Free uptime and DevOps tools

• SLA uptime calculator

  Convert an uptime percentage such as 99.9% into the downtime it allows per day, week, month, quarter and year, or work out the uptime left after an outage. Includes a reference table for the common targets from 99% to 99.999%.

• Cron expression explainer

  Paste a cron schedule and read it in plain English, with the next five run times in UTC. Handles lists, ranges, steps, month and day names and the @daily family of macros, and explains the errors in an expression that is wrong.

• SSL certificate checker

  Check any public site's certificate live on port 443: issuer, expiry date and days left, the names it covers, the chain the server sends, and whether clients will trust it.

Why these tools

Each tool answers a question we found ourselves asking while building Uptime Dog. How much downtime does a 99.95% target allow in a quarter? Will 0 9 * * 1-5 run on a bank holiday Monday? (Yes: cron has no idea what a bank holiday is.) Why does a site load in the browser while curl complains about the certificate? They're free, need no account, and every page works as a plain web form, so they work without JavaScript, in a terminal browser, and behind strict corporate proxies.

From one-off checks to monitoring

A tool tells you how things are right now. Monitoring tells you when that changes. Each tool has a matching feature:

• The SLA calculator gives you a target; uptime monitoring with checks as often as every 20 seconds and reports for any period tells you whether you're meeting it, and your status page shows customers the record.
• The cron explainer tells you when a job should run; heartbeat monitoring tells you when it didn't.
• The SSL checker shows today's certificate; SSL monitoring warns you before it expires.

Privacy

The SLA calculator and cron explainer work entirely from what you type: nothing is sent anywhere else and nothing is stored. The SSL checker connects to the host you enter to read its certificate, and doesn't keep the result. None of the tools need cookies or an account.

If you'd like a tool added, email uptimedog@makingstuffs.com. Small, useful and accurate is the bar.