Logger

Medicus provides robust support for logging during health checks. You can configure custom logger functions to centralize and customize logging for your system.

Error Logger

The errorLogger is called whenever an error occurs during health check execution, whether in an automatic background check or during a manual check.

Custom Error Logger

The errorLogger receives the error and the name of the checker where it occurred, making it easy to integrate your logging solution.

import { Medicus } from 'medicus';

const medicus = new Medicus({
  // Custom error handling implementation
  errorLogger(error, checkerName) {
    console.log('Health check failed:', checkerName, error);
  }
});

Default Error Logger

If no custom error logger is provided, Medicus uses a default implementation based on console.error. This is sufficient for development environments but can be replaced with a more robust solution for production.

import { MedicusErrorLogger } from 'medicus';

export const defaultErrorLogger: MedicusErrorLogger = (
  error,
  checkerName
) => {
  console.error(`Health check failed for ${checkerName}`, error);
};

Unhealthy Logger

The unhealthyLogger is called whenever a health check completes with a non-healthy status (either DEGRADED or UNHEALTHY). This is useful for monitoring and alerting when your system is experiencing issues.

Custom Unhealthy Logger

The unhealthyLogger receives the complete health check result, including all service details and the overall status.

import { Medicus } from 'medicus';

const medicus = new Medicus({
  // Custom unhealthy status handler
  unhealthyLogger(details) {
    console.warn('System health degraded:', {
      status: details.status,
      services: details.services
    });
  }
});

When is it Called?

The unhealthyLogger is triggered after a health check completes if:

• The overall status is UNHEALTHY, or
• The overall status is DEGRADED

It is not called when the status is HEALTHY.

Use Cases

Common use cases for the unhealthy logger include:

• Alerting: Send notifications to your team when services are degraded
• Metrics: Record degraded/unhealthy states to monitoring systems
• Audit trails: Log system health issues for later analysis
• Auto-remediation: Trigger automated recovery actions

import { Medicus, HealthStatus } from 'medicus';

const medicus = new Medicus({
  unhealthyLogger(details) {
    // Send alert if system is completely unhealthy
    if (details.status === HealthStatus.UNHEALTHY) {
      console.error('CRITICAL: System is unhealthy', details);
      // alertingService.critical('System is unhealthy', details);
    }

    // Log degraded services for monitoring
    if (details.status === HealthStatus.DEGRADED) {
      console.warn('WARNING: System is degraded', details.services);
      // metricsService.recordDegradation(details.services);
    }
  }
});

Integrations

Pino

Medicus supports integration with Pino, a high-performance JSON logger that can be used for both error logging and unhealthy status logging.

Custom Integrations

Do you need to integrate another logging tool or a custom solution? Medicus is designed to be flexible, letting you add your own loggers with ease.

Have a great implementation you’d like to share? Contribute to Medicus by opening a pull request!