Active Probing and Its Limits

Why this matters: probing is the first design anyone reaches for, it is genuinely useful, and understanding exactly where it stops working is what motivates the real solution.

Key takeaway

To verify reachability, act as clients. Deploy a prober — a service that periodically sends requests from various global vantage points — allowing you to monitor reachability from different geographic locations.

The initial design

Each prober periodically requests the service and records whether it succeeded. If European probers fail while others succeed, you have located a regional connectivity problem — from outside, which is exactly the vantage point the previous lesson showed was necessary.

This is active probing: you generate the traffic and observe the result.

Where it runs out

External probers face two significant limitations.

Incomplete coverage

The internet consists of over 100,000 autonomous systems. Placing probes in every one is cost-prohibitive and logistically difficult due to varying regulations and maintenance requirements.

Autonomous systems on the internet   > 100,000
Realistic prober deployments         tens to hundreds of vantage points
Coverage                             a tiny fraction of the paths users take

Synthetic traffic

Probes generate synthetic traffic, which may not accurately represent real user behavior or complex interaction patterns.

	A prober's request	A real user's session
Path	One endpoint, fixed	Many endpoints, varied
Client	A controlled agent	Every browser, OS, and device version
Network	A data center connection	Mobile, congested Wi-Fi, captive portals
Behavior	A simple, repeated check	Complex interaction patterns

A probe confirming that a health endpoint responds tells you little about whether a user on a mobile network behind a corporate proxy can complete a checkout.

Key takeaway

Active probing puts you outside the failure domain, which is the right instinct. Its ceiling is coverage — a few hundred vantage points cannot represent 100,000 autonomous systems, and synthetic traffic is not real traffic.

Interview signal by level

Level	What a strong answer sounds like
L4	"We'd ping the service from outside to check it's up."
L5	Designs it properly: "probers at multiple global vantage points, so we can tell a regional connectivity problem from a global outage."
Staff+	Bounds it honestly: "probing gives presence, not coverage — a few hundred vantage points against 100,000 autonomous systems, and the failures that matter are in networks we don't have a probe in. It's also synthetic, so it says nothing about a real user on a mobile network. Keep it, but it can't be the only mechanism."

Next: getting the signal from real users.