Traçage, journalisation et métriques

You've learned about logs, metrics, and traces individually. Now, let's compare them to understand their unique roles and how they work together to give you a complete picture of your system.

• Logs: Detailed events.
• Metrics: Aggregated numbers.
• Traces: End-to-end request paths.

Each serves a distinct purpose, but their true power emerges when combined.

Logs are like a system's diary entries. They capture discrete events or messages at specific points in time. When you need to understand what happened at a precise moment, logs are your go-to.

They are excellent for:

• Debugging specific errors.
• Auditing user actions.
• Providing rich context for individual occurrences.

Here's a simple example of a structured log entry. Notice how it contains specific details about an event, like a user login.

Metrics provide an aggregated, numerical view of your system's health and performance over time. Think of them as vital signs: CPU usage, request rates, error counts.

They are best for:

• Monitoring overall system health.
• Identifying trends and anomalies.
• Triggering alerts when thresholds are breached.

Metrics answer "how much" or "how often".

This conceptual code snippet shows how a counter metric might track login attempts. Instead of individual events, it focuses on the total count.

Traces reveal the end-to-end path of a single request or transaction as it flows through a distributed system. They show causality and latency across multiple services.

Traces are crucial for:

• Understanding service dependencies.
• Pinpointing performance bottlenecks in microservices.
• Debugging latency issues across an entire user journey.

They answer "why is this slow?" by showing the sequence of operations.

Unlike logs (discrete events) or metrics (aggregates), traces provide a holistic view of a single operation. They connect the dots across different services using Trace IDs and Span IDs, showing the parent-child relationships between operations.

This allows you to visualize the entire execution path, from user request to database query, even if it crosses dozens of services.

Combining logs and traces provides powerful insights. You can embed Trace IDs and Span IDs directly into your log messages.

This means:

• From a trace, you can jump to specific log messages for detailed context.
• From an error log, you can find the full trace of that problematic request.

Logs explain what happened within a span; traces show where and when in the overall flow.

Metrics can be derived from trace data (e.g., average latency of a service). When a metric alert fires (e.g., "Service X latency is high"), traces help you drill down.

You can:

• See which specific requests contributed to the high latency.
• Identify the exact span or service causing the slowdown.

Metrics tell you there's a problem; traces help you find the problem's location.

You're investigating an intermittent error where a specific user's request fails after interacting with three different microservices. Which observability signal would be MOST effective for understanding the exact sequence of operations and where the failure occurred?

Logs, metrics, and traces are distinct but interconnected signals. Logs provide detail, metrics offer aggregation, and traces map causality across services. By understanding their individual strengths and using them together, you build a comprehensive and powerful observability strategy.

This synergy is key to quickly identifying, diagnosing, and resolving issues in complex modern applications.