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Uptime monitoring, explained
Fourteen uptime questions, each answered in one sentence, then the math or the command behind it.
What is uptime monitoring
Uptime monitoring is a service outside your infrastructure that requests your URL on a fixed interval, records the status code and response time of every answer, and alerts you when a check fails, so you hear about an outage before your users do.
How much downtime is 99.9% uptime
99.9% uptime allows 8 hours 45 minutes 57.6 seconds of downtime per year, 43 minutes 49.8 seconds per average month, 10 minutes 4.8 seconds per week and 1 minute 26.4 seconds per day.
Uptime SLA
An uptime SLA is a provider's written promise that a service will be available for a set share of each month, such as 99.9% or 99.99%, with a service credit off that month's bill when it falls short.
Uptime vs downtime
Uptime is the share of a period in which your service answered correctly and downtime is the share in which it did not, so the two always add up to 100%: 99.9% uptime is 0.1% downtime, or 43.8 minutes in an average month.
Synthetic monitoring vs real user monitoring
Synthetic monitoring sends scripted requests to your app on a schedule, so it catches an outage at 3am with zero visitors, while real user monitoring records what actual visitors experience in their browsers, so it catches slow pages and errors no script ever hits.
What is heartbeat monitoring
Heartbeat monitoring turns the usual uptime check around: your job or worker calls a monitor URL every time it finishes a unit of work, and the monitor alerts you when those calls stop arriving.
What is cron monitoring
Cron monitoring tells you when a scheduled job did not run or did not succeed, by having each successful run leave a signal and alerting you when the expected signal is missing.
How to check if a website is down
Load the site from a second network, such as your phone on mobile data, and run curl against it from a terminal: if both fail it is down for everyone, and if only your connection fails the problem sits between you and the site.
How to get notified when a website is down
Point an uptime monitor at the URL and connect an alert channel you actually read: the monitor requests the site every few minutes and messages you the moment a check fails, then again when it recovers.
Why is my website down
Most outages come from six causes: a bad deploy, a crashed app or database, a full disk, an expired domain or SSL certificate, a broken DNS record, or your host having its own outage, and the error you see already narrows it to one or two.
MTTR
MTTR is the average time it takes to get a broken service working again: add up the downtime from every incident and divide by the number of incidents, so four outages of 12, 47, 6 and 31 minutes give an MTTR of 24 minutes.
Open source website monitoring
Open source website monitoring means a checker whose code you can read and run yourself: Uptime Kuma or Gatus if you want to host it, Upptime if you want GitHub Actions to run it, and Logdash if you want the code open but the monitor run for you.
Best self hosted uptime monitor
Uptime Kuma is the best self hosted uptime monitor for most people, Gatus is better if you want your checks in a YAML file in git, Upptime is better if you want no server at all, and Logdash cannot be self-hosted in production with one command yet.
Pingdom speed test alternative
PageSpeed Insights is the free Pingdom speed test alternative most people should use, WebPageTest is better for waterfalls and repeat runs, and GTmetrix works after a free signup; none of them watch your site between tests, which is what an uptime monitor like Logdash is for.