How to Monitor API Performance: Latency, Errors, and SLAs

You can't improve what you don't measure. API performance monitoring tracks latency, error rates, throughput, and availability — the metrics that determine whether your API is meeting its commitments. Here's what to measure, how to measure it, and when to alert.

The Four Golden Signals

Google SRE's four golden signals apply directly to APIs:

1. Latency

What: Time from request received to response sent.

Measure percentiles, not averages:

Percentile	Meaning	Use
p50 (median)	Half of requests are faster	Typical experience
p95	95% of requests are faster	Most users' experience
p99	99% of requests are faster	Worst-case normal experience
p99.9	99.9% are faster	Tail latency

Why not averages? An average of 100ms hides that 1% of requests take 5 seconds. p99 catches that.

Targets:

Endpoint Type	p50	p95	p99
Simple read	<50ms	<200ms	<500ms
Database query	<100ms	<500ms	<1s
Search	<200ms	<1s	<2s
Write operation	<100ms	<500ms	<1s
External API call	<500ms	<2s	<5s

2. Error Rate

What: Percentage of requests returning errors (4xx/5xx).

Metric	Healthy	Warning	Critical
5xx rate	<0.1%	0.1-1%	>1%
4xx rate	<5%	5-10%	>10%
Total error rate	<1%	1-5%	>5%

Track by status code: Distinguish between client errors (4xx — usually the client's fault) and server errors (5xx — your fault).

3. Throughput

What: Requests per second (RPS) or requests per minute (RPM).

Track throughput to:

• Capacity plan (are you approaching limits?)
• Detect anomalies (sudden spike = attack? sudden drop = outage?)
• Correlate with latency (does latency increase with load?)

4. Saturation

What: How close your system is to capacity.

Resource	Metric	Alert Threshold
CPU	Utilization %	>80% sustained
Memory	Usage / available	>85%
Database connections	Active / max pool	>80%
Disk I/O	IOPS / max IOPS	>70%
Network	Bandwidth usage	>70%

SLA / SLO / SLI

SLI (Service Level Indicator)

A measurable metric: "99.5% of requests complete in under 500ms."

SLO (Service Level Objective)

Your internal target: "p99 latency < 500ms, error rate < 0.1%."

SLA (Service Level Agreement)

Your external commitment with consequences: "99.9% uptime or service credits."

Set SLOs tighter than SLAs. If your SLA promises 99.9% uptime, set your SLO at 99.95% so you have a buffer before breaching the SLA.

Uptime Targets

Uptime	Downtime/Year	Downtime/Month
99%	3.65 days	7.3 hours
99.9%	8.77 hours	43.8 minutes
99.95%	4.38 hours	21.9 minutes
99.99%	52.6 minutes	4.38 minutes
99.999%	5.26 minutes	26.3 seconds

Alerting Strategy

Alert on Symptoms, Not Causes

Good alerts (symptoms):

• p99 latency > 2s for 5 minutes
• Error rate > 1% for 3 minutes
• Throughput dropped 50% vs same hour last week

Bad alerts (causes):

• CPU > 80% (may not affect users)
• Memory > 90% (may be normal)
• Single health check failed (transient)

Alert Severity

Severity	Criteria	Response
P1 - Critical	Service down, data loss	Page on-call, all hands
P2 - High	Degraded performance, partial outage	Page on-call, investigate
P3 - Medium	Non-critical service degraded	Next business day
P4 - Low	Cosmetic, minor issue	Backlog

Monitoring Tools

Tool	Best For	Price
Datadog	Full observability	From $5/host/mo
Grafana + Prometheus	Self-hosted, open source	Free
Better Stack	Uptime + incidents	Free (10 monitors)
Checkly	Synthetic monitoring	Free (5 checks)
Sentry	Error tracking	Free (5K events)
PostHog	Product analytics	Free (1M events)

Dashboard Essentials

Every API monitoring dashboard should show:

1. Request volume — RPS over time (detect anomalies)
2. Latency percentiles — p50, p95, p99 over time
3. Error rate — 4xx and 5xx separately
4. Top errors — most frequent error codes/messages
5. Slowest endpoints — which endpoints need optimization
6. Uptime — current and 30-day availability

---

Monitoring your API? Explore monitoring tools and best practices on APIScout — comparisons, guides, and developer resources.