRESHOLD) > 0
37                    ? Health.status("DEGRADED")
38                    : Health.up();
39 
40            return builder
41                    .withDetail("latencyMs", elapsed.toMillis())
42                    .withDetail("catalog", connection.getCatalog())
43                    .build();
44        } catch (Exception ex) {
45            return Health.down(ex)
46                    .withDetail("reason", "unable to reach replica")
47                    .build();
48        }
49    }
50}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Implementing HealthIndicator lets you fold custom probes into Actuator's aggregated health endpoint.
2A health check can report graded status, not just up or down, by distinguishing slow from failed.
3Try-with-resources guarantees the probe connection is returned even when validation or the query throws.

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