Health Checks

Metrics also provides you with a consistent, unified way of performing application health checks. A health check is basically a small self-test which your application performs to verify that a specific component or responsibility is performing correctly.

To create a health check, extend the HealthCheck class:

public class DatabaseHealthCheck extends HealthCheck {
    private final Database database;

    public DatabaseHealthCheck(Database database) {
        this.database = database;
    }

    @Override
    protected Result check() throws Exception {
        if (database.ping()) {
            return Result.healthy();
        }
        return Result.unhealthy("Can't ping database");
    }
}

In this example, we’ve created a health check for a Database class on which our application depends. Our fictitious Database class has a #ping() method, which executes a safe test query (e.g., SELECT 1). #ping() returns true if the query returns the expected result, returns false if it returns something else, and throws an exception if things have gone seriously wrong.

Our DatabaseHealthCheck, then, takes a Database instance and in its #check() method, attempts to ping the database. If it can, it returns a healthy result. If it can’t, it returns an unhealthy result.

To register a health check, either use a HealthCheckRegistry instance:

registry.register("database", new DatabaseHealthCheck(database));

You can also run the set of registered health checks:

for (Entry<String, Result> entry : registry.runHealthChecks().entrySet()) {
    if (entry.getValue().isHealthy()) {
        System.out.println(entry.getKey() + ": OK");
    } else {
        System.out.println(entry.getKey() + ": FAIL");
    }
}