If you’ve been hearing terms like “SLI,” “SLA,” or “error budgets” tossed around in tech team meetings and you’ve been nodding along while secretly Googling them under the table—don’t worry. You’re not alone. Welcome to SRE 101, your friendly intro to setting error budgets and SLIs/SLAs for your services. Whether you’re just starting with Site Reliability Engineering (SRE) or trying to make your systems more reliable without burning out your dev team, this guide breaks it all down in plain English.
What Is SRE, Anyway?
Let’s kick it off with the basics. Site Reliability Engineering (SRE) was born at Google to bridge the gap between software development and operations. Think of SRE as applying software engineering principles to infrastructure and operations problems. The goal? Make systems scalable, highly available, and—most importantly—reliable.
Now, in practice, that means SRE teams set clear expectations for how systems should perform. And that’s where the magic trio comes in: SLIs (Service Level Indicators), SLAs (Service Level Agreements), and Error Budgets.
If you’re still with me, you’re already diving into SRE 101: setting error budgets and SLIs/SLAs for your services.
Understanding SLIs, SLAs, and SLOs
To explain error budgets, we need to define their friends first.
SLI (Service Level Indicator) is a metric that tells you how well your service is doing. For example, availability, latency, or error rate.
SLO (Service Level Objective) is your goal for that metric. Say you want 99.9% availability—that’s your SLO.
SLA (Service Level Agreement) is the commitment you make to your users (usually tied to money or legal stuff). If you say your service will be up 99.9% of the time and it’s not, you might owe someone a refund.
Let’s see that in action:
- SLI: The percentage of successful requests in the last 30 days.
- SLO: 99.9% of those requests should succeed.
- SLA: If availability falls below 99.9%, the customer gets a credit.
What’s an Error Budget?
Here’s where things get interesting. Instead of aiming for perfect reliability (spoiler: it doesn’t exist), you work within an error budget.
Let’s say your SLO is 99.9% uptime per month. That gives you 0.1% of the time where things can go wrong. That 0.1%—around 43 minutes per month—is your error budget.
Why is that useful? Because it gives dev teams freedom to deploy and experiment, without feeling like they’re walking on eggshells. If you stay within the budget, you can release more. If you burn through the budget too fast, you pause releases and fix reliability.
It’s a smart, balanced way to manage innovation vs. stability.
Why Error Budgets Matter
If you only take away one thing from this SRE 101 article, let it be this: error budgets make reliability a measurable, actionable target.
Without an error budget, you either:
- Release constantly and risk breaking things.
- Never release because you’re scared of breaking things.
Neither of those ends well. Error budgets put guardrails in place. They make reliability a team responsibility, not just the ops team’s problem.
How to Set Good SLIs
Here’s the first big decision: what to measure. Not every metric is an SLI. You want something that directly impacts users.
Some great SLIs include:
- Availability: How often is the service up?
- Latency: How quickly does the service respond?
- Throughput: How many requests per second?
- Error rate: How many requests fail?
Pro tip: Focus on user-facing experiences. If your backend service is fine but users are getting 500 errors, your SLI needs fixing.
Also, be sure your metrics are measurable and queryable—if it’s too hard to get accurate numbers, you’ll stop using them.
Setting Realistic SLAs (and SLOs)
This part takes some negotiation.
SLAs are often set by business teams, while SLOs are internal goals. You might aim for 99.95% as an internal SLO but commit to 99.9% in your SLA.
Some tips:
- Don’t aim for 100%. It’s not just hard—it’s expensive.
- Look at historical data before setting targets.
- Adjust SLAs for different tiers of service. Your free plan might have 99.5% uptime, while your enterprise plan gets 99.99%.
Setting realistic SLOs and SLAs is a key lesson in SRE 101: setting error budgets and SLIs/SLAs for your services.
Calculating and Tracking Your Error Budget
Here’s a simple formula:
Error Budget = (1 – SLO Target) × Total Time
If your SLO is 99.9% over a 30-day month:
- 30 days = 43,200 minutes
- 0.1% = 43.2 minutes
- So your error budget is 43 minutes and 12 seconds
Then, you track outages and downtime:
- If you had one incident that lasted 15 minutes, you still have 28 minutes to burn.
- If you hit 44 minutes of downtime, stop releases until you fix reliability.
Many teams use monitoring tools like Datadog, Prometheus, or New Relic to track SLIs in real time.
What to Do When You Burn Your Budget
It happens. You blow through your budget before the month’s over. Now what?
- Freeze deployments (unless it’s a rollback or reliability fix)
- Postmortem the incident and learn from it
- Prioritize reliability in the next sprint
- Update runbooks and alerting thresholds
Error budgets aren’t just limits—they’re feedback loops. Burning your budget means something’s off, and your systems or processes need attention.
Real-World Example: Rolling Out a New Feature
Let’s say your dev team wants to launch a new checkout system. It’s faster and fancier. You go live, but suddenly you see a spike in failed transactions.
Your SLI (successful transactions) drops below your SLO of 99.9%. You’re at 99.7% after just two days. That means you’ve already used 57% of your monthly error budget.
Time to make a call:
- Roll back the new system?
- Patch the bug immediately?
- Stop all unrelated releases?
These decisions get easier with error budgets in place. You’re no longer debating feelings—you’re reacting to facts.
Avoid These Mistakes
Even seasoned teams get tripped up. Here are a few common mistakes:
- Too many SLIs: Focus on what truly impacts the user.
- Unrealistic SLAs: If you never hit them, they’re useless.
- No enforcement: SLAs without consequences are just vanity.
- Lack of visibility: Make error budgets visible to devs, product managers, and leadership.
Integrating SLIs and Error Budgets into Your Workflow
Want to make this all stick? Here’s how:
- Add SLIs to your dashboards
- Review error budgets in retros
- Include SLOs in OKRs
- Create alerts when error budgets near depletion
Make it part of the culture. Developers should feel empowered by these tools, not micromanaged.
Scaling Error Budgets Across Teams
In larger orgs, you might have dozens of services. Each one might need its own error budget and SLO.
Create a standard framework:
- Use templated SLIs
- Define escalation policies
- Automate tracking and reporting
This avoids chaos and ensures consistency.
FAQs about SRE 101: Setting Error Budgets and SLIs/SLAs for Your Services
1. What’s the difference between SLI, SLO, and SLA?
SLI is a measurement, SLO is a goal, SLA is a promise to users.
2. How do I know what my error budget should be?
Subtract your SLO target from 100%. Multiply by time. That’s your error budget.
3. What happens if I go over my error budget?
You should stop releases, fix reliability, and investigate the cause.
4. Can I have multiple SLIs?
Yes, but focus on a few key user-facing metrics to avoid noise.
5. Should I aim for 100% uptime?
No. It’s not realistic or cost-effective. Even Google doesn’t aim for 100%.
