Application Performance Monitoring (APM) is the practice, and the class of tools, for measuring how an application behaves from the inside. An agent or library inside the app records transactions, database queries, external calls and exceptions, then reports them to a service for analysis.
Why it matters
APM answers the question that uptime checks cannot: the site is up, but why is this endpoint slow? It breaks a request into parts, showing for instance that 80% of time is spent in one SQL query. It also groups errors by stack trace and can link a slowdown to a specific deploy. Most APM products include traces, error tracking and service maps.
Example
An APM report for POST /api/orders:
p95 latency: 1.8 s
database: 1.3 s (3 queries, 1 repeated 40 times)
external API: 0.3 s
app code: 0.2 sThe repeated query suggests an N+1 pattern, which is cheaper to fix than adding servers.
Common mistakes
- Installing APM and never reviewing it until an outage.
- Assuming APM replaces external monitoring. If the whole host or network is down, the agent cannot report.
- Ignoring agent overhead on very hot code paths.
- Alerting on averages rather than high percentiles.
Related
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