Observability (Logs, Metrics, Traces)

Also Known As

TL;DR

Explanation

Observability (as opposed to monitoring) is the degree to which a system's internal state can be inferred from its outputs. The three pillars: Logs (timestamped event records — structured JSON logs are queryable), Metrics (numeric time-series data — request rate, error rate, latency, resource usage), and Traces (end-to-end request journeys across services, correlated by trace ID). Tools: Prometheus + Grafana (metrics), ELK / Loki (logs), Jaeger / Zipkin / OpenTelemetry (traces). PHP applications emit structured logs via Monolog, expose metrics via /metrics endpoints, and propagate trace context via OpenTelemetry SDK.

Common Misconception

Why It Matters

Common Mistakes

• Logging everything at DEBUG level in production — log volume makes finding real issues impossible.
• Metrics without context — a spike in CPU is meaningless without correlated request rate and error rate.
• Structured logging not implemented — log parsing tools cannot extract fields from unstructured log lines.
• No correlation between metrics, logs, and traces — cannot connect a metric spike to its cause in logs.

Code Examples

// Unstructured log — cannot be parsed or searched reliably:
error_log('User 42 failed to login from 192.168.1.1 at ' . date('Y-m-d H:i:s'));

// Structured JSON log — searchable and filterable:
error_log(json_encode([
    'event' => 'login_failed', 'user_id' => 42,
    'ip' => '192.168.1.1', 'timestamp' => date('c')
]));

// Structured logging with context
$this->logger->info('Order placed', [
    'order_id'    => $order->id,
    'user_id'     => $order->userId,
    'total_cents' => $order->total,
    'duration_ms' => $elapsed,
]);

// Metric increment (Prometheus via StatsD)
$this->metrics->increment('orders.placed', ['status' => 'success']);
$this->metrics->histogram('orders.checkout_duration_ms', $elapsed);

// Trace span (OpenTelemetry)
$span = $tracer->spanBuilder('checkout')->startSpan();
try { /* work */ } finally { $span->end(); }

References

• ↗ https://opentelemetry.io/docs/
• ↗ https://prometheus.io/docs/introduction/overview/

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