Explain like I'm five · SRE

What is an SLO?

A Service Level Objective, told as a story about the line a park draws for itself and promises to stay inside.

isn't this good enough?no, we need to do better!the control room and the mechanic argue over this every time

The control room and the mechanic argue over this every time.Everyone's idea of fine is different, so it always ends in a fight.

90% is plentyhas to be 99%needs to be 99.99%!without a shared line, everyone's idea of fine is different

Why is this hard?Without a shared line, everyone's idea of fine is different.

today's record (SLI)OUR OWN TARGETtarget: 99.9% a monthrespond within 1 secondfree to move within itlook at today's numbers, then promise a target in advancestay inside it freely — cross it, and everyone pauses

Look at today's numbers, then promise a target in advance.Stay inside it freely — cross it, and everyone pauses.

Set the target from SLI numberstoday's record is the baseline
Too tight and there's no room to breathethe slack is the error budget
Cross it, and fixing comes firstbefore any new feature
Review it once in a whilethe target can change too
target line: 99.9%crossed the linethe real numbers wander above and below the target line

The real numbers wander above and below the target line.When a stretch crosses the line, you can see it right away.

WHAT GUESTS ACTUALLY NEEDtarget: 99.9%cost: reasonablecostTOO HIGH A TARGETtarget: 99.999%cost: never stopscostpromise only as much as guests really need

There's no such thing as perfect uptime. Instead, promise in advance.Set the target too high, and the cost never stops growing.

In one breath

An SLO is the target line a team sets for itself by looking at today's record (SLI). Stay inside it and move freely; cross it, and fixing comes before any new feature.

A target the team sets for itself, distinct from the formal SLA made with customers — usually stricter than the SLA. The slack left over is the error budget, and a burn rate tracks how fast it's being spent.

When grown-ups say it

SLO
The promise line a team sets for itself. Cross it, and everyone pauses.
SLI
The number actually measured today. This number is what sets the target. → today's record board
Error budget
The slack left over by the target. Spend it on bold new tries. → the bundle of allowed-failure tickets
Setting the target
Starts from today's record. Set it too high, and the cost never stops growing.
SLO vs SLA
What's set internally versus promised to the customer. The contract is usually looser than this. → the promise written up for the customer
Review cadence
Looked at again every so often. Because both guests and the system change.
Burn rate
How fast the slack gets spent. A fast drop triggers a warning early.
Golden signals
The four gauges that measure today's record. Latency, traffic, errors, saturation. → the four gauges on the control-room wall
Next story
The Contract We Sign With a Customer →
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