Design and run a monitoring system for a website or web app

Monitoring and Alerting

Decide what to watch, what to alert on, and how to make sure the right person finds out when things break.

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When to use

• Setting up monitoring on a new site or service
• Defining SLOs (service level objectives) and error budgets
• Choosing which alerts page someone vs which go to a quiet channel
• Designing or fixing on-call rotation
• Diagnosing alert fatigue
• Filling monitoring gaps revealed by an incident
• Migrating monitoring vendors

When NOT to use

• Responding to an active incident (use incident-response)
• Writing the post-mortem (use after-action-report)
• Designing analytics dashboards for product metrics (use analytics-strategy)
• Performance optimization itself (use performance-optimization)

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Required inputs

• The system you're monitoring (URLs, services, dependencies)
• Existing monitoring tools (uptime, errors, logs, APM)
• Business hours and team timezone(s)
• Who is on-call or available for incidents
• Existing SLOs or success metrics, if any

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The framework: 4 layers

Monitoring works in layers. Skip a layer and you'll miss a class of problems.

Layer 1: Availability

Is the site up? The simplest, most important layer.

• HTTP checks from multiple regions (every 1-5 minutes)
• DNS resolution checks
• Certificate expiration checks
• Status code checks (alert on 5xx, not just timeout)

Threshold: any sustained downtime (more than 2 consecutive failed checks) pages.

Layer 2: Correctness

The site is up, but is it serving the right thing?

• Synthetic checks (a script that loads the homepage, clicks a button, validates expected text)
• Critical user journeys (signup, checkout, search)
• Content presence checks (homepage hasn't gone blank)
• API contract checks (response shape and key fields are present)

Threshold: failures of critical-path synthetics page. Non-critical page-level synthetics alert during business hours only.

Layer 3: Performance

The site is up and correct, but is it fast enough?

• Core Web Vitals (LCP, INP, CLS) from real users (RUM)
• Synthetic performance (Lighthouse, WebPageTest, custom)
• API response times (p50, p95, p99)
• Database query times for slow queries
• Dependency response times (third-party APIs)

Threshold: regressions from baseline (e.g., p95 doubled in 5 minutes). Don't alert on absolute thresholds without baselines.

Layer 4: Errors and anomalies

The site is up, correct, and fast for most, but errors are happening.

• Error rate (% of requests returning 5xx)
• Client-side error rate (uncaught JS exceptions)
• Log error volume (unexpected spikes)
• Anomaly detection (traffic falling off a cliff)
• Background job failures
• Queue depth

Threshold: rate-based, not count-based. "Error rate above 1% for 5 minutes" beats "more than 100 errors per minute."

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SLOs and error budgets

A Service Level Objective is the target for reliability. Common form: "99.9% of homepage requests succeed in under 2 seconds, measured over 30 days."

The components:

• The thing you're measuring (homepage requests)
• The success criterion (returns 2xx in under 2 seconds)
• The target (99.9% of them)
• The window (over 30 days)

The error budget is the inverse: 0.1% of requests can fail. If you've used the whole budget, slow down on risky changes.

Picking SLOs

Don't aim for 100%. Don't aim for "five nines" (99.999%) unless you really need it. Each nine costs an order of magnitude more.

SLO	Allowed downtime per month
99%	7 hours, 18 minutes
99.9%	43 minutes
99.95%	21 minutes
99.99%	4 minutes, 22 seconds
99.999%	26 seconds

For most marketing sites, 99.9% is plenty. For SaaS, 99.95% is reasonable. Anything higher needs significant infrastructure investment.

Using error budgets

When the budget is healthy, ship aggressively. When the budget is half-spent, slow down. When the budget is exhausted, freeze risky changes until reliability recovers.

This is what makes SLOs useful: they create a feedback loop between reliability and velocity.

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Workflow

Step 1: Inventory what's already monitored

What tools are in place? What checks exist? What dashboards? What alerts?

Many teams have a tangle of half-configured tools. The first job is the inventory.

Step 2: Map the system

Draw the architecture. Front-end, back-end, database, third-party APIs, queues, workers. Each box is a candidate for monitoring.

For each box, ask:

• What does "up" mean?
• What does "correct" mean?
• What does "fast" mean?
• What's the most common failure mode?

Step 3: Define the SLOs

Pick 3-5 SLOs. They should be:

• Tied to user-visible behavior (not internal metrics)
• Achievable with current infrastructure
• Measured automatically
• Reviewed at least quarterly

Step 4: Set up checks across the 4 layers

For each box, configure checks at each layer. Some boxes won't have all four; that's fine.

Box	Availability	Correctness	Performance	Errors
Homepage	HTTP check	Synthetic	LCP/INP	JS errors
Login API	HTTP check	Synthetic flow	p95 latency	5xx rate

Step 5: Decide what pages and what doesn't

Three tiers:

1. Page (wakes someone up): site down, critical flow broken, error rate spike, security incident.
2. Notify (during business hours): non-critical synthetic failure, performance regression, slow query, dependency degradation.
3. Log (no notification): anomalies for later review, low-priority warnings, info-level events.

Anything in tier 1 must be:

• Actionable (the on-call can do something about it)
• Important (it represents real impact)
• Rare (less than 1-2 per week is the goal)

If tier 1 alerts fire frequently, alert fatigue sets in. People stop responding.

Step 6: Configure routing

Where do alerts go?

• Tier 1: paging system (e.g., PagerDuty). Do not onboard onto Opsgenie: Atlassian ended sales in June 2025 and support ends April 2027. Direct to on-call.
• Tier 2: chat channel (Slack, Teams). Tagged with the area.
• Tier 3: dashboard or log only.

Each tier should have a documented escalation path. If the on-call doesn't ack within 5-15 minutes, escalate.

Step 7: Build dashboards

One dashboard per audience:

• Real-time ops dashboard: current health, recent alerts, error rates, throughput
• SLO dashboard: SLO status and error budget consumption
• Per-service dashboards: detail for individual services or pages
• Executive dashboard: uptime over weeks/months, key business metrics

Dashboards are different from alerts. Alerts say "look now." Dashboards say "here's what's happening."

Step 8: Run an alert audit

Every quarter, audit:

• Which alerts fired? Were they actionable?
• Which alerts didn't fire when they should have?
• Are any alerts noisy (more than once a week, low actionability)?
• Are runbooks up to date?
• Have SLOs been met? Any consistently breached?

Tune the system. Monitoring drifts without active maintenance.

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Failure patterns

Alert on cause, not symptom. "CPU is high" is a cause. "Users are slow" is a symptom. Alert on symptoms; investigate causes.

Alert without a runbook. If the on-call doesn't know what to do, the alert is useless. Every paging alert needs a runbook (even a one-line one).

No baselines for "normal." Alerting on "more than 100 errors per minute" sounds reasonable but a busy day might exceed that without anything being wrong. Use rate-based and anomaly-based alerts.

Single-region monitoring. Your monitoring service in the same region as your site means you'll miss regional outages and you'll get woken up when monitoring itself has issues.

Monitoring the monitoring. Or rather, not. If your alerting platform is down, who tells you? Most paging services offer their own status feeds. Subscribe.

Too many tiers of severity. P0/P1/P2/P3/P4 with different SLAs becomes a sorting exercise. Three tiers (page, notify, log) is plenty.

Synthetics that don't match reality. A synthetic that hits the homepage every minute tests "is the homepage up." It doesn't test "is the actual user flow working." Build synthetics for the journeys that matter.

Static thresholds that never get tuned. Traffic grows, behavior changes, thresholds set last year are wrong. Review thresholds quarterly.

On-call rotation with no handoffs. Each new on-call has to figure out the system. Document. Run weekly handoff meetings or async updates.

Pager fatigue. If on-call is paged more than once or twice a week, something is wrong. Audit the alerts. Reduce, tune, or fix the underlying issues.

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Output format

A monitoring plan includes:

• System map: what's being monitored
• SLOs: the 3-5 reliability targets
• Checks per layer: availability, correctness, performance, errors
• Alert tiering: what pages, what notifies, what logs
• Routing: where alerts go, escalation paths
• Dashboards: what audiences see
• Runbooks: linked from each paging alert
• Audit cadence: when this gets reviewed

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If required data is unavailable

This skill's output depends on data, measurements, or tool results it cannot generate on its own. When a required input, tool, or data source is unavailable or unverifiable, the sanctioned output is the deliverable with the gap stated: what was needed, what was actually obtained or verified, and which parts of the output are affected. Fabricating, estimating, or interpolating a required number to complete the deliverable is never sanctioned. A stated gap is a complete answer.

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Reference files

• references/slo-design-guide.md: Detailed walkthrough of writing SLOs, error budget policies, and common SLO mistakes for web services.